ECMAScript® 2021 Language Specification
About this Specification
The document at https://tc39.es/ecma262/ is the most accurate and up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any finished proposals (those that have reached Stage 4 in the proposal process and thus are implemented in several implementations and will be in the next practical revision) since that snapshot was taken.
Contributing to this Specification
This specification is developed on GitHub with the help of the ECMAScript community. There are a number of ways to contribute to the development of this specification:
- GitHub Repository: https://github.com/tc39/ecma262
- Issues: All Issues, File a New Issue
- Pull Requests: All Pull Requests, Create a New Pull Request
- Test Suite: Test262
- Editors:
-
Community:
- Discourse: https://es.discourse.group
- IRC: #tc39 on freenode
- Mailing
List Archives: https://esdiscuss.org/
Refer to the
Introduction
This Ecma Standard defines the ECMAScript 2021 Language. It is the twelfth edition of the ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world's most widely used general-purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications.
ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.
After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. However, that work was not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security. The fifth edition was adopted by the Ecma General Assembly of December 2009.
The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June 2011.
Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. The goals for this edition included providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplemental characters in strings and regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation for regular, incremental language and library enhancements. The sixth edition was adopted by the General Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. ES2016 also included support for a new exponentiation operator and adds a new method to Array.prototype called includes.
ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and library enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous programming experience by providing syntax for promise-returning functions. Shared Memory and Atomics introduce a new Object.values, Object.entries, and Object.getOwnPropertyDescriptors.
ECMAScript 2018 introduced support for asynchronous iteration via the AsyncIterator protocol and async generators. It also included four new regular expression features: the dotAll flag, named capture groups, Unicode property escapes, and look-behind assertions. Lastly it included object rest and spread properties.
ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on Array.prototype for flattening arrays, Object.fromEntries for directly turning the return value of Object.entries into a new Object, and trimStart and trimEnd on String.prototype as better-named alternatives to the widely implemented but non-standard String.prototype.trimLeft and trimRight built-ins. In addition, it included a few minor updates to syntax and semantics. Updated syntax included optional catch binding parameters and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON. Other updates included requiring that Array.prototype.sort be a stable sort, requiring that JSON.stringify return well-formed UTF-8 regardless of input, and clarifying Function.prototype.toString by requiring that it either return the corresponding original source text or a standard placeholder.
ECMAScript 2020, the 11th edition, introduces the matchAll method for Strings, to produce an iterator for all match objects generated by a global regular expression; import(), a syntax to asynchronously import Modules with a dynamic specifier; BigInt, a new number primitive for working with arbitrary precision integers; Promise.allSettled, a new Promise combinator that does not short-circuit; globalThis, a universal way to access the global this value; dedicated export * as ns from 'module' syntax for use within modules; increased standardization of for-in enumeration order; import.meta, a null or undefined): nullish coalescing, a value selection operator; and optional chaining, a property access and function invocation operator that short-circuits if the value to access/invoke is nullish.
This specification, the 12th edition, introduces the replaceAll method for Strings; Promise.any, a Promise combinator that short-circuits when an input value is fulfilled; AggregateError, a new Error type to represent multiple errors at once; logical assignment operators (??=, &&=, ||=); WeakRef, for referring to a target object without preserving it from garbage collection, and FinalizationRegistry, to manage registration and unregistration of cleanup operations performed when target objects are garbage collected; separators for numeric literals (1_000); and Array.prototype.sort was made more precise, reducing the amount of cases that result in an
Dozens of individuals representing many organizations have made very significant contributions within Ecma TC39 to the development of this edition and to the prior editions. In addition, a vibrant community has emerged supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug reports, performed implementation experiments, contributed test suites, and educated the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and organization who has contributed to this effort.
Allen Wirfs-Brock
ECMA-262, Project Editor, 6th Edition
Brian Terlson
ECMA-262, Project Editor, 7th through 10th Editions
Jordan Harband
ECMA-262, Project Editor, 10th through 12th Editions
1 Scope
This Standard defines the ECMAScript 2021 general-purpose programming language.
2 Conformance
A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.
A conforming implementation of ECMAScript must interpret source text input in conformance with the latest version of the Unicode Standard and ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an application programming interface (API) that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.
A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may provide properties not described in this specification, and values for those properties, for objects that are described in this specification.
A conforming implementation of ECMAScript may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of any “future reserved words” noted in subclause
A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden Extension in subclause
A conforming implementation of ECMAScript must not redefine any facilities that are not
A conforming implementation of ECMAScript may choose to implement or not implement Normative Optional subclauses. If any Normative Optional behaviour is implemented, all of the behaviour in the containing Normative Optional clause must be implemented. A Normative Optional clause is denoted in this specification with the words "Normative Optional" in a coloured box, as shown below.
2.1 Example Clause Heading
Example clause contents.
3 Normative References
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 10646 Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor
ECMA-402, ECMAScript 2015 Internationalization API Specification.
https://ecma-international.org/publications/standards/Ecma-402.htm
ECMA-404, The JSON Data Interchange Format.
https://ecma-international.org/publications/standards/Ecma-404.htm
4 Overview
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a
ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general-purpose programming language. A scripting language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a
ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript is now used to provide core scripting capabilities for a variety of
ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.
4.1 Web Scripting
A web browser provides an ECMAScript
A web server provides a different
Each Web browser and server that supports ECMAScript supplies its own
4.2 Hosts and Implementations
To aid integrating ECMAScript into
An implementation is an external source that further defines facilities enumerated in Annex
An implementation-defined facility is one that defers its definition to an external source without further qualification. This specification does not make any recommendations for particular behaviours, and conforming implementations are free to choose any behaviour within the constraints put forth by this specification.
An implementation-approximated facility is one that defers its definition to an external source while recommending an ideal behaviour. While conforming implementations are free to choose any behaviour within the constraints put forth by this specification, they are encouraged to strive to approximate the ideal. Some mathematical operations, such as Math.exp
A host is an external source that further defines facilities listed in Annex
A host hook is an abstract operation that is defined in whole or in part by an external source. All
A host-defined facility is one that defers its definition to an external source without further qualification and is listed in Annex
A host environment is a particular choice of definition for all
This specification follows the editorial convention of always using the most specific term. For example, if a facility is
Both hosts and implementations may interface with this specification via the language types, specification types,
4.3 ECMAScript Overview
The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.
ECMAScript is object-based: basic language and
ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include the Object, Function, Boolean, Symbol, and various Error objects; objects that represent and manipulate numeric values including Math, Number, and Date; the text processing objects String and RegExp; objects that are indexed collections of values including Array and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections including Map and Set objects; objects supporting structured data including the JSON object, ArrayBuffer, SharedArrayBuffer, and DataView; objects supporting control abstractions including generator functions and Promise objects; and reflection objects including Proxy and Reflect.
ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.
Large ECMAScript programs are supported by modules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.
4.3.1 Objects
Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial values to their properties. Each new Date(2009, 11) creates a new Date object. Invoking a Date() produces a string representation of the current date and time rather than an object.
Every object created by a
In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, while structure, behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:
CF is a new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named
Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.
Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern of
4.3.2 The Strict Variant of ECMAScript
The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript source text units as described in
In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode source text units into a single composite program.
4.4 Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
4.4.1 implementation-approximated
an
4.4.2 implementation-defined
an
4.4.3 host-defined
same as
Editorially, see clause
4.4.4 type
set of data values as defined in clause
4.4.5 primitive value
member of one of the types Undefined, Null, Boolean, Number, BigInt, Symbol, or String as defined in clause
A primitive value is a datum that is represented directly at the lowest level of the language implementation.
4.4.6 object
member of the type Object
An object is a collection of properties and has a single prototype object. The prototype may be the null value.
4.4.7 constructor
The value of a
4.4.8 prototype
object that provides shared properties for other objects
When a constructor.prototype, and properties added to an object's prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the Object.create built-in function.
4.4.9 ordinary object
object that has the default behaviour for the essential internal methods that must be supported by all objects
4.4.10 exotic object
object that does not have the default behaviour for one or more of the essential internal methods
Any object that is not an
4.4.11 standard object
object whose semantics are defined by this specification
4.4.12 built-in object
object specified and supplied by an ECMAScript implementation
Standard built-in objects are defined in this specification. An ECMAScript implementation may specify and supply additional kinds of built-in objects. A built-in
4.4.13 undefined value
primitive value used when a variable has not been assigned a value
4.4.14 Undefined type
type whose sole value is the
4.4.15 null value
primitive value that represents the intentional absence of any object value
4.4.16 Null type
type whose sole value is the
4.4.17 Boolean value
member of the Boolean type
There are only two Boolean values,
4.4.18 Boolean type
type consisting of the primitive values
4.4.19 Boolean object
member of the Object type that is an instance of the standard built-in Boolean
A Boolean object is created by using the Boolean new expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.
4.4.20 String value
primitive value that is a finite ordered sequence of zero or more 16-bit unsigned
A String value is a member of the String type. Each
4.4.21 String type
set of all possible String values
4.4.22 String object
member of the Object type that is an instance of the standard built-in String
A String object is created by using the String new expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the String
4.4.23 Number value
primitive value corresponding to a double-precision 64-bit binary format
A
4.4.24 Number type
set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and negative infinity
4.4.25 Number object
member of the Object type that is an instance of the standard built-in Number
A Number object is created by using the Number new expression, supplying a
4.4.26 Infinity
4.4.27 NaN
4.4.28 BigInt value
primitive value corresponding to an arbitrary-precision
4.4.29 BigInt type
set of all possible BigInt values
4.4.30 BigInt object
member of the Object type that is an instance of the standard built-in BigInt
4.4.31 Symbol value
primitive value that represents a unique, non-String Object property key
4.4.32 Symbol type
set of all possible Symbol values
4.4.33 Symbol object
member of the Object type that is an instance of the standard built-in Symbol
4.4.34 function
member of the Object type that may be invoked as a subroutine
In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function's code may or may not be written in ECMAScript.
4.4.35 built-in function
built-in object that is a function
Examples of built-in functions include parseInt and Math.exp. A
4.4.36 property
part of an object that associates a key (either a String value or a Symbol value) and a value
Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a
4.4.37 method
function that is the value of a property
When a function is called as a method of an object, the object is passed to the function as its
4.4.38 built-in method
method that is a built-in function
Standard built-in methods are defined in this specification. A
4.4.39 attribute
internal value that defines some characteristic of a property
4.4.40 own property
property that is directly contained by its object
4.4.41 inherited property
property of an object that is not an own property but is a property (either own or inherited) of the object's prototype
4.5 Organization of This Specification
The remainder of this specification is organized as follows:
Clause
Clauses
Clauses
Clauses
Clause
5 Notational Conventions
5.1 Syntactic and Lexical Grammars
5.1.1 Context-Free Grammars
A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.
A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.
5.1.2 The Lexical and RegExp Grammars
A lexical grammar for ECMAScript is given in clause
Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (/*…*/ regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a
A RegExp grammar for ECMAScript is given in
Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.
5.1.3 The Numeric String Grammar
Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical grammar having to do with numeric literals and has as its terminal symbols
Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.
5.1.4 The Syntactic Grammar
The syntactic grammar for ECMAScript is given in clauses
When a stream of code points is to be parsed as an ECMAScript
When a parse is successful, it constructs a parse tree, a rooted tree structure in which each node is a Parse Node. Each Parse Node is an instance of a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The root node of the parse tree, representing the whole of the source text, is an instance of the parse's
New Parse Nodes are instantiated for each invocation of the parser and never reused between parses even of identical source text. Parse Nodes are considered the same Parse Node if and only if they represent the same span of source text, are instances of the same grammar symbol, and resulted from the same parser invocation.
Parsing the same String multiple times will lead to different Parse Nodes. For example, consider:
let str = "1 + 1;";
eval(str);
eval(str);Each call to eval converts the value of str into an ECMAScript source text and performs an independent parse that creates its own separate tree of Parse Nodes. The trees are distinct even though each parse operates upon a source text that was derived from the same String value.
Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.
The syntactic grammar as presented in clauses
In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized productions that permit token sequences that do not form a valid ECMAScript
5.1.5 Grammar Notation
Terminal symbols are shown in fixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a script exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin range, as opposed to any similar-looking code points from other Unicode ranges. A code point in a terminal symbol cannot be expressed by a \
Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:
states that the nonterminal while, followed by a left parenthesis token, followed by an
states that an
The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:
is a convenient abbreviation for:
and that:
is a convenient abbreviation for:
which in turn is an abbreviation for:
so, in this example, the nonterminal
A production may be parameterized by a subscripted annotation of the form “[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:
is a convenient abbreviation for:
and that:
is an abbreviation for:
Multiple parameters produce a combinatory number of productions, not all of which are necessarily referenced in a complete grammar.
References to nonterminals on the right-hand side of a production can also be parameterized. For example:
is equivalent to saying:
and:
is equivalent to:
A nonterminal reference may have both a parameter list and an “opt” suffix. For example:
is an abbreviation for:
Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production's left-hand side symbol. For example:
is an abbreviation for:
If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production's nonterminal symbol. This means that:
is an abbreviation for:
and that:
is an abbreviation for:
When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:
which is merely a convenient abbreviation for:
If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.
If the phrase “[lookahead = seq]” appears in the right-hand side of a production, it indicates that the production may only be used if the token sequence seq is a prefix of the immediately following input token sequence. Similarly, “[lookahead ∈ set]”, where set is a finite nonempty set of token sequences, indicates that the production may only be used if some element of set is a prefix of the immediately following token sequence. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all token sequences to which that nonterminal could expand. It is considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.
These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing production may only be used if seq is not a prefix of the immediately following input token sequence, and “[lookahead ∉ set]” indicates that the production may only be used if no element of set is a prefix of the immediately following token sequence.
As an example, given the definitions:
the definition:
matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.
Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously identify the immediately following token sequence because determining later tokens requires knowing which lexical
If the phrase “[no
indicates that the production may not be used if a throw token and the
Unless the presence of a
When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-code point token, it represents the sequence of code points that would make up such a token.
The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:
means that the nonterminal
Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:
5.2 Algorithm Conventions
The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.
Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names which may be used within the algorithm steps to reference the argument passed in that position. Optional parameters are denoted with surrounding brackets ([ , name ]) and are no different from required parameters within algorithm steps. A rest parameter may appear at the end of a parameter list, denoted with leading ellipsis (, ...name). The rest parameter captures all of the arguments provided following the required and optional parameters into a
Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:
- Top-level step
- Substep.
- Substep.
- Subsubstep.
- Subsubsubstep
- Subsubsubsubstep
- Subsubsubsubsubstep
- Subsubsubsubstep
- Subsubsubstep
- Subsubstep.
A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.
Algorithm steps may declare named aliases for any value using the form “Let x be someValue”. These aliases are reference-like in that both x and someValue refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x be a copy of someValue” creates a shallow copy of someValue.
Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior to the alias's declaration. Aliases may be modified using the form “Set x to someOtherValue”.
5.2.1 Abstract Operations
In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as OperationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.OperationName(arg1, arg2).
5.2.2 Syntax-Directed Operations
A syntax-directed operation is a named operation whose definition consists of algorithms, each of which is associated with one or more productions from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and nonterminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative definition that is matched when parsing the source text. The source text matched by a grammar production is the portion of the source text that starts at the beginning of the first terminal that participated in the match and ends at the end of the last terminal that participated in the match.
When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.
Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the conventions on steps
- Let status be SyntaxDirectedOperation of
SomeNonTerminal . - Let someParseNode be the parse of some source text.
- Perform SyntaxDirectedOperation of someParseNode.
- Perform SyntaxDirectedOperation of someParseNode passing
"value" as the argument.
Unless explicitly specified otherwise, all chain productions have an implicit definition for every operation that might be applied to that production's left-hand side nonterminal. The implicit definition simply reapplies the same operation with the same parameters, if any, to the
but the Evaluation operation does not associate an algorithm with that production. In that case, the Evaluation operation implicitly includes an association of the form:
Runtime Semantics: Evaluation
- Return the result of evaluating
StatementList .
5.2.3 Runtime Semantics
Algorithms which specify semantics that must be called at runtime are called runtime semantics. Runtime semantics are defined by
5.2.3.1 Implicit Completion Values
The algorithms of this specification often implicitly return
- Return
"Infinity" .
means the same thing as:
- Return
NormalCompletion ("Infinity" ).
However, if the value expression of a “return” statement is a
The abstract operation
Assert : completionRecord is aCompletion Record .- Return completionRecord as the
Completion Record of this abstract operation.
A “return” statement without a value in an algorithm step means the same thing as:
- Return
NormalCompletion (undefined ).
Any reference to a
5.2.3.2 Throw an Exception
Algorithms steps that say to throw an exception, such as
- Throw a
TypeError exception.
mean the same things as:
- Return
ThrowCompletion (a newly createdTypeError object).
5.2.3.3 ReturnIfAbrupt
Algorithms steps that say or are otherwise equivalent to:
ReturnIfAbrupt (argument).
mean the same thing as:
- If argument is an
abrupt completion , return argument. - Else if argument is a
Completion Record , set argument to argument.[[Value]].
Algorithms steps that say or are otherwise equivalent to:
ReturnIfAbrupt (AbstractOperation()).
mean the same thing as:
- Let hygienicTemp be AbstractOperation().
- If hygienicTemp is an
abrupt completion , return hygienicTemp. - Else if hygienicTemp is a
Completion Record , set hygienicTemp to hygienicTemp.[[Value]].
Where hygienicTemp is ephemeral and visible only in the steps pertaining to ReturnIfAbrupt.
Algorithms steps that say or are otherwise equivalent to:
- Let result be AbstractOperation(
ReturnIfAbrupt (argument)).
mean the same thing as:
- If argument is an
abrupt completion , return argument. - If argument is a
Completion Record , set argument to argument.[[Value]]. - Let result be AbstractOperation(argument).
5.2.3.4 ReturnIfAbrupt Shorthands
Invocations of ? indicate that
- ? OperationName().
is equivalent to the following step:
ReturnIfAbrupt (OperationName()).
Similarly, for method application style, the step:
- ? someValue.OperationName().
is equivalent to:
ReturnIfAbrupt (someValue.OperationName()).
Similarly, prefix ! is used to indicate that the following invocation of an abstract or syntax-directed operation will never return an
- Let val be ! OperationName().
is equivalent to the following steps:
- Let val be OperationName().
Assert : val is never anabrupt completion .- If val is a
Completion Record , set val to val.[[Value]].
Syntax-directed operations for ! or ? before the invocation of the operation:
- Perform ! SyntaxDirectedOperation of
NonTerminal .
5.2.4 Static Semantics
Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements form a valid ECMAScript
Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.
A special kind of static semantic rule is an Early Error Rule.
5.2.5 Mathematical Operations
This specification makes reference to these kinds of numeric values:
- Mathematical values: Arbitrary real numbers, used as the default numeric type.
- Extended mathematical values: Mathematical values together with +∞ and -∞.
- Numbers:
IEEE 754-2019 double-precision floating point values. - BigInts: ECMAScript values representing arbitrary integers in a one-to-one correspondence.
In the language of this specification, numerical values are distinguished among different numeric kinds using subscript suffixes. The subscript 𝔽 refers to Numbers, and the subscript ℤ refers to BigInts. Numeric values without a subscript suffix refer to mathematical values.
Numeric operators such as +, ×, =, and ≥ refer to those operations as determined by the type of the operands. When applied to mathematical values, the operators refer to the usual mathematical operations. When applied to Numbers, the operators refer to the relevant operations within
In general, when this specification refers to a numerical value, such as in the phrase, "the length of y" or "the
Numeric operators applied to mixed-type operands (such as a Number and a
This specification denotes most numeric values in base 10; it also uses numeric values of the form 0x followed by digits 0-9 or A-F as base-16 values.
When the term integer is used in this specification, it refers to a
Conversions between mathematical values and Numbers or BigInts are always explicit in this document. A conversion from a
The mathematical function
The mathematical function
The notation “
The phrase "the result of clamping x between lower and upper" (where x is an
The mathematical function
Mathematical functions min, max,
5.2.6 Value Notation
In this specification, ECMAScript language values are displayed in Function.prototype.apply or let n = 42;.
Values which are internal to the specification and not directly observable from ECMAScript code are indicated with a
6 ECMAScript Data Types and Values
Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.
Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause. When the term “empty” is used as if it was naming a value, it is equivalent to saying “no value of any type”.
6.1 ECMAScript Language Types
An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.
6.1.1 The Undefined Type
The Undefined type has exactly one value, called
6.1.2 The Null Type
The Null type has exactly one value, called
6.1.3 The Boolean Type
The Boolean type represents a logical entity having two values, called
6.1.4 The String Type
The String type is the set of all ordered sequences of zero or more 16-bit unsigned
ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not restrict the value of or relationships between these code units, so operations that further interpret String contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences. Such operations apply special treatment to every code unit with a numeric value in the inclusive range 0xD800 to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code unit) and every code unit with a numeric value in the inclusive range 0xDC00 to 0xDFFF (defined as a trailing surrogate, or more formally as a low-surrogate code unit) using the following rules:
-
A code unit that is not a
leading surrogate and not atrailing surrogate is interpreted as a code point with the same value. -
A sequence of two code units, where the first code unit c1 is a
leading surrogate and the second code unit c2 atrailing surrogate , is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See11.1.3 ) -
A code unit that is a
leading surrogate ortrailing surrogate , but is not part of asurrogate pair , is interpreted as a code point with the same value.
The function String.prototype.normalize (see String.prototype.localeCompare (see
The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source text is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.
In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code units is the concatenation of the code units (in order) of each of the arguments (in order).
The phrase "the substring of S from inclusiveStart to exclusiveEnd" (where S is a String value or a sequence of code units and inclusiveStart and exclusiveEnd are integers) denotes the String value consisting of the consecutive code units of S beginning at index inclusiveStart and ending immediately before index exclusiveEnd (which is the empty String when inclusiveStart = exclusiveEnd). If the "to" suffix is omitted, the length of S is used as the value of exclusiveEnd.
6.1.4.1 StringIndexOf ( string, searchValue, fromIndex )
The abstract operation StringIndexOf takes arguments string (a String), searchValue (a String), and fromIndex (a non-negative
Assert :Type (string) is String.Assert :Type (searchValue) is String.Assert : fromIndex is a non-negativeinteger .- Let len be the length of string.
- If searchValue is the empty String and fromIndex ≤ len, return fromIndex.
- Let searchLen be the length of searchValue.
- For each
integer i starting with fromIndex such that i ≤ len - searchLen, in ascending order, do- Let candidate be the
substring of string from i to i + searchLen. - If candidate is the same sequence of code units as searchValue, return i.
- Let candidate be the
- Return -1.
If searchValue is the empty String and fromIndex is less than or equal to the length of string, this algorithm returns fromIndex. The empty String is effectively found at every position within a string, including after the last code unit.
This algorithm always returns -1 if fromIndex > the length of string.
6.1.5 The Symbol Type
The Symbol type is the set of all non-String values that may be used as the key of an Object property (
Each possible Symbol value is unique and immutable.
Each Symbol value immutably holds an associated value called [[Description]] that is either
6.1.5.1 Well-Known Symbols
Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (
Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed in
| Specification Name | [[Description]] | Value and Purpose |
|---|---|---|
| @@asyncIterator |
|
A method that returns the default AsyncIterator for an object. Called by the semantics of the for-await-of statement.
|
| @@hasInstance |
|
A method that determines if a instanceof operator.
|
| @@isConcatSpreadable |
|
A Boolean valued property that if true indicates that an object should be flattened to its array elements by Array.prototype.concat |
| @@iterator |
|
A method that returns the default Iterator for an object. Called by the semantics of the for-of statement. |
| @@match |
|
A regular expression method that matches the regular expression against a string. Called by the String.prototype.match |
| @@matchAll |
|
A regular expression method that returns an iterator, that yields matches of the regular expression against a string. Called by the String.prototype.matchAll |
| @@replace |
|
A regular expression method that replaces matched substrings of a string. Called by the String.prototype.replace |
| @@search |
|
A regular expression method that returns the index within a string that matches the regular expression. Called by the String.prototype.search |
| @@species |
|
A function valued property that is the |
| @@split |
|
A regular expression method that splits a string at the indices that match the regular expression. Called by the String.prototype.split |
| @@toPrimitive |
|
A method that converts an object to a corresponding primitive value. Called by the |
| @@toStringTag |
|
A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method Object.prototype.toString |
| @@unscopables |
|
An object valued property whose own and inherited property names are property names that are excluded from the with environment bindings of the associated object.
|
6.1.6 Numeric Types
ECMAScript has two built-in numeric types: Number and BigInt. In this specification, every numeric type T contains a multiplicative identity value denoted T::unit. The specification types also have the following
| Invocation Synopsis | Example source | Invoked by the Evaluation semantics of ... | Result |
|---|---|---|---|
| T::unaryMinus(x) |
-x
|
- Operator |
T |
| T::bitwiseNOT(x) |
~x
|
~ ) |
T |
| T::exponentiate(x, y) |
x ** y
|
|
T, may throw |
| T::multiply(x, y) |
x * y
|
|
T |
| T::divide(x, y) |
x / y
|
|
T, may throw |
| T::remainder(x, y) |
x % y
|
|
T, may throw |
| T::add(x, y) |
x ++++ xx + y
|
+ ) |
T |
| T::subtract(x, y) |
x ---- xx - y
|
- ) |
T |
| T::leftShift(x, y) |
x << y
|
<< ) |
T |
| T::signedRightShift(x, y) |
x >> y
|
>> ) |
T |
| T::unsignedRightShift(x, y) |
x >>> y
|
>>> ) |
T, may throw |
| T::lessThan(x, y) |
x < yx > yx <= yx >= y
|
|
Boolean or |
| T::equal(x, y) |
x == yx != yx === yx !== y
|
|
Boolean |
| T::sameValue(x, y) |
Object internal methods,
via |
Boolean | |
| T::sameValueZero(x, y) |
Array, Map, and Set methods,
via |
Boolean | |
| T::bitwiseAND(x, y) |
x & y
|
|
T |
| T::bitwiseXOR(x, y) |
x ^ y
|
|
T |
| T::bitwiseOR(x, y) |
x | y
|
|
T |
| T::toString(x) |
String(x)
|
Many expressions and built-in functions, via |
String |
The T::unit value and T::op operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other
Because the numeric types are in general not convertible without loss of precision or truncation, the ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call Number and BigInt functions to convert among types when calling a function which requires another type.
The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric conversions that could lose precision or truncate. These legacy implicit conversions are maintained for backward compatibility, but not provided for BigInt in order to minimize opportunity for programmer error, and to leave open the option of generalized value types in a future edition.
6.1.6.1 The Number Type
The Number type has exactly 18,437,736,874,454,810,627 (that is, NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is
The bit pattern that might be observed in an ArrayBuffer (see
There are two other special values, called +Infinity (or simply Infinity) and -Infinity.)
The other 18,437,736,874,454,810,624 (that is,
Note that there is both a +0 (or simply 0) and -0.)
The 18,437,736,874,454,810,622 (that is,
18,428,729,675,200,069,632 (that is,
where s is 1 or -1, m is an
The remaining 9,007,199,254,740,990 (that is,
where s is 1 or -1, m is an
Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in the Number type. The
A finite number has an odd significand if it is non-zero and the
In this specification, the phrase “the Number value for x” where x represents an exact real mathematical quantity (which might even be an irrational number such as π) means a
The
Some ECMAScript operators deal only with integers in specific ranges such as
The Number::unit value is
6.1.6.1.1 Number::unaryMinus ( x )
The abstract operation Number::unaryMinus takes argument x (a Number). It performs the following steps when called:
- If x is
NaN , returnNaN . - Return the result of negating x; that is, compute a Number with the same magnitude but opposite sign.
6.1.6.1.2 Number::bitwiseNOT ( x )
The abstract operation Number::bitwiseNOT takes argument x (a Number). It performs the following steps when called:
- Let oldValue be !
ToInt32 (x). - Return the result of applying bitwise complement to oldValue. The
mathematical value of the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.3 Number::exponentiate ( base, exponent )
The abstract operation Number::exponentiate takes arguments base (a Number) and exponent (a Number). It returns an
- If exponent is
NaN , returnNaN . - If exponent is
+0 𝔽 or exponent is-0 𝔽, return1 𝔽. - If base is
NaN , returnNaN . - If base is
+∞ 𝔽, then- If exponent >
+0 𝔽, return+∞ 𝔽. Otherwise, return+0 𝔽.
- If exponent >
- If base is
-∞ 𝔽, then- If exponent >
+0 𝔽, then- If exponent is an odd
integral Number , return-∞ 𝔽. Otherwise, return+∞ 𝔽.
- If exponent is an odd
- Else,
- If exponent is an odd
integral Number , return-0 𝔽. Otherwise, return+0 𝔽.
- If exponent is an odd
- If exponent >
- If base is
+0 𝔽, then- If exponent >
+0 𝔽, return+0 𝔽. Otherwise, return+∞ 𝔽.
- If exponent >
- If base is
-0 𝔽, then- If exponent >
+0 𝔽, then- If exponent is an odd
integral Number , return-0 𝔽. Otherwise, return+0 𝔽.
- If exponent is an odd
- Else,
- If exponent is an odd
integral Number , return-∞ 𝔽. Otherwise, return+∞ 𝔽.
- If exponent is an odd
- If exponent >
Assert : base is finite and is neither+0 𝔽 nor-0 𝔽.- If exponent is
+∞ 𝔽, then - If exponent is
-∞ 𝔽, then Assert : exponent is finite and is neither+0 𝔽 nor-0 𝔽.- If base <
+0 𝔽 and exponent is not anintegral Number , returnNaN . - Return an
implementation-approximated value representing the result of raisingℝ (base) to theℝ (exponent) power.
The result of base ** exponent when base is
6.1.6.1.4 Number::multiply ( x, y )
The abstract operation Number::multiply takes arguments x (a Number) and y (a Number). It performs multiplication according to the rules of
Finite-precision multiplication is commutative, but not always associative.
6.1.6.1.5 Number::divide ( x, y )
The abstract operation Number::divide takes arguments x (a Number) and y (a Number). It performs division according to the rules of
- If x is
NaN or y isNaN , returnNaN . - If x is
+∞ 𝔽 or x is-∞ 𝔽, then- If y is
+∞ 𝔽 or y is-∞ 𝔽, returnNaN . - If y is
+0 𝔽 or y >+0 𝔽, return x. - Return -x.
- If y is
- If y is
+∞ 𝔽, then- If x is
+0 𝔽 or x >+0 𝔽, return+0 𝔽. Otherwise, return-0 𝔽.
- If x is
- If y is
-∞ 𝔽, then- If x is
+0 𝔽 or x >+0 𝔽, return-0 𝔽. Otherwise, return+0 𝔽.
- If x is
- If x is
+0 𝔽 or x is-0 𝔽, then- If y is
+0 𝔽 or y is-0 𝔽, returnNaN . - If y >
+0 𝔽, return x. - Return -x.
- If y is
- If y is
+0 𝔽, then- If x >
+0 𝔽, return+∞ 𝔽. Otherwise, return-∞ 𝔽.
- If x >
- If y is
-0 𝔽, then- If x >
+0 𝔽, return-∞ 𝔽. Otherwise, return+∞ 𝔽.
- If x >
- Return
𝔽 (ℝ (x) /ℝ (y)).
6.1.6.1.6 Number::remainder ( n, d )
The abstract operation Number::remainder takes arguments n (a Number) and d (a Number). It yields the remainder from an implied division of its operands where n is the dividend and d is the divisor. It performs the following steps when called:
- If n is
NaN or d isNaN , returnNaN . - If n is
+∞ 𝔽 or n is-∞ 𝔽, returnNaN . - If d is
+∞ 𝔽 or d is-∞ 𝔽, return n. - If d is
+0 𝔽 or d is-0 𝔽, returnNaN . - If n is
+0 𝔽 or n is-0 𝔽, return n. Assert : n and d are finite and non-zero.- Let r be
ℝ (n) - (ℝ (d) × q) where q is aninteger that is negative if and only if n and d have opposite sign, and whose magnitude is as large as possible without exceeding the magnitude ofℝ (n) /ℝ (d). - Return
𝔽 (r).
In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.
% operator is not the same as the “remainder” operation defined by % on floating-point operations to behave in a manner analogous to that of the Java 6.1.6.1.7 Number::add ( x, y )
The abstract operation Number::add takes arguments x (a Number) and y (a Number). It performs addition according to the rules of
Finite-precision addition is commutative, but not always associative.
6.1.6.1.8 Number::subtract ( x, y )
The abstract operation Number::subtract takes arguments x (a Number) and y (a Number). It performs subtraction, producing the difference of its operands; x is the minuend and y is the subtrahend. It performs the following steps when called:
- Return Number::add(x, Number::unaryMinus(y)).
It is always the case that x - y produces the same result as x + (-y).
6.1.6.1.9 Number::leftShift ( x, y )
The abstract operation Number::leftShift takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- Let lnum be !
ToInt32 (x). - Let rnum be !
ToUint32 (y). - Let shiftCount be
ℝ (rnum)modulo 32. - Return the result of left shifting lnum by shiftCount bits. The
mathematical value of the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.10 Number::signedRightShift ( x, y )
The abstract operation Number::signedRightShift takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- Let lnum be !
ToInt32 (x). - Let rnum be !
ToUint32 (y). - Let shiftCount be
ℝ (rnum)modulo 32. - Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. The
mathematical value of the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.11 Number::unsignedRightShift ( x, y )
The abstract operation Number::unsignedRightShift takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- Let lnum be !
ToUint32 (x). - Let rnum be !
ToUint32 (y). - Let shiftCount be
ℝ (rnum)modulo 32. - Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. The
mathematical value of the result is exactly representable as a 32-bit unsigned bit string.
6.1.6.1.12 Number::lessThan ( x, y )
The abstract operation Number::lessThan takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- If x is
NaN , returnundefined . - If y is
NaN , returnundefined . - If x and y are the same
Number value , returnfalse . - If x is
+0 𝔽 and y is-0 𝔽, returnfalse . - If x is
-0 𝔽 and y is+0 𝔽, returnfalse . - If x is
+∞ 𝔽, returnfalse . - If y is
+∞ 𝔽, returntrue . - If y is
-∞ 𝔽, returnfalse . - If x is
-∞ 𝔽, returntrue . Assert : x and y are finite and non-zero.- If
ℝ (x) <ℝ (y), returntrue . Otherwise, returnfalse .
6.1.6.1.13 Number::equal ( x, y )
The abstract operation Number::equal takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- If x is
NaN , returnfalse . - If y is
NaN , returnfalse . - If x is the same
Number value as y, returntrue . - If x is
+0 𝔽 and y is-0 𝔽, returntrue . - If x is
-0 𝔽 and y is+0 𝔽, returntrue . - Return
false .
6.1.6.1.14 Number::sameValue ( x, y )
The abstract operation Number::sameValue takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- If x is
NaN and y isNaN , returntrue . - If x is
+0 𝔽 and y is-0 𝔽, returnfalse . - If x is
-0 𝔽 and y is+0 𝔽, returnfalse . - If x is the same
Number value as y, returntrue . - Return
false .
6.1.6.1.15 Number::sameValueZero ( x, y )
The abstract operation Number::sameValueZero takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- If x is
NaN and y isNaN , returntrue . - If x is
+0 𝔽 and y is-0 𝔽, returntrue . - If x is
-0 𝔽 and y is+0 𝔽, returntrue . - If x is the same
Number value as y, returntrue . - Return
false .
6.1.6.1.16 NumberBitwiseOp ( op, x, y )
The abstract operation NumberBitwiseOp takes arguments op (a sequence of Unicode code points), x, and y. It performs the following steps when called:
Assert : op is&,^, or|.- Let lnum be !
ToInt32 (x). - Let rnum be !
ToInt32 (y). - Let lbits be the 32-bit two's complement bit string representing
ℝ (lnum). - Let rbits be the 32-bit two's complement bit string representing
ℝ (rnum). - If op is
&, let result be the result of applying the bitwise AND operation to lbits and rbits. - Else if op is
^, let result be the result of applying the bitwise exclusive OR (XOR) operation to lbits and rbits. - Else, op is
|. Let result be the result of applying the bitwise inclusive OR operation to lbits and rbits. - Return the
Number value for theinteger represented by the 32-bit two's complement bit string result.
6.1.6.1.17 Number::bitwiseAND ( x, y )
The abstract operation Number::bitwiseAND takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- Return
NumberBitwiseOp (&, x, y).
6.1.6.1.18 Number::bitwiseXOR ( x, y )
The abstract operation Number::bitwiseXOR takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- Return
NumberBitwiseOp (^, x, y).
6.1.6.1.19 Number::bitwiseOR ( x, y )
The abstract operation Number::bitwiseOR takes arguments x (a Number) and y (a Number). It performs the following steps when called:
- Return
NumberBitwiseOp (|, x, y).
6.1.6.1.20 Number::toString ( x )
The abstract operation Number::toString takes argument x (a Number). It converts x to String format. It performs the following steps when called:
- If x is
NaN , return the String"NaN" . - If x is
+0 𝔽 or-0 𝔽, return the String"0" . - If x <
+0 𝔽, return thestring-concatenation of"-" and !Number::toString (-x). - If x is
+∞ 𝔽, return the String"Infinity" . - Otherwise, let n, k, and s be integers such that k ≥ 1, 10k - 1 ≤ s < 10k, s × 10n - k is
ℝ (x), and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria. - If k ≤ n ≤ 21, return the
string-concatenation of:- the code units of the k digits of the decimal representation of s (in order, with no leading zeroes)
- n - k occurrences of the code unit 0x0030 (DIGIT ZERO)
- If 0 < n ≤ 21, return the
string-concatenation of:- the code units of the most significant n digits of the decimal representation of s
- the code unit 0x002E (FULL STOP)
- the code units of the remaining k - n digits of the decimal representation of s
- If -6 < n ≤ 0, return the
string-concatenation of:- the code unit 0x0030 (DIGIT ZERO)
- the code unit 0x002E (FULL STOP)
- -n occurrences of the code unit 0x0030 (DIGIT ZERO)
- the code units of the k digits of the decimal representation of s
- Otherwise, if k = 1, return the
string-concatenation of: - Return the
string-concatenation of:- the code units of the most significant digit of the decimal representation of s
- the code unit 0x002E (FULL STOP)
- the code units of the remaining k - 1 digits of the decimal representation of s
- the code unit 0x0065 (LATIN SMALL LETTER E)
- the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n - 1 is positive or negative
- the code units of the decimal representation of the
integer abs (n - 1) (with no leading zeroes)
The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:
-
If x is any
Number value other than-0 𝔽, thenToNumber (ToString (x)) is exactly the sameNumber value as x. -
The least significant digit of s is not always uniquely determined by the requirements listed in step
5 .
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step
- Otherwise, let n, k, and s be integers such that k ≥ 1, 10k - 1 ≤ s < 10k, s × 10n - k is
ℝ (x), and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10n - k is closest in value toℝ (x). If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.
Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:
Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). 30 November 1990. Available as
http://ampl.com/REFS/abstracts.html#rounding. Associated code available as
http://netlib.sandia.gov/fp/dtoa.c and as
http://netlib.sandia.gov/fp/g_fmt.c and may also be found at the various netlib mirror sites.
6.1.6.2 The BigInt Type
The BigInt type represents an
The BigInt::unit value is
6.1.6.2.1 BigInt::unaryMinus ( x )
The abstract operation BigInt::unaryMinus takes argument x (a BigInt). It performs the following steps when called:
- If x is
0 ℤ, return0 ℤ. - Return the BigInt value that represents the negation of
ℝ (x).
6.1.6.2.2 BigInt::bitwiseNOT ( x )
The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt). It returns the one's complement of x; that is, -x -
6.1.6.2.3 BigInt::exponentiate ( base, exponent )
The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and exponent (a BigInt). It performs the following steps when called:
6.1.6.2.4 BigInt::multiply ( x, y )
The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the result of multiplying x and y.
6.1.6.2.5 BigInt::divide ( x, y )
The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
6.1.6.2.6 BigInt::remainder ( n, d )
The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a BigInt). It performs the following steps when called:
- If d is
0 ℤ, throw aRangeError exception. - If n is
0 ℤ, return0 ℤ. - Let r be the BigInt defined by the mathematical relation r = n - (d × q) where q is a BigInt that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d.
- Return r.
6.1.6.2.7 BigInt::add ( x, y )
The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the sum of x and y.
6.1.6.2.8 BigInt::subtract ( x, y )
The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the difference x minus y.
6.1.6.2.9 BigInt::leftShift ( x, y )
The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
6.1.6.2.10 BigInt::signedRightShift ( x, y )
The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
- Return BigInt::leftShift(x, -y).
6.1.6.2.11 BigInt::unsignedRightShift ( x, y )
The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
- Throw a
TypeError exception.
6.1.6.2.12 BigInt::lessThan ( x, y )
The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a BigInt). It returns
6.1.6.2.13 BigInt::equal ( x, y )
The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a BigInt). It returns
6.1.6.2.14 BigInt::sameValue ( x, y )
The abstract operation BigInt::sameValue takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
- Return BigInt::equal(x, y).
6.1.6.2.15 BigInt::sameValueZero ( x, y )
The abstract operation BigInt::sameValueZero takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
- Return BigInt::equal(x, y).
6.1.6.2.16 BinaryAnd ( x, y )
The abstract operation BinaryAnd takes arguments x and y. It performs the following steps when called:
6.1.6.2.17 BinaryOr ( x, y )
The abstract operation BinaryOr takes arguments x and y. It performs the following steps when called:
6.1.6.2.18 BinaryXor ( x, y )
The abstract operation BinaryXor takes arguments x and y. It performs the following steps when called:
6.1.6.2.19 BigIntBitwiseOp ( op, x, y )
The abstract operation BigIntBitwiseOp takes arguments op (a sequence of Unicode code points), x (a BigInt), and y (a BigInt). It performs the following steps when called:
Assert : op is&,^, or|.- Set x to
ℝ (x). - Set y to
ℝ (y). - Let result be 0.
- Let shift be 0.
- Repeat, until (x = 0 or x = -1) and (y = 0 or y = -1),
- If op is
&, let tmp beBinaryAnd (xmodulo 2, ymodulo 2). - Else if op is
|, let tmp beBinaryOr (xmodulo 2, ymodulo 2). - Else,
- If tmp ≠ 0, then
- Set result to result - 2shift.
- NOTE: This extends the sign.
- Return the BigInt value for result.
6.1.6.2.20 BigInt::bitwiseAND ( x, y )
The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
- Return
BigIntBitwiseOp (&, x, y).
6.1.6.2.21 BigInt::bitwiseXOR ( x, y )
The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
- Return
BigIntBitwiseOp (^, x, y).
6.1.6.2.22 BigInt::bitwiseOR ( x, y )
The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:
- Return
BigIntBitwiseOp (|, x, y).
6.1.6.2.23 BigInt::toString ( x )
The abstract operation BigInt::toString takes argument x (a BigInt). It converts x to String format. It performs the following steps when called:
- If x <
0 ℤ, return thestring-concatenation of the String"-" and !BigInt::toString (-x). - Return the String value consisting of the code units of the digits of the decimal representation of x.
6.1.7 The Object Type
An Object is logically a collection of properties. Each property is either a data property, or an accessor property:
-
A data property associates a key value with an
ECMAScript language value and a set of Boolean attributes. -
An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve an
ECMAScript language value that is associated with the property.
Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty String, are valid as property keys. A property name is a property key that is a String value.
An integer index is a String-valued property key that is a canonical numeric String (see
Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.
All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Please see
6.1.7.1 Property Attributes
Attributes are used in this specification to define and explain the state of Object properties. A
| Attribute Name | Value Domain | Description |
|---|---|---|
| [[Value]] |
Any |
The value retrieved by a get access of the property. |
| [[Writable]] | Boolean |
If |
| [[Enumerable]] | Boolean |
If |
| [[Configurable]] | Boolean |
If |
An
| Attribute Name | Value Domain | Description |
|---|---|---|
| [[Get]] | Object | Undefined |
If the value is an Object it must be a |
| [[Set]] | Object | Undefined |
If the value is an Object it must be a |
| [[Enumerable]] | Boolean |
If |
| [[Configurable]] | Boolean |
If |
If the initial values of a property's attributes are not explicitly specified by this specification, the default value defined in
| Attribute Name | Default Value |
|---|---|
| [[Value]] |
|
| [[Get]] |
|
| [[Set]] |
|
| [[Writable]] |
|
| [[Enumerable]] |
|
| [[Configurable]] |
|
6.1.7.2 Object Internal Methods and Internal Slots
The actual semantics of objects, in ECMAScript, are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.
Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, a
Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of any
Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].
An ordinary object is an object that satisfies all of the following criteria:
-
For the internal methods listed in
Table 6 , the object uses those defined in10.1 . -
If the object has a [[Call]] internal method, it uses the one defined in
10.2.1 . -
If the object has a [[Construct]] internal method, it uses the one defined in
10.2.2 .
An exotic object is an object that is not an
This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind of
The “Signature” column of
In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.
An internal method implicitly returns a
| Internal Method | Signature | Description |
|---|---|---|
| [[GetPrototypeOf]] | ( ) → Object | Null |
Determine the object that provides inherited properties for this object. A |
| [[SetPrototypeOf]] | (Object | Null) → Boolean |
Associate this object with another object that provides inherited properties. Passing |
| [[IsExtensible]] | ( ) → Boolean | Determine whether it is permitted to add additional properties to this object. |
| [[PreventExtensions]] | ( ) → Boolean |
Control whether new properties may be added to this object. Returns |
| [[GetOwnProperty]] |
(propertyKey) → Undefined | |
Return a |
| [[DefineOwnProperty]] | (propertyKey, PropertyDescriptor) → Boolean |
Create or alter the own property, whose key is propertyKey, to have the state described by PropertyDescriptor. Return |
| [[HasProperty]] | (propertyKey) → Boolean | Return a Boolean value indicating whether this object already has either an own or inherited property whose key is propertyKey. |
| [[Get]] | (propertyKey, Receiver) → any |
Return the value of the property whose key is propertyKey from this object. If any ECMAScript code must be executed to retrieve the property value, Receiver is used as the |
| [[Set]] | (propertyKey, value, Receiver) → Boolean |
Set the value of the property whose key is propertyKey to value. If any ECMAScript code must be executed to set the property value, Receiver is used as the |
| [[Delete]] | (propertyKey) → Boolean |
Remove the own property whose key is propertyKey from this object. Return |
| [[OwnPropertyKeys]] |
( ) → |
Return a |
| Internal Method | Signature | Description |
|---|---|---|
| [[Call]] |
(any, a |
Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a |
| [[Construct]] |
(a |
Creates an object. Invoked via the new operator or a super call. The first argument to the internal method is a super call. The second argument is the object to which the new operator was initially applied. Objects that implement this internal method are called constructors. A |
The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause
6.1.7.3 Invariants of the Essential Internal Methods
The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.
An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.
Definitions:
- The target of an internal method is the object upon which the internal method is called.
-
A target is non-extensible if it has been observed to return
false from its [[IsExtensible]] internal method, ortrue from its [[PreventExtensions]] internal method. - A non-existent property is a property that does not exist as an own property on a non-extensible target.
-
All references to
SameValue are according to the definition of theSameValue algorithm.
Return value:
The value returned by any internal method must be a
- [[Type]] =
normal , [[Target]] =empty , and [[Value]] = a value of the "normal return type" shown below for that internal method, or - [[Type]] =
throw , [[Target]] =empty , and [[Value]] = anyECMAScript language value .
An internal method must not return a completion with [[Type]] =
[[GetPrototypeOf]] ( )
- The normal return type is either Object or Null.
-
If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then any future calls to [[GetPrototypeOf]] should return the
SameValue as V.
An object's prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the value
[[SetPrototypeOf]] ( V )
- The normal return type is Boolean.
-
If target is non-extensible, [[SetPrototypeOf]] must return
false , unless V is theSameValue as the target's observed [[GetPrototypeOf]] value.
[[IsExtensible]] ( )
- The normal return type is Boolean.
-
If [[IsExtensible]] returns
false , all future calls to [[IsExtensible]] on the target must returnfalse .
[[PreventExtensions]] ( )
- The normal return type is Boolean.
-
If [[PreventExtensions]] returns
true , all future calls to [[IsExtensible]] on the target must returnfalse and the target is now considered non-extensible.
[[GetOwnProperty]] ( P )
-
The normal return type is either
Property Descriptor or Undefined. -
If the Type of the return value is
Property Descriptor , the return value must be a fully populatedProperty Descriptor . -
If P is described as a non-configurable, non-writable own
data property , all future calls to [[GetOwnProperty]] ( P ) must returnProperty Descriptor whose [[Value]] isSameValue as P's [[Value]] attribute. -
If P's attributes other than [[Writable]] may change over time or if the property might be deleted, then P's [[Configurable]] attribute must be
true . -
If the [[Writable]] attribute may change from
false totrue , then the [[Configurable]] attribute must betrue . -
If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must return
undefined ).
As a consequence of the third invariant, if a property is described as a
[[DefineOwnProperty]] ( P, Desc )
- The normal return type is Boolean.
-
[[DefineOwnProperty]] must return
false if P has previously been observed as a non-configurable own property of the target, unless either:-
P is a writable
data property . A non-configurable writabledata property can be changed into a non-configurable non-writabledata property . -
All attributes of Desc are the
SameValue as P's attributes.
-
P is a writable
-
[[DefineOwnProperty]] (P, Desc) must return
false if target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.
[[HasProperty]] ( P )
- The normal return type is Boolean.
-
If P was previously observed as a non-configurable own data or
accessor property of the target, [[HasProperty]] must returntrue .
[[Get]] ( P, Receiver )
-
The normal return type is any
ECMAScript language type . -
If P was previously observed as a non-configurable, non-writable own
data property of the target with value V, then [[Get]] must return theSameValue as V. -
If P was previously observed as a non-configurable own
accessor property of the target whose [[Get]] attribute isundefined , the [[Get]] operation must returnundefined .
[[Set]] ( P, V, Receiver )
- The normal return type is Boolean.
-
If P was previously observed as a non-configurable, non-writable own
data property of the target, then [[Set]] must returnfalse unless V is theSameValue as P's [[Value]] attribute. -
If P was previously observed as a non-configurable own
accessor property of the target whose [[Set]] attribute isundefined , the [[Set]] operation must returnfalse .
[[Delete]] ( P )
- The normal return type is Boolean.
-
If P was previously observed as a non-configurable own data or
accessor property of the target, [[Delete]] must returnfalse .
[[OwnPropertyKeys]] ( )
-
The normal return type is
List . -
The returned
List must not contain any duplicate entries. -
The Type of each element of the returned
List is either String or Symbol. -
The returned
List must contain at least the keys of all non-configurable own properties that have previously been observed. -
If the target is non-extensible, the returned
List must contain only the keys of all own properties of the target that are observable using [[GetOwnProperty]].
[[Call]] ( )
-
The normal return type is any
ECMAScript language type .
[[Construct]] ( )
- The normal return type is Object.
- The target must also have a [[Call]] internal method.
6.1.7.4 Well-Known Intrinsic Objects
Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually have
Within this specification a reference such as %name% means the intrinsic object, associated with the current
| Intrinsic Name | Global Name | ECMAScript Language Association |
|---|---|---|
|
|
AggregateError
|
The AggregateError |
|
|
Array
|
The Array |
|
|
ArrayBuffer
|
The ArrayBuffer |
|
|
The prototype of Array iterator objects ( |
|
|
|
The prototype of async-from-sync iterator objects ( |
|
|
|
The |
|
|
|
The |
|
|
|
An object that all standard built-in async iterator objects indirectly inherit from | |
|
|
Atomics
|
The Atomics object ( |
|
|
BigInt
|
The BigInt |
|
|
BigInt64Array
|
The BigInt64Array |
|
|
BigUint64Array
|
The BigUint64Array |
|
|
Boolean
|
The Boolean |
|
|
DataView
|
The DataView |
|
|
Date
|
The Date |
|
|
decodeURI
|
The decodeURI function ( |
|
|
decodeURIComponent
|
The decodeURIComponent function ( |
|
|
encodeURI
|
The encodeURI function ( |
|
|
encodeURIComponent
|
The encodeURIComponent function ( |
|
|
Error
|
The Error |
|
|
eval
|
The eval function ( |
|
|
EvalError
|
The EvalError |
|
|
FinalizationRegistry
|
The |
|
|
Float32Array
|
The Float32Array |
|
|
Float64Array
|
The Float64Array |
|
|
The prototype of For-In iterator objects ( |
|
|
|
Function
|
The Function |
|
|
The |
|
|
|
Int8Array
|
The Int8Array |
|
|
Int16Array
|
The Int16Array |
|
|
Int32Array
|
The Int32Array |
|
|
isFinite
|
The isFinite function ( |
|
|
isNaN
|
The isNaN function ( |
|
|
An object that all standard built-in iterator objects indirectly inherit from | |
|
|
JSON
|
The JSON object ( |
|
|
Map
|
The Map |
|
|
The prototype of Map iterator objects ( |
|
|
|
Math
|
The Math object ( |
|
|
Number
|
The Number |
|
|
Object
|
The Object |
|
|
parseFloat
|
The parseFloat function ( |
|
|
parseInt
|
The parseInt function ( |
|
|
Promise
|
The Promise |
|
|
Proxy
|
The Proxy |
|
|
RangeError
|
The RangeError |
|
|
ReferenceError
|
The ReferenceError |
|
|
Reflect
|
The Reflect object ( |
|
|
RegExp
|
The RegExp |
|
|
The prototype of RegExp String Iterator objects ( |
|
|
|
Set
|
The Set |
|
|
The prototype of Set iterator objects ( |
|
|
|
SharedArrayBuffer
|
The SharedArrayBuffer |
|
|
String
|
The String |
|
|
The prototype of String iterator objects ( |
|
|
|
Symbol
|
The Symbol |
|
|
SyntaxError
|
The SyntaxError |
|
|
A |
|
|
|
The super class of all typed Array constructors ( |
|
|
|
TypeError
|
The TypeError |
|
|
Uint8Array
|
The Uint8Array |
|
|
Uint8ClampedArray
|
The Uint8ClampedArray |
|
|
Uint16Array
|
The Uint16Array |
|
|
Uint32Array
|
The Uint32Array |
|
|
URIError
|
The URIError |
|
|
WeakMap
|
The WeakMap |
|
|
WeakRef
|
The |
|
|
WeakSet
|
The WeakSet |
Additional entries in
6.2 ECMAScript Specification Types
A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference,
6.2.1 The List and Record Specification Types
The List type is used to explain the evaluation of argument lists (see new expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3rd element of the List arguments.
When an algorithm iterates over the elements of a List without specifying an order, the order used is the order of the elements in the List.
For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as « ».
The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is either an ECMAScript value or an abstract value represented by a name associated with the Record type. Field names are always enclosed in double brackets, for example [[Value]].
For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, { [[Field1]]: 42, [[Field2]]:
In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]:
6.2.2 The Set and Relation Specification Types
The Set type is used to explain a collection of unordered elements for use in the
The Relation type is used to explain constraints on Sets. Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a Relation on events is a set of ordered pairs of events. For a Relation R and two values a and b in the value domain of R, a R b is shorthand for saying the ordered pair (a, b) is a member of R. A Relation is least with respect to some conditions when it is the smallest Relation that satisfies those conditions.
A strict partial order is a Relation value R that satisfies the following.
-
For all a, b, and c in R's domain:
- It is not the case that a R a, and
- If a R b and b R c, then a R c.
The two properties above are called irreflexivity and transitivity, respectively.
A strict total order is a Relation value R that satisfies the following.
-
For all a, b, and c in R's domain:
- a is identical to b or a R b or b R a, and
- It is not the case that a R a, and
- If a R b and b R c, then a R c.
The three properties above are called totality, irreflexivity, and transitivity, respectively.
6.2.3 The Completion Record Specification Type
The Completion type is a break, continue, return and throw) that perform nonlocal transfers of control.
Values of the Completion type are
| Field Name | Value | Meaning |
|---|---|---|
| [[Type]] |
One of |
The type of completion that occurred. |
| [[Value]] |
any |
The value that was produced. |
| [[Target]] |
any ECMAScript string or |
The target label for directed control transfers. |
The term “abrupt completion” refers to any completion with a [[Type]] value other than
6.2.3.1 Await
Algorithm steps that say
- Let completion be
Await (value).
mean the same thing as:
- Let asyncContext be the
running execution context . - Let promise be ?
PromiseResolve (%Promise% , value). - Let stepsFulfilled be the algorithm steps defined in
Await Fulfilled Functions . - Let lengthFulfilled be the number of non-optional parameters of the function definition in
Await Fulfilled Functions . - Let onFulfilled be !
CreateBuiltinFunction (stepsFulfilled, lengthFulfilled,"" , « [[AsyncContext]] »). - Set onFulfilled.[[AsyncContext]] to asyncContext.
- Let stepsRejected be the algorithm steps defined in
Await Rejected Functions . - Let lengthRejected be the number of non-optional parameters of the function definition in
Await Rejected Functions . - Let onRejected be !
CreateBuiltinFunction (stepsRejected, lengthRejected,"" , « [[AsyncContext]] »). - Set onRejected.[[AsyncContext]] to asyncContext.
- Perform !
PerformPromiseThen (promise, onFulfilled, onRejected). - Remove asyncContext from the
execution context stack and restore theexecution context that is at the top of theexecution context stack as therunning execution context . - Set the code evaluation state of asyncContext such that when evaluation is resumed with a
Completion completion, the following steps of the algorithm that invokedAwait will be performed, with completion available. - Return.
- NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of asyncContext.
where all aliases in the above steps, with the exception of completion, are ephemeral and visible only in the steps pertaining to Await.
Await can be combined with the ? and ! prefixes, so that for example
- Let result be ?
Await (value).
means the same thing as:
- Let result be
Await (value). ReturnIfAbrupt (result).
6.2.3.1.1 Await Fulfilled Functions
An
When an
- Let F be the
active function object . - Let asyncContext be F.[[AsyncContext]].
- Let prevContext be the
running execution context . - Suspend prevContext.
- Push asyncContext onto the
execution context stack ; asyncContext is now therunning execution context . - Resume the suspended evaluation of asyncContext using
NormalCompletion (value) as the result of the operation that suspended it. Assert : When we reach this step, asyncContext has already been removed from theexecution context stack and prevContext is the currentlyrunning execution context .- Return
undefined .
The
6.2.3.1.2 Await Rejected Functions
An
When an
- Let F be the
active function object . - Let asyncContext be F.[[AsyncContext]].
- Let prevContext be the
running execution context . - Suspend prevContext.
- Push asyncContext onto the
execution context stack ; asyncContext is now therunning execution context . - Resume the suspended evaluation of asyncContext using
ThrowCompletion (reason) as the result of the operation that suspended it. Assert : When we reach this step, asyncContext has already been removed from theexecution context stack and prevContext is the currentlyrunning execution context .- Return
undefined .
The
6.2.3.2 NormalCompletion
The abstract operation NormalCompletion with a single argument, such as:
- Return
NormalCompletion (argument).
Is a shorthand that is defined as follows:
- Return
Completion { [[Type]]:normal , [[Value]]: argument, [[Target]]:empty }.
6.2.3.3 ThrowCompletion
The abstract operation ThrowCompletion with a single argument, such as:
- Return
ThrowCompletion (argument).
Is a shorthand that is defined as follows:
- Return
Completion { [[Type]]:throw , [[Value]]: argument, [[Target]]:empty }.
6.2.3.4 UpdateEmpty ( completionRecord, value )
The abstract operation UpdateEmpty takes arguments completionRecord and value. It performs the following steps when called:
Assert : If completionRecord.[[Type]] is eitherreturn orthrow , then completionRecord.[[Value]] is notempty .- If completionRecord.[[Value]] is not
empty , returnCompletion (completionRecord). - Return
Completion { [[Type]]: completionRecord.[[Type]], [[Value]]: value, [[Target]]: completionRecord.[[Target]] }.
6.2.4 The Reference Record Specification Type
The Reference Record type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the super
A Reference Record is a resolved name or property binding; its fields are defined by
| Field Name | Value | Meaning |
|---|---|---|
| [[Base]] |
One of:
|
The value or |
| [[ReferencedName]] | String or Symbol | The name of the binding. Always a String if [[Base]] value is an |
| [[Strict]] | Boolean | |
| [[ThisValue]] | any |
If not super |
The following
6.2.4.1 IsPropertyReference ( V )
The abstract operation IsPropertyReference takes argument V. It performs the following steps when called:
Assert : V is aReference Record .- If V.[[Base]] is
unresolvable , returnfalse . - If
Type (V.[[Base]]) is Boolean, String, Symbol, BigInt, Number, or Object, returntrue ; otherwise returnfalse .
6.2.4.2 IsUnresolvableReference ( V )
The abstract operation IsUnresolvableReference takes argument V. It performs the following steps when called:
Assert : V is aReference Record .- If V.[[Base]] is
unresolvable , returntrue ; otherwise returnfalse .
6.2.4.3 IsSuperReference ( V )
The abstract operation IsSuperReference takes argument V. It performs the following steps when called:
Assert : V is aReference Record .- If V.[[ThisValue]] is not
empty , returntrue ; otherwise returnfalse .
6.2.4.4 GetValue ( V )
The abstract operation GetValue takes argument V. It performs the following steps when called:
ReturnIfAbrupt (V).- If V is not a
Reference Record , return V. - If
IsUnresolvableReference (V) istrue , throw aReferenceError exception. - If
IsPropertyReference (V) istrue , then- Let baseObj be !
ToObject (V.[[Base]]). - Return ? baseObj.[[Get]](V.[[ReferencedName]],
GetThisValue (V)).
- Let baseObj be !
- Else,
- Let base be V.[[Base]].
Assert : base is anEnvironment Record .- Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see
9.1 ).
The object that may be created in step
6.2.4.5 PutValue ( V, W )
The abstract operation PutValue takes arguments V and W. It performs the following steps when called:
ReturnIfAbrupt (V).ReturnIfAbrupt (W).- If V is not a
Reference Record , throw aReferenceError exception. - If
IsUnresolvableReference (V) istrue , then- If V.[[Strict]] is
true , throw aReferenceError exception. - Let globalObj be
GetGlobalObject (). - Return ?
Set (globalObj, V.[[ReferencedName]], W,false ).
- If V.[[Strict]] is
- If
IsPropertyReference (V) istrue , then- Let baseObj be !
ToObject (V.[[Base]]). - Let succeeded be ? baseObj.[[Set]](V.[[ReferencedName]], W,
GetThisValue (V)). - If succeeded is
false and V.[[Strict]] istrue , throw aTypeError exception. - Return.
- Let baseObj be !
- Else,
- Let base be V.[[Base]].
Assert : base is anEnvironment Record .- Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see
9.1 ).
The object that may be created in step
6.2.4.6 GetThisValue ( V )
The abstract operation GetThisValue takes argument V. It performs the following steps when called:
Assert :IsPropertyReference (V) istrue .- If
IsSuperReference (V) istrue , return V.[[ThisValue]]; otherwise return V.[[Base]].
6.2.4.7 InitializeReferencedBinding ( V, W )
The abstract operation InitializeReferencedBinding takes arguments V and W. It performs the following steps when called:
ReturnIfAbrupt (V).ReturnIfAbrupt (W).Assert : V is aReference Record .Assert :IsUnresolvableReference (V) isfalse .- Let base be V.[[Base]].
Assert : base is anEnvironment Record .- Return base.InitializeBinding(V.[[ReferencedName]], W).
6.2.5 The Property Descriptor Specification Type
The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. Values of the Property Descriptor type are Records. Each field's name is an attribute name and its value is a corresponding attribute value as specified in
Property Descriptor values may be further classified as data Property Descriptors and accessor Property Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor; however, it may be neither. A generic Property Descriptor is a Property Descriptor value that is neither a data Property Descriptor nor an accessor Property Descriptor. A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the fields that correspond to the property attributes defined in either
The following
6.2.5.1 IsAccessorDescriptor ( Desc )
The abstract operation IsAccessorDescriptor takes argument Desc (a
- If Desc is
undefined , returnfalse . - If both Desc.[[Get]] and Desc.[[Set]] are absent, return
false . - Return
true .
6.2.5.2 IsDataDescriptor ( Desc )
The abstract operation IsDataDescriptor takes argument Desc (a
- If Desc is
undefined , returnfalse . - If both Desc.[[Value]] and Desc.[[Writable]] are absent, return
false . - Return
true .
6.2.5.3 IsGenericDescriptor ( Desc )
The abstract operation IsGenericDescriptor takes argument Desc (a
- If Desc is
undefined , returnfalse . - If
IsAccessorDescriptor (Desc) andIsDataDescriptor (Desc) are bothfalse , returntrue . - Return
false .
6.2.5.4 FromPropertyDescriptor ( Desc )
The abstract operation FromPropertyDescriptor takes argument Desc (a
- If Desc is
undefined , returnundefined . - Let obj be !
OrdinaryObjectCreate (%Object.prototype% ). Assert : obj is an extensibleordinary object with no own properties.- If Desc has a [[Value]] field, then
- Perform !
CreateDataPropertyOrThrow (obj,"value" , Desc.[[Value]]).
- Perform !
- If Desc has a [[Writable]] field, then
- Perform !
CreateDataPropertyOrThrow (obj,"writable" , Desc.[[Writable]]).
- Perform !
- If Desc has a [[Get]] field, then
- Perform !
CreateDataPropertyOrThrow (obj,"get" , Desc.[[Get]]).
- Perform !
- If Desc has a [[Set]] field, then
- Perform !
CreateDataPropertyOrThrow (obj,"set" , Desc.[[Set]]).
- Perform !
- If Desc has an [[Enumerable]] field, then
- Perform !
CreateDataPropertyOrThrow (obj,"enumerable" , Desc.[[Enumerable]]).
- Perform !
- If Desc has a [[Configurable]] field, then
- Perform !
CreateDataPropertyOrThrow (obj,"configurable" , Desc.[[Configurable]]).
- Perform !
- Return obj.
6.2.5.5 ToPropertyDescriptor ( Obj )
The abstract operation ToPropertyDescriptor takes argument Obj. It performs the following steps when called:
- If
Type (Obj) is not Object, throw aTypeError exception. - Let desc be a new
Property Descriptor that initially has no fields. - Let hasEnumerable be ?
HasProperty (Obj,"enumerable" ). - If hasEnumerable is
true , then - Let hasConfigurable be ?
HasProperty (Obj,"configurable" ). - If hasConfigurable is
true , then - Let hasValue be ?
HasProperty (Obj,"value" ). - If hasValue is
true , then- Let value be ?
Get (Obj,"value" ). - Set desc.[[Value]] to value.
- Let value be ?
- Let hasWritable be ?
HasProperty (Obj,"writable" ). - If hasWritable is
true , then - Let hasGet be ?
HasProperty (Obj,"get" ). - If hasGet is
true , then- Let getter be ?
Get (Obj,"get" ). - If
IsCallable (getter) isfalse and getter is notundefined , throw aTypeError exception. - Set desc.[[Get]] to getter.
- Let getter be ?
- Let hasSet be ?
HasProperty (Obj,"set" ). - If hasSet is
true , then- Let setter be ?
Get (Obj,"set" ). - If
IsCallable (setter) isfalse and setter is notundefined , throw aTypeError exception. - Set desc.[[Set]] to setter.
- Let setter be ?
- If desc.[[Get]] is present or desc.[[Set]] is present, then
- If desc.[[Value]] is present or desc.[[Writable]] is present, throw a
TypeError exception.
- If desc.[[Value]] is present or desc.[[Writable]] is present, throw a
- Return desc.
6.2.5.6 CompletePropertyDescriptor ( Desc )
The abstract operation CompletePropertyDescriptor takes argument Desc (a
Assert : Desc is aProperty Descriptor .- Let like be the
Record { [[Value]]:undefined , [[Writable]]:false , [[Get]]:undefined , [[Set]]:undefined , [[Enumerable]]:false , [[Configurable]]:false }. - If
IsGenericDescriptor (Desc) istrue orIsDataDescriptor (Desc) istrue , then- If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
- If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
- Else,
- If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
- If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
- If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
- If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
- Return Desc.
6.2.6 The Environment Record Specification Type
The
6.2.7 The Abstract Closure Specification Type
The Abstract Closure specification type is used to refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are invoked using function application style such as closure(arg1, arg2). Like
In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called, each captured value is referred to by the alias that was used to capture the value.
If an Abstract Closure returns a
Abstract Closures are created inline as part of other algorithms, shown in the following example.
- Let addend be 41.
- Let closure be a new
Abstract Closure with parameters (x) that captures addend and performs the following steps when called:- Return x + addend.
- Let val be closure(1).
Assert : val is 42.
6.2.8 Data Blocks
The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is an
For notational convenience within this specification, an array-like syntax can be used to access the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-origined
A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has an identity (for the purposes of equality testing Shared Data Block values) that is address-free: it is tied not to the virtual addresses the block is mapped to in any process, but to the set of locations in memory that the block represents. Two data blocks are equal only if the sets of the locations they contain are equal; otherwise, they are not equal and the intersection of the sets of locations they contain is empty. Finally, Shared Data Blocks can be distinguished from Data Blocks.
The semantics of Shared Data Blocks is defined using Shared Data Block events by the
Shared Data Block events are modeled by Records, defined in the
The following
6.2.8.1 CreateByteDataBlock ( size )
The abstract operation CreateByteDataBlock takes argument size (an
Assert : size ≥ 0.- Let db be a new
Data Block value consisting of size bytes. If it is impossible to create such aData Block , throw aRangeError exception. - Set all of the bytes of db to 0.
- Return db.
6.2.8.2 CreateSharedByteDataBlock ( size )
The abstract operation CreateSharedByteDataBlock takes argument size (a non-negative
Assert : size ≥ 0.- Let db be a new
Shared Data Block value consisting of size bytes. If it is impossible to create such aShared Data Block , throw aRangeError exception. - Let execution be the [[CandidateExecution]] field of the
surrounding agent 'sAgent Record . - Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier (). - Let zero be « 0 ».
- For each index i of db, do
- Append
WriteSharedMemory { [[Order]]:Init , [[NoTear]]:true , [[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero } to eventList.
- Append
- Return db.
6.2.8.3 CopyDataBlockBytes ( toBlock, toIndex, fromBlock, fromIndex, count )
The abstract operation CopyDataBlockBytes takes arguments toBlock, toIndex (a non-negative
Assert : fromBlock and toBlock are distinctData Block orShared Data Block values.- Let fromSize be the number of bytes in fromBlock.
Assert : fromIndex + count ≤ fromSize.- Let toSize be the number of bytes in toBlock.
Assert : toIndex + count ≤ toSize.- Repeat, while count > 0,
- If fromBlock is a
Shared Data Block , then- Let execution be the [[CandidateExecution]] field of the
surrounding agent 'sAgent Record . - Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier (). - Let bytes be a
List whose sole element is a nondeterministically chosenbyte value . - NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the
memory model to describe observable behaviour of hardware with weak consistency. - Let readEvent be
ReadSharedMemory { [[Order]]:Unordered , [[NoTear]]:true , [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }. - Append readEvent to eventList.
- Append
Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]]. - If toBlock is a
Shared Data Block , then- Append
WriteSharedMemory { [[Order]]:Unordered , [[NoTear]]:true , [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]: 1, [[Payload]]: bytes } to eventList.
- Append
- Else,
- Set toBlock[toIndex] to bytes[0].
- Let execution be the [[CandidateExecution]] field of the
- Else,
Assert : toBlock is not aShared Data Block .- Set toBlock[toIndex] to fromBlock[fromIndex].
- Set toIndex to toIndex + 1.
- Set fromIndex to fromIndex + 1.
- Set count to count - 1.
- If fromBlock is a
- Return
NormalCompletion (empty ).
7 Abstract Operations
These operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other, more specialized
7.1 Type Conversion
The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion
The BigInt type has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.
7.1.1 ToPrimitive ( input [ , preferredType ] )
The abstract operation ToPrimitive takes argument input and optional argument preferredType. It converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint preferredType to favour that type. It performs the following steps when called:
Assert : input is anECMAScript language value .- If
Type (input) is Object, then- Let exoticToPrim be ?
GetMethod (input,@@toPrimitive ). - If exoticToPrim is not
undefined , then - If preferredType is not present, let preferredType be
number . - Return ?
OrdinaryToPrimitive (input, preferredType).
- Let exoticToPrim be ?
- Return input.
When ToPrimitive is called with no hint, then it generally behaves as if the hint were
7.1.1.1 OrdinaryToPrimitive ( O, hint )
The abstract operation OrdinaryToPrimitive takes arguments O and hint. It performs the following steps when called:
Assert :Type (O) is Object.Assert : hint is eitherstring ornumber .- If hint is
string , then- Let methodNames be «
"toString" ,"valueOf" ».
- Let methodNames be «
- Else,
- Let methodNames be «
"valueOf" ,"toString" ».
- Let methodNames be «
- For each element name of methodNames, do
- Let method be ?
Get (O, name). - If
IsCallable (method) istrue , then
- Let method be ?
- Throw a
TypeError exception.
7.1.2 ToBoolean ( argument )
The abstract operation ToBoolean takes argument argument. It converts argument to a value of type Boolean according to
| Argument Type | Result |
|---|---|
| Undefined |
Return |
| Null |
Return |
| Boolean | Return argument. |
| Number |
If argument is |
| String |
If argument is the empty String (its length is 0), return |
| Symbol |
Return |
| BigInt |
If argument is |
| Object |
Return An alternate algorithm related to the [[IsHTMLDDA]] internal slot is mandated in section |
7.1.3 ToNumeric ( value )
The abstract operation ToNumeric takes argument value. It returns value converted to a Number or a BigInt. It performs the following steps when called:
- Let primValue be ?
ToPrimitive (value,number ). - If
Type (primValue) is BigInt, return primValue. - Return ?
ToNumber (primValue).
7.1.4 ToNumber ( argument )
The abstract operation ToNumber takes argument argument. It converts argument to a value of type Number according to
| Argument Type | Result |
|---|---|
| Undefined |
Return |
| Null |
Return |
| Boolean |
If argument is |
| Number | Return argument (no conversion). |
| String | See grammar and conversion algorithm below. |
| Symbol |
Throw a |
| BigInt |
Throw a |
| Object |
Apply the following steps:
|
7.1.4.1 ToNumber Applied to the String Type
The terminal symbols of this grammar are all composed of characters in the Unicode Basic Multilingual Plane (BMP). Therefore, the result of
Syntax
All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (
Some differences should be noted between the syntax of a
-
A
StringNumericLiteral may include leading and/or trailing white space and/or line terminators. -
A
StringNumericLiteral that is decimal may have any number of leading0digits. -
A
StringNumericLiteral that is decimal may include a+or-to indicate its sign. -
A
StringNumericLiteral that is empty or contains only white space is converted to+0 𝔽. -
Infinityand-Infinityare recognized as aStringNumericLiteral but not as aNumericLiteral . -
A
StringNumericLiteral cannot include aBigIntLiteralSuffix .
7.1.4.1.1 Runtime Semantics: MV
The conversion of a String to a
-
The MV of
StringNumericLiteral ::: [empty] -
The MV of
StringNumericLiteral ::: StrWhiteSpace -
The MV of
StringNumericLiteral ::: StrWhiteSpace opt StrNumericLiteral StrWhiteSpace opt StrNumericLiteral , no matter whether white space is present or not. -
The MV of
StrDecimalLiteral ::: - StrUnsignedDecimalLiteral StrUnsignedDecimalLiteral . (Note that if the MV ofStrUnsignedDecimalLiteral is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this signless mathematical zero to a floating-point+0 𝔽 or-0 𝔽 as appropriate.) -
The MV of
StrUnsignedDecimalLiteral ::: Infinity +∞ 𝔽). -
The MV of
StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits DecimalDigits plus (the MV of the secondDecimalDigits times 10-n), where n is the number of code points in the secondDecimalDigits . -
The MV of
StrUnsignedDecimalLiteral ::: DecimalDigits . ExponentPart DecimalDigits times 10e, where e is the MV ofExponentPart . -
The MV of
StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits ExponentPart DecimalDigits plus (the MV of the secondDecimalDigits times 10-n)) times 10e, where n is the number of code points in the secondDecimalDigits and e is the MV ofExponentPart . -
The MV of
StrUnsignedDecimalLiteral ::: . DecimalDigits DecimalDigits times 10-n, where n is the number of code points inDecimalDigits . -
The MV of
StrUnsignedDecimalLiteral ::: . DecimalDigits ExponentPart DecimalDigits times 10e - n, where n is the number of code points inDecimalDigits and e is the MV ofExponentPart . -
The MV of
StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPart DecimalDigits times 10e, where e is the MV ofExponentPart .
Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is -, in which case the rounded value is
-
it is not
0; or -
there is a non-zero digit to its left and there is a non-zero digit, not in the
ExponentPart , to its right.
7.1.5 ToIntegerOrInfinity ( argument )
The abstract operation ToIntegerOrInfinity takes argument argument. It converts argument to an
7.1.6 ToInt32 ( argument )
The abstract operation ToInt32 takes argument argument. It converts argument to one of 232
- Let number be ?
ToNumber (argument). - If number is
NaN ,+0 𝔽,-0 𝔽,+∞ 𝔽, or-∞ 𝔽, return+0 𝔽. - Let int be the
mathematical value that is the same sign as number and whose magnitude isfloor (abs (ℝ (number))). - Let int32bit be int
modulo 232. - If int32bit ≥ 231, return
𝔽 (int32bit - 232); otherwise return𝔽 (int32bit).
Given the above definition of ToInt32:
- The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
-
ToInt32(
ToUint32 (x)) is the same value as ToInt32(x) for all values of x. (It is to preserve this latter property that+∞ 𝔽 and-∞ 𝔽 are mapped to+0 𝔽.) -
ToInt32 maps
-0 𝔽 to+0 𝔽.
7.1.7 ToUint32 ( argument )
The abstract operation ToUint32 takes argument argument. It converts argument to one of 232
Given the above definition of ToUint32:
-
Step
5 is the only difference between ToUint32 andToInt32 . - The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
-
ToUint32(
ToInt32 (x)) is the same value as ToUint32(x) for all values of x. (It is to preserve this latter property that+∞ 𝔽 and-∞ 𝔽 are mapped to+0 𝔽.) -
ToUint32 maps
-0 𝔽 to+0 𝔽.
7.1.8 ToInt16 ( argument )
The abstract operation ToInt16 takes argument argument. It converts argument to one of 216
- Let number be ?
ToNumber (argument). - If number is
NaN ,+0 𝔽,-0 𝔽,+∞ 𝔽, or-∞ 𝔽, return+0 𝔽. - Let int be the
mathematical value that is the same sign as number and whose magnitude isfloor (abs (ℝ (number))). - Let int16bit be int
modulo 216. - If int16bit ≥ 215, return
𝔽 (int16bit - 216); otherwise return𝔽 (int16bit).
7.1.9 ToUint16 ( argument )
The abstract operation ToUint16 takes argument argument. It converts argument to one of 216
7.1.10 ToInt8 ( argument )
The abstract operation ToInt8 takes argument argument. It converts argument to one of 28
7.1.11 ToUint8 ( argument )
The abstract operation ToUint8 takes argument argument. It converts argument to one of 28
7.1.12 ToUint8Clamp ( argument )
The abstract operation ToUint8Clamp takes argument argument. It converts argument to one of 28
Unlike the other ECMAScript Math.round which does “round half up” tie-breaking.
7.1.13 ToBigInt ( argument )
The abstract operation ToBigInt takes argument argument. It converts argument to a BigInt value, or throws if an implicit conversion from Number would be required. It performs the following steps when called:
- Let prim be ?
ToPrimitive (argument,number ). - Return the value that prim corresponds to in
Table 13 .
| Argument Type | Result |
|---|---|
| Undefined |
Throw a |
| Null |
Throw a |
| Boolean |
Return 1n if prim is 0n if prim is |
| BigInt | Return prim. |
| Number |
Throw a |
| String |
|
| Symbol |
Throw a |
7.1.14 StringToBigInt ( argument )
Apply the algorithm in
- Replace the
StrUnsignedDecimalLiteral production withDecimalDigits to not allowInfinity , decimal points, or exponents. - If the MV is
NaN , returnNaN , otherwise return the BigInt which exactly corresponds to the MV, rather than rounding to a Number.
7.1.15 ToBigInt64 ( argument )
The abstract operation ToBigInt64 takes argument argument. It converts argument to one of 264 BigInt values in the range
7.1.16 ToBigUint64 ( argument )
The abstract operation ToBigUint64 takes argument argument. It converts argument to one of 264 BigInt values in the range
7.1.17 ToString ( argument )
The abstract operation ToString takes argument argument. It converts argument to a value of type String according to
| Argument Type | Result |
|---|---|
| Undefined |
Return |
| Null |
Return |
| Boolean |
If argument is If argument is |
| Number |
Return ! |
| String | Return argument. |
| Symbol |
Throw a |
| BigInt |
Return ! |
| Object |
Apply the following steps:
|
7.1.18 ToObject ( argument )
The abstract operation ToObject takes argument argument. It converts argument to a value of type Object according to
| Argument Type | Result |
|---|---|
| Undefined |
Throw a |
| Null |
Throw a |
| Boolean |
Return a new Boolean object whose [[BooleanData]] internal slot is set to argument. See |
| Number |
Return a new Number object whose [[NumberData]] internal slot is set to argument. See |
| String |
Return a new String object whose [[StringData]] internal slot is set to argument. See |
| Symbol |
Return a new Symbol object whose [[SymbolData]] internal slot is set to argument. See |
| BigInt |
Return a new BigInt object whose [[BigIntData]] internal slot is set to argument. See |
| Object | Return argument. |
7.1.19 ToPropertyKey ( argument )
The abstract operation ToPropertyKey takes argument argument. It converts argument to a value that can be used as a property key. It performs the following steps when called:
- Let key be ?
ToPrimitive (argument,string ). - If
Type (key) is Symbol, then- Return key.
- Return !
ToString (key).
7.1.20 ToLength ( argument )
The abstract operation ToLength takes argument argument. It converts argument to an
- Let len be ?
ToIntegerOrInfinity (argument). - If len ≤ 0, return
+0 𝔽. - Return
𝔽 (min (len, 253 - 1)).
7.1.21 CanonicalNumericIndexString ( argument )
The abstract operation CanonicalNumericIndexString takes argument argument. It returns argument converted to a
A canonical numeric string is any String value for which the CanonicalNumericIndexString abstract operation does not return
7.1.22 ToIndex ( value )
The abstract operation ToIndex takes argument value. It returns value argument converted to a non-negative
- If value is
undefined , then- Return 0.
- Else,
- Let integerIndex be
𝔽 (?ToIntegerOrInfinity (value)). - If integerIndex <
+0 𝔽, throw aRangeError exception. - Let index be !
ToLength (integerIndex). - If !
SameValue (integerIndex, index) isfalse , throw aRangeError exception. - Return
ℝ (index).
- Let integerIndex be
7.2 Testing and Comparison Operations
7.2.1 RequireObjectCoercible ( argument )
The abstract operation RequireObjectCoercible takes argument argument. It throws an error if argument is a value that cannot be converted to an Object using
| Argument Type | Result |
|---|---|
| Undefined |
Throw a |
| Null |
Throw a |
| Boolean | Return argument. |
| Number | Return argument. |
| String | Return argument. |
| Symbol | Return argument. |
| BigInt | Return argument. |
| Object | Return argument. |
7.2.2 IsArray ( argument )
The abstract operation IsArray takes argument argument. It performs the following steps when called:
- If
Type (argument) is not Object, returnfalse . - If argument is an
Array exotic object , returntrue . - If argument is a
Proxy exotic object , then- If argument.[[ProxyHandler]] is
null , throw aTypeError exception. - Let target be argument.[[ProxyTarget]].
- Return ?
IsArray (target).
- If argument.[[ProxyHandler]] is
- Return
false .
7.2.3 IsCallable ( argument )
The abstract operation IsCallable takes argument argument (an
- If
Type (argument) is not Object, returnfalse . - If argument has a [[Call]] internal method, return
true . - Return
false .
7.2.4 IsConstructor ( argument )
The abstract operation IsConstructor takes argument argument (an
- If
Type (argument) is not Object, returnfalse . - If argument has a [[Construct]] internal method, return
true . - Return
false .
7.2.5 IsExtensible ( O )
The abstract operation IsExtensible takes argument O (an Object) and returns a completion record which, if its [[Type]] is
7.2.6 IsIntegralNumber ( argument )
The abstract operation IsIntegralNumber takes argument argument. It determines if argument is a finite
7.2.7 IsPropertyKey ( argument )
The abstract operation IsPropertyKey takes argument argument (an
7.2.8 IsRegExp ( argument )
The abstract operation IsRegExp takes argument argument. It performs the following steps when called:
7.2.9 IsStringPrefix ( p, q )
The abstract operation IsStringPrefix takes arguments p (a String) and q (a String). It determines if p is a prefix of q. It performs the following steps when called:
Assert :Type (p) is String.Assert :Type (q) is String.- If q can be the
string-concatenation of p and some other String r, returntrue . Otherwise, returnfalse .
Any String is a prefix of itself, because r may be the empty String.
7.2.10 SameValue ( x, y )
The abstract operation SameValue takes arguments x (an
- If
Type (x) is different fromType (y), returnfalse . - If
Type (x) is Number or BigInt, then- Return !
Type (x)::sameValue(x, y).
- Return !
- Return !
SameValueNonNumeric (x, y).
This algorithm differs from the
7.2.11 SameValueZero ( x, y )
The abstract operation SameValueZero takes arguments x (an
- If
Type (x) is different fromType (y), returnfalse . - If
Type (x) is Number or BigInt, then- Return !
Type (x)::sameValueZero(x, y).
- Return !
- Return !
SameValueNonNumeric (x, y).
SameValueZero differs from
7.2.12 SameValueNonNumeric ( x, y )
The abstract operation SameValueNonNumeric takes arguments x (an
Assert :Type (x) is not Number or BigInt.Assert :Type (x) is the same asType (y).- If
Type (x) is Undefined, returntrue . - If
Type (x) is Null, returntrue . - If
Type (x) is String, then- If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return
true ; otherwise, returnfalse .
- If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), return
- If
Type (x) is Boolean, then- If x and y are both
true or bothfalse , returntrue ; otherwise, returnfalse .
- If x and y are both
- If
Type (x) is Symbol, then- If x and y are both the same Symbol value, return
true ; otherwise, returnfalse .
- If x and y are both the same Symbol value, return
- If x and y are the same Object value, return
true . Otherwise, returnfalse .
7.2.13 Abstract Relational Comparison
The comparison x < y, where x and y are values, produces
- If the LeftFirst flag is
true , then- Let px be ?
ToPrimitive (x,number ). - Let py be ?
ToPrimitive (y,number ).
- Let px be ?
- Else,
- NOTE: The order of evaluation needs to be reversed to preserve left to right evaluation.
- Let py be ?
ToPrimitive (y,number ). - Let px be ?
ToPrimitive (x,number ).
- If
Type (px) is String andType (py) is String, then- If
IsStringPrefix (py, px) istrue , returnfalse . - If
IsStringPrefix (px, py) istrue , returntrue . - Let k be the smallest non-negative
integer such that the code unit at index k within px is different from the code unit at index k within py. (There must be such a k, for neither String is a prefix of the other.) - Let m be the
integer that is the numeric value of the code unit at index k within px. - Let n be the
integer that is the numeric value of the code unit at index k within py. - If m < n, return
true . Otherwise, returnfalse .
- If
- Else,
- If
Type (px) is BigInt andType (py) is String, then- Let ny be !
StringToBigInt (py). - If ny is
NaN , returnundefined . - Return BigInt::lessThan(px, ny).
- Let ny be !
- If
Type (px) is String andType (py) is BigInt, then- Let nx be !
StringToBigInt (px). - If nx is
NaN , returnundefined . - Return BigInt::lessThan(nx, py).
- Let nx be !
- NOTE: Because px and py are primitive values, evaluation order is not important.
- Let nx be !
ToNumeric (px). - Let ny be !
ToNumeric (py). - If
Type (nx) is the same asType (ny), returnType (nx)::lessThan(nx, ny). Assert :Type (nx) is BigInt andType (ny) is Number, orType (nx) is Number andType (ny) is BigInt.- If nx or ny is
NaN , returnundefined . - If nx is
-∞ 𝔽 or ny is+∞ 𝔽, returntrue . - If nx is
+∞ 𝔽 or ny is-∞ 𝔽, returnfalse . - If
ℝ (nx) <ℝ (ny), returntrue ; otherwise returnfalse .
- If
The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.
7.2.14 Abstract Equality Comparison
The comparison x == y, where x and y are values, produces
- If
Type (x) is the same asType (y), then- Return the result of performing
Strict Equality Comparison x === y.
- Return the result of performing
- If x is
null and y isundefined , returntrue . - If x is
undefined and y isnull , returntrue . - NOTE: This step is replaced in section
B.3.7.2 . - If
Type (x) is Number andType (y) is String, return the result of the comparison x == !ToNumber (y). - If
Type (x) is String andType (y) is Number, return the result of the comparison !ToNumber (x) == y. - If
Type (x) is BigInt andType (y) is String, then- Let n be !
StringToBigInt (y). - If n is
NaN , returnfalse . - Return the result of the comparison x == n.
- Let n be !
- If
Type (x) is String andType (y) is BigInt, return the result of the comparison y == x. - If
Type (x) is Boolean, return the result of the comparison !ToNumber (x) == y. - If
Type (y) is Boolean, return the result of the comparison x == !ToNumber (y). - If
Type (x) is either String, Number, BigInt, or Symbol andType (y) is Object, return the result of the comparison x == ?ToPrimitive (y). - If
Type (x) is Object andType (y) is either String, Number, BigInt, or Symbol, return the result of the comparison ?ToPrimitive (x) == y. - If
Type (x) is BigInt andType (y) is Number, or ifType (x) is Number andType (y) is BigInt, then - Return
false .
7.2.15 Strict Equality Comparison
The comparison x === y, where x and y are values, produces
- If
Type (x) is different fromType (y), returnfalse . - If
Type (x) is Number or BigInt, then- Return !
Type (x)::equal(x, y).
- Return !
- Return !
SameValueNonNumeric (x, y).
This algorithm differs from the
7.3 Operations on Objects
7.3.1 MakeBasicObject ( internalSlotsList )
The abstract operation MakeBasicObject takes argument internalSlotsList. It is the source of all ECMAScript objects that are created algorithmically, including both ordinary objects and exotic objects. It factors out common steps used in creating all objects, and centralizes object creation. It performs the following steps when called:
Assert : internalSlotsList is aList of internal slot names.- Let obj be a newly created object with an internal slot for each name in internalSlotsList.
- Set obj's essential internal methods to the default
ordinary object definitions specified in10.1 . Assert : If the caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList contains [[Prototype]].Assert : If the caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential internal methods, then internalSlotsList contains [[Extensible]].- If internalSlotsList contains [[Extensible]], set obj.[[Extensible]] to
true . - Return obj.
Within this specification, exotic objects are created in
7.3.2 Get ( O, P )
The abstract operation Get takes arguments O (an Object) and P (a property key). It is used to retrieve the value of a specific property of an object. It performs the following steps when called:
Assert :Type (O) is Object.Assert :IsPropertyKey (P) istrue .- Return ? O.[[Get]](P, O).
7.3.3 GetV ( V, P )
The abstract operation GetV takes arguments V (an
Assert :IsPropertyKey (P) istrue .- Let O be ?
ToObject (V). - Return ? O.[[Get]](P, V).
7.3.4 Set ( O, P, V, Throw )
The abstract operation Set takes arguments O (an Object), P (a property key), V (an
7.3.5 CreateDataProperty ( O, P, V )
The abstract operation CreateDataProperty takes arguments O (an Object), P (a property key), and V (an
Assert :Type (O) is Object.Assert :IsPropertyKey (P) istrue .- Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true , [[Enumerable]]:true , [[Configurable]]:true }. - Return ? O.[[DefineOwnProperty]](P, newDesc).
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return
7.3.6 CreateMethodProperty ( O, P, V )
The abstract operation CreateMethodProperty takes arguments O (an Object), P (a property key), and V (an
Assert :Type (O) is Object.Assert :IsPropertyKey (P) istrue .- Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true , [[Enumerable]]:false , [[Configurable]]:true }. - Return ? O.[[DefineOwnProperty]](P, newDesc).
This abstract operation creates a property whose attributes are set to the same defaults used for built-in methods and methods defined using class declaration syntax. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return
7.3.7 CreateDataPropertyOrThrow ( O, P, V )
The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an
Assert :Type (O) is Object.Assert :IsPropertyKey (P) istrue .- Let success be ?
CreateDataProperty (O, P, V). - If success is
false , throw aTypeError exception. - Return success.
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return
7.3.8 DefinePropertyOrThrow ( O, P, desc )
The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a property key), and desc (a
Assert :Type (O) is Object.Assert :IsPropertyKey (P) istrue .- Let success be ? O.[[DefineOwnProperty]](P, desc).
- If success is
false , throw aTypeError exception. - Return success.
7.3.9 DeletePropertyOrThrow ( O, P )
The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a property key). It is used to remove a specific own property of an object. It throws an exception if the property is not configurable. It performs the following steps when called:
Assert :Type (O) is Object.Assert :IsPropertyKey (P) istrue .- Let success be ? O.[[Delete]](P).
- If success is
false , throw aTypeError exception. - Return success.
7.3.10 GetMethod ( V, P )
The abstract operation GetMethod takes arguments V (an
Assert :IsPropertyKey (P) istrue .- Let func be ?
GetV (V, P). - If func is either
undefined ornull , returnundefined . - If
IsCallable (func) isfalse , throw aTypeError exception. - Return func.
7.3.11 HasProperty ( O, P )
The abstract operation HasProperty takes arguments O (an Object) and P (a property key) and returns a completion record which, if its [[Type]] is
Assert :Type (O) is Object.Assert :IsPropertyKey (P) istrue .- Return ? O.[[HasProperty]](P).
7.3.12 HasOwnProperty ( O, P )
The abstract operation HasOwnProperty takes arguments O (an Object) and P (a property key) and returns a completion record which, if its [[Type]] is
Assert :Type (O) is Object.Assert :IsPropertyKey (P) istrue .- Let desc be ? O.[[GetOwnProperty]](P).
- If desc is
undefined , returnfalse . - Return
true .
7.3.13 Call ( F, V [ , argumentsList ] )
The abstract operation Call takes arguments F (an
- If argumentsList is not present, set argumentsList to a new empty
List . - If
IsCallable (F) isfalse , throw aTypeError exception. - Return ? F.[[Call]](V, argumentsList).
7.3.14 Construct ( F [ , argumentsList [ , newTarget ] ] )
The abstract operation Construct takes argument F (a
- If newTarget is not present, set newTarget to F.
- If argumentsList is not present, set argumentsList to a new empty
List . Assert :IsConstructor (F) istrue .Assert :IsConstructor (newTarget) istrue .- Return ? F.[[Construct]](argumentsList, newTarget).
If newTarget is not present, this operation is equivalent to: new F(...argumentsList)
7.3.15 SetIntegrityLevel ( O, level )
The abstract operation SetIntegrityLevel takes arguments O and level. It is used to fix the set of own properties of an object. It performs the following steps when called:
Assert :Type (O) is Object.Assert : level is eithersealed orfrozen .- Let status be ? O.[[PreventExtensions]]().
- If status is
false , returnfalse . - Let keys be ? O.[[OwnPropertyKeys]]().
- If level is
sealed , then- For each element k of keys, do
- Perform ?
DefinePropertyOrThrow (O, k, PropertyDescriptor { [[Configurable]]:false }).
- Perform ?
- For each element k of keys, do
- Else,
Assert : level isfrozen .- For each element k of keys, do
- Let currentDesc be ? O.[[GetOwnProperty]](k).
- If currentDesc is not
undefined , then- If
IsAccessorDescriptor (currentDesc) istrue , then- Let desc be the PropertyDescriptor { [[Configurable]]:
false }.
- Let desc be the PropertyDescriptor { [[Configurable]]:
- Else,
- Let desc be the PropertyDescriptor { [[Configurable]]:
false , [[Writable]]:false }.
- Let desc be the PropertyDescriptor { [[Configurable]]:
- Perform ?
DefinePropertyOrThrow (O, k, desc).
- If
- Return
true .
7.3.16 TestIntegrityLevel ( O, level )
The abstract operation TestIntegrityLevel takes arguments O and level. It is used to determine if the set of own properties of an object are fixed. It performs the following steps when called:
Assert :Type (O) is Object.Assert : level is eithersealed orfrozen .- Let extensible be ?
IsExtensible (O). - If extensible is
true , returnfalse . - NOTE: If the object is extensible, none of its properties are examined.
- Let keys be ? O.[[OwnPropertyKeys]]().
- For each element k of keys, do
- Let currentDesc be ? O.[[GetOwnProperty]](k).
- If currentDesc is not
undefined , then- If currentDesc.[[Configurable]] is
true , returnfalse . - If level is
frozen andIsDataDescriptor (currentDesc) istrue , then- If currentDesc.[[Writable]] is
true , returnfalse .
- If currentDesc.[[Writable]] is
- If currentDesc.[[Configurable]] is
- Return
true .
7.3.17 CreateArrayFromList ( elements )
The abstract operation CreateArrayFromList takes argument elements (a
Assert : elements is aList whose elements are all ECMAScript language values.- Let array be !
ArrayCreate (0). - Let n be 0.
- For each element e of elements, do
- Perform !
CreateDataPropertyOrThrow (array, !ToString (𝔽 (n)), e). - Set n to n + 1.
- Perform !
- Return array.
7.3.18 LengthOfArrayLike ( obj )
The abstract operation LengthOfArrayLike takes argument obj. It returns the value of the
An array-like object is any object for which this operation returns an
7.3.19 CreateListFromArrayLike ( obj [ , elementTypes ] )
The abstract operation CreateListFromArrayLike takes argument obj and optional argument elementTypes (a
- If elementTypes is not present, set elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, BigInt, Object ».
- If
Type (obj) is not Object, throw aTypeError exception. - Let len be ?
LengthOfArrayLike (obj). - Let list be a new empty
List . - Let index be 0.
- Repeat, while index < len,
- Return list.
7.3.20 Invoke ( V, P [ , argumentsList ] )
The abstract operation Invoke takes arguments V (an
Assert :IsPropertyKey (P) istrue .- If argumentsList is not present, set argumentsList to a new empty
List . - Let func be ?
GetV (V, P). - Return ?
Call (func, V, argumentsList).
7.3.21 OrdinaryHasInstance ( C, O )
The abstract operation OrdinaryHasInstance takes arguments C (an
- If
IsCallable (C) isfalse , returnfalse . - If C has a [[BoundTargetFunction]] internal slot, then
- Let BC be C.[[BoundTargetFunction]].
- Return ?
InstanceofOperator (O, BC).
- If
Type (O) is not Object, returnfalse . - Let P be ?
Get (C,"prototype" ). - If
Type (P) is not Object, throw aTypeError exception. - Repeat,
- Set O to ? O.[[GetPrototypeOf]]().
- If O is
null , returnfalse . - If
SameValue (P, O) istrue , returntrue .
7.3.22 SpeciesConstructor ( O, defaultConstructor )
The abstract operation SpeciesConstructor takes arguments O (an Object) and defaultConstructor (a
Assert :Type (O) is Object.- Let C be ?
Get (O,"constructor" ). - If C is
undefined , return defaultConstructor. - If
Type (C) is not Object, throw aTypeError exception. - Let S be ?
Get (C,@@species ). - If S is either
undefined ornull , return defaultConstructor. - If
IsConstructor (S) istrue , return S. - Throw a
TypeError exception.
7.3.23 EnumerableOwnPropertyNames ( O, kind )
The abstract operation EnumerableOwnPropertyNames takes arguments O (an Object) and kind (one of
Assert :Type (O) is Object.- Let ownKeys be ? O.[[OwnPropertyKeys]]().
- Let properties be a new empty
List . - For each element key of ownKeys, do
- If
Type (key) is String, then- Let desc be ? O.[[GetOwnProperty]](key).
- If desc is not
undefined and desc.[[Enumerable]] istrue , then- If kind is
key , append key to properties. - Else,
- Let value be ?
Get (O, key). - If kind is
value , append value to properties. - Else,
Assert : kind iskey+value .- Let entry be !
CreateArrayFromList (« key, value »). - Append entry to properties.
- Let value be ?
- If kind is
- If
- Return properties.
7.3.24 GetFunctionRealm ( obj )
The abstract operation GetFunctionRealm takes argument obj. It performs the following steps when called:
Assert : !IsCallable (obj) istrue .- If obj has a [[Realm]] internal slot, then
- Return obj.[[Realm]].
- If obj is a
bound function exotic object , then- Let target be obj.[[BoundTargetFunction]].
- Return ?
GetFunctionRealm (target).
- If obj is a
Proxy exotic object , then- If obj.[[ProxyHandler]] is
null , throw aTypeError exception. - Let proxyTarget be obj.[[ProxyTarget]].
- Return ?
GetFunctionRealm (proxyTarget).
- If obj.[[ProxyHandler]] is
- Return
the current Realm Record .
Step
7.3.25 CopyDataProperties ( target, source, excludedItems )
The abstract operation CopyDataProperties takes arguments target, source, and excludedItems. It performs the following steps when called:
Assert :Type (target) is Object.Assert : excludedItems is aList of property keys.- If source is
undefined ornull , return target. - Let from be !
ToObject (source). - Let keys be ? from.[[OwnPropertyKeys]]().
- For each element nextKey of keys, do
- Let excluded be
false . - For each element e of excludedItems, do
- If
SameValue (e, nextKey) istrue , then- Set excluded to
true .
- Set excluded to
- If
- If excluded is
false , then- Let desc be ? from.[[GetOwnProperty]](nextKey).
- If desc is not
undefined and desc.[[Enumerable]] istrue , then- Let propValue be ?
Get (from, nextKey). - Perform !
CreateDataPropertyOrThrow (target, nextKey, propValue).
- Let propValue be ?
- Let excluded be
- Return target.
The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.
7.4 Operations on Iterator Objects
See Common Iteration Interfaces (
7.4.1 GetIterator ( obj [ , hint [ , method ] ] )
The abstract operation GetIterator takes argument obj and optional arguments hint and method. It performs the following steps when called:
- If hint is not present, set hint to
sync . Assert : hint is eithersync orasync .- If method is not present, then
- If hint is
async , then- Set method to ?
GetMethod (obj,@@asyncIterator ). - If method is
undefined , then- Let syncMethod be ?
GetMethod (obj,@@iterator ). - Let syncIteratorRecord be ?
GetIterator (obj,sync , syncMethod). - Return !
CreateAsyncFromSyncIterator (syncIteratorRecord).
- Let syncMethod be ?
- Set method to ?
- Otherwise, set method to ?
GetMethod (obj,@@iterator ).
- If hint is
- Let iterator be ?
Call (method, obj). - If
Type (iterator) is not Object, throw aTypeError exception. - Let nextMethod be ?
GetV (iterator,"next" ). - Let iteratorRecord be the
Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]:false }. - Return iteratorRecord.
7.4.2 IteratorNext ( iteratorRecord [ , value ] )
The abstract operation IteratorNext takes argument iteratorRecord and optional argument value. It performs the following steps when called:
7.4.3 IteratorComplete ( iterResult )
The abstract operation IteratorComplete takes argument iterResult. It performs the following steps when called:
7.4.4 IteratorValue ( iterResult )
The abstract operation IteratorValue takes argument iterResult. It performs the following steps when called:
7.4.5 IteratorStep ( iteratorRecord )
The abstract operation IteratorStep takes argument iteratorRecord. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either
- Let result be ?
IteratorNext (iteratorRecord). - Let done be ?
IteratorComplete (result). - If done is
true , returnfalse . - Return result.
7.4.6 IteratorClose ( iteratorRecord, completion )
The abstract operation IteratorClose takes arguments iteratorRecord and completion. It is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:
Assert :Type (iteratorRecord.[[Iterator]]) is Object.Assert : completion is aCompletion Record .- Let iterator be iteratorRecord.[[Iterator]].
- Let innerResult be
GetMethod (iterator,"return" ). - If innerResult.[[Type]] is
normal , then- Let return be innerResult.[[Value]].
- If return is
undefined , returnCompletion (completion). - Set innerResult to
Call (return, iterator).
- If completion.[[Type]] is
throw , returnCompletion (completion). - If innerResult.[[Type]] is
throw , returnCompletion (innerResult). - If
Type (innerResult.[[Value]]) is not Object, throw aTypeError exception. - Return
Completion (completion).
7.4.7 AsyncIteratorClose ( iteratorRecord, completion )
The abstract operation AsyncIteratorClose takes arguments iteratorRecord and completion. It is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:
Assert :Type (iteratorRecord.[[Iterator]]) is Object.Assert : completion is aCompletion Record .- Let iterator be iteratorRecord.[[Iterator]].
- Let innerResult be
GetMethod (iterator,"return" ). - If innerResult.[[Type]] is
normal , then- Let return be innerResult.[[Value]].
- If return is
undefined , returnCompletion (completion). - Set innerResult to
Call (return, iterator). - If innerResult.[[Type]] is
normal , set innerResult toAwait (innerResult.[[Value]]).
- If completion.[[Type]] is
throw , returnCompletion (completion). - If innerResult.[[Type]] is
throw , returnCompletion (innerResult). - If
Type (innerResult.[[Value]]) is not Object, throw aTypeError exception. - Return
Completion (completion).
7.4.8 CreateIterResultObject ( value, done )
The abstract operation CreateIterResultObject takes arguments value and done. It creates an object that supports the IteratorResult interface. It performs the following steps when called:
Assert :Type (done) is Boolean.- Let obj be !
OrdinaryObjectCreate (%Object.prototype% ). - Perform !
CreateDataPropertyOrThrow (obj,"value" , value). - Perform !
CreateDataPropertyOrThrow (obj,"done" , done). - Return obj.
7.4.9 CreateListIteratorRecord ( list )
The abstract operation CreateListIteratorRecord takes argument list. It creates an Iterator (
- Let closure be a new
Abstract Closure with no parameters that captures list and performs the following steps when called:- For each element E of list, do
- Perform ?
Yield (E).
- Perform ?
- Return
undefined .
- For each element E of list, do
- Let iterator be !
CreateIteratorFromClosure (closure,empty ,%IteratorPrototype% ). - Return
Record { [[Iterator]]: iterator, [[NextMethod]]: %GeneratorFunction.prototype.prototype.next%, [[Done]]:false }.
The list iterator object is never directly accessible to ECMAScript code.
7.4.10 IterableToList ( items [ , method ] )
The abstract operation IterableToList takes argument items and optional argument method. It performs the following steps when called:
- If method is present, then
- Let iteratorRecord be ?
GetIterator (items,sync , method).
- Let iteratorRecord be ?
- Else,
- Let iteratorRecord be ?
GetIterator (items,sync ).
- Let iteratorRecord be ?
- Let values be a new empty
List . - Let next be
true . - Repeat, while next is not
false ,- Set next to ?
IteratorStep (iteratorRecord). - If next is not
false , then- Let nextValue be ?
IteratorValue (next). - Append nextValue to the end of the
List values.
- Let nextValue be ?
- Set next to ?
- Return values.
8 Syntax-Directed Operations
In addition to those defined in this section, specialized syntax-directed operations are defined throughout this specification.
8.1 Scope Analysis
8.1.1 Static Semantics: BoundNames
- Return a
List whose sole element is theStringValue ofIdentifier .
- Return a
List whose sole element is"yield" .
- Return a
List whose sole element is"await" .
- Return the
BoundNames ofBindingList .
- Let names be the
BoundNames ofBindingList . - Append to names the elements of the
BoundNames ofLexicalBinding . - Return names.
- Return the
BoundNames ofBindingIdentifier .
- Return the
BoundNames ofBindingPattern .
- Let names be
BoundNames ofVariableDeclarationList . - Append to names the elements of
BoundNames ofVariableDeclaration . - Return names.
- Return the
BoundNames ofBindingIdentifier .
- Return the
BoundNames ofBindingPattern .
- Return a new empty
List .
- Let names be
BoundNames ofBindingPropertyList . - Append to names the elements of
BoundNames ofBindingRestProperty . - Return names.
- Return a new empty
List .
- Return the
BoundNames ofBindingRestElement .
- Return the
BoundNames ofBindingElementList .
- Let names be
BoundNames ofBindingElementList . - Append to names the elements of
BoundNames ofBindingRestElement . - Return names.
- Let names be
BoundNames ofBindingPropertyList . - Append to names the elements of
BoundNames ofBindingProperty . - Return names.
- Let names be
BoundNames ofBindingElementList . - Append to names the elements of
BoundNames ofBindingElisionElement . - Return names.
- Return
BoundNames ofBindingElement .
- Return the
BoundNames ofBindingElement .
- Return the
BoundNames ofBindingIdentifier .
- Return the
BoundNames ofBindingPattern .
- Return the
BoundNames ofForBinding .
- Return the
BoundNames ofBindingIdentifier .
- Return «
"*default*" ».
- Return a new empty
List .
- Let names be
BoundNames ofFormalParameterList . - Append to names the
BoundNames ofFunctionRestParameter . - Return names.
- Let names be
BoundNames ofFormalParameterList . - Append to names the
BoundNames ofFormalParameter . - Return names.
- Let formals be
CoveredFormalsList ofCoverParenthesizedExpressionAndArrowParameterList . - Return the
BoundNames of formals.
- Return the
BoundNames ofBindingIdentifier .
- Return «
"*default*" ».
- Return the
BoundNames ofBindingIdentifier .
- Return «
"*default*" ».
- Return the
BoundNames ofBindingIdentifier .
- Return «
"*default*" ».
- Return the
BoundNames ofBindingIdentifier .
- Return «
"*default*" ».
- Let head be
CoveredAsyncArrowHead ofCoverCallExpressionAndAsyncArrowHead . - Return the
BoundNames of head.
- Return the
BoundNames ofImportClause .
- Return a new empty
List .
- Let names be the
BoundNames ofImportedDefaultBinding . - Append to names the elements of the
BoundNames ofNameSpaceImport . - Return names.
- Let names be the
BoundNames ofImportedDefaultBinding . - Append to names the elements of the
BoundNames ofNamedImports . - Return names.
- Return a new empty
List .
- Let names be the
BoundNames ofImportsList . - Append to names the elements of the
BoundNames ofImportSpecifier . - Return names.
- Return the
BoundNames ofImportedBinding .
- Return a new empty
List .
- Return the
BoundNames ofVariableStatement .
- Return the
BoundNames ofDeclaration .
- Let declarationNames be the
BoundNames ofHoistableDeclaration . - If declarationNames does not include the element
"*default*" , append"*default*" to declarationNames. - Return declarationNames.
- Let declarationNames be the
BoundNames ofClassDeclaration . - If declarationNames does not include the element
"*default*" , append"*default*" to declarationNames. - Return declarationNames.
- Return «
"*default*" ».
8.1.2 Static Semantics: DeclarationPart
- Return
FunctionDeclaration .
- Return
GeneratorDeclaration .
- Return
AsyncFunctionDeclaration .
- Return
AsyncGeneratorDeclaration .
- Return
ClassDeclaration .
- Return
LexicalDeclaration .
8.1.3 Static Semantics: IsConstantDeclaration
- Return
IsConstantDeclaration ofLetOrConst .
- Return
false .
- Return
true .
- Return
false .
- Return
false .
- Return
false .
It is not necessary to treat export default
8.1.4 Static Semantics: LexicallyDeclaredNames
- Return a new empty
List .
- Let names be
LexicallyDeclaredNames ofStatementList . - Append to names the elements of the
LexicallyDeclaredNames ofStatementListItem . - Return names.
- If
Statement isStatement : LabelledStatement LexicallyDeclaredNames ofLabelledStatement . - Return a new empty
List .
- Return the
BoundNames ofDeclaration .
- Return a new empty
List .
- If the first
CaseClauses is present, let names be theLexicallyDeclaredNames of the firstCaseClauses . - Else, let names be a new empty
List . - Append to names the elements of the
LexicallyDeclaredNames ofDefaultClause . - If the second
CaseClauses is not present, return names. - Return the result of appending to names the elements of the
LexicallyDeclaredNames of the secondCaseClauses .
- Let names be
LexicallyDeclaredNames ofCaseClauses . - Append to names the elements of the
LexicallyDeclaredNames ofCaseClause . - Return names.
- If the
StatementList is present, return theLexicallyDeclaredNames ofStatementList . - Return a new empty
List .
- If the
StatementList is present, return theLexicallyDeclaredNames ofStatementList . - Return a new empty
List .
- Return the
LexicallyDeclaredNames ofLabelledItem .
- Return a new empty
List .
- Return
BoundNames ofFunctionDeclaration .
- Return a new empty
List .
- Return
TopLevelLexicallyDeclaredNames ofStatementList .
- Return a new empty
List .
- Return a new empty
List .
- Return
TopLevelLexicallyDeclaredNames ofStatementList .
At the top level of a
The LexicallyDeclaredNames of a
- Let names be
LexicallyDeclaredNames ofModuleItemList . - Append to names the elements of the
LexicallyDeclaredNames ofModuleItem . - Return names.
- Return the
BoundNames ofImportDeclaration .
- If
ExportDeclaration isexportVariableStatement , return a new emptyList . - Return the
BoundNames ofExportDeclaration .
- Return
LexicallyDeclaredNames ofStatementListItem .
At the top level of a
8.1.5 Static Semantics: LexicallyScopedDeclarations
- Let declarations be
LexicallyScopedDeclarations ofStatementList . - Append to declarations the elements of the
LexicallyScopedDeclarations ofStatementListItem . - Return declarations.
- If
Statement isStatement : LabelledStatement LexicallyScopedDeclarations ofLabelledStatement . - Return a new empty
List .
- Return a
List whose sole element isDeclarationPart ofDeclaration .
- Return a new empty
List .
- If the first
CaseClauses is present, let declarations be theLexicallyScopedDeclarations of the firstCaseClauses . - Else, let declarations be a new empty
List . - Append to declarations the elements of the
LexicallyScopedDeclarations ofDefaultClause . - If the second
CaseClauses is not present, return declarations. - Return the result of appending to declarations the elements of the
LexicallyScopedDeclarations of the secondCaseClauses .
- Let declarations be
LexicallyScopedDeclarations ofCaseClauses . - Append to declarations the elements of the
LexicallyScopedDeclarations ofCaseClause . - Return declarations.
- If the
StatementList is present, return theLexicallyScopedDeclarations ofStatementList . - Return a new empty
List .
- If the
StatementList is present, return theLexicallyScopedDeclarations ofStatementList . - Return a new empty
List .
- Return the
LexicallyScopedDeclarations ofLabelledItem .
- Return a new empty
List .
- Return a
List whose sole element isFunctionDeclaration .
- Return a new empty
List .
- Return the
TopLevelLexicallyScopedDeclarations ofStatementList .
- Return a new empty
List .
- Return a new empty
List .
- Return
TopLevelLexicallyScopedDeclarations ofStatementList .
- Return a new empty
List .
- Let declarations be
LexicallyScopedDeclarations ofModuleItemList . - Append to declarations the elements of the
LexicallyScopedDeclarations ofModuleItem . - Return declarations.
- Return a new empty
List .
- Return a new empty
List .
- Return a
List whose sole element isDeclarationPart ofDeclaration .
- Return a
List whose sole element isDeclarationPart ofHoistableDeclaration .
- Return a
List whose sole element isClassDeclaration .
- Return a
List whose sole element is thisExportDeclaration .
8.1.6 Static Semantics: VarDeclaredNames
- Return a new empty
List .
- Return a new empty
List .
- Let names be
VarDeclaredNames ofStatementList . - Append to names the elements of the
VarDeclaredNames ofStatementListItem . - Return names.
- Return a new empty
List .
- Return
BoundNames ofVariableDeclarationList .
- Let names be
VarDeclaredNames of the firstStatement . - Append to names the elements of the
VarDeclaredNames of the secondStatement . - Return names.
- Return the
VarDeclaredNames ofStatement .
- Return the
VarDeclaredNames ofStatement .
- Return the
VarDeclaredNames ofStatement .
- Return the
VarDeclaredNames ofStatement .
- Let names be
BoundNames ofVariableDeclarationList . - Append to names the elements of the
VarDeclaredNames ofStatement . - Return names.
- Return the
VarDeclaredNames ofStatement .
- Return the
VarDeclaredNames ofStatement .
- Let names be the
BoundNames ofForBinding . - Append to names the elements of the
VarDeclaredNames ofStatement . - Return names.
This section is extended by Annex
- Return the
VarDeclaredNames ofStatement .
- Return the
VarDeclaredNames ofCaseBlock .
- Return a new empty
List .
- If the first
CaseClauses is present, let names be theVarDeclaredNames of the firstCaseClauses . - Else, let names be a new empty
List . - Append to names the elements of the
VarDeclaredNames ofDefaultClause . - If the second
CaseClauses is not present, return names. - Return the result of appending to names the elements of the
VarDeclaredNames of the secondCaseClauses .
- Let names be
VarDeclaredNames ofCaseClauses . - Append to names the elements of the
VarDeclaredNames ofCaseClause . - Return names.
- If the
StatementList is present, return theVarDeclaredNames ofStatementList . - Return a new empty
List .
- If the
StatementList is present, return theVarDeclaredNames ofStatementList . - Return a new empty
List .
- Return the
VarDeclaredNames ofLabelledItem .
- Return a new empty
List .
- Let names be
VarDeclaredNames ofBlock . - Append to names the elements of the
VarDeclaredNames ofCatch . - Return names.
- Let names be
VarDeclaredNames ofBlock . - Append to names the elements of the
VarDeclaredNames ofFinally . - Return names.
- Let names be
VarDeclaredNames ofBlock . - Append to names the elements of the
VarDeclaredNames ofCatch . - Append to names the elements of the
VarDeclaredNames ofFinally . - Return names.
- Return the
VarDeclaredNames ofBlock .
- Return a new empty
List .
- Return
TopLevelVarDeclaredNames ofStatementList .
- Return a new empty
List .
- Return a new empty
List .
- Return
TopLevelVarDeclaredNames ofStatementList .
- Return a new empty
List .
- Let names be
VarDeclaredNames ofModuleItemList . - Append to names the elements of the
VarDeclaredNames ofModuleItem . - Return names.
- Return a new empty
List .
- If
ExportDeclaration isexportVariableStatement , returnBoundNames ofExportDeclaration . - Return a new empty
List .
8.1.7 Static Semantics: VarScopedDeclarations
- Return a new empty
List .
- Return a new empty
List .
- Let declarations be
VarScopedDeclarations ofStatementList . - Append to declarations the elements of the
VarScopedDeclarations ofStatementListItem . - Return declarations.
- Return a new empty
List .
- Return a
List whose sole element isVariableDeclaration .
- Let declarations be
VarScopedDeclarations ofVariableDeclarationList . - Append
VariableDeclaration to declarations. - Return declarations.
- Let declarations be
VarScopedDeclarations of the firstStatement . - Append to declarations the elements of the
VarScopedDeclarations of the secondStatement . - Return declarations.
- Return the
VarScopedDeclarations ofStatement .
- Return the
VarScopedDeclarations ofStatement .
- Return the
VarScopedDeclarations ofStatement .
- Return the
VarScopedDeclarations ofStatement .
- Let declarations be
VarScopedDeclarations ofVariableDeclarationList . - Append to declarations the elements of the
VarScopedDeclarations ofStatement . - Return declarations.
- Return the
VarScopedDeclarations ofStatement .
- Return the
VarScopedDeclarations ofStatement .
- Let declarations be a
List whose sole element isForBinding . - Append to declarations the elements of the
VarScopedDeclarations ofStatement . - Return declarations.
This section is extended by Annex
- Return the
VarScopedDeclarations ofStatement .
- Return the
VarScopedDeclarations ofCaseBlock .
- Return a new empty
List .
- If the first
CaseClauses is present, let declarations be theVarScopedDeclarations of the firstCaseClauses . - Else, let declarations be a new empty
List . - Append to declarations the elements of the
VarScopedDeclarations ofDefaultClause . - If the second
CaseClauses is not present, return declarations. - Return the result of appending to declarations the elements of the
VarScopedDeclarations of the secondCaseClauses .
- Let declarations be
VarScopedDeclarations ofCaseClauses . - Append to declarations the elements of the
VarScopedDeclarations ofCaseClause . - Return declarations.
- If the
StatementList is present, return theVarScopedDeclarations ofStatementList . - Return a new empty
List .
- If the
StatementList is present, return theVarScopedDeclarations ofStatementList . - Return a new empty
List .
- Return the
VarScopedDeclarations ofLabelledItem .
- Return a new empty
List .
- Let declarations be
VarScopedDeclarations ofBlock . - Append to declarations the elements of the
VarScopedDeclarations ofCatch . - Return declarations.
- Let declarations be
VarScopedDeclarations ofBlock . - Append to declarations the elements of the
VarScopedDeclarations ofFinally . - Return declarations.
- Let declarations be
VarScopedDeclarations ofBlock . - Append to declarations the elements of the
VarScopedDeclarations ofCatch . - Append to declarations the elements of the
VarScopedDeclarations ofFinally . - Return declarations.
- Return the
VarScopedDeclarations ofBlock .
- Return a new empty
List .
- Return the
TopLevelVarScopedDeclarations ofStatementList .
- Return a new empty
List .
- Return a new empty
List .
- Return
TopLevelVarScopedDeclarations ofStatementList .
- Return a new empty
List .
- Let declarations be
VarScopedDeclarations ofModuleItemList . - Append to declarations the elements of the
VarScopedDeclarations ofModuleItem . - Return declarations.
- Return a new empty
List .
- If
ExportDeclaration isexportVariableStatement , returnVarScopedDeclarations ofVariableStatement . - Return a new empty
List .
8.1.8 Static Semantics: TopLevelLexicallyDeclaredNames
- Let names be
TopLevelLexicallyDeclaredNames ofStatementList . - Append to names the elements of the
TopLevelLexicallyDeclaredNames ofStatementListItem . - Return names.
- Return a new empty
List .
- If
Declaration isDeclaration : HoistableDeclaration - Return « ».
- Return the
BoundNames ofDeclaration .
At the top level of a function, or script, function declarations are treated like var declarations rather than like lexical declarations.
- Return a new empty
List .
8.1.9 Static Semantics: TopLevelLexicallyScopedDeclarations
- Return a new empty
List .
- Let declarations be
TopLevelLexicallyScopedDeclarations ofStatementList . - Append to declarations the elements of the
TopLevelLexicallyScopedDeclarations ofStatementListItem . - Return declarations.
- Return a new empty
List .
- If
Declaration isDeclaration : HoistableDeclaration - Return « ».
- Return a
List whose sole element isDeclaration .
- Return a new empty
List .
8.1.10 Static Semantics: TopLevelVarDeclaredNames
- Return a new empty
List .
- Let names be
TopLevelVarDeclaredNames ofStatementList . - Append to names the elements of the
TopLevelVarDeclaredNames ofStatementListItem . - Return names.
- If
Declaration isDeclaration : HoistableDeclaration - Return the
BoundNames ofHoistableDeclaration .
- Return the
- Return a new empty
List .
- If
Statement isStatement : LabelledStatement TopLevelVarDeclaredNames ofStatement . - Return
VarDeclaredNames ofStatement .
At the top level of a function or script, inner function declarations are treated like var declarations.
- Return the
TopLevelVarDeclaredNames ofLabelledItem .
- If
Statement isStatement : LabelledStatement TopLevelVarDeclaredNames ofStatement . - Return
VarDeclaredNames ofStatement .
- Return
BoundNames ofFunctionDeclaration .
8.1.11 Static Semantics: TopLevelVarScopedDeclarations
- Return a new empty
List .
- Let declarations be
TopLevelVarScopedDeclarations ofStatementList . - Append to declarations the elements of the
TopLevelVarScopedDeclarations ofStatementListItem . - Return declarations.
- If
Statement isStatement : LabelledStatement TopLevelVarScopedDeclarations ofStatement . - Return
VarScopedDeclarations ofStatement .
- If
Declaration isDeclaration : HoistableDeclaration - Let declaration be
DeclarationPart ofHoistableDeclaration . - Return « declaration ».
- Let declaration be
- Return a new empty
List .
- Return the
TopLevelVarScopedDeclarations ofLabelledItem .
- If
Statement isStatement : LabelledStatement TopLevelVarScopedDeclarations ofStatement . - Return
VarScopedDeclarations ofStatement .
- Return a
List whose sole element isFunctionDeclaration .
8.2 Labels
8.2.1 Static Semantics: ContainsDuplicateLabels
With parameter labelSet.
- Return
false .
- Let hasDuplicates be
ContainsDuplicateLabels ofStatementList with argument labelSet. - If hasDuplicates is
true , returntrue . - Return
ContainsDuplicateLabels ofStatementListItem with argument labelSet.
- Let hasDuplicate be
ContainsDuplicateLabels of the firstStatement with argument labelSet. - If hasDuplicate is
true , returntrue . - Return
ContainsDuplicateLabels of the secondStatement with argument labelSet.
- Return
ContainsDuplicateLabels ofStatement with argument labelSet.
- Return
ContainsDuplicateLabels ofStatement with argument labelSet.
- Return
ContainsDuplicateLabels ofStatement with argument labelSet.
- Return
ContainsDuplicateLabels ofStatement with argument labelSet.
- Return
ContainsDuplicateLabels ofStatement with argument labelSet.
This section is extended by Annex
- Return
ContainsDuplicateLabels ofStatement with argument labelSet.
- Return
ContainsDuplicateLabels ofCaseBlock with argument labelSet.
- Return
false .
- If the first
CaseClauses is present, then- Let hasDuplicates be
ContainsDuplicateLabels of the firstCaseClauses with argument labelSet. - If hasDuplicates is
true , returntrue .
- Let hasDuplicates be
- Let hasDuplicates be
ContainsDuplicateLabels ofDefaultClause with argument labelSet. - If hasDuplicates is
true , returntrue . - If the second
CaseClauses is not present, returnfalse . - Return
ContainsDuplicateLabels of the secondCaseClauses with argument labelSet.
- Let hasDuplicates be
ContainsDuplicateLabels ofCaseClauses with argument labelSet. - If hasDuplicates is
true , returntrue . - Return
ContainsDuplicateLabels ofCaseClause with argument labelSet.
- If the
StatementList is present, returnContainsDuplicateLabels ofStatementList with argument labelSet. - Return
false .
- If the
StatementList is present, returnContainsDuplicateLabels ofStatementList with argument labelSet. - Return
false .
- Let label be the
StringValue ofLabelIdentifier . - If label is an element of labelSet, return
true . - Let newLabelSet be a copy of labelSet with label appended.
- Return
ContainsDuplicateLabels ofLabelledItem with argument newLabelSet.
- Return
false .
- Let hasDuplicates be
ContainsDuplicateLabels ofBlock with argument labelSet. - If hasDuplicates is
true , returntrue . - Return
ContainsDuplicateLabels ofCatch with argument labelSet.
- Let hasDuplicates be
ContainsDuplicateLabels ofBlock with argument labelSet. - If hasDuplicates is
true , returntrue . - Return
ContainsDuplicateLabels ofFinally with argument labelSet.
- Let hasDuplicates be
ContainsDuplicateLabels ofBlock with argument labelSet. - If hasDuplicates is
true , returntrue . - Let hasDuplicates be
ContainsDuplicateLabels ofCatch with argument labelSet. - If hasDuplicates is
true , returntrue . - Return
ContainsDuplicateLabels ofFinally with argument labelSet.
- Return
ContainsDuplicateLabels ofBlock with argument labelSet.
- Return
false .
- Let hasDuplicates be
ContainsDuplicateLabels ofModuleItemList with argument labelSet. - If hasDuplicates is
true , returntrue . - Return
ContainsDuplicateLabels ofModuleItem with argument labelSet.
- Return
false .
8.2.2 Static Semantics: ContainsUndefinedBreakTarget
With parameter labelSet.
- Return
false .
- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget ofStatementList with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedBreakTarget ofStatementListItem with argument labelSet.
- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget of the firstStatement with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedBreakTarget of the secondStatement with argument labelSet.
- Return
ContainsUndefinedBreakTarget ofStatement with argument labelSet.
- Return
ContainsUndefinedBreakTarget ofStatement with argument labelSet.
- Return
ContainsUndefinedBreakTarget ofStatement with argument labelSet.
- Return
ContainsUndefinedBreakTarget ofStatement with argument labelSet.
- Return
ContainsUndefinedBreakTarget ofStatement with argument labelSet.
This section is extended by Annex
- Return
false .
- If the
StringValue ofLabelIdentifier is not an element of labelSet, returntrue . - Return
false .
- Return
ContainsUndefinedBreakTarget ofStatement with argument labelSet.
- Return
ContainsUndefinedBreakTarget ofCaseBlock with argument labelSet.
- Return
false .
- If the first
CaseClauses is present, then- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget of the firstCaseClauses with argument labelSet. - If hasUndefinedLabels is
true , returntrue .
- Let hasUndefinedLabels be
- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget ofDefaultClause with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - If the second
CaseClauses is not present, returnfalse . - Return
ContainsUndefinedBreakTarget of the secondCaseClauses with argument labelSet.
- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget ofCaseClauses with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedBreakTarget ofCaseClause with argument labelSet.
- If the
StatementList is present, returnContainsUndefinedBreakTarget ofStatementList with argument labelSet. - Return
false .
- If the
StatementList is present, returnContainsUndefinedBreakTarget ofStatementList with argument labelSet. - Return
false .
- Let label be the
StringValue ofLabelIdentifier . - Let newLabelSet be a copy of labelSet with label appended.
- Return
ContainsUndefinedBreakTarget ofLabelledItem with argument newLabelSet.
- Return
false .
- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget ofBlock with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedBreakTarget ofCatch with argument labelSet.
- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget ofBlock with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedBreakTarget ofFinally with argument labelSet.
- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget ofBlock with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - Let hasUndefinedLabels be
ContainsUndefinedBreakTarget ofCatch with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedBreakTarget ofFinally with argument labelSet.
- Return
ContainsUndefinedBreakTarget ofBlock with argument labelSet.
- Return
false .
- Let hasUndefinedLabels be
ContainsUndefinedBreakTarget ofModuleItemList with argument labelSet. - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedBreakTarget ofModuleItem with argument labelSet.
- Return
false .
8.2.3 Static Semantics: ContainsUndefinedContinueTarget
With parameters iterationSet and labelSet.
- Return
false .
- Let newIterationSet be a copy of iterationSet with all the elements of labelSet appended.
- Return
ContainsUndefinedContinueTarget ofIterationStatement with arguments newIterationSet and « ».
- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget ofStatementList with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedContinueTarget ofStatementListItem with arguments iterationSet and « ».
- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget of the firstStatement with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedContinueTarget of the secondStatement with arguments iterationSet and « ».
- Return
ContainsUndefinedContinueTarget ofStatement with arguments iterationSet and « ».
- Return
ContainsUndefinedContinueTarget ofStatement with arguments iterationSet and « ».
- Return
ContainsUndefinedContinueTarget ofStatement with arguments iterationSet and « ».
- Return
ContainsUndefinedContinueTarget ofStatement with arguments iterationSet and « ».
- Return
ContainsUndefinedContinueTarget ofStatement with arguments iterationSet and « ».
This section is extended by Annex
- Return
false .
- If the
StringValue ofLabelIdentifier is not an element of iterationSet, returntrue . - Return
false .
- Return
ContainsUndefinedContinueTarget ofStatement with arguments iterationSet and « ».
- Return
ContainsUndefinedContinueTarget ofCaseBlock with arguments iterationSet and « ».
- Return
false .
- If the first
CaseClauses is present, then- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget of the firstCaseClauses with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue .
- Let hasUndefinedLabels be
- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget ofDefaultClause with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - If the second
CaseClauses is not present, returnfalse . - Return
ContainsUndefinedContinueTarget of the secondCaseClauses with arguments iterationSet and « ».
- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget ofCaseClauses with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedContinueTarget ofCaseClause with arguments iterationSet and « ».
- If the
StatementList is present, returnContainsUndefinedContinueTarget ofStatementList with arguments iterationSet and « ». - Return
false .
- If the
StatementList is present, returnContainsUndefinedContinueTarget ofStatementList with arguments iterationSet and « ». - Return
false .
- Let label be the
StringValue ofLabelIdentifier . - Let newLabelSet be a copy of labelSet with label appended.
- Return
ContainsUndefinedContinueTarget ofLabelledItem with arguments iterationSet and newLabelSet.
- Return
false .
- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget ofBlock with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedContinueTarget ofCatch with arguments iterationSet and « ».
- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget ofBlock with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedContinueTarget ofFinally with arguments iterationSet and « ».
- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget ofBlock with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - Let hasUndefinedLabels be
ContainsUndefinedContinueTarget ofCatch with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedContinueTarget ofFinally with arguments iterationSet and « ».
- Return
ContainsUndefinedContinueTarget ofBlock with arguments iterationSet and « ».
- Return
false .
- Let hasUndefinedLabels be
ContainsUndefinedContinueTarget ofModuleItemList with arguments iterationSet and « ». - If hasUndefinedLabels is
true , returntrue . - Return
ContainsUndefinedContinueTarget ofModuleItem with arguments iterationSet and « ».
- Return
false .
8.3 Function Name Inference
8.3.1 Static Semantics: HasName
- Let expr be
CoveredParenthesizedExpression ofCoverParenthesizedExpressionAndArrowParameterList . - If
IsFunctionDefinition of expr isfalse , returnfalse . - Return
HasName of expr.
- Return
false .
- Return
true .
8.3.2 Static Semantics: IsFunctionDefinition
- Let expr be
CoveredParenthesizedExpression ofCoverParenthesizedExpressionAndArrowParameterList . - Return
IsFunctionDefinition of expr.
- Return
false .
- Return
true .
8.3.3 Static Semantics: IsAnonymousFunctionDefinition ( expr )
The abstract operation IsAnonymousFunctionDefinition takes argument expr (a
- If
IsFunctionDefinition of expr isfalse , returnfalse . - Let hasName be
HasName of expr. - If hasName is
true , returnfalse . - Return
true .
8.3.4 Static Semantics: IsIdentifierRef
- Return
true .
- Return
false .
8.3.5 Runtime Semantics: NamedEvaluation
With parameter name.
- Let expr be
CoveredParenthesizedExpression ofCoverParenthesizedExpressionAndArrowParameterList . - Return the result of performing
NamedEvaluation for expr with argument name.
Assert :IsAnonymousFunctionDefinition (Expression ) istrue .- Return the result of performing
NamedEvaluation forExpression with argument name.
- Return
InstantiateOrdinaryFunctionExpression ofFunctionExpression with argument name.
- Return
InstantiateGeneratorFunctionExpression ofGeneratorExpression with argument name.
- Return
InstantiateAsyncGeneratorFunctionExpression ofAsyncGeneratorExpression with argument name.
- Return
InstantiateAsyncFunctionExpression ofAsyncFunctionExpression with argument name.
- Return
InstantiateArrowFunctionExpression ofArrowFunction with argument name.
- Return
InstantiateAsyncArrowFunctionExpression ofAsyncArrowFunction with argument name.
- Let value be the result of
ClassDefinitionEvaluation ofClassTail with argumentsundefined and name. ReturnIfAbrupt (value).- Set value.[[SourceText]] to the source text matched by
ClassExpression . - Return value.
8.4 Contains
8.4.1 Static Semantics: Contains
With parameter symbol.
Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of Contains:
- For each child node child of this
Parse Node , do- If child is an instance of symbol, return
true . - If child is an instance of a nonterminal, then
- Let contained be the result of child
Contains symbol. - If contained is
true , returntrue .
- Let contained be the result of child
- If child is an instance of symbol, return
- Return
false .
- Return
false .
Static semantic rules that depend upon substructure generally do not look into function definitions.
- If symbol is
ClassBody , returntrue . - If symbol is
ClassHeritage , then- If
ClassHeritage is present, returntrue ; otherwise returnfalse .
- If
- Let inHeritage be
ClassHeritage Contains symbol. - If inHeritage is
true , returntrue . - Return the result of
ComputedPropertyContains forClassBody with argument symbol.
Static semantic rules that depend upon substructure generally do not look into class bodies except for
- If symbol is not one of
NewTarget ,SuperProperty ,SuperCall ,superorthis, returnfalse . - If
ArrowParameters Contains symbol istrue , returntrue . - Return
ConciseBody Contains symbol.
- Let formals be
CoveredFormalsList ofCoverParenthesizedExpressionAndArrowParameterList . - Return formals
Contains symbol.
- If symbol is not one of
NewTarget ,SuperProperty ,SuperCall ,super, orthis, returnfalse . - Return
AsyncConciseBody Contains symbol.
- If symbol is not one of
NewTarget ,SuperProperty ,SuperCall ,super, orthis, returnfalse . - Let head be
CoveredAsyncArrowHead ofCoverCallExpressionAndAsyncArrowHead . - If head
Contains symbol istrue , returntrue . - Return
AsyncConciseBody Contains symbol.
Contains is used to detect new.target, this, and super usage within an
- If symbol is
MethodDefinition , returntrue . - Return the result of
ComputedPropertyContains forMethodDefinition with argument symbol.
- Return
false .
- If
MemberExpression Contains symbol istrue , returntrue . - Return
false .
- If symbol is the
ReservedWord super, returntrue . - Return
false .
- If
CallExpression Contains symbol istrue , returntrue . - Return
false .
- Return
false .
- If
OptionalChain Contains symbol istrue , returntrue . - Return
false .
8.4.2 Static Semantics: ComputedPropertyContains
With parameter symbol.
- Return
false .
- Return the result of
ComputedPropertyName Contains symbol.
- Return the result of
ComputedPropertyContains forPropertyName with argument symbol.
- Return the result of
ComputedPropertyContains forPropertyName with argument symbol.
- Return the result of
ComputedPropertyContains forPropertyName with argument symbol.
- Let inList be
ComputedPropertyContains ofClassElementList with argument symbol. - If inList is
true , returntrue . - Return the result of
ComputedPropertyContains forClassElement with argument symbol.
- Return
false .
- Return the result of
ComputedPropertyContains forPropertyName with argument symbol.
8.5 Miscellaneous
These operations are used in multiple places throughout the specification.
8.5.1 Runtime Semantics: InstantiateFunctionObject
With parameter scope.
- Return ?
InstantiateOrdinaryFunctionObject ofFunctionDeclaration with argument scope.
- Return ?
InstantiateGeneratorFunctionObject ofGeneratorDeclaration with argument scope.
- Return ?
InstantiateAsyncGeneratorFunctionObject ofAsyncGeneratorDeclaration with argument scope.
- Return ?
InstantiateAsyncFunctionObject ofAsyncFunctionDeclaration with argument scope.
8.5.2 Runtime Semantics: BindingInitialization
With parameters value and environment.
var statements and formal parameter lists of some non-strict functions (See
- Let name be
StringValue ofIdentifier . - Return ?
InitializeBoundName (name, value, environment).
- Return ?
InitializeBoundName ("yield" , value, environment).
- Return ?
InitializeBoundName ("await" , value, environment).
- Perform ?
RequireObjectCoercible (value). - Return the result of performing
BindingInitialization forObjectBindingPattern using value and environment as arguments.
- Let iteratorRecord be ?
GetIterator (value). - Let result be
IteratorBindingInitialization ofArrayBindingPattern with arguments iteratorRecord and environment. - If iteratorRecord.[[Done]] is
false , return ?IteratorClose (iteratorRecord, result). - Return result.
- Return
NormalCompletion (empty ).
- Perform ?
PropertyBindingInitialization forBindingPropertyList using value and environment as the arguments. - Return
NormalCompletion (empty ).
- Let excludedNames be a new empty
List . - Return the result of performing
RestBindingInitialization ofBindingRestProperty with value, environment, and excludedNames as the arguments.
- Let excludedNames be ?
PropertyBindingInitialization ofBindingPropertyList with arguments value and environment. - Return the result of performing
RestBindingInitialization ofBindingRestProperty with arguments value, environment, and excludedNames.
8.5.2.1 InitializeBoundName ( name, value, environment )
The abstract operation InitializeBoundName takes arguments name, value, and environment. It performs the following steps when called:
Assert :Type (name) is String.- If environment is not
undefined , then- Perform environment.InitializeBinding(name, value).
- Return
NormalCompletion (undefined ).
- Else,
- Let lhs be
ResolveBinding (name). - Return ?
PutValue (lhs, value).
- Let lhs be
8.5.3 Runtime Semantics: IteratorBindingInitialization
With parameters iteratorRecord and environment.
When
- Return
NormalCompletion (empty ).
- Return the result of performing
IteratorDestructuringAssignmentEvaluation ofElision with iteratorRecord as the argument.
- If
Elision is present, then- Perform ?
IteratorDestructuringAssignmentEvaluation ofElision with iteratorRecord as the argument.
- Perform ?
- Return the result of performing
IteratorBindingInitialization forBindingRestElement with iteratorRecord and environment as arguments.
- Perform ?
IteratorBindingInitialization forBindingElementList with iteratorRecord and environment as arguments. - Return the result of performing
IteratorDestructuringAssignmentEvaluation ofElision with iteratorRecord as the argument.
- Perform ?
IteratorBindingInitialization forBindingElementList with iteratorRecord and environment as arguments. - If
Elision is present, then- Perform ?
IteratorDestructuringAssignmentEvaluation ofElision with iteratorRecord as the argument.
- Perform ?
- Return the result of performing
IteratorBindingInitialization forBindingRestElement with iteratorRecord and environment as arguments.
- Perform ?
IteratorBindingInitialization forBindingElementList with iteratorRecord and environment as arguments. - Return the result of performing
IteratorBindingInitialization forBindingElisionElement using iteratorRecord and environment as arguments.
- Perform ?
IteratorDestructuringAssignmentEvaluation ofElision with iteratorRecord as the argument. - Return the result of performing
IteratorBindingInitialization ofBindingElement with iteratorRecord and environment as the arguments.
- Let bindingId be
StringValue ofBindingIdentifier . - Let lhs be ?
ResolveBinding (bindingId, environment). - If iteratorRecord.[[Done]] is
false , then- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue . - Else,
- Let v be
IteratorValue (next). - If v is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (v).
- Let v be
- Let next be
- If iteratorRecord.[[Done]] is
true , let v beundefined . - If
Initializer is present and v isundefined , then- If
IsAnonymousFunctionDefinition (Initializer ) istrue , then- Set v to the result of performing
NamedEvaluation forInitializer with argument bindingId.
- Set v to the result of performing
- Else,
- Let defaultValue be the result of evaluating
Initializer . - Set v to ?
GetValue (defaultValue).
- Let defaultValue be the result of evaluating
- If
- If environment is
undefined , return ?PutValue (lhs, v). - Return
InitializeReferencedBinding (lhs, v).
- If iteratorRecord.[[Done]] is
false , then- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue . - Else,
- Let v be
IteratorValue (next). - If v is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (v).
- Let v be
- Let next be
- If iteratorRecord.[[Done]] is
true , let v beundefined . - If
Initializer is present and v isundefined , then- Let defaultValue be the result of evaluating
Initializer . - Set v to ?
GetValue (defaultValue).
- Let defaultValue be the result of evaluating
- Return the result of performing
BindingInitialization ofBindingPattern with v and environment as the arguments.
- Let lhs be ?
ResolveBinding (StringValue ofBindingIdentifier , environment). - Let A be !
ArrayCreate (0). - Let n be 0.
- Repeat,
- If iteratorRecord.[[Done]] is
false , then- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue .
- Let next be
- If iteratorRecord.[[Done]] is
true , then- If environment is
undefined , return ?PutValue (lhs, A). - Return
InitializeReferencedBinding (lhs, A).
- If environment is
- Let nextValue be
IteratorValue (next). - If nextValue is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (nextValue).- Perform !
CreateDataPropertyOrThrow (A, !ToString (𝔽 (n)), nextValue). - Set n to n + 1.
- If iteratorRecord.[[Done]] is
- Let A be !
ArrayCreate (0). - Let n be 0.
- Repeat,
- If iteratorRecord.[[Done]] is
false , then- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue .
- Let next be
- If iteratorRecord.[[Done]] is
true , then- Return the result of performing
BindingInitialization ofBindingPattern with A and environment as the arguments.
- Return the result of performing
- Let nextValue be
IteratorValue (next). - If nextValue is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (nextValue).- Perform !
CreateDataPropertyOrThrow (A, !ToString (𝔽 (n)), nextValue). - Set n to n + 1.
- If iteratorRecord.[[Done]] is
- Return
NormalCompletion (empty ).
- Perform ?
IteratorBindingInitialization forFormalParameterList using iteratorRecord and environment as the arguments. - Return the result of performing
IteratorBindingInitialization forFunctionRestParameter using iteratorRecord and environment as the arguments.
- Perform ?
IteratorBindingInitialization forFormalParameterList using iteratorRecord and environment as the arguments. - Return the result of performing
IteratorBindingInitialization forFormalParameter using iteratorRecord and environment as the arguments.
Assert : iteratorRecord.[[Done]] isfalse .- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue . - Else,
- Let v be
IteratorValue (next). - If v is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (v).
- Let v be
- If iteratorRecord.[[Done]] is
true , let v beundefined . - Return the result of performing
BindingInitialization forBindingIdentifier using v and environment as the arguments.
- Let formals be
CoveredFormalsList ofCoverParenthesizedExpressionAndArrowParameterList . - Return
IteratorBindingInitialization of formals with arguments iteratorRecord and environment.
Assert : iteratorRecord.[[Done]] isfalse .- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue . - Else,
- Let v be
IteratorValue (next). - If v is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (v).
- Let v be
- If iteratorRecord.[[Done]] is
true , let v beundefined . - Return the result of performing
BindingInitialization forBindingIdentifier using v and environment as the arguments.
8.5.4 Static Semantics: AssignmentTargetType
- If this
IdentifierReference is contained instrict mode code andStringValue ofIdentifier is"eval" or"arguments" , returninvalid . - Return
simple .
- Return
simple .
- Let expr be
CoveredParenthesizedExpression ofCoverParenthesizedExpressionAndArrowParameterList . - Return
AssignmentTargetType of expr.
- Return
invalid .
8.5.5 Static Semantics: PropName
- Return
StringValue ofIdentifierReference .
- Return
empty .
- Return
PropName ofPropertyName .
- Return
StringValue ofIdentifierName .
- Return the
SV ofStringLiteral .
- Let nbr be the
NumericValue ofNumericLiteral . - Return !
ToString (nbr).
- Return
empty .
- Return
PropName ofPropertyName .
- Return
PropName ofPropertyName .
- Return
PropName ofPropertyName .
- Return
empty .
- Return
PropName ofPropertyName .
9 Executable Code and Execution Contexts
9.1 Environment Records
Environment Record is a specification type used to define the association of
Every Environment Record has an [[OuterEnv]] field, which is either
Environment Records are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.
9.1.1 The Environment Record Type Hierarchy
Environment Records can be thought of as existing in a simple object-oriented hierarchy where
-
Environment Record (abstract)-
A
declarative Environment Record is used to define the effect of ECMAScript language syntactic elements such asFunctionDeclaration s,VariableDeclaration s, andCatch clauses that directly associate identifier bindings with ECMAScript language values.-
A
function Environment Record corresponds to the invocation of an ECMAScriptfunction object , and contains bindings for the top-level declarations within that function. It may establish a newthisbinding. It also captures the state necessary to supportsupermethod invocations. -
A
module Environment Record contains the bindings for the top-level declarations of aModule . It also contains the bindings that are explicitly imported by theModule . Its [[OuterEnv]] is aglobal Environment Record .
-
-
An
object Environment Record is used to define the effect of ECMAScript elements such asWithStatement that associate identifier bindings with the properties of some object. -
A
global Environment Record is used forScript global declarations. It does not have an outer environment; its [[OuterEnv]] isnull . It may be prepopulated with identifier bindings and it includes an associatedglobal object whose properties provide some of the global environment's identifier bindings. As ECMAScript code is executed, additional properties may be added to theglobal object and the initial properties may be modified.
-
The
| Method | Purpose |
|---|---|
| HasBinding(N) |
Determine if an |
| CreateMutableBinding(N, D) |
Create a new but uninitialized mutable binding in an |
| CreateImmutableBinding(N, S) |
Create a new but uninitialized immutable binding in an |
| InitializeBinding(N, V) |
Set the value of an already existing but uninitialized binding in an |
| SetMutableBinding(N, V, S) |
Set the value of an already existing mutable binding in an |
| GetBindingValue(N, S) |
Returns the value of an already existing binding from an |
| DeleteBinding(N) |
Delete a binding from an |
| HasThisBinding() |
Determine if an this binding. Return |
| HasSuperBinding() |
Determine if an super method binding. Return |
| WithBaseObject() |
If this with statement, return the with object. Otherwise, return |
9.1.1.1 Declarative Environment Records
Each declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.
The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.
9.1.1.1.1 HasBinding ( N )
The HasBinding concrete method of a
- If envRec has a binding for the name that is the value of N, return
true . - Return
false .
9.1.1.1.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of a
Assert : envRec does not already have a binding for N.- Create a mutable binding in envRec for N and record that it is uninitialized. If D is
true , record that the newly created binding may be deleted by a subsequent DeleteBinding call. - Return
NormalCompletion (empty ).
9.1.1.1.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of a
Assert : envRec does not already have a binding for N.- Create an immutable binding in envRec for N and record that it is uninitialized. If S is
true , record that the newly created binding is a strict binding. - Return
NormalCompletion (empty ).
9.1.1.1.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a
Assert : envRec must have an uninitialized binding for N.- Set the bound value for N in envRec to V.
Record that the binding for N in envRec has been initialized.- Return
NormalCompletion (empty ).
9.1.1.1.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of a
- If envRec does not have a binding for N, then
- If S is
true , throw aReferenceError exception. - Perform envRec.CreateMutableBinding(N,
true ). - Perform envRec.InitializeBinding(N, V).
- Return
NormalCompletion (empty ).
- If S is
- If the binding for N in envRec is a strict binding, set S to
true . - If the binding for N in envRec has not yet been initialized, throw a
ReferenceError exception. - Else if the binding for N in envRec is a mutable binding, change its bound value to V.
- Else,
Assert : This is an attempt to change the value of an immutable binding.- If S is
true , throw aTypeError exception.
- Return
NormalCompletion (empty ).
An example of ECMAScript code that results in a missing binding at step
function f() { eval("var x; x = (delete x, 0);"); }9.1.1.1.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a
Assert : envRec has a binding for N.- If the binding for N in envRec is an uninitialized binding, throw a
ReferenceError exception. - Return the value currently bound to N in envRec.
9.1.1.1.7 DeleteBinding ( N )
The DeleteBinding concrete method of a
Assert : envRec has a binding for the name that is the value of N.- If the binding for N in envRec cannot be deleted, return
false . - Remove the binding for N from envRec.
- Return
true .
9.1.1.1.8 HasThisBinding ( )
The HasThisBinding concrete method of a
- Return
false .
A regular this binding.
9.1.1.1.9 HasSuperBinding ( )
The HasSuperBinding concrete method of a
- Return
false .
A regular super binding.
9.1.1.1.10 WithBaseObject ( )
The WithBaseObject concrete method of a
- Return
undefined .
9.1.1.2 Object Environment Records
Each object Environment Record is associated with an object called its binding object. An object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an
Object Environment Records created for with statements (
The behaviour of the concrete specification methods for object Environment Records is defined by the following algorithms.
9.1.1.2.1 HasBinding ( N )
The HasBinding concrete method of an
- Let bindings be the binding object for envRec.
- Let foundBinding be ?
HasProperty (bindings, N). - If foundBinding is
false , returnfalse . - If the withEnvironment flag of envRec is
false , returntrue . - Let unscopables be ?
Get (bindings,@@unscopables ). - If
Type (unscopables) is Object, then - Return
true .
9.1.1.2.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of an
- Let bindings be the binding object for envRec.
- Return ?
DefinePropertyOrThrow (bindings, N, PropertyDescriptor { [[Value]]:undefined , [[Writable]]:true , [[Enumerable]]:true , [[Configurable]]: D }).
Normally envRec will not have a binding for N but if it does, the semantics of
9.1.1.2.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of an
9.1.1.2.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of an
- Return ? envRec.SetMutableBinding(N, V,
false ).
In this specification, all uses of CreateMutableBinding for object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, this specification does not explicitly track the initialization state of bindings in object Environment Records.
9.1.1.2.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of an
- Let bindings be the binding object for envRec.
- Let stillExists be ?
HasProperty (bindings, N). - If stillExists is
false and S istrue , throw aReferenceError exception. - Return ?
Set (bindings, N, V, S).
9.1.1.2.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of an
- Let bindings be the binding object for envRec.
- Let value be ?
HasProperty (bindings, N). - If value is
false , then- If S is
false , return the valueundefined ; otherwise throw aReferenceError exception.
- If S is
- Return ?
Get (bindings, N).
9.1.1.2.7 DeleteBinding ( N )
The DeleteBinding concrete method of an
- Let bindings be the binding object for envRec.
- Return ? bindings.[[Delete]](N).
9.1.1.2.8 HasThisBinding ( )
The HasThisBinding concrete method of an
- Return
false .
Object Environment Records do not provide a this binding.
9.1.1.2.9 HasSuperBinding ( )
The HasSuperBinding concrete method of an
- Return
false .
Object Environment Records do not provide a super binding.
9.1.1.2.10 WithBaseObject ( )
The WithBaseObject concrete method of an
- If the withEnvironment flag of envRec is
true , return the binding object for envRec. - Otherwise, return
undefined .
9.1.1.3 Function Environment Records
A function Environment Record is a this binding. If a function is not an super, its function Environment Record also contains the state that is used to perform super method invocations from within the function.
Function Environment Records have the additional state fields listed in
| Field Name | Value | Meaning |
|---|---|---|
| [[ThisValue]] | Any |
This is the |
| [[ThisBindingStatus]] |
|
If the value is |
| [[FunctionObject]] | Object |
The |
| [[NewTarget]] |
Object | |
If this |
Function Environment Records support all of the
| Method | Purpose |
|---|---|
| BindThisValue(V) | Set the [[ThisValue]] and record that it has been initialized. |
| GetThisBinding() |
Return the value of this this binding. Throws a this binding has not been initialized.
|
| GetSuperBase() |
Return the object that is the base for super property accesses bound in this super property accesses will produce runtime errors.
|
The behaviour of the additional concrete specification methods for function Environment Records is defined by the following algorithms:
9.1.1.3.1 BindThisValue ( V )
The BindThisValue concrete method of a
Assert : envRec.[[ThisBindingStatus]] is notlexical .- If envRec.[[ThisBindingStatus]] is
initialized , throw aReferenceError exception. - Set envRec.[[ThisValue]] to V.
- Set envRec.[[ThisBindingStatus]] to
initialized . - Return V.
9.1.1.3.2 HasThisBinding ( )
The HasThisBinding concrete method of a
- If envRec.[[ThisBindingStatus]] is
lexical , returnfalse ; otherwise, returntrue .
9.1.1.3.3 HasSuperBinding ( )
The HasSuperBinding concrete method of a
- If envRec.[[ThisBindingStatus]] is
lexical , returnfalse . - If envRec.[[FunctionObject]].[[HomeObject]] has the value
undefined , returnfalse ; otherwise, returntrue .
9.1.1.3.4 GetThisBinding ( )
The GetThisBinding concrete method of a
Assert : envRec.[[ThisBindingStatus]] is notlexical .- If envRec.[[ThisBindingStatus]] is
uninitialized , throw aReferenceError exception. - Return envRec.[[ThisValue]].
9.1.1.3.5 GetSuperBase ( )
The GetSuperBase concrete method of a
9.1.1.4 Global Environment Records
A global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript
A global Environment Record is logically a single record but it is specified as a composite encapsulating an
Properties may be created directly on a
Global Environment Records have the additional fields listed in
| Field Name | Value | Meaning |
|---|---|---|
| [[ObjectRecord]] |
|
Binding object is the |
| [[GlobalThisValue]] | Object |
The value returned by this in global scope. Hosts may provide any ECMAScript Object value.
|
| [[DeclarativeRecord]] |
|
|
| [[VarNames]] |
|
The string names bound by |
| Method | Purpose |
|---|---|
| GetThisBinding() |
Return the value of this this binding.
|
| HasVarDeclaration (N) |
Determines if the argument identifier has a binding in this |
| HasLexicalDeclaration (N) |
Determines if the argument identifier has a binding in this |
| HasRestrictedGlobalProperty (N) |
Determines if the argument is the name of a |
| CanDeclareGlobalVar (N) | Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. |
| CanDeclareGlobalFunction (N) | Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N. |
| CreateGlobalVarBinding(N, D) |
Used to create and initialize to var binding in the [[ObjectRecord]] component of a var. The String value N is the bound name. If D is |
| CreateGlobalFunctionBinding(N, V, D) |
Create and initialize a global function binding in the [[ObjectRecord]] component of a function. The String value N is the bound name. V is the initialization value. If the Boolean argument D is |
The behaviour of the concrete specification methods for global Environment Records is defined by the following algorithms.
9.1.1.4.1 HasBinding ( N )
The HasBinding concrete method of a
- Let DclRec be envRec.[[DeclarativeRecord]].
- If DclRec.HasBinding(N) is
true , returntrue . - Let ObjRec be envRec.[[ObjectRecord]].
- Return ? ObjRec.HasBinding(N).
9.1.1.4.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of a
- Let DclRec be envRec.[[DeclarativeRecord]].
- If DclRec.HasBinding(N) is
true , throw aTypeError exception. - Return DclRec.CreateMutableBinding(N, D).
9.1.1.4.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of a
- Let DclRec be envRec.[[DeclarativeRecord]].
- If DclRec.HasBinding(N) is
true , throw aTypeError exception. - Return DclRec.CreateImmutableBinding(N, S).
9.1.1.4.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a
- Let DclRec be envRec.[[DeclarativeRecord]].
- If DclRec.HasBinding(N) is
true , then- Return DclRec.InitializeBinding(N, V).
Assert : If the binding exists, it must be in theobject Environment Record .- Let ObjRec be envRec.[[ObjectRecord]].
- Return ? ObjRec.InitializeBinding(N, V).
9.1.1.4.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of a
- Let DclRec be envRec.[[DeclarativeRecord]].
- If DclRec.HasBinding(N) is
true , then- Return DclRec.SetMutableBinding(N, V, S).
- Let ObjRec be envRec.[[ObjectRecord]].
- Return ? ObjRec.SetMutableBinding(N, V, S).
9.1.1.4.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a
- Let DclRec be envRec.[[DeclarativeRecord]].
- If DclRec.HasBinding(N) is
true , then- Return DclRec.GetBindingValue(N, S).
- Let ObjRec be envRec.[[ObjectRecord]].
- Return ? ObjRec.GetBindingValue(N, S).
9.1.1.4.7 DeleteBinding ( N )
The DeleteBinding concrete method of a
- Let DclRec be envRec.[[DeclarativeRecord]].
- If DclRec.HasBinding(N) is
true , then- Return DclRec.DeleteBinding(N).
- Let ObjRec be envRec.[[ObjectRecord]].
- Let globalObject be the binding object for ObjRec.
- Let existingProp be ?
HasOwnProperty (globalObject, N). - If existingProp is
true , then- Let status be ? ObjRec.DeleteBinding(N).
- If status is
true , then- Let varNames be envRec.[[VarNames]].
- If N is an element of varNames, remove that element from the varNames.
- Return status.
- Return
true .
9.1.1.4.8 HasThisBinding ( )
The HasThisBinding concrete method of a
- Return
true .
Global Environment Records always provide a this binding.
9.1.1.4.9 HasSuperBinding ( )
The HasSuperBinding concrete method of a
- Return
false .
Global Environment Records do not provide a super binding.
9.1.1.4.10 WithBaseObject ( )
The WithBaseObject concrete method of a
- Return
undefined .
9.1.1.4.11 GetThisBinding ( )
The GetThisBinding concrete method of a
- Return envRec.[[GlobalThisValue]].
9.1.1.4.12 HasVarDeclaration ( N )
The HasVarDeclaration concrete method of a
- Let varDeclaredNames be envRec.[[VarNames]].
- If varDeclaredNames contains N, return
true . - Return
false .
9.1.1.4.13 HasLexicalDeclaration ( N )
The HasLexicalDeclaration concrete method of a
- Let DclRec be envRec.[[DeclarativeRecord]].
- Return DclRec.HasBinding(N).
9.1.1.4.14 HasRestrictedGlobalProperty ( N )
The HasRestrictedGlobalProperty concrete method of a
- Let ObjRec be envRec.[[ObjectRecord]].
- Let globalObject be the binding object for ObjRec.
- Let existingProp be ? globalObject.[[GetOwnProperty]](N).
- If existingProp is
undefined , returnfalse . - If existingProp.[[Configurable]] is
true , returnfalse . - Return
true .
Properties may exist upon a
9.1.1.4.15 CanDeclareGlobalVar ( N )
The CanDeclareGlobalVar concrete method of a
- Let ObjRec be envRec.[[ObjectRecord]].
- Let globalObject be the binding object for ObjRec.
- Let hasProperty be ?
HasOwnProperty (globalObject, N). - If hasProperty is
true , returntrue . - Return ?
IsExtensible (globalObject).
9.1.1.4.16 CanDeclareGlobalFunction ( N )
The CanDeclareGlobalFunction concrete method of a
- Let ObjRec be envRec.[[ObjectRecord]].
- Let globalObject be the binding object for ObjRec.
- Let existingProp be ? globalObject.[[GetOwnProperty]](N).
- If existingProp is
undefined , return ?IsExtensible (globalObject). - If existingProp.[[Configurable]] is
true , returntrue . - If
IsDataDescriptor (existingProp) istrue and existingProp has attribute values { [[Writable]]:true , [[Enumerable]]:true }, returntrue . - Return
false .
9.1.1.4.17 CreateGlobalVarBinding ( N, D )
The CreateGlobalVarBinding concrete method of a
- Let ObjRec be envRec.[[ObjectRecord]].
- Let globalObject be the binding object for ObjRec.
- Let hasProperty be ?
HasOwnProperty (globalObject, N). - Let extensible be ?
IsExtensible (globalObject). - If hasProperty is
false and extensible istrue , then- Perform ? ObjRec.CreateMutableBinding(N, D).
- Perform ? ObjRec.InitializeBinding(N,
undefined ).
- Let varDeclaredNames be envRec.[[VarNames]].
- If varDeclaredNames does not contain N, then
- Append N to varDeclaredNames.
- Return
NormalCompletion (empty ).
9.1.1.4.18 CreateGlobalFunctionBinding ( N, V, D )
The CreateGlobalFunctionBinding concrete method of a
- Let ObjRec be envRec.[[ObjectRecord]].
- Let globalObject be the binding object for ObjRec.
- Let existingProp be ? globalObject.[[GetOwnProperty]](N).
- If existingProp is
undefined or existingProp.[[Configurable]] istrue , then- Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true , [[Enumerable]]:true , [[Configurable]]: D }.
- Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
- Else,
- Let desc be the PropertyDescriptor { [[Value]]: V }.
- Perform ?
DefinePropertyOrThrow (globalObject, N, desc). - Perform ?
Set (globalObject, N, V,false ). - Let varDeclaredNames be envRec.[[VarNames]].
- If varDeclaredNames does not contain N, then
- Append N to varDeclaredNames.
- Return
NormalCompletion (empty ).
Global function declarations are always represented as own properties of the
9.1.1.5 Module Environment Records
A module Environment Record is a
Module Environment Records support all of the
| Method | Purpose |
|---|---|
| CreateImportBinding(N, M, N2) |
Create an immutable indirect binding in a |
| GetThisBinding() |
Return the value of this this binding.
|
The behaviour of the additional concrete specification methods for module Environment Records are defined by the following algorithms:
9.1.1.5.1 GetBindingValue ( N, S )
The GetBindingValue concrete method of a
Assert : S istrue .Assert : envRec has a binding for N.- If the binding for N is an indirect binding, then
- Let M and N2 be the indirection values provided when this binding for N was created.
- Let targetEnv be M.[[Environment]].
- If targetEnv is
undefined , throw aReferenceError exception. - Return ? targetEnv.GetBindingValue(N2,
true ).
- If the binding for N in envRec is an uninitialized binding, throw a
ReferenceError exception. - Return the value currently bound to N in envRec.
S will always be
9.1.1.5.2 DeleteBinding ( N )
The DeleteBinding concrete method of a
Module Environment Records are only used within strict code and an
9.1.1.5.3 HasThisBinding ( )
The HasThisBinding concrete method of a
- Return
true .
Module Environment Records always provide a this binding.
9.1.1.5.4 GetThisBinding ( )
The GetThisBinding concrete method of a
- Return
undefined .
9.1.1.5.5 CreateImportBinding ( N, M, N2 )
The CreateImportBinding concrete method of a
Assert : envRec does not already have a binding for N.Assert : M is aModule Record .Assert : When M.[[Environment]] is instantiated it will have a direct binding for N2.- Create an immutable indirect binding in envRec for N that references M and N2 as its target binding and record that the binding is initialized.
- Return
NormalCompletion (empty ).
9.1.2 Environment Record Operations
The following
9.1.2.1 GetIdentifierReference ( env, name, strict )
The abstract operation GetIdentifierReference takes arguments env (an
- If env is the value
null , then- Return the
Reference Record { [[Base]]:unresolvable , [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]:empty }.
- Return the
- Let exists be ? env.HasBinding(name).
- If exists is
true , then- Return the
Reference Record { [[Base]]: env, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]:empty }.
- Return the
- Else,
- Let outer be env.[[OuterEnv]].
- Return ?
GetIdentifierReference (outer, name, strict).
9.1.2.2 NewDeclarativeEnvironment ( E )
The abstract operation NewDeclarativeEnvironment takes argument E (an
- Let env be a new
declarative Environment Record containing no bindings. - Set env.[[OuterEnv]] to E.
- Return env.
9.1.2.3 NewObjectEnvironment ( O, E )
The abstract operation NewObjectEnvironment takes arguments O (an Object) and E (an
- Let env be a new
object Environment Record containing O as the binding object. - Set env.[[OuterEnv]] to E.
- Return env.
9.1.2.4 NewFunctionEnvironment ( F, newTarget )
The abstract operation NewFunctionEnvironment takes arguments F and newTarget. It performs the following steps when called:
Assert : F is an ECMAScript function.Assert :Type (newTarget) is Undefined or Object.- Let env be a new
function Environment Record containing no bindings. - Set env.[[FunctionObject]] to F.
- If F.[[ThisMode]] is
lexical , set env.[[ThisBindingStatus]] tolexical . - Else, set env.[[ThisBindingStatus]] to
uninitialized . - Set env.[[NewTarget]] to newTarget.
- Set env.[[OuterEnv]] to F.[[Environment]].
- Return env.
9.1.2.5 NewGlobalEnvironment ( G, thisValue )
The abstract operation NewGlobalEnvironment takes arguments G and thisValue. It performs the following steps when called:
- Let objRec be a new
object Environment Record containing G as the binding object. - Let dclRec be a new
declarative Environment Record containing no bindings. - Let env be a new
global Environment Record . - Set env.[[ObjectRecord]] to objRec.
- Set env.[[GlobalThisValue]] to thisValue.
- Set env.[[DeclarativeRecord]] to dclRec.
- Set env.[[VarNames]] to a new empty
List . - Set env.[[OuterEnv]] to
null . - Return env.
9.1.2.6 NewModuleEnvironment ( E )
The abstract operation NewModuleEnvironment takes argument E (an
- Let env be a new
module Environment Record containing no bindings. - Set env.[[OuterEnv]] to E.
- Return env.
9.2 Realms
Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a
A
| Field Name | Value | Meaning |
|---|---|---|
| [[Intrinsics]] |
|
The intrinsic values used by code associated with this |
| [[GlobalObject]] | Object |
The |
| [[GlobalEnv]] |
|
The global environment for this |
| [[TemplateMap]] |
A |
Template objects are canonicalized separately for each Once a |
| [[HostDefined]] |
Any, default value is |
Field reserved for use by hosts that need to associate additional information with a |
9.2.1 CreateRealm ( )
The abstract operation CreateRealm takes no arguments. It performs the following steps when called:
- Let realmRec be a new
Realm Record . - Perform
CreateIntrinsics (realmRec). - Set realmRec.[[GlobalObject]] to
undefined . - Set realmRec.[[GlobalEnv]] to
undefined . - Set realmRec.[[TemplateMap]] to a new empty
List . - Return realmRec.
9.2.2 CreateIntrinsics ( realmRec )
The abstract operation CreateIntrinsics takes argument realmRec. It performs the following steps when called:
- Let intrinsics be a new
Record . - Set realmRec.[[Intrinsics]] to intrinsics.
- Set fields of intrinsics with the values listed in
Table 8 . The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses19 through28 . All object property values are newly created object values. All values that are built-in function objects are created by performingCreateBuiltinFunction (steps, length, name, slots, realmRec, prototype) where steps is the definition of that function provided by this specification, name is the initial value of the function'snameproperty, length is the initial value of the function'slengthproperty, slots is a list of the names, if any, of the function's specified internal slots, and prototype is the specified value of the function's [[Prototype]] internal slot. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created. - Perform
AddRestrictedFunctionProperties (intrinsics.[[%Function.prototype% ]], realmRec). - Return intrinsics.
9.2.3 SetRealmGlobalObject ( realmRec, globalObj, thisValue )
The abstract operation SetRealmGlobalObject takes arguments realmRec, globalObj, and thisValue. It performs the following steps when called:
- If globalObj is
undefined , then- Let intrinsics be realmRec.[[Intrinsics]].
- Set globalObj to !
OrdinaryObjectCreate (intrinsics.[[%Object.prototype% ]]).
Assert :Type (globalObj) is Object.- If thisValue is
undefined , set thisValue to globalObj. - Set realmRec.[[GlobalObject]] to globalObj.
- Let newGlobalEnv be
NewGlobalEnvironment (globalObj, thisValue). - Set realmRec.[[GlobalEnv]] to newGlobalEnv.
- Return realmRec.
9.2.4 SetDefaultGlobalBindings ( realmRec )
The abstract operation SetDefaultGlobalBindings takes argument realmRec. It performs the following steps when called:
- Let global be realmRec.[[GlobalObject]].
- For each property of the Global Object specified in clause
19 , do- Let name be the String value of the
property name . - Let desc be the fully populated data
Property Descriptor for the property, containing the specified attributes for the property. For properties listed in19.2 ,19.3 , or19.4 the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec. - Perform ?
DefinePropertyOrThrow (global, name, desc).
- Let name be the String value of the
- Return global.
9.3 Execution Contexts
An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context per
The execution context stack is used to track execution contexts. The
An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in
| Component | Purpose |
|---|---|
| code evaluation state |
Any state needed to perform, suspend, and resume evaluation of the code associated with this |
| Function |
If this |
|
|
The |
| ScriptOrModule |
The |
Evaluation of code by the
The value of the
Execution contexts for ECMAScript code have the additional state components listed in
| Component | Purpose |
|---|---|
| LexicalEnvironment |
Identifies the |
| VariableEnvironment |
Identifies the |
The LexicalEnvironment and VariableEnvironment components of an execution context are always Environment Records.
Execution contexts representing the evaluation of generator objects have the additional state components listed in
| Component | Purpose |
|---|---|
| Generator |
The generator object that this |
In most situations only the
An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.
9.3.1 GetActiveScriptOrModule ( )
The abstract operation GetActiveScriptOrModule takes no arguments. It is used to determine the running script or module, based on the
- If the
execution context stack is empty, returnnull . - Let ec be the topmost
execution context on theexecution context stack whose ScriptOrModule component is notnull . - If no such
execution context exists, returnnull . Otherwise, return ec's ScriptOrModule.
9.3.2 ResolveBinding ( name [ , env ] )
The abstract operation ResolveBinding takes argument name (a String) and optional argument env (an
- If env is not present or if env is
undefined , then- Set env to the
running execution context 's LexicalEnvironment.
- Set env to the
Assert : env is anEnvironment Record .- If the code matching the syntactic production that is being evaluated is contained in
strict mode code , let strict betrue ; else let strict befalse . - Return ?
GetIdentifierReference (env, name, strict).
The result of ResolveBinding is always a
9.3.3 GetThisEnvironment ( )
The abstract operation GetThisEnvironment takes no arguments. It finds the this. It performs the following steps when called:
- Let env be the
running execution context 's LexicalEnvironment. - Repeat,
- Let exists be env.HasThisBinding().
- If exists is
true , return env. - Let outer be env.[[OuterEnv]].
Assert : outer is notnull .- Set env to outer.
The loop in step this binding.
9.3.4 ResolveThisBinding ( )
The abstract operation ResolveThisBinding takes no arguments. It determines the binding of the this using the LexicalEnvironment of the
- Let envRec be
GetThisEnvironment (). - Return ? envRec.GetThisBinding().
9.3.5 GetNewTarget ( )
The abstract operation GetNewTarget takes no arguments. It determines the NewTarget value using the LexicalEnvironment of the
- Let envRec be
GetThisEnvironment (). Assert : envRec has a [[NewTarget]] field.- Return envRec.[[NewTarget]].
9.3.6 GetGlobalObject ( )
The abstract operation GetGlobalObject takes no arguments. It returns the
- Let currentRealm be
the current Realm Record . - Return currentRealm.[[GlobalObject]].
9.4 Jobs and Host Operations to Enqueue Jobs
A Job is an
Jobs are scheduled for execution by ECMAScript
- At some future point in time, when there is no
running execution context and theexecution context stack is empty, the implementation must:- Perform any
host-defined preparation steps. Invoke theJob Abstract Closure .- Perform any
host-defined cleanup steps, after which theexecution context stack must be empty.
- Perform any
- Only one
Job may be actively undergoing evaluation at any point in time. - Once evaluation of a
Job starts, it must run to completion before evaluation of any otherJob starts. - The
Abstract Closure must return a normal completion, implementing its own handling of errors.
At any particular time, scriptOrModule (a
GetActiveScriptOrModule () is scriptOrModule.- If scriptOrModule is a
Script Record orModule Record , let ec be the topmostexecution context on theexecution context stack whose ScriptOrModule component is scriptOrModule. TheRealm component of ec is scriptOrModule.[[Realm]].
At any particular time, an execution is prepared to evaluate ECMAScript code if all of the following conditions are true:
- The
execution context stack is not empty. - The
Realm component of the topmostexecution context on theexecution context stack is aRealm Record .
The specific choice of
Particular kinds of Jobs have additional conformance requirements.
9.4.1 JobCallback Records
A JobCallback Record is a
The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses the
JobCallback Records have the fields listed in
| Field Name | Value | Meaning |
|---|---|---|
| [[Callback]] |
A |
The function to invoke when the |
| [[HostDefined]] |
Any, default value is |
Field reserved for use by hosts. |
9.4.2 HostMakeJobCallback ( callback )
The
The implementation of HostMakeJobCallback must conform to the following requirements:
- It must always complete normally (i.e., not return an
abrupt completion ). - It must always return a
JobCallback Record whose [[Callback]] field is callback.
The default implementation of HostMakeJobCallback performs the following steps when called:
Assert :IsCallable (callback) istrue .- Return the
JobCallback Record { [[Callback]]: callback, [[HostDefined]]:empty }.
ECMAScript hosts that are not web browsers must use the default implementation of HostMakeJobCallback.
This is called at the time that the callback is passed to the function that is responsible for its being eventually scheduled and run. For example, promise.then(thenAction) calls MakeJobCallback on thenAction at the time of invoking Promise.prototype.then, not at the time of scheduling the reaction
9.4.3 HostCallJobCallback ( jobCallback, V, argumentsList )
The
The implementation of HostCallJobCallback must conform to the following requirements:
- It must always perform and return the result of
Call (jobCallback.[[Callback]], V, argumentsList).
This requirement means that hosts cannot change the [[Call]] behaviour of function objects defined in this specification.
The default implementation of HostCallJobCallback performs the following steps when called:
Assert :IsCallable (jobCallback.[[Callback]]) istrue .- Return ?
Call (jobCallback.[[Callback]], V, argumentsList).
ECMAScript hosts that are not web browsers must use the default implementation of HostCallJobCallback.
9.4.4 HostEnqueuePromiseJob ( job, realm )
The
The implementation of HostEnqueuePromiseJob must conform to the requirements in
- If realm is not
null , each time job is invoked the implementation must performimplementation-defined steps such that execution isprepared to evaluate ECMAScript code at the time of job's invocation. - Let scriptOrModule be
GetActiveScriptOrModule () at the time HostEnqueuePromiseJob is invoked. If realm is notnull , each time job is invoked the implementation must performimplementation-defined steps such that scriptOrModule is theactive script or module at the time of job's invocation. - Jobs must run in the same order as the HostEnqueuePromiseJob invocations that scheduled them.
The realm for Jobs returned by
9.5 InitializeHostDefinedRealm ( )
The abstract operation InitializeHostDefinedRealm takes no arguments. It performs the following steps when called:
- Let realm be
CreateRealm (). - Let newContext be a new
execution context . - Set the Function of newContext to
null . - Set the
Realm of newContext to realm. - Set the ScriptOrModule of newContext to
null . - Push newContext onto the
execution context stack ; newContext is now therunning execution context . - If the
host requires use of anexotic object to serve as realm'sglobal object , let global be such an object created in ahost-defined manner. Otherwise, let global beundefined , indicating that anordinary object should be created as theglobal object . - If the
host requires that thethisbinding in realm's global scope return an object other than theglobal object , let thisValue be such an object created in ahost-defined manner. Otherwise, let thisValue beundefined , indicating that realm's globalthisbinding should be theglobal object . - Perform
SetRealmGlobalObject (realm, global, thisValue). - Let globalObj be ?
SetDefaultGlobalBindings (realm). - Create any
host-defined global object properties on globalObj. - Return
NormalCompletion (empty ).
9.6 Agents
An agent comprises a set of ECMAScript execution contexts, an
An
Some web browsers share a single
While an
| Field Name | Value | Meaning |
|---|---|---|
| [[LittleEndian]] | Boolean | The default value computed for the isLittleEndian parameter when it is needed by the algorithms |
| [[CanBlock]] | Boolean | Determines whether the |
| [[Signifier]] | Any globally-unique value | Uniquely identifies the |
| [[IsLockFree1]] | Boolean | |
| [[IsLockFree2]] | Boolean | |
| [[IsLockFree8]] | Boolean | |
| [[CandidateExecution]] | A |
See the |
| [[KeptAlive]] | Initially a new empty |
Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any
The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but may also reflect implementation choices that can vary over time and between ECMAScript implementations.
There is no [[IsLockFree4]] property: 4-byte atomic operations are always lock-free.
In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to complete a lock-free atomic operation.
That an atomic access of size n is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. See
An
9.6.1 AgentSignifier ( )
The abstract operation AgentSignifier takes no arguments. It performs the following steps when called:
- Let AR be the
Agent Record of thesurrounding agent . - Return AR.[[Signifier]].
9.6.2 AgentCanSuspend ( )
The abstract operation AgentCanSuspend takes no arguments. It performs the following steps when called:
- Let AR be the
Agent Record of thesurrounding agent . - Return AR.[[CanBlock]].
In some environments it may not be reasonable for a given
9.7 Agent Clusters
An agent cluster is a maximal set of agents that can communicate by operating on shared memory.
Programs within different agents may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffer objects can be shared among the agents in the cluster.
There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.
Every
All agents within a cluster must have the same value for the [[LittleEndian]] property in their respective
If different agents within an agent cluster have different values of [[LittleEndian]] it becomes hard to use shared memory for multi-byte data.
All agents within a cluster must have the same values for the [[IsLockFree1]] property in their respective
All agents within a cluster must have different values for the [[Signifier]] property in their respective
An embedding may deactivate (stop forward progress) or activate (resume forward progress) an
The purpose of the preceding restriction is to avoid a situation where an
The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same suspend/wake collective within the embedding.
An embedding may terminate an
Prior to any evaluation of any ECMAScript code by any
All agents in an agent cluster share the same
An agent cluster is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.
9.8 Forward Progress
For an
An
Implementations must ensure that:
- every unblocked
agent with a dedicatedexecuting thread eventually makes forward progress - in a set of agents that share an
executing thread , oneagent eventually makes forward progress - an
agent does not cause anotheragent to become blocked except via explicit APIs that provide blocking.
This, along with the liveness guarantee in the
9.9 Processing Model of WeakRef and FinalizationRegistry Objects
9.9.1 Objectives
This specification does not make any guarantees that any object will be garbage collected. Objects which are not
The semantics of
-
When
WeakRef.prototype.derefis called, the referent (ifundefined is not returned) is kept alive so that subsequent, synchronous accesses also return the object. This list is reset when synchronous work is done using theClearKeptObjects abstract operation. -
When an object which is registered with a
FinalizationRegistry becomes unreachable, a call of theFinalizationRegistry 's cleanup callback may eventually be made, after synchronous ECMAScript execution completes. TheFinalizationRegistry cleanup is performed with theCleanupFinalizationRegistry abstract operation.
Neither of these actions (
Some ECMAScript implementations include garbage collector implementations which run in the background, including when ECMAScript is idle. Letting the
9.9.2 Liveness
For some set of objects S, a hypothetical WeakRef-oblivious execution with respect to S is an execution whereby the abstract operation
At any point during evaluation, a set of objects S is considered live if either of the following conditions is met:
-
Any element in S is included in any
agent 's [[KeptAlive]]List . - There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that observes the Object value of any object in S.
Presence of an object in a field, an internal slot, or a property does not imply that the object is live. For example if the object in question is never passed back to the program, then it cannot be observed.
This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of a
The above definition implies that, if a key in a WeakMap is not live, then its corresponding value is not necessarily live either.
9.9.3 Execution
At any time, if a set of objects S is not
- For each element obj of S, do
- For each
WeakRef ref such that ref.[[WeakRefTarget]] is obj, do- Set ref.[[WeakRefTarget]] to
empty .
- Set ref.[[WeakRefTarget]] to
- For each
FinalizationRegistry fg such that fg.[[Cells]] contains aRecord cell such that cell.[[WeakRefTarget]] is obj, do- Set cell.[[WeakRefTarget]] to
empty . - Optionally, perform !
HostEnqueueFinalizationRegistryCleanupJob (fg).
- Set cell.[[WeakRefTarget]] to
- For each WeakMap map such that map.[[WeakMapData]] contains a
Record r such that r.[[Key]] is obj, do- Set r.[[Key]] to
empty . - Set r.[[Value]] to
empty .
- Set r.[[Key]] to
- For each WeakSet set such that set.[[WeakSetData]] contains obj, do
- Replace the element of set.[[WeakSetData]] whose value is obj with an element whose value is
empty .
- Replace the element of set.[[WeakSetData]] whose value is obj with an element whose value is
- For each
Together with the definition of liveness, this clause prescribes legal optimizations that an implementation may apply regarding WeakRefs.
It is possible to access an object without observing its identity. Optimizations such as dead variable elimination and scalar replacement on properties of non-escaping objects whose identity is not observed are allowed. These optimizations are thus allowed to observably empty WeakRefs that point to such objects.
On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of a
Because calling
Implementations are not obligated to empty WeakRefs for maximal sets of non-
If an implementation chooses a non-
9.9.4 Host Hooks
9.9.4.1 HostEnqueueFinalizationRegistryCleanupJob ( finalizationRegistry )
The abstract operation HostEnqueueFinalizationRegistryCleanupJob takes argument finalizationRegistry (a
9.10 ClearKeptObjects ( )
The abstract operation ClearKeptObjects takes no arguments. ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of ECMAScript executions completes. It performs the following steps when called:
- Let agentRecord be the
surrounding agent 'sAgent Record . - Set agentRecord.[[KeptAlive]] to a new empty
List .
9.11 AddToKeptObjects ( object )
The abstract operation AddToKeptObjects takes argument object (an Object). It performs the following steps when called:
- Let agentRecord be the
surrounding agent 'sAgent Record . - Append object to agentRecord.[[KeptAlive]].
9.12 CleanupFinalizationRegistry ( finalizationRegistry )
The abstract operation CleanupFinalizationRegistry takes argument finalizationRegistry (a
Assert : finalizationRegistry has [[Cells]] and [[CleanupCallback]] internal slots.- Let callback be finalizationRegistry.[[CleanupCallback]].
- While finalizationRegistry.[[Cells]] contains a
Record cell such that cell.[[WeakRefTarget]] isempty , an implementation may perform the following steps:- Choose any such cell.
- Remove cell from finalizationRegistry.[[Cells]].
- Perform ?
Call (callback,undefined , « cell.[[HeldValue]] »).
- Return
NormalCompletion (undefined ).
10 Ordinary and Exotic Objects Behaviours
10.1 Ordinary Object Internal Methods and Internal Slots
All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either
Every
In the following algorithm descriptions, assume O is an
Each
10.1.1 [[GetPrototypeOf]] ( )
The [[GetPrototypeOf]] internal method of an
- Return !
OrdinaryGetPrototypeOf (O).
10.1.1.1 OrdinaryGetPrototypeOf ( O )
The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object). It performs the following steps when called:
- Return O.[[Prototype]].
10.1.2 [[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of an
- Return !
OrdinarySetPrototypeOf (O, V).
10.1.2.1 OrdinarySetPrototypeOf ( O, V )
The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V (an
Assert : EitherType (V) is Object orType (V) is Null.- Let current be O.[[Prototype]].
- If
SameValue (V, current) istrue , returntrue . - Let extensible be O.[[Extensible]].
- If extensible is
false , returnfalse . - Let p be V.
- Let done be
false . - Repeat, while done is
false ,- If p is
null , set done totrue . - Else if
SameValue (p, O) istrue , returnfalse . - Else,
- If p.[[GetPrototypeOf]] is not the
ordinary object internal method defined in10.1.1 , set done totrue . - Else, set p to p.[[Prototype]].
- If p.[[GetPrototypeOf]] is not the
- If p is
- Set O.[[Prototype]] to V.
- Return
true .
The loop in step
10.1.3 [[IsExtensible]] ( )
The [[IsExtensible]] internal method of an
- Return !
OrdinaryIsExtensible (O).
10.1.3.1 OrdinaryIsExtensible ( O )
The abstract operation OrdinaryIsExtensible takes argument O (an Object). It performs the following steps when called:
- Return O.[[Extensible]].
10.1.4 [[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of an
- Return !
OrdinaryPreventExtensions (O).
10.1.4.1 OrdinaryPreventExtensions ( O )
The abstract operation OrdinaryPreventExtensions takes argument O (an Object). It performs the following steps when called:
- Set O.[[Extensible]] to
false . - Return
true .
10.1.5 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of an
- Return !
OrdinaryGetOwnProperty (O, P).
10.1.5.1 OrdinaryGetOwnProperty ( O, P )
The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and P (a property key). It performs the following steps when called:
Assert :IsPropertyKey (P) istrue .- If O does not have an own property with key P, return
undefined . - Let D be a newly created
Property Descriptor with no fields. - Let X be O's own property whose key is P.
- If X is a
data property , then- Set D.[[Value]] to the value of X's [[Value]] attribute.
- Set D.[[Writable]] to the value of X's [[Writable]] attribute.
- Else,
Assert : X is anaccessor property .- Set D.[[Get]] to the value of X's [[Get]] attribute.
- Set D.[[Set]] to the value of X's [[Set]] attribute.
- Set D.[[Enumerable]] to the value of X's [[Enumerable]] attribute.
- Set D.[[Configurable]] to the value of X's [[Configurable]] attribute.
- Return D.
10.1.6 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of an
- Return ?
OrdinaryDefineOwnProperty (O, P, Desc).
10.1.6.1 OrdinaryDefineOwnProperty ( O, P, Desc )
The abstract operation OrdinaryDefineOwnProperty takes arguments O (an Object), P (a property key), and Desc (a
- Let current be ? O.[[GetOwnProperty]](P).
- Let extensible be ?
IsExtensible (O). - Return
ValidateAndApplyPropertyDescriptor (O, P, extensible, Desc, current).
10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible, Desc, Current )
The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean), Desc (a
- Return
ValidateAndApplyPropertyDescriptor (undefined ,undefined , Extensible, Desc, Current).
10.1.6.3 ValidateAndApplyPropertyDescriptor ( O, P, extensible, Desc, current )
The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object or
If
Assert : If O is notundefined , thenIsPropertyKey (P) istrue .- If current is
undefined , then- If extensible is
false , returnfalse . Assert : extensible istrue .- If
IsGenericDescriptor (Desc) istrue orIsDataDescriptor (Desc) istrue , then- If O is not
undefined , create an owndata property named P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to itsdefault value .
- If O is not
- Else,
Assert : !IsAccessorDescriptor (Desc) istrue .- If O is not
undefined , create an ownaccessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to itsdefault value .
- Return
true .
- If extensible is
- If every field in Desc is absent, return
true . - If current.[[Configurable]] is
false , then- If Desc.[[Configurable]] is present and its value is
true , returnfalse . - If Desc.[[Enumerable]] is present and !
SameValue (Desc.[[Enumerable]], current.[[Enumerable]]) isfalse , returnfalse .
- If Desc.[[Configurable]] is present and its value is
- If !
IsGenericDescriptor (Desc) istrue , then- NOTE: No further validation is required.
- Else if !
SameValue (!IsDataDescriptor (current), !IsDataDescriptor (Desc)) isfalse , then- If current.[[Configurable]] is
false , returnfalse . - If
IsDataDescriptor (current) istrue , then- If O is not
undefined , convert the property named P of object O from adata property to anaccessor property . Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to theirdefault values .
- If O is not
- Else,
- If O is not
undefined , convert the property named P of object O from anaccessor property to adata property . Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to theirdefault values .
- If O is not
- If current.[[Configurable]] is
- Else if
IsDataDescriptor (current) andIsDataDescriptor (Desc) are bothtrue , then- If current.[[Configurable]] is
false and current.[[Writable]] isfalse , then- If Desc.[[Writable]] is present and Desc.[[Writable]] is
true , returnfalse . - If Desc.[[Value]] is present and
SameValue (Desc.[[Value]], current.[[Value]]) isfalse , returnfalse . - Return
true .
- If Desc.[[Writable]] is present and Desc.[[Writable]] is
- If current.[[Configurable]] is
- Else,
Assert : !IsAccessorDescriptor (current) and !IsAccessorDescriptor (Desc) are bothtrue .- If current.[[Configurable]] is
false , then
- If O is not
undefined , then- For each field of Desc that is present, set the corresponding attribute of the property named P of object O to the value of the field.
- Return
true .
10.1.7 [[HasProperty]] ( P )
The [[HasProperty]] internal method of an
- Return ?
OrdinaryHasProperty (O, P).
10.1.7.1 OrdinaryHasProperty ( O, P )
The abstract operation OrdinaryHasProperty takes arguments O (an Object) and P (a property key). It performs the following steps when called:
Assert :IsPropertyKey (P) istrue .- Let hasOwn be ? O.[[GetOwnProperty]](P).
- If hasOwn is not
undefined , returntrue . - Let parent be ? O.[[GetPrototypeOf]]().
- If parent is not
null , then- Return ? parent.[[HasProperty]](P).
- Return
false .
10.1.8 [[Get]] ( P, Receiver )
The [[Get]] internal method of an
- Return ?
OrdinaryGet (O, P, Receiver).
10.1.8.1 OrdinaryGet ( O, P, Receiver )
The abstract operation OrdinaryGet takes arguments O (an Object), P (a property key), and Receiver (an
Assert :IsPropertyKey (P) istrue .- Let desc be ? O.[[GetOwnProperty]](P).
- If desc is
undefined , then- Let parent be ? O.[[GetPrototypeOf]]().
- If parent is
null , returnundefined . - Return ? parent.[[Get]](P, Receiver).
- If
IsDataDescriptor (desc) istrue , return desc.[[Value]]. Assert :IsAccessorDescriptor (desc) istrue .- Let getter be desc.[[Get]].
- If getter is
undefined , returnundefined . - Return ?
Call (getter, Receiver).
10.1.9 [[Set]] ( P, V, Receiver )
The [[Set]] internal method of an
- Return ?
OrdinarySet (O, P, V, Receiver).
10.1.9.1 OrdinarySet ( O, P, V, Receiver )
The abstract operation OrdinarySet takes arguments O (an Object), P (a property key), V (an
Assert :IsPropertyKey (P) istrue .- Let ownDesc be ? O.[[GetOwnProperty]](P).
- Return
OrdinarySetWithOwnDescriptor (O, P, V, Receiver, ownDesc).
10.1.9.2 OrdinarySetWithOwnDescriptor ( O, P, V, Receiver, ownDesc )
The abstract operation OrdinarySetWithOwnDescriptor takes arguments O (an Object), P (a property key), V (an
Assert :IsPropertyKey (P) istrue .- If ownDesc is
undefined , then- Let parent be ? O.[[GetPrototypeOf]]().
- If parent is not
null , then- Return ? parent.[[Set]](P, V, Receiver).
- Else,
- Set ownDesc to the PropertyDescriptor { [[Value]]:
undefined , [[Writable]]:true , [[Enumerable]]:true , [[Configurable]]:true }.
- Set ownDesc to the PropertyDescriptor { [[Value]]:
- If
IsDataDescriptor (ownDesc) istrue , then- If ownDesc.[[Writable]] is
false , returnfalse . - If
Type (Receiver) is not Object, returnfalse . - Let existingDescriptor be ? Receiver.[[GetOwnProperty]](P).
- If existingDescriptor is not
undefined , then- If
IsAccessorDescriptor (existingDescriptor) istrue , returnfalse . - If existingDescriptor.[[Writable]] is
false , returnfalse . - Let valueDesc be the PropertyDescriptor { [[Value]]: V }.
- Return ? Receiver.[[DefineOwnProperty]](P, valueDesc).
- If
- Else,
Assert : Receiver does not currently have a property P.- Return ?
CreateDataProperty (Receiver, P, V).
- If ownDesc.[[Writable]] is
Assert :IsAccessorDescriptor (ownDesc) istrue .- Let setter be ownDesc.[[Set]].
- If setter is
undefined , returnfalse . - Perform ?
Call (setter, Receiver, « V »). - Return
true .
10.1.10 [[Delete]] ( P )
The [[Delete]] internal method of an
- Return ?
OrdinaryDelete (O, P).
10.1.10.1 OrdinaryDelete ( O, P )
The abstract operation OrdinaryDelete takes arguments O (an Object) and P (a property key). It performs the following steps when called:
Assert :IsPropertyKey (P) istrue .- Let desc be ? O.[[GetOwnProperty]](P).
- If desc is
undefined , returntrue . - If desc.[[Configurable]] is
true , then- Remove the own property with name P from O.
- Return
true .
- Return
false .
10.1.11 [[OwnPropertyKeys]] ( )
The [[OwnPropertyKeys]] internal method of an
- Return !
OrdinaryOwnPropertyKeys (O).
10.1.11.1 OrdinaryOwnPropertyKeys ( O )
The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object). It performs the following steps when called:
- Let keys be a new empty
List . - For each own property key P of O such that P is an
array index , in ascending numeric index order, do- Add P as the last element of keys.
- For each own property key P of O such that
Type (P) is String and P is not anarray index , in ascending chronological order of property creation, do- Add P as the last element of keys.
- For each own property key P of O such that
Type (P) is Symbol, in ascending chronological order of property creation, do- Add P as the last element of keys.
- Return keys.
10.1.12 OrdinaryObjectCreate ( proto [ , additionalInternalSlotsList ] )
The abstract operation OrdinaryObjectCreate takes argument proto (an Object or
- Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
- If additionalInternalSlotsList is present, append each of its elements to internalSlotsList.
- Let O be !
MakeBasicObject (internalSlotsList). - Set O.[[Prototype]] to proto.
- Return O.
Although OrdinaryObjectCreate does little more than call
10.1.13 OrdinaryCreateFromConstructor ( constructor, intrinsicDefaultProto [ , internalSlotsList ] )
The abstract operation OrdinaryCreateFromConstructor takes arguments constructor and intrinsicDefaultProto and optional argument internalSlotsList (a
Assert : intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.- Let proto be ?
GetPrototypeFromConstructor (constructor, intrinsicDefaultProto). - Return !
OrdinaryObjectCreate (proto, internalSlotsList).
10.1.14 GetPrototypeFromConstructor ( constructor, intrinsicDefaultProto )
The abstract operation GetPrototypeFromConstructor takes arguments constructor and intrinsicDefaultProto. It determines the [[Prototype]] value that should be used to create an object corresponding to a specific
Assert : intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.Assert :IsCallable (constructor) istrue .- Let proto be ?
Get (constructor,"prototype" ). - If
Type (proto) is not Object, then- Let realm be ?
GetFunctionRealm (constructor). - Set proto to realm's intrinsic object named intrinsicDefaultProto.
- Let realm be ?
- Return proto.
If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the
10.1.15 RequireInternalSlot ( O, internalSlot )
The abstract operation RequireInternalSlot takes arguments O and internalSlot. It throws an exception unless O is an Object and has the given internal slot. It performs the following steps when called:
- If
Type (O) is not Object, throw aTypeError exception. - If O does not have an internalSlot internal slot, throw a
TypeError exception.
10.2 ECMAScript Function Objects
ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScript
In addition to [[Extensible]] and [[Prototype]], ECMAScript function objects also have the internal slots listed in
| Internal Slot | Type | Description |
|---|---|---|
| [[Environment]] |
|
The |
| [[FormalParameters]] |
|
The root parse node of the source text that defines the function's formal parameter list. |
| [[ECMAScriptCode]] |
|
The root parse node of the source text that defines the function's body. |
| [[ConstructorKind]] |
|
Whether or not the function is a derived class |
| [[Realm]] |
|
The |
| [[ScriptOrModule]] |
|
The script or module in which the function was created. |
| [[ThisMode]] |
|
Defines how this references are interpreted within the formal parameters and code body of the function. this refers to the |
| [[Strict]] | Boolean |
|
| [[HomeObject]] | Object |
If the function uses super, this is the object whose [[GetPrototypeOf]] provides the object where super property lookups begin.
|
| [[SourceText]] | sequence of Unicode code points |
The |
| [[IsClassConstructor]] | Boolean |
Indicates whether the function is a class |
All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.
10.2.1 [[Call]] ( thisArgument, argumentsList )
The [[Call]] internal method of an ECMAScript
Assert : F is an ECMAScriptfunction object .- Let callerContext be the
running execution context . - Let calleeContext be
PrepareForOrdinaryCall (F,undefined ). Assert : calleeContext is now therunning execution context .- If F.[[IsClassConstructor]] is
true , then- Let error be a newly created
TypeError object. - NOTE: error is created in calleeContext with F's associated
Realm Record . - Remove calleeContext from the
execution context stack and restore callerContext as therunning execution context . - Return
ThrowCompletion (error).
- Let error be a newly created
- Perform
OrdinaryCallBindThis (F, calleeContext, thisArgument). - Let result be
OrdinaryCallEvaluateBody (F, argumentsList). - Remove calleeContext from the
execution context stack and restore callerContext as therunning execution context . - If result.[[Type]] is
return , returnNormalCompletion (result.[[Value]]). ReturnIfAbrupt (result).- Return
NormalCompletion (undefined ).
When calleeContext is removed from the
10.2.1.1 PrepareForOrdinaryCall ( F, newTarget )
The abstract operation PrepareForOrdinaryCall takes arguments F (a
Assert :Type (newTarget) is Undefined or Object.- Let callerContext be the
running execution context . - Let calleeContext be a new ECMAScript code
execution context . - Set the Function of calleeContext to F.
- Let calleeRealm be F.[[Realm]].
- Set the
Realm of calleeContext to calleeRealm. - Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
- Let localEnv be
NewFunctionEnvironment (F, newTarget). - Set the LexicalEnvironment of calleeContext to localEnv.
- Set the VariableEnvironment of calleeContext to localEnv.
- If callerContext is not already suspended, suspend callerContext.
- Push calleeContext onto the
execution context stack ; calleeContext is now therunning execution context . - NOTE: Any exception objects produced after this point are associated with calleeRealm.
- Return calleeContext.
10.2.1.2 OrdinaryCallBindThis ( F, calleeContext, thisArgument )
The abstract operation OrdinaryCallBindThis takes arguments F (a
- Let thisMode be F.[[ThisMode]].
- If thisMode is
lexical , returnNormalCompletion (undefined ). - Let calleeRealm be F.[[Realm]].
- Let localEnv be the LexicalEnvironment of calleeContext.
- If thisMode is
strict , let thisValue be thisArgument. - Else,
- If thisArgument is
undefined ornull , then- Let globalEnv be calleeRealm.[[GlobalEnv]].
Assert : globalEnv is aglobal Environment Record .- Let thisValue be globalEnv.[[GlobalThisValue]].
- Else,
- If thisArgument is
Assert : localEnv is afunction Environment Record .Assert : The next step never returns anabrupt completion because localEnv.[[ThisBindingStatus]] is notinitialized .- Return localEnv.BindThisValue(thisValue).
10.2.1.3 Runtime Semantics: EvaluateBody
With parameters functionObject and argumentsList (a
- Return ?
EvaluateFunctionBody ofFunctionBody with arguments functionObject and argumentsList.
- Return ?
EvaluateConciseBody ofConciseBody with arguments functionObject and argumentsList.
- Return ?
EvaluateGeneratorBody ofGeneratorBody with arguments functionObject and argumentsList.
- Return ?
EvaluateAsyncGeneratorBody ofAsyncGeneratorBody with arguments functionObject and argumentsList.
- Return ?
EvaluateAsyncFunctionBody ofAsyncFunctionBody with arguments functionObject and argumentsList.
- Return ?
EvaluateAsyncConciseBody ofAsyncConciseBody with arguments functionObject and argumentsList.
10.2.1.4 OrdinaryCallEvaluateBody ( F, argumentsList )
The abstract operation OrdinaryCallEvaluateBody takes arguments F (a
- Return the result of
EvaluateBody of the parsed code that is F.[[ECMAScriptCode]] passing F and argumentsList as the arguments.
10.2.2 [[Construct]] ( argumentsList, newTarget )
The [[Construct]] internal method of an ECMAScript
Assert : F is an ECMAScriptfunction object .Assert :Type (newTarget) is Object.- Let callerContext be the
running execution context . - Let kind be F.[[ConstructorKind]].
- If kind is
base , then- Let thisArgument be ?
OrdinaryCreateFromConstructor (newTarget,"%Object.prototype%" ).
- Let thisArgument be ?
- Let calleeContext be
PrepareForOrdinaryCall (F, newTarget). Assert : calleeContext is now therunning execution context .- If kind is
base , performOrdinaryCallBindThis (F, calleeContext, thisArgument). - Let constructorEnv be the LexicalEnvironment of calleeContext.
- Let result be
OrdinaryCallEvaluateBody (F, argumentsList). - Remove calleeContext from the
execution context stack and restore callerContext as therunning execution context . - If result.[[Type]] is
return , then- If
Type (result.[[Value]]) is Object, returnNormalCompletion (result.[[Value]]). - If kind is
base , returnNormalCompletion (thisArgument). - If result.[[Value]] is not
undefined , throw aTypeError exception.
- If
- Else,
ReturnIfAbrupt (result). - Return ? constructorEnv.GetThisBinding().
10.2.3 OrdinaryFunctionCreate ( functionPrototype, sourceText, ParameterList, Body, thisMode, Scope )
The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object), sourceText (a sequence of Unicode code points), ParameterList (a
Assert :Type (functionPrototype) is Object.- Let internalSlotsList be the internal slots listed in
Table 29 . - Let F be !
OrdinaryObjectCreate (functionPrototype, internalSlotsList). - Set F.[[Call]] to the definition specified in
10.2.1 . - Set F.[[SourceText]] to sourceText.
- Set F.[[FormalParameters]] to ParameterList.
- Set F.[[ECMAScriptCode]] to Body.
- If the source text matching Body is
strict mode code , let Strict betrue ; else let Strict befalse . - Set F.[[Strict]] to Strict.
- If thisMode is
lexical-this , set F.[[ThisMode]] tolexical . - Else if Strict is
true , set F.[[ThisMode]] tostrict . - Else, set F.[[ThisMode]] to
global . - Set F.[[IsClassConstructor]] to
false . - Set F.[[Environment]] to Scope.
- Set F.[[ScriptOrModule]] to
GetActiveScriptOrModule (). - Set F.[[Realm]] to
the current Realm Record . - Set F.[[HomeObject]] to
undefined . - Let len be the
ExpectedArgumentCount of ParameterList. - Perform !
SetFunctionLength (F, len). - Return F.
10.2.4 AddRestrictedFunctionProperties ( F, realm )
The abstract operation AddRestrictedFunctionProperties takes arguments F (a
Assert : realm.[[Intrinsics]].[[%ThrowTypeError% ]] exists and has been initialized.- Let thrower be realm.[[Intrinsics]].[[
%ThrowTypeError% ]]. - Perform !
DefinePropertyOrThrow (F,"caller" , PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]:false , [[Configurable]]:true }). - Return !
DefinePropertyOrThrow (F,"arguments" , PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]:false , [[Configurable]]:true }).
10.2.4.1 %ThrowTypeError% ( )
The %ThrowTypeError% intrinsic is an anonymous built-in
- Throw a
TypeError exception.
The value of the [[Extensible]] internal slot of a %ThrowTypeError% function is
The
The
10.2.5 MakeConstructor ( F [ , writablePrototype [ , prototype ] ] )
The abstract operation MakeConstructor takes argument F (a
Assert : F is an ECMAScriptfunction object or a built-infunction object .- If F is an ECMAScript
function object , thenAssert :IsConstructor (F) isfalse .Assert : F is an extensible object that does not have a"prototype" own property.- Set F.[[Construct]] to the definition specified in
10.2.2 .
- Set F.[[ConstructorKind]] to
base . - If writablePrototype is not present, set writablePrototype to
true . - If prototype is not present, then
- Set prototype to !
OrdinaryObjectCreate (%Object.prototype% ). - Perform !
DefinePropertyOrThrow (prototype,"constructor" , PropertyDescriptor { [[Value]]: F, [[Writable]]: writablePrototype, [[Enumerable]]:false , [[Configurable]]:true }).
- Set prototype to !
- Perform !
DefinePropertyOrThrow (F,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]:false , [[Configurable]]:false }). - Return
NormalCompletion (undefined ).
10.2.6 MakeClassConstructor ( F )
The abstract operation MakeClassConstructor takes argument F. It performs the following steps when called:
Assert : F is an ECMAScriptfunction object .Assert : F.[[IsClassConstructor]] isfalse .- Set F.[[IsClassConstructor]] to
true . - Return
NormalCompletion (undefined ).
10.2.7 MakeMethod ( F, homeObject )
The abstract operation MakeMethod takes arguments F and homeObject. It configures F as a method. It performs the following steps when called:
Assert : F is an ECMAScriptfunction object .Assert :Type (homeObject) is Object.- Set F.[[HomeObject]] to homeObject.
- Return
NormalCompletion (undefined ).
10.2.8 SetFunctionName ( F, name [ , prefix ] )
The abstract operation SetFunctionName takes arguments F (a
Assert : F is an extensible object that does not have a"name" own property.Assert :Type (name) is either Symbol or String.Assert : If prefix is present, thenType (prefix) is String.- If
Type (name) is Symbol, then- Let description be name's [[Description]] value.
- If description is
undefined , set name to the empty String. - Else, set name to the
string-concatenation of"[" , description, and"]" .
- If F has an [[InitialName]] internal slot, then
- Set F.[[InitialName]] to name.
- If prefix is present, then
- Set name to the
string-concatenation of prefix, the code unit 0x0020 (SPACE), and name. - If F has an [[InitialName]] internal slot, then
- Optionally, set F.[[InitialName]] to name.
- Set name to the
- Return !
DefinePropertyOrThrow (F,"name" , PropertyDescriptor { [[Value]]: name, [[Writable]]:false , [[Enumerable]]:false , [[Configurable]]:true }).
10.2.9 SetFunctionLength ( F, length )
The abstract operation SetFunctionLength takes arguments F (a
Assert : F is an extensible object that does not have a"length" own property.- Return !
DefinePropertyOrThrow (F,"length" , PropertyDescriptor { [[Value]]:𝔽 (length), [[Writable]]:false , [[Enumerable]]:false , [[Configurable]]:true }).
10.2.10 FunctionDeclarationInstantiation ( func, argumentsList )
When an
The abstract operation FunctionDeclarationInstantiation takes arguments func (a
- Let calleeContext be the
running execution context . - Let code be func.[[ECMAScriptCode]].
- Let strict be func.[[Strict]].
- Let formals be func.[[FormalParameters]].
- Let parameterNames be the
BoundNames of formals. - If parameterNames has any duplicate entries, let hasDuplicates be
true . Otherwise, let hasDuplicates befalse . - Let simpleParameterList be
IsSimpleParameterList of formals. - Let hasParameterExpressions be
ContainsExpression of formals. - Let varNames be the
VarDeclaredNames of code. - Let varDeclarations be the
VarScopedDeclarations of code. - Let lexicalNames be the
LexicallyDeclaredNames of code. - Let functionNames be a new empty
List . - Let functionsToInitialize be a new empty
List . - For each element d of varDeclarations, in reverse
List order, do- If d is neither a
VariableDeclaration nor aForBinding nor aBindingIdentifier , thenAssert : d is either aFunctionDeclaration , aGeneratorDeclaration , anAsyncFunctionDeclaration , or anAsyncGeneratorDeclaration .- Let fn be the sole element of the
BoundNames of d. - If fn is not an element of functionNames, then
- Insert fn as the first element of functionNames.
- NOTE: If there are multiple function declarations for the same name, the last declaration is used.
- Insert d as the first element of functionsToInitialize.
- If d is neither a
- Let argumentsObjectNeeded be
true . - If func.[[ThisMode]] is
lexical , then- NOTE: Arrow functions never have an arguments objects.
- Set argumentsObjectNeeded to
false .
- Else if
"arguments" is an element of parameterNames, then- Set argumentsObjectNeeded to
false .
- Set argumentsObjectNeeded to
- Else if hasParameterExpressions is
false , then- If
"arguments" is an element of functionNames or if"arguments" is an element of lexicalNames, then- Set argumentsObjectNeeded to
false .
- Set argumentsObjectNeeded to
- If
- If strict is
true or if hasParameterExpressions isfalse , then- NOTE: Only a single
Environment Record is needed for the parameters and top-level vars. - Let env be the LexicalEnvironment of calleeContext.
- NOTE: Only a single
- Else,
- NOTE: A separate
Environment Record is needed to ensure that bindings created bydirect eval calls in the formal parameter list are outside the environment where parameters are declared. - Let calleeEnv be the LexicalEnvironment of calleeContext.
- Let env be
NewDeclarativeEnvironment (calleeEnv). Assert : The VariableEnvironment of calleeContext is calleeEnv.- Set the LexicalEnvironment of calleeContext to env.
- NOTE: A separate
- For each String paramName of parameterNames, do
- Let alreadyDeclared be env.HasBinding(paramName).
- NOTE: Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
- If alreadyDeclared is
false , then- Perform ! env.CreateMutableBinding(paramName,
false ). - If hasDuplicates is
true , then- Perform ! env.InitializeBinding(paramName,
undefined ).
- Perform ! env.InitializeBinding(paramName,
- Perform ! env.CreateMutableBinding(paramName,
- If argumentsObjectNeeded is
true , then- If strict is
true or if simpleParameterList isfalse , then- Let ao be
CreateUnmappedArgumentsObject (argumentsList).
- Let ao be
- Else,
- NOTE: A mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
- Let ao be
CreateMappedArgumentsObject (func, formals, argumentsList, env).
- If strict is
true , then- Perform ! env.CreateImmutableBinding(
"arguments" ,false ).
- Perform ! env.CreateImmutableBinding(
- Else,
- Perform ! env.CreateMutableBinding(
"arguments" ,false ).
- Perform ! env.CreateMutableBinding(
- Call env.InitializeBinding(
"arguments" , ao). - Let parameterBindings be a
List whose elements are the elements of parameterNames, followed by"arguments" .
- If strict is
- Else,
- Let parameterBindings be parameterNames.
- Let iteratorRecord be
CreateListIteratorRecord (argumentsList). - If hasDuplicates is
true , then- Perform ?
IteratorBindingInitialization for formals with iteratorRecord andundefined as arguments.
- Perform ?
- Else,
- Perform ?
IteratorBindingInitialization for formals with iteratorRecord and env as arguments.
- Perform ?
- If hasParameterExpressions is
false , then- NOTE: Only a single
Environment Record is needed for the parameters and top-level vars. - Let instantiatedVarNames be a copy of the
List parameterBindings. - For each element n of varNames, do
- If n is not an element of instantiatedVarNames, then
- Append n to instantiatedVarNames.
- Perform ! env.CreateMutableBinding(n,
false ). - Call env.InitializeBinding(n,
undefined ).
- If n is not an element of instantiatedVarNames, then
- Let varEnv be env.
- NOTE: Only a single
- Else,
- NOTE: A separate
Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body. - Let varEnv be
NewDeclarativeEnvironment (env). - Set the VariableEnvironment of calleeContext to varEnv.
- Let instantiatedVarNames be a new empty
List . - For each element n of varNames, do
- If n is not an element of instantiatedVarNames, then
- Append n to instantiatedVarNames.
- Perform ! varEnv.CreateMutableBinding(n,
false ). - If n is not an element of parameterBindings or if n is an element of functionNames, let initialValue be
undefined . - Else,
- Let initialValue be ! env.GetBindingValue(n,
false ).
- Let initialValue be ! env.GetBindingValue(n,
- Call varEnv.InitializeBinding(n, initialValue).
- NOTE: A var with the same name as a formal parameter initially has the same value as the corresponding initialized parameter.
- If n is not an element of instantiatedVarNames, then
- NOTE: A separate
- NOTE: Annex
B.3.3.1 adds additional steps at this point. - If strict is
false , then- Let lexEnv be
NewDeclarativeEnvironment (varEnv). - NOTE: Non-strict functions use a separate
Environment Record for top-level lexical declarations so that adirect eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strictdirect eval always places all declarations into a newEnvironment Record .
- Let lexEnv be
- Else, let lexEnv be varEnv.
- Set the LexicalEnvironment of calleeContext to lexEnv.
- Let lexDeclarations be the
LexicallyScopedDeclarations of code. - For each element d of lexDeclarations, do
- NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
- For each element dn of the
BoundNames of d, do- If
IsConstantDeclaration of d istrue , then- Perform ! lexEnv.CreateImmutableBinding(dn,
true ).
- Perform ! lexEnv.CreateImmutableBinding(dn,
- Else,
- Perform ! lexEnv.CreateMutableBinding(dn,
false ).
- Perform ! lexEnv.CreateMutableBinding(dn,
- If
- For each
Parse Node f of functionsToInitialize, do- Let fn be the sole element of the
BoundNames of f. - Let fo be
InstantiateFunctionObject of f with argument lexEnv. - Perform ! varEnv.SetMutableBinding(fn, fo,
false ).
- Let fn be the sole element of the
- Return
NormalCompletion (empty ).
Parameter
10.3 Built-in Function Objects
The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (
If a built-in
Unless otherwise specified every built-in
The behaviour specified for each built-in function via algorithm steps or other means is the specification of the function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]] thisArgument provides the
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-in new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-in
Built-in functions that are not constructors do not have a
Built-in functions have an [[InitialName]] internal slot.
If a built-in
10.3.1 [[Call]] ( thisArgument, argumentsList )
The [[Call]] internal method of a built-in
- Let callerContext be the
running execution context . - If callerContext is not already suspended, suspend callerContext.
- Let calleeContext be a new
execution context . - Set the Function of calleeContext to F.
- Let calleeRealm be F.[[Realm]].
- Set the
Realm of calleeContext to calleeRealm. - Set the ScriptOrModule of calleeContext to
null . - Perform any necessary
implementation-defined initialization of calleeContext. - Push calleeContext onto the
execution context stack ; calleeContext is now therunning execution context . - Let result be the
Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. thisArgument is thethis value, argumentsList provides the named parameters, and the NewTarget value isundefined . - Remove calleeContext from the
execution context stack and restore callerContext as therunning execution context . - Return result.
When calleeContext is removed from the
10.3.2 [[Construct]] ( argumentsList, newTarget )
The [[Construct]] internal method of a built-in
- Let result be the
Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. Thethis value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.
10.3.3 CreateBuiltinFunction ( steps, length, name, internalSlotsList [ , realm [ , prototype [ , prefix ] ] ] )
The abstract operation CreateBuiltinFunction takes arguments steps, length, name, and internalSlotsList (a
Assert : steps is either a set of algorithm steps or other definition of a function's behaviour provided in this specification.- If realm is not present or realm is
empty , set realm tothe current Realm Record . Assert : realm is aRealm Record .- If prototype is not present, set prototype to realm.[[Intrinsics]].[[
%Function.prototype% ]]. - Let func be a new built-in
function object that when called performs the action described by steps. The newfunction object has internal slots whose names are the elements of internalSlotsList, and an [[InitialName]] internal slot. - Set func.[[Realm]] to realm.
- Set func.[[Prototype]] to prototype.
- Set func.[[Extensible]] to
true . - Set func.[[InitialName]] to
null . - Perform !
SetFunctionLength (func, length). - If prefix is not present, then
- Perform !
SetFunctionName (func, name).
- Perform !
- Else,
- Perform !
SetFunctionName (func, name, prefix).
- Perform !
- Return func.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.
10.4 Built-in Exotic Object Internal Methods and Slots
This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the
10.4.1 Bound Function Exotic Objects
A
An object is a bound function exotic object if its [[Call]] and (if applicable) [[Construct]] internal methods use the following implementations, and its other essential internal methods use the definitions found in
Bound function exotic objects do not have the internal slots of ECMAScript function objects listed in
| Internal Slot | Type | Description |
|---|---|---|
| [[BoundTargetFunction]] | Callable Object |
The wrapped |
| [[BoundThis]] | Any |
The value that is always passed as the |
| [[BoundArguments]] |
|
A list of values whose elements are used as the first arguments to any call to the wrapped function. |
10.4.1.1 [[Call]] ( thisArgument, argumentsList )
The [[Call]] internal method of a
10.4.1.2 [[Construct]] ( argumentsList, newTarget )
The [[Construct]] internal method of a
- Let target be F.[[BoundTargetFunction]].
Assert :IsConstructor (target) istrue .- Let boundArgs be F.[[BoundArguments]].
- Let args be a
List whose elements are the elements of boundArgs, followed by the elements of argumentsList. - If
SameValue (F, newTarget) istrue , set newTarget to target. - Return ?
Construct (target, args, newTarget).
10.4.1.3 BoundFunctionCreate ( targetFunction, boundThis, boundArgs )
The abstract operation BoundFunctionCreate takes arguments targetFunction, boundThis, and boundArgs. It is used to specify the creation of new bound function exotic objects. It performs the following steps when called:
Assert :Type (targetFunction) is Object.- Let proto be ? targetFunction.[[GetPrototypeOf]]().
- Let internalSlotsList be the internal slots listed in
Table 30 , plus [[Prototype]] and [[Extensible]]. - Let obj be !
MakeBasicObject (internalSlotsList). - Set obj.[[Prototype]] to proto.
- Set obj.[[Call]] as described in
10.4.1.1 . - If
IsConstructor (targetFunction) istrue , then- Set obj.[[Construct]] as described in
10.4.1.2 .
- Set obj.[[Construct]] as described in
- Set obj.[[BoundTargetFunction]] to targetFunction.
- Set obj.[[BoundThis]] to boundThis.
- Set obj.[[BoundArguments]] to boundArgs.
- Return obj.
10.4.2 Array Exotic Objects
An Array object is an
A String
An object is an Array exotic object (or simply, an Array object) if its [[DefineOwnProperty]] internal method uses the following implementation, and its other essential internal methods use the definitions found in
10.4.2.1 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of an
Assert :IsPropertyKey (P) istrue .- If P is
"length" , then- Return ?
ArraySetLength (A, Desc).
- Return ?
- Else if P is an
array index , then- Let oldLenDesc be
OrdinaryGetOwnProperty (A,"length" ). Assert : !IsDataDescriptor (oldLenDesc) istrue .Assert : oldLenDesc.[[Configurable]] isfalse .- Let oldLen be oldLenDesc.[[Value]].
Assert : oldLen is a non-negativeintegral Number .- Let index be !
ToUint32 (P). - If index ≥ oldLen and oldLenDesc.[[Writable]] is
false , returnfalse . - Let succeeded be !
OrdinaryDefineOwnProperty (A, P, Desc). - If succeeded is
false , returnfalse . - If index ≥ oldLen, then
- Set oldLenDesc.[[Value]] to index +
1 𝔽. - Let succeeded be
OrdinaryDefineOwnProperty (A,"length" , oldLenDesc). Assert : succeeded istrue .
- Set oldLenDesc.[[Value]] to index +
- Return
true .
- Let oldLenDesc be
- Return
OrdinaryDefineOwnProperty (A, P, Desc).
10.4.2.2 ArrayCreate ( length [ , proto ] )
The abstract operation ArrayCreate takes argument length (a non-negative
- If length > 232 - 1, throw a
RangeError exception. - If proto is not present, set proto to
%Array.prototype% . - Let A be !
MakeBasicObject (« [[Prototype]], [[Extensible]] »). - Set A.[[Prototype]] to proto.
- Set A.[[DefineOwnProperty]] as specified in
10.4.2.1 . - Perform !
OrdinaryDefineOwnProperty (A,"length" , PropertyDescriptor { [[Value]]:𝔽 (length), [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Return A.
10.4.2.3 ArraySpeciesCreate ( originalArray, length )
The abstract operation ArraySpeciesCreate takes arguments originalArray and length (a non-negative
- Let isArray be ?
IsArray (originalArray). - If isArray is
false , return ?ArrayCreate (length). - Let C be ?
Get (originalArray,"constructor" ). - If
IsConstructor (C) istrue , then- Let thisRealm be
the current Realm Record . - Let realmC be ?
GetFunctionRealm (C). - If thisRealm and realmC are not the same
Realm Record , then
- Let thisRealm be
- If
Type (C) is Object, then - If C is
undefined , return ?ArrayCreate (length). - If
IsConstructor (C) isfalse , throw aTypeError exception. - Return ?
Construct (C, «𝔽 (length) »).
If originalArray was created using the standard built-in Array Array.prototype methods that now are defined using ArraySpeciesCreate.
10.4.2.4 ArraySetLength ( A, Desc )
The abstract operation ArraySetLength takes arguments A (an Array object) and Desc (a
- If Desc.[[Value]] is absent, then
- Return
OrdinaryDefineOwnProperty (A,"length" , Desc).
- Return
- Let newLenDesc be a copy of Desc.
- Let newLen be ?
ToUint32 (Desc.[[Value]]). - Let numberLen be ?
ToNumber (Desc.[[Value]]). - If newLen is not the same value as numberLen, throw a
RangeError exception. - Set newLenDesc.[[Value]] to newLen.
- Let oldLenDesc be
OrdinaryGetOwnProperty (A,"length" ). Assert : !IsDataDescriptor (oldLenDesc) istrue .Assert : oldLenDesc.[[Configurable]] isfalse .- Let oldLen be oldLenDesc.[[Value]].
- If newLen ≥ oldLen, then
- Return
OrdinaryDefineOwnProperty (A,"length" , newLenDesc).
- Return
- If oldLenDesc.[[Writable]] is
false , returnfalse . - If newLenDesc.[[Writable]] is absent or has the value
true , let newWritable betrue . - Else,
- NOTE: Setting the [[Writable]] attribute to
false is deferred in case any elements cannot be deleted. - Let newWritable be
false . - Set newLenDesc.[[Writable]] to
true .
- NOTE: Setting the [[Writable]] attribute to
- Let succeeded be !
OrdinaryDefineOwnProperty (A,"length" , newLenDesc). - If succeeded is
false , returnfalse . - For each own property key P of A that is an
array index , whose numeric value is greater than or equal to newLen, in descending numeric index order, do- Let deleteSucceeded be ! A.[[Delete]](P).
- If deleteSucceeded is
false , then- Set newLenDesc.[[Value]] to !
ToUint32 (P) +1 𝔽. - If newWritable is
false , set newLenDesc.[[Writable]] tofalse . - Perform !
OrdinaryDefineOwnProperty (A,"length" , newLenDesc). - Return
false .
- Set newLenDesc.[[Value]] to !
- If newWritable is
false , then- Let succeeded be !
OrdinaryDefineOwnProperty (A,"length" , PropertyDescriptor { [[Writable]]:false }). Assert : succeeded istrue .
- Let succeeded be !
- Return
true .
10.4.3 String Exotic Objects
A String object is an
An object is a String exotic object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal methods use the following implementations, and its other essential internal methods use the definitions found in
String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.
10.4.3.1 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of a
Assert :IsPropertyKey (P) istrue .- Let desc be
OrdinaryGetOwnProperty (S, P). - If desc is not
undefined , return desc. - Return !
StringGetOwnProperty (S, P).
10.4.3.2 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of a
Assert :IsPropertyKey (P) istrue .- Let stringDesc be !
StringGetOwnProperty (S, P). - If stringDesc is not
undefined , then- Let extensible be S.[[Extensible]].
- Return !
IsCompatiblePropertyDescriptor (extensible, Desc, stringDesc).
- Return !
OrdinaryDefineOwnProperty (S, P, Desc).
10.4.3.3 [[OwnPropertyKeys]] ( )
The [[OwnPropertyKeys]] internal method of a
- Let keys be a new empty
List . - Let str be O.[[StringData]].
Assert :Type (str) is String.- Let len be the length of str.
- For each
integer i starting with 0 such that i < len, in ascending order, do - For each own property key P of O such that P is an
array index and !ToIntegerOrInfinity (P) ≥ len, in ascending numeric index order, do- Add P as the last element of keys.
- For each own property key P of O such that
Type (P) is String and P is not anarray index , in ascending chronological order of property creation, do- Add P as the last element of keys.
- For each own property key P of O such that
Type (P) is Symbol, in ascending chronological order of property creation, do- Add P as the last element of keys.
- Return keys.
10.4.3.4 StringCreate ( value, prototype )
The abstract operation StringCreate takes arguments value (a String) and prototype. It is used to specify the creation of new String exotic objects. It performs the following steps when called:
- Let S be !
MakeBasicObject (« [[Prototype]], [[Extensible]], [[StringData]] »). - Set S.[[Prototype]] to prototype.
- Set S.[[StringData]] to value.
- Set S.[[GetOwnProperty]] as specified in
10.4.3.1 . - Set S.[[DefineOwnProperty]] as specified in
10.4.3.2 . - Set S.[[OwnPropertyKeys]] as specified in
10.4.3.3 . - Let length be the number of code unit elements in value.
- Perform !
DefinePropertyOrThrow (S,"length" , PropertyDescriptor { [[Value]]:𝔽 (length), [[Writable]]:false , [[Enumerable]]:false , [[Configurable]]:false }). - Return S.
10.4.3.5 StringGetOwnProperty ( S, P )
The abstract operation StringGetOwnProperty takes arguments S and P. It performs the following steps when called:
Assert : S is an Object that has a [[StringData]] internal slot.Assert :IsPropertyKey (P) istrue .- If
Type (P) is not String, returnundefined . - Let index be !
CanonicalNumericIndexString (P). - If index is
undefined , returnundefined . - If
IsIntegralNumber (index) isfalse , returnundefined . - If index is
-0 𝔽, returnundefined . - Let str be S.[[StringData]].
Assert :Type (str) is String.- Let len be the length of str.
- If
ℝ (index) < 0 or len ≤ℝ (index), returnundefined . - Let resultStr be the String value of length 1, containing one code unit from str, specifically the code unit at index
ℝ (index). - Return the PropertyDescriptor { [[Value]]: resultStr, [[Writable]]:
false , [[Enumerable]]:true , [[Configurable]]:false }.
10.4.4 Arguments Exotic Objects
Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either an
An object is an arguments exotic object if its internal methods use the following implementations, with the ones not specified here using those found in
While
Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (
The
The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.
Ordinary arguments objects define a non-configurable
ECMAScript implementations of arguments exotic objects have historically contained an
10.4.4.1 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of an
- Let desc be
OrdinaryGetOwnProperty (args, P). - If desc is
undefined , return desc. - Let map be args.[[ParameterMap]].
- Let isMapped be !
HasOwnProperty (map, P). - If isMapped is
true , then- Set desc.[[Value]] to
Get (map, P).
- Set desc.[[Value]] to
- Return desc.
10.4.4.2 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of an
- Let map be args.[[ParameterMap]].
- Let isMapped be
HasOwnProperty (map, P). - Let newArgDesc be Desc.
- If isMapped is
true andIsDataDescriptor (Desc) istrue , then- If Desc.[[Value]] is not present and Desc.[[Writable]] is present and its value is
false , then- Set newArgDesc to a copy of Desc.
- Set newArgDesc.[[Value]] to
Get (map, P).
- If Desc.[[Value]] is not present and Desc.[[Writable]] is present and its value is
- Let allowed be ?
OrdinaryDefineOwnProperty (args, P, newArgDesc). - If allowed is
false , returnfalse . - If isMapped is
true , then- If
IsAccessorDescriptor (Desc) istrue , then- Call map.[[Delete]](P).
- Else,
- If
- Return
true .
10.4.4.3 [[Get]] ( P, Receiver )
The [[Get]] internal method of an
- Let map be args.[[ParameterMap]].
- Let isMapped be !
HasOwnProperty (map, P). - If isMapped is
false , then- Return ?
OrdinaryGet (args, P, Receiver).
- Return ?
- Else,
10.4.4.4 [[Set]] ( P, V, Receiver )
The [[Set]] internal method of an
- If
SameValue (args, Receiver) isfalse , then- Let isMapped be
false .
- Let isMapped be
- Else,
- Let map be args.[[ParameterMap]].
- Let isMapped be !
HasOwnProperty (map, P).
- If isMapped is
true , then - Return ?
OrdinarySet (args, P, V, Receiver).
10.4.4.5 [[Delete]] ( P )
The [[Delete]] internal method of an
- Let map be args.[[ParameterMap]].
- Let isMapped be !
HasOwnProperty (map, P). - Let result be ?
OrdinaryDelete (args, P). - If result is
true and isMapped istrue , then- Call map.[[Delete]](P).
- Return result.
10.4.4.6 CreateUnmappedArgumentsObject ( argumentsList )
The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList. It performs the following steps when called:
- Let len be the number of elements in argumentsList.
- Let obj be !
OrdinaryObjectCreate (%Object.prototype% , « [[ParameterMap]] »). - Set obj.[[ParameterMap]] to
undefined . - Perform
DefinePropertyOrThrow (obj,"length" , PropertyDescriptor { [[Value]]:𝔽 (len), [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:true }). - Let index be 0.
- Repeat, while index < len,
- Let val be argumentsList[index].
- Perform !
CreateDataPropertyOrThrow (obj, !ToString (𝔽 (index)), val). - Set index to index + 1.
- Perform !
DefinePropertyOrThrow (obj,@@iterator , PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:true }). - Perform !
DefinePropertyOrThrow (obj,"callee" , PropertyDescriptor { [[Get]]:%ThrowTypeError% , [[Set]]:%ThrowTypeError% , [[Enumerable]]:false , [[Configurable]]:false }). - Return obj.
10.4.4.7 CreateMappedArgumentsObject ( func, formals, argumentsList, env )
The abstract operation CreateMappedArgumentsObject takes arguments func (an Object), formals (a
Assert : formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.- Let len be the number of elements in argumentsList.
- Let obj be !
MakeBasicObject (« [[Prototype]], [[Extensible]], [[ParameterMap]] »). - Set obj.[[GetOwnProperty]] as specified in
10.4.4.1 . - Set obj.[[DefineOwnProperty]] as specified in
10.4.4.2 . - Set obj.[[Get]] as specified in
10.4.4.3 . - Set obj.[[Set]] as specified in
10.4.4.4 . - Set obj.[[Delete]] as specified in
10.4.4.5 . - Set obj.[[Prototype]] to
%Object.prototype% . - Let map be !
OrdinaryObjectCreate (null ). - Set obj.[[ParameterMap]] to map.
- Let parameterNames be the
BoundNames of formals. - Let numberOfParameters be the number of elements in parameterNames.
- Let index be 0.
- Repeat, while index < len,
- Let val be argumentsList[index].
- Perform !
CreateDataPropertyOrThrow (obj, !ToString (𝔽 (index)), val). - Set index to index + 1.
- Perform !
DefinePropertyOrThrow (obj,"length" , PropertyDescriptor { [[Value]]:𝔽 (len), [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:true }). - Let mappedNames be a new empty
List . - Let index be numberOfParameters - 1.
- Repeat, while index ≥ 0,
- Let name be parameterNames[index].
- If name is not an element of mappedNames, then
- Add name as an element of the list mappedNames.
- If index < len, then
- Let g be
MakeArgGetter (name, env). - Let p be
MakeArgSetter (name, env). - Perform map.[[DefineOwnProperty]](!
ToString (𝔽 (index)), PropertyDescriptor { [[Set]]: p, [[Get]]: g, [[Enumerable]]:false , [[Configurable]]:true }).
- Let g be
- Set index to index - 1.
- Perform !
DefinePropertyOrThrow (obj,@@iterator , PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:true }). - Perform !
DefinePropertyOrThrow (obj,"callee" , PropertyDescriptor { [[Value]]: func, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:true }). - Return obj.
10.4.4.7.1 MakeArgGetter ( name, env )
The abstract operation MakeArgGetter takes arguments name (a String) and env (an
- Let steps be the steps of an ArgGetter function as specified below.
- Let length be the number of non-optional parameters of an ArgGetter function as specified below.
- Let getter be !
CreateBuiltinFunction (steps, length,"" , « [[Name]], [[Env]] »). - Set getter.[[Name]] to name.
- Set getter.[[Env]] to env.
- Return getter.
An ArgGetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgGetter function that expects no arguments is called it performs the following steps:
- Let f be the
active function object . - Let name be f.[[Name]].
- Let env be f.[[Env]].
- Return env.GetBindingValue(name,
false ).
ArgGetter functions are never directly accessible to ECMAScript code.
10.4.4.7.2 MakeArgSetter ( name, env )
The abstract operation MakeArgSetter takes arguments name (a String) and env (an
- Let steps be the steps of an ArgSetter function as specified below.
- Let length be the number of non-optional parameters of an ArgSetter function as specified below.
- Let setter be !
CreateBuiltinFunction (steps, length,"" , « [[Name]], [[Env]] »). - Set setter.[[Name]] to name.
- Set setter.[[Env]] to env.
- Return setter.
An ArgSetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgSetter function is called with argument value it performs the following steps:
- Let f be the
active function object . - Let name be f.[[Name]].
- Let env be f.[[Env]].
- Return env.SetMutableBinding(name, value,
false ).
ArgSetter functions are never directly accessible to ECMAScript code.
10.4.5 Integer-Indexed Exotic Objects
An
An object is an Integer-Indexed exotic object if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in
10.4.5.1 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of an
Assert :IsPropertyKey (P) istrue .Assert : O is anInteger-Indexed exotic object .- If
Type (P) is String, then- Let numericIndex be !
CanonicalNumericIndexString (P). - If numericIndex is not
undefined , then- Let value be !
IntegerIndexedElementGet (O, numericIndex). - If value is
undefined , returnundefined . - Return the PropertyDescriptor { [[Value]]: value, [[Writable]]:
true , [[Enumerable]]:true , [[Configurable]]:true }.
- Let value be !
- Let numericIndex be !
- Return
OrdinaryGetOwnProperty (O, P).
10.4.5.2 [[HasProperty]] ( P )
The [[HasProperty]] internal method of an
Assert :IsPropertyKey (P) istrue .Assert : O is anInteger-Indexed exotic object .- If
Type (P) is String, then- Let numericIndex be !
CanonicalNumericIndexString (P). - If numericIndex is not
undefined , return !IsValidIntegerIndex (O, numericIndex).
- Let numericIndex be !
- Return ?
OrdinaryHasProperty (O, P).
10.4.5.3 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of an
Assert :IsPropertyKey (P) istrue .Assert : O is anInteger-Indexed exotic object .- If
Type (P) is String, then- Let numericIndex be !
CanonicalNumericIndexString (P). - If numericIndex is not
undefined , then- If !
IsValidIntegerIndex (O, numericIndex) isfalse , returnfalse . - If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is
false , returnfalse . - If Desc has an [[Enumerable]] field and if Desc.[[Enumerable]] is
false , returnfalse . - If !
IsAccessorDescriptor (Desc) istrue , returnfalse . - If Desc has a [[Writable]] field and if Desc.[[Writable]] is
false , returnfalse . - If Desc has a [[Value]] field, perform ?
IntegerIndexedElementSet (O, numericIndex, Desc.[[Value]]). - Return
true .
- If !
- Let numericIndex be !
- Return !
OrdinaryDefineOwnProperty (O, P, Desc).
10.4.5.4 [[Get]] ( P, Receiver )
The [[Get]] internal method of an
Assert :IsPropertyKey (P) istrue .- If
Type (P) is String, then- Let numericIndex be !
CanonicalNumericIndexString (P). - If numericIndex is not
undefined , then- Return !
IntegerIndexedElementGet (O, numericIndex).
- Return !
- Let numericIndex be !
- Return ?
OrdinaryGet (O, P, Receiver).
10.4.5.5 [[Set]] ( P, V, Receiver )
The [[Set]] internal method of an
Assert :IsPropertyKey (P) istrue .- If
Type (P) is String, then- Let numericIndex be !
CanonicalNumericIndexString (P). - If numericIndex is not
undefined , then- Perform ?
IntegerIndexedElementSet (O, numericIndex, V). - Return
true .
- Perform ?
- Let numericIndex be !
- Return ?
OrdinarySet (O, P, V, Receiver).
10.4.5.6 [[Delete]] ( P )
The [[Delete]] internal method of an
Assert :IsPropertyKey (P) istrue .Assert : O is anInteger-Indexed exotic object .- If
Type (P) is String, then- Let numericIndex be !
CanonicalNumericIndexString (P). - If numericIndex is not
undefined , then- If !
IsValidIntegerIndex (O, numericIndex) isfalse , returntrue ; else returnfalse .
- If !
- Let numericIndex be !
- Return ?
OrdinaryDelete (O, P).
10.4.5.7 [[OwnPropertyKeys]] ( )
The [[OwnPropertyKeys]] internal method of an
- Let keys be a new empty
List . Assert : O is anInteger-Indexed exotic object .- If
IsDetachedBuffer (O.[[ViewedArrayBuffer]]) isfalse , then - For each own property key P of O such that
Type (P) is String and P is not aninteger index , in ascending chronological order of property creation, do- Add P as the last element of keys.
- For each own property key P of O such that
Type (P) is Symbol, in ascending chronological order of property creation, do- Add P as the last element of keys.
- Return keys.
10.4.5.8 IntegerIndexedObjectCreate ( prototype )
The abstract operation IntegerIndexedObjectCreate takes argument prototype. It is used to specify the creation of new
- Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
- Let A be !
MakeBasicObject (internalSlotsList). - Set A.[[GetOwnProperty]] as specified in
10.4.5.1 . - Set A.[[HasProperty]] as specified in
10.4.5.2 . - Set A.[[DefineOwnProperty]] as specified in
10.4.5.3 . - Set A.[[Get]] as specified in
10.4.5.4 . - Set A.[[Set]] as specified in
10.4.5.5 . - Set A.[[Delete]] as specified in
10.4.5.6 . - Set A.[[OwnPropertyKeys]] as specified in
10.4.5.7 . - Set A.[[Prototype]] to prototype.
- Return A.
10.4.5.9 IsValidIntegerIndex ( O, index )
The abstract operation IsValidIntegerIndex takes arguments O and index (a Number). It performs the following steps when called:
Assert : O is anInteger-Indexed exotic object .- If
IsDetachedBuffer (O.[[ViewedArrayBuffer]]) istrue , returnfalse . - If !
IsIntegralNumber (index) isfalse , returnfalse . - If index is
-0 𝔽, returnfalse . - If
ℝ (index) < 0 orℝ (index) ≥ O.[[ArrayLength]], returnfalse . - Return
true .
10.4.5.10 IntegerIndexedElementGet ( O, index )
The abstract operation IntegerIndexedElementGet takes arguments O and index (a Number). It performs the following steps when called:
Assert : O is anInteger-Indexed exotic object .- If !
IsValidIntegerIndex (O, index) isfalse , returnundefined . - Let offset be O.[[ByteOffset]].
- Let arrayTypeName be the String value of O.[[TypedArrayName]].
- Let elementSize be the Element Size value specified in
Table 60 for arrayTypeName. - Let indexedPosition be (
ℝ (index) × elementSize) + offset. - Let elementType be the Element Type value in
Table 60 for arrayTypeName. - Return
GetValueFromBuffer (O.[[ViewedArrayBuffer]], indexedPosition, elementType,true ,Unordered ).
10.4.5.11 IntegerIndexedElementSet ( O, index, value )
The abstract operation IntegerIndexedElementSet takes arguments O, index (a Number), and value. It performs the following steps when called:
Assert : O is anInteger-Indexed exotic object .- If O.[[ContentType]] is
BigInt , let numValue be ?ToBigInt (value). - Otherwise, let numValue be ?
ToNumber (value). - If !
IsValidIntegerIndex (O, index) istrue , then- Let offset be O.[[ByteOffset]].
- Let arrayTypeName be the String value of O.[[TypedArrayName]].
- Let elementSize be the Element Size value specified in
Table 60 for arrayTypeName. - Let indexedPosition be (
ℝ (index) × elementSize) + offset. - Let elementType be the Element Type value in
Table 60 for arrayTypeName. - Perform
SetValueInBuffer (O.[[ViewedArrayBuffer]], indexedPosition, elementType, numValue,true ,Unordered ).
- Return
NormalCompletion (undefined ).
This operation always appears to succeed, but it has no effect when attempting to write past the end of a TypedArray or to a TypedArray which is backed by a detached ArrayBuffer.
10.4.6 Module Namespace Exotic Objects
A export * export items. Each String-valued own property key is the
An object is a module namespace exotic object if its [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in
Module namespace exotic objects have the internal slots defined in
| Internal Slot | Type | Description |
|---|---|---|
| [[Module]] |
|
The |
| [[Exports]] |
|
A |
| [[Prototype]] | Null |
This slot always contains the value |
Module namespace exotic objects provide alternative definitions for all of the internal methods except [[GetPrototypeOf]], which behaves as defined in
10.4.6.1 [[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of a
- Return ?
SetImmutablePrototype (O, V).
10.4.6.2 [[IsExtensible]] ( )
The [[IsExtensible]] internal method of a
- Return
false .
10.4.6.3 [[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of a
- Return
true .
10.4.6.4 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of a
- If
Type (P) is Symbol, returnOrdinaryGetOwnProperty (O, P). - Let exports be O.[[Exports]].
- If P is not an element of exports, return
undefined . - Let value be ? O.[[Get]](P, O).
- Return PropertyDescriptor { [[Value]]: value, [[Writable]]:
true , [[Enumerable]]:true , [[Configurable]]:false }.
10.4.6.5 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of a
- If
Type (P) is Symbol, returnOrdinaryDefineOwnProperty (O, P, Desc). - Let current be ? O.[[GetOwnProperty]](P).
- If current is
undefined , returnfalse . - If Desc.[[Configurable]] is present and has value
true , returnfalse . - If Desc.[[Enumerable]] is present and has value
false , returnfalse . - If !
IsAccessorDescriptor (Desc) istrue , returnfalse . - If Desc.[[Writable]] is present and has value
false , returnfalse . - If Desc.[[Value]] is present, return
SameValue (Desc.[[Value]], current.[[Value]]). - Return
true .
10.4.6.6 [[HasProperty]] ( P )
The [[HasProperty]] internal method of a
- If
Type (P) is Symbol, returnOrdinaryHasProperty (O, P). - Let exports be O.[[Exports]].
- If P is an element of exports, return
true . - Return
false .
10.4.6.7 [[Get]] ( P, Receiver )
The [[Get]] internal method of a
Assert :IsPropertyKey (P) istrue .- If
Type (P) is Symbol, then- Return ?
OrdinaryGet (O, P, Receiver).
- Return ?
- Let exports be O.[[Exports]].
- If P is not an element of exports, return
undefined . - Let m be O.[[Module]].
- Let binding be ! m.ResolveExport(P).
Assert : binding is aResolvedBinding Record .- Let targetModule be binding.[[Module]].
Assert : targetModule is notundefined .- If binding.[[BindingName]] is
"*namespace*" , then- Return ?
GetModuleNamespace (targetModule).
- Return ?
- Let targetEnv be targetModule.[[Environment]].
- If targetEnv is
undefined , throw aReferenceError exception. - Return ? targetEnv.GetBindingValue(binding.[[BindingName]],
true ).
ResolveExport is side-effect free. Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each
10.4.6.8 [[Set]] ( P, V, Receiver )
The [[Set]] internal method of a
- Return
false .
10.4.6.9 [[Delete]] ( P )
The [[Delete]] internal method of a
Assert :IsPropertyKey (P) istrue .- If
Type (P) is Symbol, then- Return ?
OrdinaryDelete (O, P).
- Return ?
- Let exports be O.[[Exports]].
- If P is an element of exports, return
false . - Return
true .
10.4.6.10 [[OwnPropertyKeys]] ( )
The [[OwnPropertyKeys]] internal method of a
- Let exports be a copy of O.[[Exports]].
- Let symbolKeys be !
OrdinaryOwnPropertyKeys (O). - Append all the entries of symbolKeys to the end of exports.
- Return exports.
10.4.6.11 ModuleNamespaceCreate ( module, exports )
The abstract operation ModuleNamespaceCreate takes arguments module and exports. It is used to specify the creation of new module namespace exotic objects. It performs the following steps when called:
Assert : module is aModule Record .Assert : module.[[Namespace]] isundefined .Assert : exports is aList of String values.- Let internalSlotsList be the internal slots listed in
Table 31 . - Let M be !
MakeBasicObject (internalSlotsList). - Set M's essential internal methods to the definitions specified in
10.4.6 . - Set M.[[Prototype]] to
null . - Set M.[[Module]] to module.
- Let sortedExports be a
List whose elements are the elements of exports ordered as if an Array of the same values had been sorted using %Array.prototype.sort% usingundefined as comparefn. - Set M.[[Exports]] to sortedExports.
- Create own properties of M corresponding to the definitions in
28.3 . - Set module.[[Namespace]] to M.
- Return M.
10.4.7 Immutable Prototype Exotic Objects
An
An object is an immutable prototype exotic object if its [[SetPrototypeOf]] internal method uses the following implementation. (Its other essential internal methods may use any implementation, depending on the specific
Unlike other exotic objects, there is not a dedicated creation abstract operation provided for immutable prototype exotic objects. This is because they are only used by
10.4.7.1 [[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of an
- Return ?
SetImmutablePrototype (O, V).
10.4.7.2 SetImmutablePrototype ( O, V )
The abstract operation SetImmutablePrototype takes arguments O and V. It performs the following steps when called:
10.5 Proxy Object Internal Methods and Internal Slots
A proxy object is an
An object is a Proxy exotic object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal methods are installed in
| Internal Method | Handler Method |
|---|---|
| [[GetPrototypeOf]] |
getPrototypeOf
|
| [[SetPrototypeOf]] |
setPrototypeOf
|
| [[IsExtensible]] |
isExtensible
|
| [[PreventExtensions]] |
preventExtensions
|
| [[GetOwnProperty]] |
getOwnPropertyDescriptor
|
| [[DefineOwnProperty]] |
defineProperty
|
| [[HasProperty]] |
has
|
| [[Get]] |
get
|
| [[Set]] |
set
|
| [[Delete]] |
deleteProperty
|
| [[OwnPropertyKeys]] |
ownKeys
|
| [[Call]] |
apply
|
| [[Construct]] |
construct
|
When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to
Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a proxy object whose handler methods violates the invariants defined in
In the following algorithm descriptions, assume O is an ECMAScript proxy object, P is a property key value, V is any
10.5.1 [[GetPrototypeOf]] ( )
The [[GetPrototypeOf]] internal method of a
- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"getPrototypeOf" ). - If trap is
undefined , then- Return ? target.[[GetPrototypeOf]]().
- Let handlerProto be ?
Call (trap, handler, « target »). - If
Type (handlerProto) is neither Object nor Null, throw aTypeError exception. - Let extensibleTarget be ?
IsExtensible (target). - If extensibleTarget is
true , return handlerProto. - Let targetProto be ? target.[[GetPrototypeOf]]().
- If
SameValue (handlerProto, targetProto) isfalse , throw aTypeError exception. - Return handlerProto.
[[GetPrototypeOf]] for proxy objects enforces the following invariants:
-
The result of [[GetPrototypeOf]] must be either an Object or
null . - If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf]] applied to the proxy object's target object.
10.5.2 [[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of a
Assert : EitherType (V) is Object orType (V) is Null.- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"setPrototypeOf" ). - If trap is
undefined , then- Return ? target.[[SetPrototypeOf]](V).
- Let booleanTrapResult be !
ToBoolean (?Call (trap, handler, « target, V »)). - If booleanTrapResult is
false , returnfalse . - Let extensibleTarget be ?
IsExtensible (target). - If extensibleTarget is
true , returntrue . - Let targetProto be ? target.[[GetPrototypeOf]]().
- If
SameValue (V, targetProto) isfalse , throw aTypeError exception. - Return
true .
[[SetPrototypeOf]] for proxy objects enforces the following invariants:
- The result of [[SetPrototypeOf]] is a Boolean value.
- If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.
10.5.3 [[IsExtensible]] ( )
The [[IsExtensible]] internal method of a
- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"isExtensible" ). - If trap is
undefined , then- Return ?
IsExtensible (target).
- Return ?
- Let booleanTrapResult be !
ToBoolean (?Call (trap, handler, « target »)). - Let targetResult be ?
IsExtensible (target). - If
SameValue (booleanTrapResult, targetResult) isfalse , throw aTypeError exception. - Return booleanTrapResult.
[[IsExtensible]] for proxy objects enforces the following invariants:
- The result of [[IsExtensible]] is a Boolean value.
- [[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object's target object with the same argument.
10.5.4 [[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of a
- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"preventExtensions" ). - If trap is
undefined , then- Return ? target.[[PreventExtensions]]().
- Let booleanTrapResult be !
ToBoolean (?Call (trap, handler, « target »)). - If booleanTrapResult is
true , then- Let extensibleTarget be ?
IsExtensible (target). - If extensibleTarget is
true , throw aTypeError exception.
- Let extensibleTarget be ?
- Return booleanTrapResult.
[[PreventExtensions]] for proxy objects enforces the following invariants:
- The result of [[PreventExtensions]] is a Boolean value.
-
[[PreventExtensions]] applied to the proxy object only returns
true if [[IsExtensible]] applied to the proxy object's target object isfalse .
10.5.5 [[GetOwnProperty]] ( P )
The [[GetOwnProperty]] internal method of a
Assert :IsPropertyKey (P) istrue .- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"getOwnPropertyDescriptor" ). - If trap is
undefined , then- Return ? target.[[GetOwnProperty]](P).
- Let trapResultObj be ?
Call (trap, handler, « target, P »). - If
Type (trapResultObj) is neither Object nor Undefined, throw aTypeError exception. - Let targetDesc be ? target.[[GetOwnProperty]](P).
- If trapResultObj is
undefined , then- If targetDesc is
undefined , returnundefined . - If targetDesc.[[Configurable]] is
false , throw aTypeError exception. - Let extensibleTarget be ?
IsExtensible (target). - If extensibleTarget is
false , throw aTypeError exception. - Return
undefined .
- If targetDesc is
- Let extensibleTarget be ?
IsExtensible (target). - Let resultDesc be ?
ToPropertyDescriptor (trapResultObj). - Call
CompletePropertyDescriptor (resultDesc). - Let valid be
IsCompatiblePropertyDescriptor (extensibleTarget, resultDesc, targetDesc). - If valid is
false , throw aTypeError exception. - If resultDesc.[[Configurable]] is
false , then- If targetDesc is
undefined or targetDesc.[[Configurable]] istrue , then- Throw a
TypeError exception.
- Throw a
- If resultDesc has a [[Writable]] field and resultDesc.[[Writable]] is
false , then- If targetDesc.[[Writable]] is
true , throw aTypeError exception.
- If targetDesc.[[Writable]] is
- If targetDesc is
- Return resultDesc.
[[GetOwnProperty]] for proxy objects enforces the following invariants:
-
The result of [[GetOwnProperty]] must be either an Object or
undefined . - A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
- A property cannot be reported as non-existent, if the target object is not extensible, unless it does not exist as an own property of the target object.
- A property cannot be reported as existent, if the target object is not extensible, unless it exists as an own property of the target object.
- A property cannot be reported as non-configurable, unless it exists as a non-configurable own property of the target object.
- A property cannot be reported as both non-configurable and non-writable, unless it exists as a non-configurable, non-writable own property of the target object.
10.5.6 [[DefineOwnProperty]] ( P, Desc )
The [[DefineOwnProperty]] internal method of a
Assert :IsPropertyKey (P) istrue .- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"defineProperty" ). - If trap is
undefined , then- Return ? target.[[DefineOwnProperty]](P, Desc).
- Let descObj be
FromPropertyDescriptor (Desc). - Let booleanTrapResult be !
ToBoolean (?Call (trap, handler, « target, P, descObj »)). - If booleanTrapResult is
false , returnfalse . - Let targetDesc be ? target.[[GetOwnProperty]](P).
- Let extensibleTarget be ?
IsExtensible (target). - If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is
false , then- Let settingConfigFalse be
true .
- Let settingConfigFalse be
- Else, let settingConfigFalse be
false . - If targetDesc is
undefined , then- If extensibleTarget is
false , throw aTypeError exception. - If settingConfigFalse is
true , throw aTypeError exception.
- If extensibleTarget is
- Else,
- If
IsCompatiblePropertyDescriptor (extensibleTarget, Desc, targetDesc) isfalse , throw aTypeError exception. - If settingConfigFalse is
true and targetDesc.[[Configurable]] istrue , throw aTypeError exception. - If
IsDataDescriptor (targetDesc) istrue , targetDesc.[[Configurable]] isfalse , and targetDesc.[[Writable]] istrue , then- If Desc has a [[Writable]] field and Desc.[[Writable]] is
false , throw aTypeError exception.
- If Desc has a [[Writable]] field and Desc.[[Writable]] is
- If
- Return
true .
[[DefineOwnProperty]] for proxy objects enforces the following invariants:
- The result of [[DefineOwnProperty]] is a Boolean value.
- A property cannot be added, if the target object is not extensible.
- A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
- A non-configurable property cannot be non-writable, unless there exists a corresponding non-configurable, non-writable own property of the target object.
-
If a property has a corresponding target object property then applying the
Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
10.5.7 [[HasProperty]] ( P )
The [[HasProperty]] internal method of a
Assert :IsPropertyKey (P) istrue .- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"has" ). - If trap is
undefined , then- Return ? target.[[HasProperty]](P).
- Let booleanTrapResult be !
ToBoolean (?Call (trap, handler, « target, P »)). - If booleanTrapResult is
false , then- Let targetDesc be ? target.[[GetOwnProperty]](P).
- If targetDesc is not
undefined , then- If targetDesc.[[Configurable]] is
false , throw aTypeError exception. - Let extensibleTarget be ?
IsExtensible (target). - If extensibleTarget is
false , throw aTypeError exception.
- If targetDesc.[[Configurable]] is
- Return booleanTrapResult.
[[HasProperty]] for proxy objects enforces the following invariants:
- The result of [[HasProperty]] is a Boolean value.
- A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
- A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
10.5.8 [[Get]] ( P, Receiver )
The [[Get]] internal method of a
Assert :IsPropertyKey (P) istrue .- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"get" ). - If trap is
undefined , then- Return ? target.[[Get]](P, Receiver).
- Let trapResult be ?
Call (trap, handler, « target, P, Receiver »). - Let targetDesc be ? target.[[GetOwnProperty]](P).
- If targetDesc is not
undefined and targetDesc.[[Configurable]] isfalse , then- If
IsDataDescriptor (targetDesc) istrue and targetDesc.[[Writable]] isfalse , then- If
SameValue (trapResult, targetDesc.[[Value]]) isfalse , throw aTypeError exception.
- If
- If
IsAccessorDescriptor (targetDesc) istrue and targetDesc.[[Get]] isundefined , then- If trapResult is not
undefined , throw aTypeError exception.
- If trapResult is not
- If
- Return trapResult.
[[Get]] for proxy objects enforces the following invariants:
-
The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own
data property . -
The value reported for a property must be
undefined if the corresponding target object property is a non-configurable ownaccessor property that hasundefined as its [[Get]] attribute.
10.5.9 [[Set]] ( P, V, Receiver )
The [[Set]] internal method of a
Assert :IsPropertyKey (P) istrue .- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"set" ). - If trap is
undefined , then- Return ? target.[[Set]](P, V, Receiver).
- Let booleanTrapResult be !
ToBoolean (?Call (trap, handler, « target, P, V, Receiver »)). - If booleanTrapResult is
false , returnfalse . - Let targetDesc be ? target.[[GetOwnProperty]](P).
- If targetDesc is not
undefined and targetDesc.[[Configurable]] isfalse , then- If
IsDataDescriptor (targetDesc) istrue and targetDesc.[[Writable]] isfalse , then- If
SameValue (V, targetDesc.[[Value]]) isfalse , throw aTypeError exception.
- If
- If
IsAccessorDescriptor (targetDesc) istrue , then- If targetDesc.[[Set]] is
undefined , throw aTypeError exception.
- If targetDesc.[[Set]] is
- If
- Return
true .
[[Set]] for proxy objects enforces the following invariants:
- The result of [[Set]] is a Boolean value.
-
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own
data property . -
Cannot set the value of a property if the corresponding target object property is a non-configurable own
accessor property that hasundefined as its [[Set]] attribute.
10.5.10 [[Delete]] ( P )
The [[Delete]] internal method of a
Assert :IsPropertyKey (P) istrue .- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"deleteProperty" ). - If trap is
undefined , then- Return ? target.[[Delete]](P).
- Let booleanTrapResult be !
ToBoolean (?Call (trap, handler, « target, P »)). - If booleanTrapResult is
false , returnfalse . - Let targetDesc be ? target.[[GetOwnProperty]](P).
- If targetDesc is
undefined , returntrue . - If targetDesc.[[Configurable]] is
false , throw aTypeError exception. - Let extensibleTarget be ?
IsExtensible (target). - If extensibleTarget is
false , throw aTypeError exception. - Return
true .
[[Delete]] for proxy objects enforces the following invariants:
- The result of [[Delete]] is a Boolean value.
- A property cannot be reported as deleted, if it exists as a non-configurable own property of the target object.
- A property cannot be reported as deleted, if it exists as an own property of the target object and the target object is non-extensible.
10.5.11 [[OwnPropertyKeys]] ( )
The [[OwnPropertyKeys]] internal method of a
- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"ownKeys" ). - If trap is
undefined , then- Return ? target.[[OwnPropertyKeys]]().
- Let trapResultArray be ?
Call (trap, handler, « target »). - Let trapResult be ?
CreateListFromArrayLike (trapResultArray, « String, Symbol »). - If trapResult contains any duplicate entries, throw a
TypeError exception. - Let extensibleTarget be ?
IsExtensible (target). - Let targetKeys be ? target.[[OwnPropertyKeys]]().
Assert : targetKeys is aList whose elements are only String and Symbol values.Assert : targetKeys contains no duplicate entries.- Let targetConfigurableKeys be a new empty
List . - Let targetNonconfigurableKeys be a new empty
List . - For each element key of targetKeys, do
- Let desc be ? target.[[GetOwnProperty]](key).
- If desc is not
undefined and desc.[[Configurable]] isfalse , then- Append key as an element of targetNonconfigurableKeys.
- Else,
- Append key as an element of targetConfigurableKeys.
- If extensibleTarget is
true and targetNonconfigurableKeys is empty, then- Return trapResult.
- Let uncheckedResultKeys be a
List whose elements are the elements of trapResult. - For each element key of targetNonconfigurableKeys, do
- If key is not an element of uncheckedResultKeys, throw a
TypeError exception. - Remove key from uncheckedResultKeys.
- If key is not an element of uncheckedResultKeys, throw a
- If extensibleTarget is
true , return trapResult. - For each element key of targetConfigurableKeys, do
- If key is not an element of uncheckedResultKeys, throw a
TypeError exception. - Remove key from uncheckedResultKeys.
- If key is not an element of uncheckedResultKeys, throw a
- If uncheckedResultKeys is not empty, throw a
TypeError exception. - Return trapResult.
[[OwnPropertyKeys]] for proxy objects enforces the following invariants:
-
The result of [[OwnPropertyKeys]] is a
List . -
The returned
List contains no duplicate entries. -
The Type of each result
List element is either String or Symbol. -
The result
List must contain the keys of all non-configurable own properties of the target object. -
If the target object is not extensible, then the result
List must contain all the keys of the own properties of the target object and no other values.
10.5.12 [[Call]] ( thisArgument, argumentsList )
The [[Call]] internal method of a
- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
- Let trap be ?
GetMethod (handler,"apply" ). - If trap is
undefined , then- Return ?
Call (target, thisArgument, argumentsList).
- Return ?
- Let argArray be !
CreateArrayFromList (argumentsList). - Return ?
Call (trap, handler, « target, thisArgument, argArray »).
A
10.5.13 [[Construct]] ( argumentsList, newTarget )
The [[Construct]] internal method of a
- Let handler be O.[[ProxyHandler]].
- If handler is
null , throw aTypeError exception. Assert :Type (handler) is Object.- Let target be O.[[ProxyTarget]].
Assert :IsConstructor (target) istrue .- Let trap be ?
GetMethod (handler,"construct" ). - If trap is
undefined , then- Return ?
Construct (target, argumentsList, newTarget).
- Return ?
- Let argArray be !
CreateArrayFromList (argumentsList). - Let newObj be ?
Call (trap, handler, « target, argArray, newTarget »). - If
Type (newObj) is not Object, throw aTypeError exception. - Return newObj.
A
[[Construct]] for proxy objects enforces the following invariants:
- The result of [[Construct]] must be an Object.
10.5.14 ProxyCreate ( target, handler )
The abstract operation ProxyCreate takes arguments target and handler. It is used to specify the creation of new Proxy exotic objects. It performs the following steps when called:
- If
Type (target) is not Object, throw aTypeError exception. - If
Type (handler) is not Object, throw aTypeError exception. - Let P be !
MakeBasicObject (« [[ProxyHandler]], [[ProxyTarget]] »). - Set P's essential internal methods, except for [[Call]] and [[Construct]], to the definitions specified in
10.5 . - If
IsCallable (target) istrue , then- Set P.[[Call]] as specified in
10.5.12 . - If
IsConstructor (target) istrue , then- Set P.[[Construct]] as specified in
10.5.13 .
- Set P.[[Construct]] as specified in
- Set P.[[Call]] as specified in
- Set P.[[ProxyTarget]] to target.
- Set P.[[ProxyHandler]] to handler.
- Return P.
11 ECMAScript Language: Source Code
11.1 Source Text
Syntax
ECMAScript code is expressed using Unicode. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of
The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.
In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.
11.1.1 Static Semantics: UTF16EncodeCodePoint ( cp )
The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code point). It performs the following steps when called:
Assert : 0 ≤ cp ≤ 0x10FFFF.- If cp ≤ 0xFFFF, return the String value consisting of the code unit whose value is cp.
- Let cu1 be the code unit whose value is
floor ((cp - 0x10000) / 0x400) + 0xD800. - Let cu2 be the code unit whose value is ((cp - 0x10000)
modulo 0x400) + 0xDC00. - Return the
string-concatenation of cu1 and cu2.
11.1.2 Static Semantics: CodePointsToString ( text )
The abstract operation CodePointsToString takes argument text (a sequence of Unicode code points). It converts text into a String value, as described in
- Let result be the empty String.
- For each code point cp of text, do
- Set result to the
string-concatenation of result and !UTF16EncodeCodePoint (cp).
- Set result to the
- Return result.
11.1.3 Static Semantics: UTF16SurrogatePairToCodePoint ( lead, trail )
The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code unit) and trail (a code unit). Two code units that form a UTF-16
Assert : lead is aleading surrogate and trail is atrailing surrogate .- Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
- Return the code point cp.
11.1.4 Static Semantics: CodePointAt ( string, position )
The abstract operation CodePointAt takes arguments string (a String) and position (a non-negative
- Let size be the length of string.
Assert : position ≥ 0 and position < size.- Let first be the code unit at index position within string.
- Let cp be the code point whose numeric value is that of first.
- If first is not a
leading surrogate ortrailing surrogate , then- Return the
Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:false }.
- Return the
- If first is a
trailing surrogate or position + 1 = size, then- Return the
Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:true }.
- Return the
- Let second be the code unit at index position + 1 within string.
- If second is not a
trailing surrogate , then- Return the
Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:true }.
- Return the
- Set cp to !
UTF16SurrogatePairToCodePoint (first, second). - Return the
Record { [[CodePoint]]: cp, [[CodeUnitCount]]: 2, [[IsUnpairedSurrogate]]:false }.
11.1.5 Static Semantics: StringToCodePoints ( string )
The abstract operation StringToCodePoints takes argument string (a String). It returns the sequence of Unicode code points that results from interpreting string as UTF-16 encoded Unicode text as described in
- Let codePoints be a new empty
List . - Let size be the length of string.
- Let position be 0.
- Repeat, while position < size,
- Let cp be !
CodePointAt (string, position). - Append cp.[[CodePoint]] to codePoints.
- Set position to position + cp.[[CodeUnitCount]].
- Let cp be !
- Return codePoints.
11.1.6 Static Semantics: ParseText ( sourceText, goalSymbol )
The abstract operation ParseText takes arguments sourceText (a sequence of Unicode code points) and goalSymbol (a nonterminal in one of the ECMAScript grammars). It performs the following steps when called:
- Attempt to parse sourceText using goalSymbol as the
goal symbol , and analyse the parse result for anyearly error conditions. Parsing andearly error detection may be interleaved in animplementation-defined manner. - If the parse succeeded and no early errors were found, return the
Parse Node (an instance of goalSymbol) at the root of the parse tree resulting from the parse. - Otherwise, return a
List of one or moreSyntaxError objects representing the parsing errors and/or early errors. If more than one parsing error orearly error is present, the number and ordering of error objects in the list isimplementation-defined , but at least one must be present.
Consider a text that has an
See also clause
11.2 Types of Source Code
There are four types of ECMAScript code:
-
Global code is source text that is treated as an ECMAScript
Script . The global code of a particularScript does not include any source text that is parsed as part of aFunctionDeclaration ,FunctionExpression ,GeneratorDeclaration ,GeneratorExpression ,AsyncFunctionDeclaration ,AsyncFunctionExpression ,AsyncGeneratorDeclaration ,AsyncGeneratorExpression ,MethodDefinition ,ArrowFunction ,AsyncArrowFunction ,ClassDeclaration , orClassExpression . -
Eval code is the source text supplied to the built-in
evalfunction. More precisely, if the parameter to the built-inevalfunction is a String, it is treated as an ECMAScriptScript . The eval code for a particular invocation ofevalis the global code portion of thatScript . -
Function code is source text that is parsed to supply the value of the [[ECMAScriptCode]] and [[FormalParameters]] internal slots (see
10.2 ) of an ECMAScriptfunction object . The function code of a particular ECMAScript function does not include any source text that is parsed as the function code of a nestedFunctionDeclaration ,FunctionExpression ,GeneratorDeclaration ,GeneratorExpression ,AsyncFunctionDeclaration ,AsyncFunctionExpression ,AsyncGeneratorDeclaration ,AsyncGeneratorExpression ,MethodDefinition ,ArrowFunction ,AsyncArrowFunction ,ClassDeclaration , orClassExpression .In addition, if the source text referred to above is parsed as:
- the
FormalParameters andFunctionBody of aFunctionDeclaration orFunctionExpression , - the
FormalParameters andGeneratorBody of aGeneratorDeclaration orGeneratorExpression , - the
FormalParameters andAsyncFunctionBody of anAsyncFunctionDeclaration orAsyncFunctionExpression , or - the
FormalParameters andAsyncGeneratorBody of anAsyncGeneratorDeclaration orAsyncGeneratorExpression ,
then the source text matching the
BindingIdentifier (if any) of that declaration or expression is also included in the function code of the corresponding function. - the
-
Module code is source text that is code that is provided as a
ModuleBody . It is the code that is directly evaluated when a module is initialized. The module code of a particular module does not include any source text that is parsed as part of a nestedFunctionDeclaration ,FunctionExpression ,GeneratorDeclaration ,GeneratorExpression ,AsyncFunctionDeclaration ,AsyncFunctionExpression ,AsyncGeneratorDeclaration ,AsyncGeneratorExpression ,MethodDefinition ,ArrowFunction ,AsyncArrowFunction ,ClassDeclaration , orClassExpression .
Function code is generally provided as the bodies of Function Definitions (
The practical effect of including the
11.2.1 Directive Prologues and the Use Strict Directive
A Directive Prologue is the longest sequence of
A Use Strict Directive is an "use strict" or 'use strict'. A
A
The
11.2.2 Strict Mode Code
An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics (
-
Global code is strict mode code if it begins with a
Directive Prologue that contains aUse Strict Directive . - Module code is always strict mode code.
-
All parts of a
ClassDeclaration or aClassExpression are strict mode code. -
Eval code is strict mode code if it begins with a
Directive Prologue that contains aUse Strict Directive or if the call toevalis adirect eval that is contained in strict mode code. -
Function code is strict mode code if the associated
FunctionDeclaration ,FunctionExpression ,GeneratorDeclaration ,GeneratorExpression ,AsyncFunctionDeclaration ,AsyncFunctionExpression ,AsyncGeneratorDeclaration ,AsyncGeneratorExpression ,MethodDefinition ,ArrowFunction , orAsyncArrowFunction is contained in strict mode code or if the code that produces the value of the function's [[ECMAScriptCode]] internal slot begins with aDirective Prologue that contains aUse Strict Directive . -
Function code that is supplied as the arguments to the built-in Function, Generator, AsyncFunction, and AsyncGenerator constructors is strict mode code if the last argument is a String that when processed is a
FunctionBody that begins with aDirective Prologue that contains aUse Strict Directive .
ECMAScript code that is not strict mode code is called non-strict code.
11.2.3 Non-ECMAScript Functions
An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in some
12 ECMAScript Language: Lexical Grammar
The source text of an ECMAScript
There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The
The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading
a = b
/hi/g.exec(c).map(d);where the first non-whitespace, non-comment code point after a
a = b / hi / g.exec(c).map(d);Syntax
12.1 Unicode Format-Control Characters
The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.
U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see
The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in
| Code Point | Name | Abbreviation | Usage |
|---|---|---|---|
U+200C
|
ZERO WIDTH NON-JOINER | <ZWNJ> |
|
U+200D
|
ZERO WIDTH JOINER | <ZWJ> |
|
U+FEFF
|
ZERO WIDTH NO-BREAK SPACE | <ZWNBSP> |
|
12.2 White Space
White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a
The ECMAScript white space code points are listed in
| Code Point | Name | Abbreviation |
|---|---|---|
U+0009
|
CHARACTER TABULATION | <TAB> |
U+000B
|
LINE TABULATION | <VT> |
U+000C
|
FORM FEED (FF) | <FF> |
U+0020
|
SPACE | <SP> |
U+00A0
|
NO-BREAK SPACE | <NBSP> |
U+FEFF
|
ZERO WIDTH NO-BREAK SPACE | <ZWNBSP> |
| Other category “Zs” | Any other Unicode “Space_Separator” code point | <USP> |
ECMAScript implementations must recognize as
Other than for the code points listed in
Syntax
12.3 Line Terminators
Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (
A line terminator can occur within a
Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.
The ECMAScript line terminator code points are listed in
| Code Point | Unicode Name | Abbreviation |
|---|---|---|
U+000A
|
LINE FEED (LF) | <LF> |
U+000D
|
CARRIAGE RETURN (CR) | <CR> |
U+2028
|
LINE SEPARATOR | <LS> |
U+2029
|
PARAGRAPH SEPARATOR | <PS> |
Only the Unicode code points in
Syntax
12.4 Comments
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a // marker to the end of the line. However, the
Comments behave like white space and are discarded except that, if a
Syntax
A number of productions in this section are given alternative definitions in section
12.5 Tokens
Syntax
The
12.6 Names and Keywords
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an
Unicode escape sequences are permitted in an \ preceding the u and { } code units, if they appear, do not contribute code points to the \
Two
Syntax
The definitions of the nonterminal
The nonterminal _ via
The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.
12.6.1 Identifier Names
12.6.1.1 Static Semantics: Early Errors
-
It is a Syntax Error if the
SV ofUnicodeEscapeSequence is none of"$" , or"_" , or !UTF16EncodeCodePoint (cp) for some Unicode code point cp matched by theUnicodeIDStart lexical grammar production.
-
It is a Syntax Error if the
SV ofUnicodeEscapeSequence is none of"$" ,"_" , !UTF16EncodeCodePoint (<ZWNJ>), !UTF16EncodeCodePoint (<ZWJ>), or !UTF16EncodeCodePoint (cp) for some Unicode code point cp that would be matched by theUnicodeIDContinue lexical grammar production.
12.6.2 Keywords and Reserved Words
A keyword is a token that matches fixed width font, in some syntactic production. The keywords of ECMAScript include if, while, async, await, and many others.
A reserved word is an if and while are reserved words. await is reserved only inside async functions and modules. async is not reserved; it can be used as a variable name or statement label without restriction.
This specification uses a combination of grammatical productions and await and yield, are unconditionally reserved. Exceptions for await and yield are specified in
-
Those that are always allowed as identifiers, and are not keywords, such as
Math,window,toString, and_; -
Those that are never allowed as identifiers, namely the
ReservedWord s listed below exceptawaitandyield; -
Those that are contextually allowed as identifiers, namely
awaitandyield; -
Those that are contextually disallowed as identifiers, in
strict mode code :let,static,implements,interface,package,private,protected, andpublic; -
Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places where
Identifier is not allowed:as,async,from,get,of,set, andtarget.
The term conditional keyword, or contextual keyword, is sometimes used to refer to the keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as keywords in others.
Syntax
Per \
An \ els\u{65}. The
enum is not currently used as a keyword in this specification. It is a future reserved word, set aside for use as a keyword in future language extensions.
Similarly, implements, interface, package, private, protected, and public are future reserved words in
12.7 Punctuators
Syntax
12.8 Literals
12.8.1 Null Literals
Syntax
12.8.2 Boolean Literals
Syntax
12.8.3 Numeric Literals
Syntax
The
For example: 3in is an error and not the two input elements 3 and in.
A conforming implementation, when processing
12.8.3.1 Static Semantics: MV
A numeric literal stands for a value of the Number type or the BigInt type.
-
The MV of
NumericLiteral :: DecimalLiteral DecimalLiteral . -
The MV of
NonDecimalIntegerLiteral :: BinaryIntegerLiteral BinaryIntegerLiteral . -
The MV of
NonDecimalIntegerLiteral :: OctalIntegerLiteral OctalIntegerLiteral . -
The MV of
NonDecimalIntegerLiteral :: HexIntegerLiteral HexIntegerLiteral . -
The MV of
DecimalLiteral :: DecimalIntegerLiteral . DecimalIntegerLiteral . -
The MV of
DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits DecimalIntegerLiteral plus (the MV ofDecimalDigits × 10-n), where n is the number of code points inDecimalDigits , excluding all occurrences ofNumericLiteralSeparator . -
The MV of
DecimalLiteral :: DecimalIntegerLiteral . ExponentPart DecimalIntegerLiteral × 10e, where e is the MV ofExponentPart . -
The MV of
DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits ExponentPart DecimalIntegerLiteral plus (the MV ofDecimalDigits × 10-n)) × 10e, where n is the number of code points inDecimalDigits , excluding all occurrences ofNumericLiteralSeparator and e is the MV ofExponentPart . -
The MV of
DecimalLiteral :: . DecimalDigits DecimalDigits × 10-n, where n is the number of code points inDecimalDigits , excluding all occurrences ofNumericLiteralSeparator . -
The MV of
DecimalLiteral :: . DecimalDigits ExponentPart DecimalDigits × 10e - n, where n is the number of code points inDecimalDigits , excluding all occurrences ofNumericLiteralSeparator , and e is the MV ofExponentPart . -
The MV of
DecimalLiteral :: DecimalIntegerLiteral DecimalIntegerLiteral . -
The MV of
DecimalLiteral :: DecimalIntegerLiteral ExponentPart DecimalIntegerLiteral × 10e, where e is the MV ofExponentPart . -
The MV of
DecimalIntegerLiteral :: 0 -
The MV of
DecimalIntegerLiteral :: NonZeroDigit NonZeroDigit . -
The MV of
DecimalIntegerLiteral :: NonZeroDigit NumericLiteralSeparator opt DecimalDigits NonZeroDigit × 10n) plus the MV ofDecimalDigits , where n is the number of code points inDecimalDigits , excluding all occurrences ofNumericLiteralSeparator . -
The MV of
DecimalDigits :: DecimalDigit DecimalDigit . -
The MV of
DecimalDigits :: DecimalDigits DecimalDigit DecimalDigits × 10) plus the MV ofDecimalDigit . -
The MV of
DecimalDigits :: DecimalDigits NumericLiteralSeparator DecimalDigit DecimalDigits × 10) plus the MV ofDecimalDigit . -
The MV of
ExponentPart :: ExponentIndicator SignedInteger SignedInteger . -
The MV of
SignedInteger :: DecimalDigits DecimalDigits . -
The MV of
SignedInteger :: + DecimalDigits DecimalDigits . -
The MV of
SignedInteger :: - DecimalDigits DecimalDigits . -
The MV of
DecimalDigit :: 0 HexDigit :: 0 OctalDigit :: 0 BinaryDigit :: 0 -
The MV of
DecimalDigit :: 1 NonZeroDigit :: 1 HexDigit :: 1 OctalDigit :: 1 BinaryDigit :: 1 -
The MV of
DecimalDigit :: 2 NonZeroDigit :: 2 HexDigit :: 2 OctalDigit :: 2 -
The MV of
DecimalDigit :: 3 NonZeroDigit :: 3 HexDigit :: 3 OctalDigit :: 3 -
The MV of
DecimalDigit :: 4 NonZeroDigit :: 4 HexDigit :: 4 OctalDigit :: 4 -
The MV of
DecimalDigit :: 5 NonZeroDigit :: 5 HexDigit :: 5 OctalDigit :: 5 -
The MV of
DecimalDigit :: 6 NonZeroDigit :: 6 HexDigit :: 6 OctalDigit :: 6 -
The MV of
DecimalDigit :: 7 NonZeroDigit :: 7 HexDigit :: 7 OctalDigit :: 7 -
The MV of
DecimalDigit :: 8 NonZeroDigit :: 8 HexDigit :: 8 -
The MV of
DecimalDigit :: 9 NonZeroDigit :: 9 HexDigit :: 9 -
The MV of
HexDigit :: a HexDigit :: A -
The MV of
HexDigit :: b HexDigit :: B -
The MV of
HexDigit :: c HexDigit :: C -
The MV of
HexDigit :: d HexDigit :: D -
The MV of
HexDigit :: e HexDigit :: E -
The MV of
HexDigit :: f HexDigit :: F -
The MV of
BinaryIntegerLiteral :: 0b BinaryDigits BinaryDigits . -
The MV of
BinaryIntegerLiteral :: 0B BinaryDigits BinaryDigits . -
The MV of
BinaryDigits :: BinaryDigit BinaryDigit . -
The MV of
BinaryDigits :: BinaryDigits BinaryDigit BinaryDigits × 2) plus the MV ofBinaryDigit . -
The MV of
BinaryDigits :: BinaryDigits NumericLiteralSeparator BinaryDigit BinaryDigits × 2) plus the MV ofBinaryDigit . -
The MV of
OctalIntegerLiteral :: 0o OctalDigits OctalDigits . -
The MV of
OctalIntegerLiteral :: 0O OctalDigits OctalDigits . -
The MV of
OctalDigits :: OctalDigit OctalDigit . -
The MV of
OctalDigits :: OctalDigits OctalDigit OctalDigits × 8) plus the MV ofOctalDigit . -
The MV of
OctalDigits :: OctalDigits NumericLiteralSeparator OctalDigit OctalDigits × 8) plus the MV ofOctalDigit . -
The MV of
HexIntegerLiteral :: 0x HexDigits HexDigits . -
The MV of
HexIntegerLiteral :: 0X HexDigits HexDigits . -
The MV of
HexDigits :: HexDigit HexDigit . -
The MV of
HexDigits :: HexDigits HexDigit HexDigits × 16) plus the MV ofHexDigit . -
The MV of
HexDigits :: HexDigits NumericLiteralSeparator HexDigit HexDigits × 16) plus the MV ofHexDigit .
12.8.3.2 Static Semantics: NumericValue
- Return the
Number value that results from rounding the MV ofDecimalLiteral as described below.
- Return the
Number value that results from rounding the MV ofNonDecimalIntegerLiteral as described below.
Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is 0 digit or the 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an
-
it is not
0; or -
there is a non-zero digit to its left and there is a non-zero digit, not in the
ExponentPart , to its right.
- Return the BigInt value that represents the MV of
NonDecimalIntegerLiteral .
- Return
0 ℤ.
- Return the BigInt value that represents the MV of
NonZeroDigit .
- Let n be the number of code points in
DecimalDigits , excluding all occurrences ofNumericLiteralSeparator . - Let mv be (the MV of
NonZeroDigit × 10) plus the MV ofDecimalDigits . - Return
ℤ (mv).
12.8.4 String Literals
A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in
Syntax
A conforming implementation, when processing
The definition of the nonterminal
<LF> and <CR> cannot appear in a string literal, except as part of a \n or \u000A.
12.8.4.1 Static Semantics: SV
A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of String values contributed by the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a
-
The SV of
StringLiteral :: " " -
The SV of
StringLiteral :: ' ' -
The SV of
DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters string-concatenation of the SV ofDoubleStringCharacter and the SV ofDoubleStringCharacters . -
The SV of
SingleStringCharacters :: SingleStringCharacter SingleStringCharacters string-concatenation of the SV ofSingleStringCharacter and the SV ofSingleStringCharacters . -
The SV of
DoubleStringCharacter :: SourceCharacter but not one of " or\ orLineTerminator UTF16EncodeCodePoint on the code point value ofSourceCharacter . -
The SV of
DoubleStringCharacter :: <LS> -
The SV of
DoubleStringCharacter :: <PS> -
The SV of
DoubleStringCharacter :: LineContinuation -
The SV of
SingleStringCharacter :: SourceCharacter but not one of ' or\ orLineTerminator UTF16EncodeCodePoint on the code point value ofSourceCharacter . -
The SV of
SingleStringCharacter :: <LS> -
The SV of
SingleStringCharacter :: <PS> -
The SV of
SingleStringCharacter :: LineContinuation -
The SV of
EscapeSequence :: 0 -
The SV of
CharacterEscapeSequence :: SingleEscapeCharacter SingleEscapeCharacter according toTable 36 .
| Escape Sequence | Code Unit Value | Unicode Character Name | Symbol |
|---|---|---|---|
\b
|
0x0008
|
BACKSPACE | <BS> |
\t
|
0x0009
|
CHARACTER TABULATION | <HT> |
\n
|
0x000A
|
LINE FEED (LF) | <LF> |
\v
|
0x000B
|
LINE TABULATION | <VT> |
\f
|
0x000C
|
FORM FEED (FF) | <FF> |
\r
|
0x000D
|
CARRIAGE RETURN (CR) | <CR> |
\"
|
0x0022
|
QUOTATION MARK |
"
|
\'
|
0x0027
|
APOSTROPHE |
'
|
\\
|
0x005C
|
REVERSE SOLIDUS |
\
|
-
The SV of
NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter orLineTerminator UTF16EncodeCodePoint on the code point value ofSourceCharacter . -
The SV of
HexEscapeSequence :: x HexDigit HexDigit HexEscapeSequence . -
The SV of
Hex4Digits :: HexDigit HexDigit HexDigit HexDigit Hex4Digits . -
The SV of
UnicodeEscapeSequence :: u{ CodePoint } UTF16EncodeCodePoint on the MV ofCodePoint .
12.8.4.2 Static Semantics: MV
-
The MV of
HexEscapeSequence :: x HexDigit HexDigit HexDigit ) plus the MV of the secondHexDigit . -
The MV of
Hex4Digits :: HexDigit HexDigit HexDigit HexDigit HexDigit ) plus (0x100 × the MV of the secondHexDigit ) plus (0x10 × the MV of the thirdHexDigit ) plus the MV of the fourthHexDigit .
12.8.5 Regular Expression Literals
A regular expression literal is an input element that is converted to a RegExp object (see === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExp
The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the
An implementation may extend the ECMAScript Regular Expression grammar defined in
Syntax
Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.
12.8.5.1 Static Semantics: Early Errors
-
It is a Syntax Error if
IdentifierPart contains a Unicode escape sequence.
12.8.5.2 Static Semantics: BodyText
- Return the source text that was recognized as
RegularExpressionBody .
12.8.5.3 Static Semantics: FlagText
- Return the source text that was recognized as
RegularExpressionFlags .
12.8.6 Template Literal Lexical Components
Syntax
A conforming implementation must not use the extended definition of
12.8.6.1 Static Semantics: TV and TRV
A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of String values (
-
The TV and TRV of
NoSubstitutionTemplate :: ` ` -
The TV and TRV of
TemplateHead :: ` ${ -
The TV and TRV of
TemplateMiddle :: } ${ -
The TV and TRV of
TemplateTail :: } ` -
The TV of
TemplateCharacters :: TemplateCharacter TemplateCharacters undefined if either the TV ofTemplateCharacter isundefined or the TV ofTemplateCharacters isundefined . Otherwise, it is thestring-concatenation of the TV ofTemplateCharacter and the TV ofTemplateCharacters . -
The TV of
TemplateCharacter :: SourceCharacter but not one of ` or\ or$ orLineTerminator UTF16EncodeCodePoint on the code point value ofSourceCharacter . -
The TV of
TemplateCharacter :: $ -
The TV of
TemplateCharacter :: \ EscapeSequence SV ofEscapeSequence . -
The TV of
TemplateCharacter :: \ NotEscapeSequence undefined . -
The TV of
TemplateCharacter :: LineTerminatorSequence LineTerminatorSequence . -
The TV of
LineContinuation :: \ LineTerminatorSequence -
The TRV of
TemplateCharacters :: TemplateCharacter TemplateCharacters string-concatenation of the TRV ofTemplateCharacter and the TRV ofTemplateCharacters . -
The TRV of
TemplateCharacter :: SourceCharacter but not one of ` or\ or$ orLineTerminator UTF16EncodeCodePoint on the code point value ofSourceCharacter . -
The TRV of
TemplateCharacter :: $ -
The TRV of
TemplateCharacter :: \ EscapeSequence string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV ofEscapeSequence . -
The TRV of
TemplateCharacter :: \ NotEscapeSequence string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV ofNotEscapeSequence . -
The TRV of
EscapeSequence :: 0 -
The TRV of
NotEscapeSequence :: 0 DecimalDigit string-concatenation of the code unit 0x0030 (DIGIT ZERO) and the TRV ofDecimalDigit . -
The TRV of
NotEscapeSequence :: x [lookahead ∉ HexDigit ] -
The TRV of
NotEscapeSequence :: x HexDigit [lookahead ∉ HexDigit ]string-concatenation of the code unit 0x0078 (LATIN SMALL LETTER X) and the TRV ofHexDigit . -
The TRV of
NotEscapeSequence :: u [lookahead ∉ HexDigit ][lookahead ≠ { ] -
The TRV of
NotEscapeSequence :: u HexDigit [lookahead ∉ HexDigit ]string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U) and the TRV ofHexDigit . -
The TRV of
NotEscapeSequence :: u HexDigit HexDigit [lookahead ∉ HexDigit ]string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the TRV of the firstHexDigit , and the TRV of the secondHexDigit . -
The TRV of
NotEscapeSequence :: u HexDigit HexDigit HexDigit [lookahead ∉ HexDigit ]string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the TRV of the firstHexDigit , the TRV of the secondHexDigit , and the TRV of the thirdHexDigit . -
The TRV of
NotEscapeSequence :: u { [lookahead ∉ HexDigit ]string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U) and the code unit 0x007B (LEFT CURLY BRACKET). -
The TRV of
NotEscapeSequence :: u { NotCodePoint [lookahead ∉ HexDigit ]string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the code unit 0x007B (LEFT CURLY BRACKET), and the TRV ofNotCodePoint . -
The TRV of
NotEscapeSequence :: u { CodePoint [lookahead ∉ HexDigit ][lookahead ≠ } ]string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the code unit 0x007B (LEFT CURLY BRACKET), and the TRV ofCodePoint . -
The TRV of
DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9 UTF16EncodeCodePoint on the single code point matched by this production. -
The TRV of
CharacterEscapeSequence :: NonEscapeCharacter SV ofNonEscapeCharacter . -
The TRV of
SingleEscapeCharacter :: one of ' " \ b f n r t v UTF16EncodeCodePoint on the single code point matched by this production. -
The TRV of
HexEscapeSequence :: x HexDigit HexDigit string-concatenation of the code unit 0x0078 (LATIN SMALL LETTER X), the TRV of the firstHexDigit , and the TRV of the secondHexDigit . -
The TRV of
UnicodeEscapeSequence :: u Hex4Digits string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U) and the TRV ofHex4Digits . -
The TRV of
UnicodeEscapeSequence :: u{ CodePoint } string-concatenation of the code unit 0x0075 (LATIN SMALL LETTER U), the code unit 0x007B (LEFT CURLY BRACKET), the TRV ofCodePoint , and the code unit 0x007D (RIGHT CURLY BRACKET). -
The TRV of
Hex4Digits :: HexDigit HexDigit HexDigit HexDigit string-concatenation of the TRV of the firstHexDigit , the TRV of the secondHexDigit , the TRV of the thirdHexDigit , and the TRV of the fourthHexDigit . -
The TRV of
HexDigits :: HexDigits HexDigit string-concatenation of the TRV ofHexDigits and the TRV ofHexDigit . -
The TRV of
HexDigit :: one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F UTF16EncodeCodePoint on the single code point matched by this production. -
The TRV of
LineContinuation :: \ LineTerminatorSequence string-concatenation of the code unit 0x005C (REVERSE SOLIDUS) and the TRV ofLineTerminatorSequence . -
The TRV of
LineTerminatorSequence :: <LF> -
The TRV of
LineTerminatorSequence :: <CR> -
The TRV of
LineTerminatorSequence :: <LS> -
The TRV of
LineTerminatorSequence :: <PS> -
The TRV of
LineTerminatorSequence :: <CR> <LF>
TV excludes the code units of
12.9 Automatic Semicolon Insertion
Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.
12.9.1 Rules of Automatic Semicolon Insertion
In the following rules, “token” means the actual recognized lexical token determined using the current lexical
There are three basic rules of semicolon insertion:
-
When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
-
The offending token is separated from the previous token by at least one
LineTerminator . -
The offending token is
}. -
The previous token is
)and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (14.7.2 ).
-
The offending token is separated from the previous token by at least one
- When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
-
When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no
LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least oneLineTerminator , then a semicolon is automatically inserted before the restricted token.
However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see
The following are the only restricted productions in the grammar:
The practical effect of these restricted productions is as follows:
-
When a
++or--token is encountered where the parser would treat it as a postfix operator, and at least oneLineTerminator occurred between the preceding token and the++or--token, then a semicolon is automatically inserted before the++or--token. -
When a
continue,break,return,throw, oryieldtoken is encountered and aLineTerminator is encountered before the next token, a semicolon is automatically inserted after thecontinue,break,return,throw, oryieldtoken.
The resulting practical advice to ECMAScript programmers is:
-
A postfix
++or--operator should appear on the same line as its operand. -
An
Expression in areturnorthrowstatement or anAssignmentExpression in ayieldexpression should start on the same line as thereturn,throw, oryieldtoken. -
A
LabelIdentifier in abreakorcontinuestatement should be on the same line as thebreakorcontinuetoken.
12.9.2 Examples of Automatic Semicolon Insertion
This section is non-normative.The source
{ 1 2 } 3is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source
{ 1
2 } 3is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:
{ 1
;2 ;} 3;which is a valid ECMAScript sentence.
The source
for (a; b
)is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.
The source
return
a + bis transformed by automatic semicolon insertion into the following:
return;
a + b;The expression a + b is not treated as a value to be returned by the return statement, because a return.
The source
a = b
++cis transformed by automatic semicolon insertion into the following:
a = b;
++c;The token ++ is not treated as a postfix operator applying to the variable b, because a b and ++.
The source
if (a > b)
else c = dis not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.
The source
a = b + c
(d + e).print()is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:
a = b + c(d + e).print()In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.
12.9.3 Interesting Cases of Automatic Semicolon Insertion
This section is non-normative.ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon insertion can depend on multiple tokens across line terminators.
As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.
For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The rest of this section describes a number of interesting cases of automatic semicolon insertion in this version of ECMAScript.
12.9.3.1 Interesting Cases of Automatic Semicolon Insertion in Statement Lists
In a
- An opening parenthesis (
(). Without a semicolon, the two lines together are treated as aCallExpression . - An opening square bracket (
[). Without a semicolon, the two lines together are treated as property access, rather than anArrayLiteral orArrayAssignmentPattern . - A template literal (
`). Without a semicolon, the two lines together are interpreted as a tagged Template (13.3.11 ), with the previous expression as theMemberExpression . - Unary
+or-. Without a semicolon, the two lines together are interpreted as a usage of the corresponding binary operator. - A RegExp literal. Without a semicolon, the two lines together may be parsed instead as the
/MultiplicativeOperator , for example if the RegExp has flags.
12.9.3.2 Cases of Automatic Semicolon Insertion and “[no LineTerminator here]”
This section is non-normative.
ECMAScript contains grammar productions which include “[no
The rest of this section describes a number of productions using “[no
12.9.3.2.1 List of Grammar Productions with Optional Operands and “[no LineTerminator here]”
UpdateExpression .ContinueStatement .BreakStatement .ReturnStatement .YieldExpression .- Async Function Definitions (
15.8 ) with relation to Function Definitions (15.2 )
13 ECMAScript Language: Expressions
13.1 Identifiers
Syntax
yield and await are permitted as
let
await 0;13.1.1 Static Semantics: Early Errors
-
It is a Syntax Error if the code matched by this production is contained in
strict mode code and theStringValue ofIdentifier is"arguments" or"eval" .
-
It is a Syntax Error if the code matched by this production is contained in
strict mode code .
-
It is a Syntax Error if the
goal symbol of the syntactic grammar isModule .
- It is a Syntax Error if this production has a [Yield] parameter.
- It is a Syntax Error if this production has an [Await] parameter.
-
It is a Syntax Error if this production has a [Yield] parameter and
StringValue ofIdentifier is"yield" . -
It is a Syntax Error if this production has an [Await] parameter and
StringValue ofIdentifier is"await" .
-
It is a Syntax Error if this phrase is contained in
strict mode code and theStringValue ofIdentifierName is:"implements" ,"interface" ,"let" ,"package" ,"private" ,"protected" ,"public" ,"static" , or"yield" . -
It is a Syntax Error if the
goal symbol of the syntactic grammar isModule and theStringValue ofIdentifierName is"await" . -
It is a Syntax Error if
StringValue ofIdentifierName is the same String value as theStringValue of anyReservedWord except foryieldorawait.
13.1.2 Static Semantics: StringValue
- Let idText be the source text matched by
IdentifierName . - Let idTextUnescaped be the result of replacing any occurrences of
\UnicodeEscapeSequence in idText with the code point represented by theUnicodeEscapeSequence . - Return !
CodePointsToString (idTextUnescaped).
- Return
"yield" .
- Return
"await" .
- Return the
StringValue ofIdentifierName .
13.1.3 Runtime Semantics: Evaluation
- Return ?
ResolveBinding (StringValue ofIdentifier ).
- Return ?
ResolveBinding ("yield" ).
- Return ?
ResolveBinding ("await" ).
The result of evaluating an
In yield may be used as an identifier. Evaluating the yield as if it was an
13.2 Primary Expression
Syntax
Supplemental Syntax
When processing an instance of the production
the interpretation of
13.2.1 Semantics
13.2.1.1 Static Semantics: CoveredParenthesizedExpression
- Return the
ParenthesizedExpression that iscovered byCoverParenthesizedExpressionAndArrowParameterList .
13.2.2 The this Keyword
13.2.2.1 Runtime Semantics: Evaluation
- Return ?
ResolveThisBinding ().
13.2.3 Identifier Reference
See
13.2.4 Literals
Syntax
13.2.4.1 Runtime Semantics: Evaluation
- Return
null .
- If
BooleanLiteral is the tokenfalse, returnfalse . - If
BooleanLiteral is the tokentrue, returntrue .
- Return the
NumericValue ofNumericLiteral as defined in12.8.3 .
- Return the
SV ofStringLiteral as defined in12.8.4.1 .
13.2.5 Array Initializer
An
Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an
Syntax
13.2.5.1 Runtime Semantics: ArrayAccumulation
With parameters array and nextIndex.
- Return the result of performing
ArrayAccumulation forElision with arguments array and nextIndex + 1.
- If
Elision is present, then- Set nextIndex to the result of performing
ArrayAccumulation forElision with arguments array and nextIndex. ReturnIfAbrupt (nextIndex).
- Set nextIndex to the result of performing
- Let initResult be the result of evaluating
AssignmentExpression . - Let initValue be ?
GetValue (initResult). - Let created be !
CreateDataPropertyOrThrow (array, !ToString (𝔽 (nextIndex)), initValue). - Return nextIndex + 1.
- If
Elision is present, then- Set nextIndex to the result of performing
ArrayAccumulation forElision with arguments array and nextIndex. ReturnIfAbrupt (nextIndex).
- Set nextIndex to the result of performing
- Return the result of performing
ArrayAccumulation forSpreadElement with arguments array and nextIndex.
- Set nextIndex to the result of performing
ArrayAccumulation forElementList with arguments array and nextIndex. ReturnIfAbrupt (nextIndex).- If
Elision is present, then- Set nextIndex to the result of performing
ArrayAccumulation forElision with arguments array and nextIndex. ReturnIfAbrupt (nextIndex).
- Set nextIndex to the result of performing
- Let initResult be the result of evaluating
AssignmentExpression . - Let initValue be ?
GetValue (initResult). - Let created be !
CreateDataPropertyOrThrow (array, !ToString (𝔽 (nextIndex)), initValue). - Return nextIndex + 1.
- Set nextIndex to the result of performing
ArrayAccumulation forElementList with arguments array and nextIndex. ReturnIfAbrupt (nextIndex).- If
Elision is present, then- Set nextIndex to the result of performing
ArrayAccumulation forElision with arguments array and nextIndex. ReturnIfAbrupt (nextIndex).
- Set nextIndex to the result of performing
- Return the result of performing
ArrayAccumulation forSpreadElement with arguments array and nextIndex.
- Let spreadRef be the result of evaluating
AssignmentExpression . - Let spreadObj be ?
GetValue (spreadRef). - Let iteratorRecord be ?
GetIterator (spreadObj). - Repeat,
- Let next be ?
IteratorStep (iteratorRecord). - If next is
false , return nextIndex. - Let nextValue be ?
IteratorValue (next). - Perform !
CreateDataPropertyOrThrow (array, !ToString (𝔽 (nextIndex)), nextValue). - Set nextIndex to nextIndex + 1.
- Let next be ?
13.2.5.2 Runtime Semantics: Evaluation
- Let array be !
ArrayCreate (0). - If
Elision is present, then- Let len be the result of performing
ArrayAccumulation forElision with arguments array and 0. ReturnIfAbrupt (len).
- Let len be the result of performing
- Return array.
- Let array be !
ArrayCreate (0). - Let len be the result of performing
ArrayAccumulation forElementList with arguments array and 0. ReturnIfAbrupt (len).- Return array.
- Let array be !
ArrayCreate (0). - Let nextIndex be the result of performing
ArrayAccumulation forElementList with arguments array and 0. ReturnIfAbrupt (nextIndex).- If
Elision is present, then- Let len be the result of performing
ArrayAccumulation forElision with arguments array and nextIndex. ReturnIfAbrupt (len).
- Let len be the result of performing
- Return array.
13.2.6 Object Initializer
An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.
Syntax
In certain contexts,
13.2.6.1 Static Semantics: Early Errors
-
It is a Syntax Error if
HasDirectSuper ofMethodDefinition istrue .
In addition to describing an actual object initializer the
- Always throw a Syntax Error if code matches this production.
This production exists so that
13.2.6.2 Static Semantics: IsComputedPropertyKey
- Return
false .
- Return
true .
13.2.6.3 Static Semantics: PropertyNameList
- If
PropName ofPropertyDefinition isempty , return a new emptyList . - Return a
List whose sole element isPropName ofPropertyDefinition .
- Let list be
PropertyNameList ofPropertyDefinitionList . - If
PropName ofPropertyDefinition isempty , return list. - Append
PropName ofPropertyDefinition to the end of list. - Return list.
13.2.6.4 Runtime Semantics: Evaluation
- Return !
OrdinaryObjectCreate (%Object.prototype% ).
- Let obj be !
OrdinaryObjectCreate (%Object.prototype% ). - Perform ?
PropertyDefinitionEvaluation ofPropertyDefinitionList with arguments obj andtrue . - Return obj.
- Return
StringValue ofIdentifierName .
- Return the
SV ofStringLiteral .
- Let nbr be the
NumericValue ofNumericLiteral . - Return !
ToString (nbr).
- Let exprValue be the result of evaluating
AssignmentExpression . - Let propName be ?
GetValue (exprValue). - Return ?
ToPropertyKey (propName).
13.2.6.5 Runtime Semantics: PropertyDefinitionEvaluation
With parameters object and enumerable.
- Perform ?
PropertyDefinitionEvaluation ofPropertyDefinitionList with arguments object and enumerable. - Return the result of performing
PropertyDefinitionEvaluation ofPropertyDefinition with arguments object and enumerable.
- Let exprValue be the result of evaluating
AssignmentExpression . - Let fromValue be ?
GetValue (exprValue). - Let excludedNames be a new empty
List . - Return ?
CopyDataProperties (object, fromValue, excludedNames).
- Let propName be
StringValue ofIdentifierReference . - Let exprValue be the result of evaluating
IdentifierReference . - Let propValue be ?
GetValue (exprValue). Assert : enumerable istrue .Assert : object is an ordinary, extensible object with no non-configurable properties.- Return !
CreateDataPropertyOrThrow (object, propName, propValue).
- Let propKey be the result of evaluating
PropertyName . ReturnIfAbrupt (propKey).- If
IsAnonymousFunctionDefinition (AssignmentExpression ) istrue , then- Let propValue be ?
NamedEvaluation ofAssignmentExpression with argument propKey.
- Let propValue be ?
- Else,
- Let exprValueRef be the result of evaluating
AssignmentExpression . - Let propValue be ?
GetValue (exprValueRef).
- Let exprValueRef be the result of evaluating
Assert : enumerable istrue .Assert : object is an ordinary, extensible object with no non-configurable properties.- Return !
CreateDataPropertyOrThrow (object, propKey, propValue).
An alternative semantics for this production is given in
- Return ?
MethodDefinitionEvaluation ofMethodDefinition with arguments object and enumerable.
- Return ?
MethodDefinitionEvaluation ofGeneratorMethod with arguments object and enumerable.
- Return ?
MethodDefinitionEvaluation ofAsyncGeneratorMethod with arguments object and enumerable.
- Return ?
MethodDefinitionEvaluation ofAsyncMethod with arguments object and enumerable.
13.2.7 Function Defining Expressions
See
See
See
See
See
13.2.8 Regular Expression Literals
Syntax
See
13.2.8.1 Static Semantics: Early Errors
-
It is a Syntax Error if
IsValidRegularExpressionLiteral (RegularExpressionLiteral ) isfalse .
13.2.8.2 Static Semantics: IsValidRegularExpressionLiteral ( literal )
The abstract operation IsValidRegularExpressionLiteral takes argument literal. It determines if its argument is a valid regular expression literal. It performs the following steps when called:
Assert : literal is aRegularExpressionLiteral .- If
FlagText of literal contains any code points other thang,i,m,s,u, ory, or if it contains the same code point more than once, returnfalse . - Let patternText be
BodyText of literal. - If
FlagText of literal containsu, let u betrue ; else let u befalse . - If u is
false , then- Let stringValue be
CodePointsToString (patternText). - Set patternText to the sequence of code points resulting from interpreting each of the 16-bit elements of stringValue as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
- Let stringValue be
- Let parseResult be
ParsePattern (patternText, u). - If parseResult is a
Parse Node , returntrue ; else returnfalse .
13.2.8.3 Runtime Semantics: Evaluation
- Let pattern be !
CodePointsToString (BodyText ofRegularExpressionLiteral ). - Let flags be !
CodePointsToString (FlagText ofRegularExpressionLiteral ). - Return
RegExpCreate (pattern, flags).
13.2.9 Template Literals
Syntax
13.2.9.1 Static Semantics: Early Errors
-
It is a Syntax Error if the [Tagged] parameter was not set and
NoSubstitutionTemplate Contains NotEscapeSequence .
-
It is a Syntax Error if the number of elements in the result of
TemplateStrings ofTemplateLiteral with argumentfalse is greater than 232 - 1.
-
It is a Syntax Error if the [Tagged] parameter was not set and
TemplateHead Contains NotEscapeSequence .
-
It is a Syntax Error if the [Tagged] parameter was not set and
TemplateTail Contains NotEscapeSequence .
-
It is a Syntax Error if the [Tagged] parameter was not set and
TemplateMiddle Contains NotEscapeSequence .
13.2.9.2 Static Semantics: TemplateStrings
With parameter raw.
- If raw is
false , then- Let string be the TV of
NoSubstitutionTemplate .
- Let string be the TV of
- Else,
- Let string be the TRV of
NoSubstitutionTemplate .
- Let string be the TRV of
- Return a
List whose sole element is string.
- If raw is
false , then- Let head be the TV of
TemplateHead .
- Let head be the TV of
- Else,
- Let head be the TRV of
TemplateHead .
- Let head be the TRV of
- Let tail be
TemplateStrings ofTemplateSpans with argument raw. - Return a
List whose elements are head followed by the elements of tail.
- If raw is
false , then- Let tail be the TV of
TemplateTail .
- Let tail be the TV of
- Else,
- Let tail be the TRV of
TemplateTail .
- Let tail be the TRV of
- Return a
List whose sole element is tail.
- Let middle be
TemplateStrings ofTemplateMiddleList with argument raw. - If raw is
false , then- Let tail be the TV of
TemplateTail .
- Let tail be the TV of
- Else,
- Let tail be the TRV of
TemplateTail .
- Let tail be the TRV of
- Return a
List whose elements are the elements of middle followed by tail.
- If raw is
false , then- Let string be the TV of
TemplateMiddle .
- Let string be the TV of
- Else,
- Let string be the TRV of
TemplateMiddle .
- Let string be the TRV of
- Return a
List whose sole element is string.
- Let front be
TemplateStrings ofTemplateMiddleList with argument raw. - If raw is
false , then- Let last be the TV of
TemplateMiddle .
- Let last be the TV of
- Else,
- Let last be the TRV of
TemplateMiddle .
- Let last be the TRV of
- Append last as the last element of the
List front. - Return front.
13.2.9.3 GetTemplateObject ( templateLiteral )
The abstract operation GetTemplateObject takes argument templateLiteral (a
- Let realm be
the current Realm Record . - Let templateRegistry be realm.[[TemplateMap]].
- For each element e of templateRegistry, do
- If e.[[Site]] is
the same Parse Node as templateLiteral, then- Return e.[[Array]].
- If e.[[Site]] is
- Let rawStrings be
TemplateStrings of templateLiteral with argumenttrue . - Let cookedStrings be
TemplateStrings of templateLiteral with argumentfalse . - Let count be the number of elements in the
List cookedStrings. Assert : count ≤ 232 - 1.- Let template be !
ArrayCreate (count). - Let rawObj be !
ArrayCreate (count). - Let index be 0.
- Repeat, while index < count,
- Let prop be !
ToString (𝔽 (index)). - Let cookedValue be cookedStrings[index].
- Perform !
DefinePropertyOrThrow (template, prop, PropertyDescriptor { [[Value]]: cookedValue, [[Writable]]:false , [[Enumerable]]:true , [[Configurable]]:false }). - Let rawValue be the String value rawStrings[index].
- Perform !
DefinePropertyOrThrow (rawObj, prop, PropertyDescriptor { [[Value]]: rawValue, [[Writable]]:false , [[Enumerable]]:true , [[Configurable]]:false }). - Set index to index + 1.
- Let prop be !
- Perform !
SetIntegrityLevel (rawObj,frozen ). - Perform !
DefinePropertyOrThrow (template,"raw" , PropertyDescriptor { [[Value]]: rawObj, [[Writable]]:false , [[Enumerable]]:false , [[Configurable]]:false }). - Perform !
SetIntegrityLevel (template,frozen ). - Append the
Record { [[Site]]: templateLiteral, [[Array]]: template } to templateRegistry. - Return template.
The creation of a template object cannot result in an
Each
Future editions of this specification may define additional non-enumerable properties of template objects.
13.2.9.4 Runtime Semantics: SubstitutionEvaluation
- Return a new empty
List .
- Return the result of
SubstitutionEvaluation ofTemplateMiddleList .
- Let subRef be the result of evaluating
Expression . - Let sub be ?
GetValue (subRef). - Return a
List whose sole element is sub.
- Let preceding be ?
SubstitutionEvaluation ofTemplateMiddleList . - Let nextRef be the result of evaluating
Expression . - Let next be ?
GetValue (nextRef). - Append next as the last element of the
List preceding. - Return preceding.
13.2.9.5 Runtime Semantics: Evaluation
- Return the TV of
NoSubstitutionTemplate as defined in12.8.6 .
- Let head be the TV of
TemplateHead as defined in12.8.6 . - Let subRef be the result of evaluating
Expression . - Let sub be ?
GetValue (subRef). - Let middle be ?
ToString (sub). - Let tail be the result of evaluating
TemplateSpans . ReturnIfAbrupt (tail).- Return the
string-concatenation of head, middle, and tail.
The string conversion semantics applied to the String.prototype.concat rather than the + operator.
- Return the TV of
TemplateTail as defined in12.8.6 .
- Let head be the result of evaluating
TemplateMiddleList . ReturnIfAbrupt (head).- Let tail be the TV of
TemplateTail as defined in12.8.6 . - Return the
string-concatenation of head and tail.
- Let head be the TV of
TemplateMiddle as defined in12.8.6 . - Let subRef be the result of evaluating
Expression . - Let sub be ?
GetValue (subRef). - Let middle be ?
ToString (sub). - Return the
string-concatenation of head and middle.
The string conversion semantics applied to the String.prototype.concat rather than the + operator.
- Let rest be the result of evaluating
TemplateMiddleList . ReturnIfAbrupt (rest).- Let middle be the TV of
TemplateMiddle as defined in12.8.6 . - Let subRef be the result of evaluating
Expression . - Let sub be ?
GetValue (subRef). - Let last be ?
ToString (sub). - Return the
string-concatenation of rest, middle, and last.
The string conversion semantics applied to the String.prototype.concat rather than the + operator.
13.2.10 The Grouping Operator
13.2.10.1 Static Semantics: Early Errors
-
It is a Syntax Error if
CoverParenthesizedExpressionAndArrowParameterList is notcovering aParenthesizedExpression . -
All Early Error rules for
ParenthesizedExpression and its derived productions also apply toCoveredParenthesizedExpression ofCoverParenthesizedExpressionAndArrowParameterList .
13.2.10.2 Runtime Semantics: Evaluation
- Let expr be
CoveredParenthesizedExpression ofCoverParenthesizedExpressionAndArrowParameterList . - Return the result of evaluating expr.
- Return the result of evaluating
Expression . This may be of type Reference.
This algorithm does not apply delete and typeof may be applied to parenthesized expressions.
13.3 Left-Hand-Side Expressions
Syntax
Supplemental Syntax
When processing an instance of the production
the interpretation of
13.3.1 Static Semantics
13.3.1.1 Static Semantics: Early Errors
- It is a Syntax Error if any code matches this production.
This production exists in order to prevent automatic semicolon insertion rules (
a?.b
`c`so that it would be interpreted as two valid statements. The purpose is to maintain consistency with similar code without optional chaining:
a.b
`c`which is a valid statement and where automatic semicolon insertion does not apply.
-
It is a Syntax Error if the syntactic
goal symbol is notModule .
13.3.1.2 Static Semantics: CoveredCallExpression
- Return the
CallMemberExpression that iscovered byCoverCallExpressionAndAsyncArrowHead .
13.3.2 Property Accessors
Properties are accessed by name, using either the dot notation:
or the bracket notation:
The dot notation is explained by the following syntactic conversion:
is identical in its behaviour to
and similarly
is identical in its behaviour to
where <identifier-name-string> is the result of evaluating
13.3.2.1 Runtime Semantics: Evaluation
- Let baseReference be the result of evaluating
MemberExpression . - Let baseValue be ?
GetValue (baseReference). - If the code matched by this
MemberExpression isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
EvaluatePropertyAccessWithExpressionKey (baseValue,Expression , strict).
- Let baseReference be the result of evaluating
MemberExpression . - Let baseValue be ?
GetValue (baseReference). - If the code matched by this
MemberExpression isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
EvaluatePropertyAccessWithIdentifierKey (baseValue,IdentifierName , strict).
- Let baseReference be the result of evaluating
CallExpression . - Let baseValue be ?
GetValue (baseReference). - If the code matched by this
CallExpression isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
EvaluatePropertyAccessWithExpressionKey (baseValue,Expression , strict).
- Let baseReference be the result of evaluating
CallExpression . - Let baseValue be ?
GetValue (baseReference). - If the code matched by this
CallExpression isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
EvaluatePropertyAccessWithIdentifierKey (baseValue,IdentifierName , strict).
13.3.3 EvaluatePropertyAccessWithExpressionKey ( baseValue, expression, strict )
The abstract operation EvaluatePropertyAccessWithExpressionKey takes arguments baseValue (an
- Let propertyNameReference be the result of evaluating expression.
- Let propertyNameValue be ?
GetValue (propertyNameReference). - Let bv be ?
RequireObjectCoercible (baseValue). - Let propertyKey be ?
ToPropertyKey (propertyNameValue). - Return the
Reference Record { [[Base]]: bv, [[ReferencedName]]: propertyKey, [[Strict]]: strict, [[ThisValue]]:empty }.
13.3.4 EvaluatePropertyAccessWithIdentifierKey ( baseValue, identifierName, strict )
The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (an
Assert : identifierName is anIdentifierName .- Let bv be ?
RequireObjectCoercible (baseValue). - Let propertyNameString be
StringValue of identifierName. - Return the
Reference Record { [[Base]]: bv, [[ReferencedName]]: propertyNameString, [[Strict]]: strict, [[ThisValue]]:empty }.
13.3.5 The new Operator
13.3.5.1 Runtime Semantics: Evaluation
- Return ?
EvaluateNew (NewExpression ,empty ).
- Return ?
EvaluateNew (MemberExpression ,Arguments ).
13.3.5.1.1 EvaluateNew ( constructExpr, arguments )
The abstract operation EvaluateNew takes arguments constructExpr and arguments. It performs the following steps when called:
Assert : constructExpr is either aNewExpression or aMemberExpression .Assert : arguments is eitherempty or anArguments .- Let ref be the result of evaluating constructExpr.
- Let constructor be ?
GetValue (ref). - If arguments is
empty , let argList be a new emptyList . - Else,
- Let argList be ?
ArgumentListEvaluation of arguments.
- Let argList be ?
- If
IsConstructor (constructor) isfalse , throw aTypeError exception. - Return ?
Construct (constructor, argList).
13.3.6 Function Calls
13.3.6.1 Runtime Semantics: Evaluation
- Let expr be
CoveredCallExpression ofCoverCallExpressionAndAsyncArrowHead . - Let memberExpr be the
MemberExpression of expr. - Let arguments be the
Arguments of expr. - Let ref be the result of evaluating memberExpr.
- Let func be ?
GetValue (ref). - If ref is a
Reference Record ,IsPropertyReference (ref) isfalse , and ref.[[ReferencedName]] is"eval" , then- If
SameValue (func,%eval% ) istrue , then- Let argList be ?
ArgumentListEvaluation of arguments. - If argList has no elements, return
undefined . - Let evalArg be the first element of argList.
- If the source code matching this
CallExpression isstrict mode code , let strictCaller betrue . Otherwise let strictCaller befalse . - Let evalRealm be
the current Realm Record . - Return ?
PerformEval (evalArg, evalRealm, strictCaller,true ).
- Let argList be ?
- If
- Let thisCall be this
CallExpression . - Let tailCall be
IsInTailPosition (thisCall). - Return ?
EvaluateCall (func, ref, arguments, tailCall).
A
- Let ref be the result of evaluating
CallExpression . - Let func be ?
GetValue (ref). - Let thisCall be this
CallExpression . - Let tailCall be
IsInTailPosition (thisCall). - Return ?
EvaluateCall (func, ref,Arguments , tailCall).
13.3.6.2 EvaluateCall ( func, ref, arguments, tailPosition )
The abstract operation EvaluateCall takes arguments func (an
- If ref is a
Reference Record , then- If
IsPropertyReference (ref) istrue , then- Let thisValue be
GetThisValue (ref).
- Let thisValue be
- Else,
- Let refEnv be ref.[[Base]].
Assert : refEnv is anEnvironment Record .- Let thisValue be refEnv.WithBaseObject().
- If
- Else,
- Let thisValue be
undefined .
- Let thisValue be
- Let argList be ?
ArgumentListEvaluation of arguments. - If
Type (func) is not Object, throw aTypeError exception. - If
IsCallable (func) isfalse , throw aTypeError exception. - If tailPosition is
true , performPrepareForTailCall (). - Let result be
Call (func, thisValue, argList). Assert : If tailPosition istrue , the above call will not return here, but instead evaluation will continue as if the following return has already occurred.Assert : If result is not anabrupt completion , thenType (result) is anECMAScript language type .- Return result.
13.3.7 The super Keyword
13.3.7.1 Runtime Semantics: Evaluation
- Let env be
GetThisEnvironment (). - Let actualThis be ? env.GetThisBinding().
- Let propertyNameReference be the result of evaluating
Expression . - Let propertyNameValue be ?
GetValue (propertyNameReference). - Let propertyKey be ?
ToPropertyKey (propertyNameValue). - If the code matched by this
SuperProperty isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
MakeSuperPropertyReference (actualThis, propertyKey, strict).
- Let env be
GetThisEnvironment (). - Let actualThis be ? env.GetThisBinding().
- Let propertyKey be
StringValue ofIdentifierName . - If the code matched by this
SuperProperty isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
MakeSuperPropertyReference (actualThis, propertyKey, strict).
- Let newTarget be
GetNewTarget (). Assert :Type (newTarget) is Object.- Let func be !
GetSuperConstructor (). - Let argList be ?
ArgumentListEvaluation ofArguments . - If
IsConstructor (func) isfalse , throw aTypeError exception. - Let result be ?
Construct (func, argList, newTarget). - Let thisER be
GetThisEnvironment (). - Return ? thisER.BindThisValue(result).
13.3.7.2 GetSuperConstructor ( )
The abstract operation GetSuperConstructor takes no arguments. It performs the following steps when called:
- Let envRec be
GetThisEnvironment (). Assert : envRec is afunction Environment Record .- Let activeFunction be envRec.[[FunctionObject]].
Assert : activeFunction is an ECMAScriptfunction object .- Let superConstructor be ! activeFunction.[[GetPrototypeOf]]().
- Return superConstructor.
13.3.7.3 MakeSuperPropertyReference ( actualThis, propertyKey, strict )
The abstract operation MakeSuperPropertyReference takes arguments actualThis, propertyKey, and strict. It performs the following steps when called:
- Let env be
GetThisEnvironment (). Assert : env.HasSuperBinding() istrue .- Let baseValue be ? env.GetSuperBase().
- Let bv be ?
RequireObjectCoercible (baseValue). - Return the
Reference Record { [[Base]]: bv, [[ReferencedName]]: propertyKey, [[Strict]]: strict, [[ThisValue]]: actualThis }. - NOTE: This returns a
Super Reference Record .
13.3.8 Argument Lists
The evaluation of an argument list produces a
13.3.8.1 Runtime Semantics: ArgumentListEvaluation
- Return a new empty
List .
- Let ref be the result of evaluating
AssignmentExpression . - Let arg be ?
GetValue (ref). - Return a
List whose sole element is arg.
- Let list be a new empty
List . - Let spreadRef be the result of evaluating
AssignmentExpression . - Let spreadObj be ?
GetValue (spreadRef). - Let iteratorRecord be ?
GetIterator (spreadObj). - Repeat,
- Let next be ?
IteratorStep (iteratorRecord). - If next is
false , return list. - Let nextArg be ?
IteratorValue (next). - Append nextArg as the last element of list.
- Let next be ?
- Let precedingArgs be ?
ArgumentListEvaluation ofArgumentList . - Let ref be the result of evaluating
AssignmentExpression . - Let arg be ?
GetValue (ref). - Append arg to the end of precedingArgs.
- Return precedingArgs.
- Let precedingArgs be ?
ArgumentListEvaluation ofArgumentList . - Let spreadRef be the result of evaluating
AssignmentExpression . - Let iteratorRecord be ?
GetIterator (?GetValue (spreadRef)). - Repeat,
- Let next be ?
IteratorStep (iteratorRecord). - If next is
false , return precedingArgs. - Let nextArg be ?
IteratorValue (next). - Append nextArg as the last element of precedingArgs.
- Let next be ?
- Let templateLiteral be this
TemplateLiteral . - Let siteObj be
GetTemplateObject (templateLiteral). - Return a
List whose sole element is siteObj.
- Let templateLiteral be this
TemplateLiteral . - Let siteObj be
GetTemplateObject (templateLiteral). - Let remaining be ?
ArgumentListEvaluation ofSubstitutionTemplate . - Return a
List whose first element is siteObj and whose subsequent elements are the elements of remaining.
- Let firstSubRef be the result of evaluating
Expression . - Let firstSub be ?
GetValue (firstSubRef). - Let restSub be ?
SubstitutionEvaluation ofTemplateSpans . Assert : restSub is aList .- Return a
List whose first element is firstSub and whose subsequent elements are the elements of restSub. restSub may contain no elements.
13.3.9 Optional Chains
?..13.3.9.1 Runtime Semantics: Evaluation
- Let baseReference be the result of evaluating
MemberExpression . - Let baseValue be ?
GetValue (baseReference). - If baseValue is
undefined ornull , then- Return
undefined .
- Return
- Return the result of performing
ChainEvaluation ofOptionalChain with arguments baseValue and baseReference.
- Let baseReference be the result of evaluating
CallExpression . - Let baseValue be ?
GetValue (baseReference). - If baseValue is
undefined ornull , then- Return
undefined .
- Return
- Return the result of performing
ChainEvaluation ofOptionalChain with arguments baseValue and baseReference.
- Let baseReference be the result of evaluating
OptionalExpression . - Let baseValue be ?
GetValue (baseReference). - If baseValue is
undefined ornull , then- Return
undefined .
- Return
- Return the result of performing
ChainEvaluation ofOptionalChain with arguments baseValue and baseReference.
13.3.9.2 Runtime Semantics: ChainEvaluation
With parameters baseValue and baseReference.
- Let thisChain be this
OptionalChain . - Let tailCall be
IsInTailPosition (thisChain). - Return ?
EvaluateCall (baseValue, baseReference,Arguments , tailCall).
- If the code matched by this
OptionalChain isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
EvaluatePropertyAccessWithExpressionKey (baseValue,Expression , strict).
- If the code matched by this
OptionalChain isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
EvaluatePropertyAccessWithIdentifierKey (baseValue,IdentifierName , strict).
- Let optionalChain be
OptionalChain . - Let newReference be ?
ChainEvaluation of optionalChain with arguments baseValue and baseReference. - Let newValue be ?
GetValue (newReference). - Let thisChain be this
OptionalChain . - Let tailCall be
IsInTailPosition (thisChain). - Return ?
EvaluateCall (newValue, newReference,Arguments , tailCall).
- Let optionalChain be
OptionalChain . - Let newReference be ?
ChainEvaluation of optionalChain with arguments baseValue and baseReference. - Let newValue be ?
GetValue (newReference). - If the code matched by this
OptionalChain isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
EvaluatePropertyAccessWithExpressionKey (newValue,Expression , strict).
- Let optionalChain be
OptionalChain . - Let newReference be ?
ChainEvaluation of optionalChain with arguments baseValue and baseReference. - Let newValue be ?
GetValue (newReference). - If the code matched by this
OptionalChain isstrict mode code , let strict betrue ; else let strict befalse . - Return ?
EvaluatePropertyAccessWithIdentifierKey (newValue,IdentifierName , strict).
13.3.10 Import Calls
13.3.10.1 Runtime Semantics: Evaluation
- Let referencingScriptOrModule be !
GetActiveScriptOrModule (). - Let argRef be the result of evaluating
AssignmentExpression . - Let specifier be ?
GetValue (argRef). - Let promiseCapability be !
NewPromiseCapability (%Promise% ). - Let specifierString be
ToString (specifier). IfAbruptRejectPromise (specifierString, promiseCapability).- Perform !
HostImportModuleDynamically (referencingScriptOrModule, specifierString, promiseCapability). - Return promiseCapability.[[Promise]].
13.3.11 Tagged Templates
A tagged template is a function call where the arguments of the call are derived from a
13.3.11.1 Runtime Semantics: Evaluation
- Let tagRef be the result of evaluating
MemberExpression . - Let tagFunc be ?
GetValue (tagRef). - Let thisCall be this
MemberExpression . - Let tailCall be
IsInTailPosition (thisCall). - Return ?
EvaluateCall (tagFunc, tagRef,TemplateLiteral , tailCall).
- Let tagRef be the result of evaluating
CallExpression . - Let tagFunc be ?
GetValue (tagRef). - Let thisCall be this
CallExpression . - Let tailCall be
IsInTailPosition (thisCall). - Return ?
EvaluateCall (tagFunc, tagRef,TemplateLiteral , tailCall).
13.3.12 Meta Properties
13.3.12.1 Runtime Semantics: Evaluation
- Return
GetNewTarget ().
- Let module be !
GetActiveScriptOrModule (). Assert : module is aSource Text Module Record .- Let importMeta be module.[[ImportMeta]].
- If importMeta is
empty , then- Set importMeta to !
OrdinaryObjectCreate (null ). - Let importMetaValues be !
HostGetImportMetaProperties (module). - For each
Record { [[Key]], [[Value]] } p of importMetaValues, do- Perform !
CreateDataPropertyOrThrow (importMeta, p.[[Key]], p.[[Value]]).
- Perform !
- Perform !
HostFinalizeImportMeta (importMeta, module). - Set module.[[ImportMeta]] to importMeta.
- Return importMeta.
- Set importMeta to !
- Else,
13.3.12.1.1 HostGetImportMetaProperties ( moduleRecord )
The import.meta.
The implementation of HostGetImportMetaProperties must conform to the following requirements:
- It must return a
List , whose values are all Records with two fields, [[Key]] and [[Value]]. - Each such
Record 's [[Key]] field must be a property key, i.e.,IsPropertyKey must returntrue when applied to it. - Each such
Record 's [[Value]] field must be an ECMAScript value. - It must always complete normally (i.e., not return an
abrupt completion ).
The default implementation of HostGetImportMetaProperties is to return a new empty
13.3.12.1.2 HostFinalizeImportMeta ( importMeta, moduleRecord )
The import.meta.
Most hosts will be able to simply define
The implementation of HostFinalizeImportMeta must conform to the following requirements:
- It must always complete normally (i.e., not return an
abrupt completion ).
The default implementation of HostFinalizeImportMeta is to return
13.4 Update Expressions
Syntax
13.4.1 Static Semantics: Early Errors
-
It is an early Syntax Error if
AssignmentTargetType ofLeftHandSideExpression is notsimple .
-
It is an early Syntax Error if
AssignmentTargetType ofUnaryExpression is notsimple .
13.4.2 Postfix Increment Operator
13.4.2.1 Runtime Semantics: Evaluation
13.4.3 Postfix Decrement Operator
13.4.3.1 Runtime Semantics: Evaluation
13.4.4 Prefix Increment Operator
13.4.4.1 Runtime Semantics: Evaluation
13.4.5 Prefix Decrement Operator
13.4.5.1 Runtime Semantics: Evaluation
13.5 Unary Operators
Syntax
13.5.1 The delete Operator
13.5.1.1 Static Semantics: Early Errors
-
It is a Syntax Error if the
UnaryExpression is contained instrict mode code and the derivedUnaryExpression isPrimaryExpression : IdentifierReference -
It is a Syntax Error if the derived
UnaryExpression is
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
andCoverParenthesizedExpressionAndArrowParameterList ultimately derives a phrase that, if used in place ofUnaryExpression , would produce a Syntax Error according to these rules. This rule is recursively applied.
The last rule means that expressions such as delete (((foo))) produce early errors because of recursive application of the first rule.
13.5.1.2 Runtime Semantics: Evaluation
- Let ref be the result of evaluating
UnaryExpression . ReturnIfAbrupt (ref).- If ref is not a
Reference Record , returntrue . - If
IsUnresolvableReference (ref) istrue , thenAssert : ref.[[Strict]] isfalse .- Return
true .
- If
IsPropertyReference (ref) istrue , then- If
IsSuperReference (ref) istrue , throw aReferenceError exception. - Let baseObj be !
ToObject (ref.[[Base]]). - Let deleteStatus be ? baseObj.[[Delete]](ref.[[ReferencedName]]).
- If deleteStatus is
false and ref.[[Strict]] istrue , throw aTypeError exception. - Return deleteStatus.
- If
- Else,
- Let base be ref.[[Base]].
Assert : base is anEnvironment Record .- Return ? base.DeleteBinding(ref.[[ReferencedName]]).
When a delete operator occurs within delete operator occurs within
The object that may be created in step
13.5.2 The void Operator
13.5.2.1 Runtime Semantics: Evaluation
- Let expr be the result of evaluating
UnaryExpression . - Perform ?
GetValue (expr). - Return
undefined .
13.5.3 The typeof Operator
13.5.3.1 Runtime Semantics: Evaluation
- Let val be the result of evaluating
UnaryExpression . - If val is a
Reference Record , then- If
IsUnresolvableReference (val) istrue , return"undefined" .
- If
- Set val to ?
GetValue (val). - Return a String according to
Table 37 .
| Type of val | Result |
|---|---|
| Undefined |
|
| Null |
|
| Boolean |
|
| Number |
|
| String |
|
| Symbol |
|
| BigInt |
|
| Object (does not implement [[Call]]) |
|
| Object (implements [[Call]]) |
|
An additional entry related to [[IsHTMLDDA]] Internal Slot can be found in
13.5.4 Unary + Operator
The unary + operator converts its operand to Number type.
13.5.4.1 Runtime Semantics: Evaluation
- Let expr be the result of evaluating
UnaryExpression . - Return ?
ToNumber (?GetValue (expr)).
13.5.5 Unary - Operator
The unary - operator converts its operand to Number type and then negates it. Negating
13.5.5.1 Runtime Semantics: Evaluation
- Let expr be the result of evaluating
UnaryExpression . - Let oldValue be ?
ToNumeric (?GetValue (expr)). - Let T be
Type (oldValue). - Return ! T::unaryMinus(oldValue).
13.5.6 Bitwise NOT Operator ( ~ )
13.5.6.1 Runtime Semantics: Evaluation
- Let expr be the result of evaluating
UnaryExpression . - Let oldValue be ?
ToNumeric (?GetValue (expr)). - Let T be
Type (oldValue). - Return ! T::bitwiseNOT(oldValue).
13.5.7 Logical NOT Operator ( ! )
13.5.7.1 Runtime Semantics: Evaluation
- Let expr be the result of evaluating
UnaryExpression . - Let oldValue be !
ToBoolean (?GetValue (expr)). - If oldValue is
true , returnfalse . - Return
true .
13.6 Exponentiation Operator
Syntax
13.6.1 Runtime Semantics: Evaluation
13.7 Multiplicative Operators
Syntax
- The
*operator performs multiplication, producing the product of its operands. - The
/operator performs division, producing the quotient of its operands. - The
%operator yields the remainder of its operands from an implied division.
13.7.1 Runtime Semantics: Evaluation
- Let opText be the source text matched by
MultiplicativeOperator . - Return ?
EvaluateStringOrNumericBinaryExpression (MultiplicativeExpression , opText,ExponentiationExpression ).
13.8 Additive Operators
Syntax
13.8.1 The Addition Operator ( + )
The addition operator either performs string concatenation or numeric addition.
13.8.1.1 Runtime Semantics: Evaluation
13.8.2 The Subtraction Operator ( - )
The - operator performs subtraction, producing the difference of its operands.
13.8.2.1 Runtime Semantics: Evaluation
13.9 Bitwise Shift Operators
Syntax
13.9.1 The Left Shift Operator ( << )
Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.
13.9.1.1 Runtime Semantics: Evaluation
- Return ?
EvaluateStringOrNumericBinaryExpression (ShiftExpression ,<<,AdditiveExpression ).
13.9.2 The Signed Right Shift Operator ( >> )
Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.
13.9.2.1 Runtime Semantics: Evaluation
- Return ?
EvaluateStringOrNumericBinaryExpression (ShiftExpression ,>>,AdditiveExpression ).
13.9.3 The Unsigned Right Shift Operator ( >>> )
Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.
13.9.3.1 Runtime Semantics: Evaluation
- Return ?
EvaluateStringOrNumericBinaryExpression (ShiftExpression ,>>>,AdditiveExpression ).
13.10 Relational Operators
The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
Syntax
The [In] grammar parameter is needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.
13.10.1 Runtime Semantics: Evaluation
- Let lref be the result of evaluating
RelationalExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
ShiftExpression . - Let rval be ?
GetValue (rref). - Let r be the result of performing
Abstract Relational Comparison lval < rval. ReturnIfAbrupt (r).- If r is
undefined , returnfalse . Otherwise, return r.
- Let lref be the result of evaluating
RelationalExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
ShiftExpression . - Let rval be ?
GetValue (rref). - Let r be the result of performing
Abstract Relational Comparison rval < lval with LeftFirst equal tofalse . ReturnIfAbrupt (r).- If r is
undefined , returnfalse . Otherwise, return r.
- Let lref be the result of evaluating
RelationalExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
ShiftExpression . - Let rval be ?
GetValue (rref). - Let r be the result of performing
Abstract Relational Comparison rval < lval with LeftFirst equal tofalse . ReturnIfAbrupt (r).- If r is
true orundefined , returnfalse . Otherwise, returntrue .
- Let lref be the result of evaluating
RelationalExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
ShiftExpression . - Let rval be ?
GetValue (rref). - Let r be the result of performing
Abstract Relational Comparison lval < rval. ReturnIfAbrupt (r).- If r is
true orundefined , returnfalse . Otherwise, returntrue .
- Let lref be the result of evaluating
RelationalExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
ShiftExpression . - Let rval be ?
GetValue (rref). - Return ?
InstanceofOperator (lval, rval).
- Let lref be the result of evaluating
RelationalExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
ShiftExpression . - Let rval be ?
GetValue (rref). - If
Type (rval) is not Object, throw aTypeError exception. - Return ?
HasProperty (rval, ?ToPropertyKey (lval)).
13.10.2 InstanceofOperator ( V, target )
The abstract operation InstanceofOperator takes arguments V (an
- If
Type (target) is not Object, throw aTypeError exception. - Let instOfHandler be ?
GetMethod (target,@@hasInstance ). - If instOfHandler is not
undefined , then - If
IsCallable (target) isfalse , throw aTypeError exception. - Return ?
OrdinaryHasInstance (target, V).
Steps instanceof operator semantics. If an object does not define or inherit instanceof semantics.
13.11 Equality Operators
The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
Syntax
13.11.1 Runtime Semantics: Evaluation
- Let lref be the result of evaluating
EqualityExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
RelationalExpression . - Let rval be ?
GetValue (rref). - Return the result of performing
Abstract Equality Comparison rval == lval.
- Let lref be the result of evaluating
EqualityExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
RelationalExpression . - Let rval be ?
GetValue (rref). - Let r be the result of performing
Abstract Equality Comparison rval == lval. ReturnIfAbrupt (r).- If r is
true , returnfalse . Otherwise, returntrue .
- Let lref be the result of evaluating
EqualityExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
RelationalExpression . - Let rval be ?
GetValue (rref). - Return the result of performing
Strict Equality Comparison rval === lval.
- Let lref be the result of evaluating
EqualityExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
RelationalExpression . - Let rval be ?
GetValue (rref). - Let r be the result of performing
Strict Equality Comparison rval === lval. Assert : r is a normal completion.- If r.[[Value]] is
true , returnfalse . Otherwise, returntrue .
Given the above definition of equality:
-
String comparison can be forced by:
`${a}` == `${b}`. -
Numeric comparison can be forced by:
+a == +b. -
Boolean comparison can be forced by:
!a == !b.
The equality operators maintain the following invariants:
-
A != Bis equivalent to!(A == B). -
A == Bis equivalent toB == A, except in the order of evaluation ofAandB.
The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:
-
new String("a") == "a"and"a" == new String("a")are bothtrue . -
new String("a") == new String("a")isfalse .
Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.
13.12 Binary Bitwise Operators
Syntax
13.12.1 Runtime Semantics: Evaluation
13.13 Binary Logical Operators
Syntax
The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.
13.13.1 Runtime Semantics: Evaluation
- Let lref be the result of evaluating
LogicalANDExpression . - Let lval be ?
GetValue (lref). - Let lbool be !
ToBoolean (lval). - If lbool is
false , return lval. - Let rref be the result of evaluating
BitwiseORExpression . - Return ?
GetValue (rref).
- Let lref be the result of evaluating
LogicalORExpression . - Let lval be ?
GetValue (lref). - Let lbool be !
ToBoolean (lval). - If lbool is
true , return lval. - Let rref be the result of evaluating
LogicalANDExpression . - Return ?
GetValue (rref).
- Let lref be the result of evaluating
CoalesceExpressionHead . - Let lval be ?
GetValue (lref). - If lval is
undefined ornull , then- Let rref be the result of evaluating
BitwiseORExpression . - Return ?
GetValue (rref).
- Let rref be the result of evaluating
- Otherwise, return lval.
13.14 Conditional Operator ( ? : )
Syntax
The grammar for a
13.14.1 Runtime Semantics: Evaluation
- Let lref be the result of evaluating
ShortCircuitExpression . - Let lval be !
ToBoolean (?GetValue (lref)). - If lval is
true , then- Let trueRef be the result of evaluating the first
AssignmentExpression . - Return ?
GetValue (trueRef).
- Let trueRef be the result of evaluating the first
- Else,
- Let falseRef be the result of evaluating the second
AssignmentExpression . - Return ?
GetValue (falseRef).
- Let falseRef be the result of evaluating the second
13.15 Assignment Operators
Syntax
13.15.1 Static Semantics: Early Errors
If
-
It is a Syntax Error if
LeftHandSideExpression is notcovering anAssignmentPattern . -
All Early Error rules for
AssignmentPattern and its derived productions also apply to theAssignmentPattern that iscovered byLeftHandSideExpression .
If
-
It is a Syntax Error if
AssignmentTargetType ofLeftHandSideExpression is notsimple .
-
It is a Syntax Error if
AssignmentTargetType ofLeftHandSideExpression is notsimple .
13.15.2 Runtime Semantics: Evaluation
- If
LeftHandSideExpression is neither anObjectLiteral nor anArrayLiteral , then- Let lref be the result of evaluating
LeftHandSideExpression . ReturnIfAbrupt (lref).- If
IsAnonymousFunctionDefinition (AssignmentExpression ) andIsIdentifierRef ofLeftHandSideExpression are bothtrue , then- Let rval be
NamedEvaluation ofAssignmentExpression with argument lref.[[ReferencedName]].
- Let rval be
- Else,
- Let rref be the result of evaluating
AssignmentExpression . - Let rval be ?
GetValue (rref).
- Let rref be the result of evaluating
- Perform ?
PutValue (lref, rval). - Return rval.
- Let lref be the result of evaluating
- Let assignmentPattern be the
AssignmentPattern that iscovered byLeftHandSideExpression . - Let rref be the result of evaluating
AssignmentExpression . - Let rval be ?
GetValue (rref). - Perform ?
DestructuringAssignmentEvaluation of assignmentPattern using rval as the argument. - Return rval.
- Let lref be the result of evaluating
LeftHandSideExpression . - Let lval be ?
GetValue (lref). - Let rref be the result of evaluating
AssignmentExpression . - Let rval be ?
GetValue (rref). - Let assignmentOpText be the source text matched by
AssignmentOperator . - Let opText be the sequence of Unicode code points associated with assignmentOpText in the following table:
assignmentOpText opText **=***=*/=/%=%+=+-=-<<=<<>>=>>>>>=>>>&=&^=^|=| - Let r be
ApplyStringOrNumericBinaryOperator (lval, opText, rval). - Perform ?
PutValue (lref, r). - Return r.
- Let lref be the result of evaluating
LeftHandSideExpression . - Let lval be ?
GetValue (lref). - Let lbool be !
ToBoolean (lval). - If lbool is
false , return lval. - If
IsAnonymousFunctionDefinition (AssignmentExpression ) istrue andIsIdentifierRef ofLeftHandSideExpression istrue , then- Let rval be
NamedEvaluation ofAssignmentExpression with argument lref.[[ReferencedName]].
- Let rval be
- Else,
- Let rref be the result of evaluating
AssignmentExpression . - Let rval be ?
GetValue (rref).
- Let rref be the result of evaluating
- Perform ?
PutValue (lref, rval). - Return rval.
- Let lref be the result of evaluating
LeftHandSideExpression . - Let lval be ?
GetValue (lref). - Let lbool be !
ToBoolean (lval). - If lbool is
true , return lval. - If
IsAnonymousFunctionDefinition (AssignmentExpression ) istrue andIsIdentifierRef ofLeftHandSideExpression istrue , then- Let rval be
NamedEvaluation ofAssignmentExpression with argument lref.[[ReferencedName]].
- Let rval be
- Else,
- Let rref be the result of evaluating
AssignmentExpression . - Let rval be ?
GetValue (rref).
- Let rref be the result of evaluating
- Perform ?
PutValue (lref, rval). - Return rval.
- Let lref be the result of evaluating
LeftHandSideExpression . - Let lval be ?
GetValue (lref). - If lval is neither
undefined nornull , return lval. - If
IsAnonymousFunctionDefinition (AssignmentExpression ) istrue andIsIdentifierRef ofLeftHandSideExpression istrue , then- Let rval be
NamedEvaluation ofAssignmentExpression with argument lref.[[ReferencedName]].
- Let rval be
- Else,
- Let rref be the result of evaluating
AssignmentExpression . - Let rval be ?
GetValue (rref).
- Let rref be the result of evaluating
- Perform ?
PutValue (lref, rval). - Return rval.
When this expression occurs within
13.15.3 ApplyStringOrNumericBinaryOperator ( lval, opText, rval )
The abstract operation ApplyStringOrNumericBinaryOperator takes arguments lval (an
Assert : opText is present in the table in step8 .- If opText is
+, then- Let lprim be ?
ToPrimitive (lval). - Let rprim be ?
ToPrimitive (rval). - If
Type (lprim) is String orType (rprim) is String, then- Let lstr be ?
ToString (lprim). - Let rstr be ?
ToString (rprim). - Return the
string-concatenation of lstr and rstr.
- Let lstr be ?
- Set lval to lprim.
- Set rval to rprim.
- Let lprim be ?
- NOTE: At this point, it must be a numeric operation.
- Let lnum be ?
ToNumeric (lval). - Let rnum be ?
ToNumeric (rval). - If
Type (lnum) is different fromType (rnum), throw aTypeError exception. - Let T be
Type (lnum). - Let operation be the abstract operation associated with opText in the following table:
opText operation **T::exponentiate *T::multiply /T::divide %T::remainder +T::add -T::subtract <<T::leftShift >>T::signedRightShift >>>T::unsignedRightShift &T::bitwiseAND ^T::bitwiseXOR |T::bitwiseOR - Return ? operation(lnum, rnum).
No hint is provided in the calls to
Step
13.15.4 EvaluateStringOrNumericBinaryExpression ( leftOperand, opText, rightOperand )
The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (a
- Let lref be the result of evaluating leftOperand.
- Let lval be ?
GetValue (lref). - Let rref be the result of evaluating rightOperand.
- Let rval be ?
GetValue (rref). - Return ?
ApplyStringOrNumericBinaryOperator (lval, opText, rval).
13.15.5 Destructuring Assignment
Supplemental Syntax
In certain circumstances when processing an instance of the production
the interpretation of
13.15.5.1 Static Semantics: Early Errors
-
It is a Syntax Error if
AssignmentTargetType ofIdentifierReference is notsimple .
-
It is a Syntax Error if
DestructuringAssignmentTarget is anArrayLiteral or anObjectLiteral .
If
-
It is a Syntax Error if
LeftHandSideExpression is notcovering anAssignmentPattern . -
All Early Error rules for
AssignmentPattern and its derived productions also apply to theAssignmentPattern that iscovered byLeftHandSideExpression .
If
-
It is a Syntax Error if
AssignmentTargetType ofLeftHandSideExpression is notsimple .
13.15.5.2 Runtime Semantics: DestructuringAssignmentEvaluation
With parameter value.
- Perform ?
RequireObjectCoercible (value). - Return
NormalCompletion (empty ).
- Perform ?
RequireObjectCoercible (value). - Perform ?
PropertyDestructuringAssignmentEvaluation forAssignmentPropertyList using value as the argument. - Return
NormalCompletion (empty ).
- Let iteratorRecord be ?
GetIterator (value). - Return ?
IteratorClose (iteratorRecord,NormalCompletion (empty )).
- Let iteratorRecord be ?
GetIterator (value). - Let result be
IteratorDestructuringAssignmentEvaluation ofElision with argument iteratorRecord. - If iteratorRecord.[[Done]] is
false , return ?IteratorClose (iteratorRecord, result). - Return result.
- Let iteratorRecord be ?
GetIterator (value). - If
Elision is present, then- Let status be
IteratorDestructuringAssignmentEvaluation ofElision with argument iteratorRecord. - If status is an
abrupt completion , thenAssert : iteratorRecord.[[Done]] istrue .- Return
Completion (status).
- Let status be
- Let result be
IteratorDestructuringAssignmentEvaluation ofAssignmentRestElement with argument iteratorRecord. - If iteratorRecord.[[Done]] is
false , return ?IteratorClose (iteratorRecord, result). - Return result.
- Let iteratorRecord be ?
GetIterator (value). - Let result be
IteratorDestructuringAssignmentEvaluation ofAssignmentElementList with argument iteratorRecord. - If iteratorRecord.[[Done]] is
false , return ?IteratorClose (iteratorRecord, result). - Return result.
- Let iteratorRecord be ?
GetIterator (value). - Let status be
IteratorDestructuringAssignmentEvaluation ofAssignmentElementList with argument iteratorRecord. - If status is an
abrupt completion , then- If iteratorRecord.[[Done]] is
false , return ?IteratorClose (iteratorRecord, status). - Return
Completion (status).
- If iteratorRecord.[[Done]] is
- If
Elision is present, then- Set status to the result of performing
IteratorDestructuringAssignmentEvaluation ofElision with iteratorRecord as the argument. - If status is an
abrupt completion , thenAssert : iteratorRecord.[[Done]] istrue .- Return
Completion (status).
- Set status to the result of performing
- If
AssignmentRestElement is present, then- Set status to the result of performing
IteratorDestructuringAssignmentEvaluation ofAssignmentRestElement with iteratorRecord as the argument.
- Set status to the result of performing
- If iteratorRecord.[[Done]] is
false , return ?IteratorClose (iteratorRecord, status). - Return
Completion (status).
- Perform ?
RequireObjectCoercible (value). - Let excludedNames be a new empty
List . - Return the result of performing
RestDestructuringAssignmentEvaluation ofAssignmentRestProperty with value and excludedNames as the arguments.
- Perform ?
RequireObjectCoercible (value). - Let excludedNames be ?
PropertyDestructuringAssignmentEvaluation ofAssignmentPropertyList with argument value. - Return the result of performing
RestDestructuringAssignmentEvaluation ofAssignmentRestProperty with arguments value and excludedNames.
13.15.5.3 Runtime Semantics: PropertyDestructuringAssignmentEvaluation
With parameter value.
- Let propertyNames be ?
PropertyDestructuringAssignmentEvaluation ofAssignmentPropertyList with argument value. - Let nextNames be ?
PropertyDestructuringAssignmentEvaluation ofAssignmentProperty with argument value. - Append each item in nextNames to the end of propertyNames.
- Return propertyNames.
- Let P be
StringValue ofIdentifierReference . - Let lref be ?
ResolveBinding (P). - Let v be ?
GetV (value, P). - If
Initializer is present and v isopt undefined , then- If
IsAnonymousFunctionDefinition (Initializer ) istrue , then- Set v to the result of performing
NamedEvaluation forInitializer with argument P.
- Set v to the result of performing
- Else,
- Let defaultValue be the result of evaluating
Initializer . - Set v to ?
GetValue (defaultValue).
- Let defaultValue be the result of evaluating
- If
- Perform ?
PutValue (lref, v). - Return a
List whose sole element is P.
- Let name be the result of evaluating
PropertyName . ReturnIfAbrupt (name).- Perform ?
KeyedDestructuringAssignmentEvaluation ofAssignmentElement with value and name as the arguments. - Return a
List whose sole element is name.
13.15.5.4 Runtime Semantics: RestDestructuringAssignmentEvaluation
With parameters value and excludedNames.
- Let lref be the result of evaluating
DestructuringAssignmentTarget . ReturnIfAbrupt (lref).- Let restObj be !
OrdinaryObjectCreate (%Object.prototype% ). - Perform ?
CopyDataProperties (restObj, value, excludedNames). - Return
PutValue (lref, restObj).
13.15.5.5 Runtime Semantics: IteratorDestructuringAssignmentEvaluation
With parameter iteratorRecord.
- Return the result of performing
IteratorDestructuringAssignmentEvaluation ofAssignmentElisionElement using iteratorRecord as the argument.
- Perform ?
IteratorDestructuringAssignmentEvaluation ofAssignmentElementList using iteratorRecord as the argument. - Return the result of performing
IteratorDestructuringAssignmentEvaluation ofAssignmentElisionElement using iteratorRecord as the argument.
- Return the result of performing
IteratorDestructuringAssignmentEvaluation ofAssignmentElement with iteratorRecord as the argument.
- Perform ?
IteratorDestructuringAssignmentEvaluation ofElision with iteratorRecord as the argument. - Return the result of performing
IteratorDestructuringAssignmentEvaluation ofAssignmentElement with iteratorRecord as the argument.
- If iteratorRecord.[[Done]] is
false , then- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue .
- Let next be
- Return
NormalCompletion (empty ).
- Perform ?
IteratorDestructuringAssignmentEvaluation ofElision with iteratorRecord as the argument. - If iteratorRecord.[[Done]] is
false , then- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue .
- Let next be
- Return
NormalCompletion (empty ).
- If
DestructuringAssignmentTarget is neither anObjectLiteral nor anArrayLiteral , then- Let lref be the result of evaluating
DestructuringAssignmentTarget . ReturnIfAbrupt (lref).
- Let lref be the result of evaluating
- If iteratorRecord.[[Done]] is
false , then- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue . - Else,
- Let value be
IteratorValue (next). - If value is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (value).
- Let value be
- Let next be
- If iteratorRecord.[[Done]] is
true , let value beundefined . - If
Initializer is present and value isundefined , then- If
IsAnonymousFunctionDefinition (Initializer ) istrue andIsIdentifierRef ofDestructuringAssignmentTarget istrue , then- Let v be ?
NamedEvaluation ofInitializer with argument lref.[[ReferencedName]].
- Let v be ?
- Else,
- Let defaultValue be the result of evaluating
Initializer . - Let v be ?
GetValue (defaultValue).
- Let defaultValue be the result of evaluating
- If
- Else, let v be value.
- If
DestructuringAssignmentTarget is anObjectLiteral or anArrayLiteral , then- Let nestedAssignmentPattern be the
AssignmentPattern that iscovered byDestructuringAssignmentTarget . - Return the result of performing
DestructuringAssignmentEvaluation of nestedAssignmentPattern with v as the argument.
- Let nestedAssignmentPattern be the
- Return ?
PutValue (lref, v).
Left to right evaluation order is maintained by evaluating a
- If
DestructuringAssignmentTarget is neither anObjectLiteral nor anArrayLiteral , then- Let lref be the result of evaluating
DestructuringAssignmentTarget . ReturnIfAbrupt (lref).
- Let lref be the result of evaluating
- Let A be !
ArrayCreate (0). - Let n be 0.
- Repeat, while iteratorRecord.[[Done]] is
false ,- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , set iteratorRecord.[[Done]] totrue . - Else,
- Let nextValue be
IteratorValue (next). - If nextValue is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (nextValue).- Perform !
CreateDataPropertyOrThrow (A, !ToString (𝔽 (n)), nextValue). - Set n to n + 1.
- Let nextValue be
- Let next be
- If
DestructuringAssignmentTarget is neither anObjectLiteral nor anArrayLiteral , then- Return ?
PutValue (lref, A).
- Return ?
- Let nestedAssignmentPattern be the
AssignmentPattern that iscovered byDestructuringAssignmentTarget . - Return the result of performing
DestructuringAssignmentEvaluation of nestedAssignmentPattern with A as the argument.
13.15.5.6 Runtime Semantics: KeyedDestructuringAssignmentEvaluation
With parameters value and propertyName.
- If
DestructuringAssignmentTarget is neither anObjectLiteral nor anArrayLiteral , then- Let lref be the result of evaluating
DestructuringAssignmentTarget . ReturnIfAbrupt (lref).
- Let lref be the result of evaluating
- Let v be ?
GetV (value, propertyName). - If
Initializer is present and v isundefined , then- If
IsAnonymousFunctionDefinition (Initializer ) andIsIdentifierRef ofDestructuringAssignmentTarget are bothtrue , then- Let rhsValue be ?
NamedEvaluation ofInitializer with argument lref.[[ReferencedName]].
- Let rhsValue be ?
- Else,
- Let defaultValue be the result of evaluating
Initializer . - Let rhsValue be ?
GetValue (defaultValue).
- Let defaultValue be the result of evaluating
- If
- Else, let rhsValue be v.
- If
DestructuringAssignmentTarget is anObjectLiteral or anArrayLiteral , then- Let assignmentPattern be the
AssignmentPattern that iscovered byDestructuringAssignmentTarget . - Return the result of performing
DestructuringAssignmentEvaluation of assignmentPattern with rhsValue as the argument.
- Let assignmentPattern be the
- Return ?
PutValue (lref, rhsValue).
13.16 Comma Operator ( , )
Syntax
13.16.1 Runtime Semantics: Evaluation
- Let lref be the result of evaluating
Expression . - Perform ?
GetValue (lref). - Let rref be the result of evaluating
AssignmentExpression . - Return ?
GetValue (rref).
14 ECMAScript Language: Statements and Declarations
Syntax
14.1 Statement Semantics
14.1.1 Runtime Semantics: Evaluation
- Return
NormalCompletion (empty ).
- Return the result of evaluating
FunctionDeclaration .
- Let newLabelSet be a new empty
List . - Return the result of performing
LabelledEvaluation of thisBreakableStatement with argument newLabelSet.
14.2 Block
Syntax
14.2.1 Static Semantics: Early Errors
-
It is a Syntax Error if the
LexicallyDeclaredNames ofStatementList contains any duplicate entries. -
It is a Syntax Error if any element of the
LexicallyDeclaredNames ofStatementList also occurs in theVarDeclaredNames ofStatementList .
14.2.2 Runtime Semantics: Evaluation
- Return
NormalCompletion (empty ).
- Let oldEnv be the
running execution context 's LexicalEnvironment. - Let blockEnv be
NewDeclarativeEnvironment (oldEnv). - Perform
BlockDeclarationInstantiation (StatementList , blockEnv). - Set the
running execution context 's LexicalEnvironment to blockEnv. - Let blockValue be the result of evaluating
StatementList . - Set the
running execution context 's LexicalEnvironment to oldEnv. - Return blockValue.
No matter how control leaves the
- Let sl be the result of evaluating
StatementList . ReturnIfAbrupt (sl).- Let s be the result of evaluating
StatementListItem . - Return
Completion (UpdateEmpty (s, sl)).
The value of a eval function all return the value 1:
eval("1;;;;;")
eval("1;{}")
eval("1;var a;")14.2.3 BlockDeclarationInstantiation ( code, env )
When a
The abstract operation BlockDeclarationInstantiation takes arguments code (a
Assert : env is adeclarative Environment Record .- Let declarations be the
LexicallyScopedDeclarations of code. - For each element d of declarations, do
- For each element dn of the
BoundNames of d, do- If
IsConstantDeclaration of d istrue , then- Perform ! env.CreateImmutableBinding(dn,
true ).
- Perform ! env.CreateImmutableBinding(dn,
- Else,
- Perform ! env.CreateMutableBinding(dn,
false ). NOTE: This step is replaced in sectionB.3.3.6 .
- Perform ! env.CreateMutableBinding(dn,
- If
- If d is a
FunctionDeclaration , aGeneratorDeclaration , anAsyncFunctionDeclaration , or anAsyncGeneratorDeclaration , then- Let fn be the sole element of the
BoundNames of d. - Let fo be
InstantiateFunctionObject of d with argument env. - Perform env.InitializeBinding(fn, fo). NOTE: This step is replaced in section
B.3.3.6 .
- Let fn be the sole element of the
- For each element dn of the
14.3 Declarations and the Variable Statement
14.3.1 Let and Const Declarations
let and const declarations define variables that are scoped to the let declaration does not have an
Syntax
14.3.1.1 Static Semantics: Early Errors
-
It is a Syntax Error if the
BoundNames ofBindingList contains"let" . -
It is a Syntax Error if the
BoundNames ofBindingList contains any duplicate entries.
-
It is a Syntax Error if
Initializer is not present andIsConstantDeclaration of theLexicalDeclaration containing thisLexicalBinding istrue .
14.3.1.2 Runtime Semantics: Evaluation
- Let next be the result of evaluating
BindingList . ReturnIfAbrupt (next).- Return
NormalCompletion (empty ).
- Let next be the result of evaluating
BindingList . ReturnIfAbrupt (next).- Return the result of evaluating
LexicalBinding .
- Let lhs be
ResolveBinding (StringValue ofBindingIdentifier ). - Return
InitializeReferencedBinding (lhs,undefined ).
A const declaration.
- Let bindingId be
StringValue ofBindingIdentifier . - Let lhs be
ResolveBinding (bindingId). - If
IsAnonymousFunctionDefinition (Initializer ) istrue , then- Let value be
NamedEvaluation ofInitializer with argument bindingId.
- Let value be
- Else,
- Let rhs be the result of evaluating
Initializer . - Let value be ?
GetValue (rhs).
- Let rhs be the result of evaluating
- Return
InitializeReferencedBinding (lhs, value).
- Let rhs be the result of evaluating
Initializer . - Let value be ?
GetValue (rhs). - Let env be the
running execution context 's LexicalEnvironment. - Return the result of performing
BindingInitialization forBindingPattern using value and env as the arguments.
14.3.2 Variable Statement
A var statement declares variables that are scoped to the
Syntax
14.3.2.1 Runtime Semantics: Evaluation
- Let next be the result of evaluating
VariableDeclarationList . ReturnIfAbrupt (next).- Return
NormalCompletion (empty ).
- Let next be the result of evaluating
VariableDeclarationList . ReturnIfAbrupt (next).- Return the result of evaluating
VariableDeclaration .
- Return
NormalCompletion (empty ).
- Let bindingId be
StringValue ofBindingIdentifier . - Let lhs be ?
ResolveBinding (bindingId). - If
IsAnonymousFunctionDefinition (Initializer ) istrue , then- Let value be
NamedEvaluation ofInitializer with argument bindingId.
- Let value be
- Else,
- Let rhs be the result of evaluating
Initializer . - Let value be ?
GetValue (rhs).
- Let rhs be the result of evaluating
- Return ?
PutValue (lhs, value).
If a
- Let rhs be the result of evaluating
Initializer . - Let rval be ?
GetValue (rhs). - Return the result of performing
BindingInitialization forBindingPattern passing rval andundefined as arguments.
14.3.3 Destructuring Binding Patterns
Syntax
14.3.3.1 Runtime Semantics: PropertyBindingInitialization
With parameters value and environment.
- Let boundNames be ?
PropertyBindingInitialization ofBindingPropertyList with arguments value and environment. - Let nextNames be ?
PropertyBindingInitialization ofBindingProperty with arguments value and environment. - Append each item in nextNames to the end of boundNames.
- Return boundNames.
- Let name be the string that is the only element of
BoundNames ofSingleNameBinding . - Perform ?
KeyedBindingInitialization forSingleNameBinding using value, environment, and name as the arguments. - Return a
List whose sole element is name.
- Let P be the result of evaluating
PropertyName . ReturnIfAbrupt (P).- Perform ?
KeyedBindingInitialization ofBindingElement with value, environment, and P as the arguments. - Return a
List whose sole element is P.
14.3.3.2 Runtime Semantics: RestBindingInitialization
With parameters value, environment, and excludedNames.
- Let lhs be ?
ResolveBinding (StringValue ofBindingIdentifier , environment). - Let restObj be !
OrdinaryObjectCreate (%Object.prototype% ). - Perform ?
CopyDataProperties (restObj, value, excludedNames). - If environment is
undefined , returnPutValue (lhs, restObj). - Return
InitializeReferencedBinding (lhs, restObj).
14.3.3.3 Runtime Semantics: KeyedBindingInitialization
With parameters value, environment, and propertyName.
When
- Let v be ?
GetV (value, propertyName). - If
Initializer is present and v isundefined , then- Let defaultValue be the result of evaluating
Initializer . - Set v to ?
GetValue (defaultValue).
- Let defaultValue be the result of evaluating
- Return the result of performing
BindingInitialization forBindingPattern passing v and environment as arguments.
- Let bindingId be
StringValue ofBindingIdentifier . - Let lhs be ?
ResolveBinding (bindingId, environment). - Let v be ?
GetV (value, propertyName). - If
Initializer is present and v isundefined , then- If
IsAnonymousFunctionDefinition (Initializer ) istrue , then- Set v to the result of performing
NamedEvaluation forInitializer with argument bindingId.
- Set v to the result of performing
- Else,
- Let defaultValue be the result of evaluating
Initializer . - Set v to ?
GetValue (defaultValue).
- Let defaultValue be the result of evaluating
- If
- If environment is
undefined , return ?PutValue (lhs, v). - Return
InitializeReferencedBinding (lhs, v).
14.4 Empty Statement
Syntax
14.4.1 Runtime Semantics: Evaluation
- Return
NormalCompletion (empty ).
14.5 Expression Statement
Syntax
An function or class keywords because that would make it ambiguous with a async function because that would make it ambiguous with an let [ because that would make it ambiguous with a let
14.5.1 Runtime Semantics: Evaluation
- Let exprRef be the result of evaluating
Expression . - Return ?
GetValue (exprRef).
14.6 The if Statement
Syntax
else] resolves the classic "dangling else" problem in the usual way. That is, when the choice of associated if is otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate ifs14.6.1 Static Semantics: Early Errors
-
It is a Syntax Error if
IsLabelledFunction (Statement ) istrue .
It is only necessary to apply this rule if the extension specified in
14.6.2 Runtime Semantics: Evaluation
- Let exprRef be the result of evaluating
Expression . - Let exprValue be !
ToBoolean (?GetValue (exprRef)). - If exprValue is
true , then- Let stmtCompletion be the result of evaluating the first
Statement .
- Let stmtCompletion be the result of evaluating the first
- Else,
- Let stmtCompletion be the result of evaluating the second
Statement .
- Let stmtCompletion be the result of evaluating the second
- Return
Completion (UpdateEmpty (stmtCompletion,undefined )).
- Let exprRef be the result of evaluating
Expression . - Let exprValue be !
ToBoolean (?GetValue (exprRef)). - If exprValue is
false , then- Return
NormalCompletion (undefined ).
- Return
- Else,
- Let stmtCompletion be the result of evaluating
Statement . - Return
Completion (UpdateEmpty (stmtCompletion,undefined )).
- Let stmtCompletion be the result of evaluating
14.7 Iteration Statements
Syntax
14.7.1 Semantics
14.7.1.1 LoopContinues ( completion, labelSet )
The abstract operation LoopContinues takes arguments completion and labelSet. It performs the following steps when called:
- If completion.[[Type]] is
normal , returntrue . - If completion.[[Type]] is not
continue , returnfalse . - If completion.[[Target]] is
empty , returntrue . - If completion.[[Target]] is an element of labelSet, return
true . - Return
false .
Within the
14.7.1.2 Runtime Semantics: LoopEvaluation
With parameter labelSet.
- Return ?
DoWhileLoopEvaluation ofDoWhileStatement with argument labelSet.
- Return ?
WhileLoopEvaluation ofWhileStatement with argument labelSet.
- Return ?
ForLoopEvaluation ofForStatement with argument labelSet.
- Return ?
ForInOfLoopEvaluation ofForInOfStatement with argument labelSet.
14.7.2 The do-while Statement
Syntax
14.7.2.1 Static Semantics: Early Errors
-
It is a Syntax Error if
IsLabelledFunction (Statement ) istrue .
It is only necessary to apply this rule if the extension specified in
14.7.2.2 Runtime Semantics: DoWhileLoopEvaluation
With parameter labelSet.
- Let V be
undefined . - Repeat,
- Let stmtResult be the result of evaluating
Statement . - If
LoopContinues (stmtResult, labelSet) isfalse , returnCompletion (UpdateEmpty (stmtResult, V)). - If stmtResult.[[Value]] is not
empty , set V to stmtResult.[[Value]]. - Let exprRef be the result of evaluating
Expression . - Let exprValue be ?
GetValue (exprRef). - If !
ToBoolean (exprValue) isfalse , returnNormalCompletion (V).
- Let stmtResult be the result of evaluating
14.7.3 The while Statement
Syntax
14.7.3.1 Static Semantics: Early Errors
-
It is a Syntax Error if
IsLabelledFunction (Statement ) istrue .
It is only necessary to apply this rule if the extension specified in
14.7.3.2 Runtime Semantics: WhileLoopEvaluation
With parameter labelSet.
- Let V be
undefined . - Repeat,
- Let exprRef be the result of evaluating
Expression . - Let exprValue be ?
GetValue (exprRef). - If !
ToBoolean (exprValue) isfalse , returnNormalCompletion (V). - Let stmtResult be the result of evaluating
Statement . - If
LoopContinues (stmtResult, labelSet) isfalse , returnCompletion (UpdateEmpty (stmtResult, V)). - If stmtResult.[[Value]] is not
empty , set V to stmtResult.[[Value]].
- Let exprRef be the result of evaluating
14.7.4 The for Statement
Syntax
14.7.4.1 Static Semantics: Early Errors
-
It is a Syntax Error if
IsLabelledFunction (Statement ) istrue .
It is only necessary to apply this rule if the extension specified in
-
It is a Syntax Error if any element of the
BoundNames ofLexicalDeclaration also occurs in theVarDeclaredNames ofStatement .
14.7.4.2 Runtime Semantics: ForLoopEvaluation
With parameter labelSet.
- If the first
Expression is present, then- Let exprRef be the result of evaluating the first
Expression . - Perform ?
GetValue (exprRef).
- Let exprRef be the result of evaluating the first
- Return ?
ForBodyEvaluation (the secondExpression , the thirdExpression ,Statement , « », labelSet).
- Let varDcl be the result of evaluating
VariableDeclarationList . ReturnIfAbrupt (varDcl).- Return ?
ForBodyEvaluation (the firstExpression , the secondExpression ,Statement , « », labelSet).
- Let oldEnv be the
running execution context 's LexicalEnvironment. - Let loopEnv be
NewDeclarativeEnvironment (oldEnv). - Let isConst be
IsConstantDeclaration ofLexicalDeclaration . - Let boundNames be the
BoundNames ofLexicalDeclaration . - For each element dn of boundNames, do
- If isConst is
true , then- Perform ! loopEnv.CreateImmutableBinding(dn,
true ).
- Perform ! loopEnv.CreateImmutableBinding(dn,
- Else,
- Perform ! loopEnv.CreateMutableBinding(dn,
false ).
- Perform ! loopEnv.CreateMutableBinding(dn,
- If isConst is
- Set the
running execution context 's LexicalEnvironment to loopEnv. - Let forDcl be the result of evaluating
LexicalDeclaration . - If forDcl is an
abrupt completion , then- Set the
running execution context 's LexicalEnvironment to oldEnv. - Return
Completion (forDcl).
- Set the
- If isConst is
false , let perIterationLets be boundNames; otherwise let perIterationLets be « ». - Let bodyResult be
ForBodyEvaluation (the firstExpression , the secondExpression ,Statement , perIterationLets, labelSet). - Set the
running execution context 's LexicalEnvironment to oldEnv. - Return
Completion (bodyResult).
14.7.4.3 ForBodyEvaluation ( test, increment, stmt, perIterationBindings, labelSet )
The abstract operation ForBodyEvaluation takes arguments test, increment, stmt, perIterationBindings, and labelSet. It performs the following steps when called:
- Let V be
undefined . - Perform ?
CreatePerIterationEnvironment (perIterationBindings). - Repeat,
- If test is not
[empty] , then- Let testRef be the result of evaluating test.
- Let testValue be ?
GetValue (testRef). - If !
ToBoolean (testValue) isfalse , returnNormalCompletion (V).
- Let result be the result of evaluating stmt.
- If
LoopContinues (result, labelSet) isfalse , returnCompletion (UpdateEmpty (result, V)). - If result.[[Value]] is not
empty , set V to result.[[Value]]. - Perform ?
CreatePerIterationEnvironment (perIterationBindings). - If increment is not
[empty] , then- Let incRef be the result of evaluating increment.
- Perform ?
GetValue (incRef).
- If test is not
14.7.4.4 CreatePerIterationEnvironment ( perIterationBindings )
The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings. It performs the following steps when called:
- If perIterationBindings has any elements, then
- Let lastIterationEnv be the
running execution context 's LexicalEnvironment. - Let outer be lastIterationEnv.[[OuterEnv]].
Assert : outer is notnull .- Let thisIterationEnv be
NewDeclarativeEnvironment (outer). - For each element bn of perIterationBindings, do
- Perform ! thisIterationEnv.CreateMutableBinding(bn,
false ). - Let lastValue be ? lastIterationEnv.GetBindingValue(bn,
true ). - Perform thisIterationEnv.InitializeBinding(bn, lastValue).
- Perform ! thisIterationEnv.CreateMutableBinding(bn,
- Set the
running execution context 's LexicalEnvironment to thisIterationEnv.
- Let lastIterationEnv be the
- Return
undefined .
14.7.5 The for-in, for-of, and for-await-of Statements
Syntax
This section is extended by Annex
14.7.5.1 Static Semantics: Early Errors
-
It is a Syntax Error if
IsLabelledFunction (Statement ) istrue .
It is only necessary to apply this rule if the extension specified in
If
-
It is a Syntax Error if
LeftHandSideExpression is notcovering anAssignmentPattern . -
All Early Error rules for
AssignmentPattern and its derived productions also apply to theAssignmentPattern that iscovered byLeftHandSideExpression .
If
-
It is a Syntax Error if
AssignmentTargetType ofLeftHandSideExpression is notsimple .
-
It is a Syntax Error if the
BoundNames ofForDeclaration contains"let" . -
It is a Syntax Error if any element of the
BoundNames ofForDeclaration also occurs in theVarDeclaredNames ofStatement . -
It is a Syntax Error if the
BoundNames ofForDeclaration contains any duplicate entries.
14.7.5.2 Static Semantics: IsDestructuring
- If
PrimaryExpression is either anObjectLiteral or anArrayLiteral , returntrue . - Return
false .
- Return
false .
- Return
IsDestructuring ofForBinding .
- Return
false .
- Return
true .
This section is extended by Annex
14.7.5.3 Runtime Semantics: ForDeclarationBindingInitialization
With parameters value and environment.
var statements and the formal parameter lists of some non-strict functions (see
- Return the result of performing
BindingInitialization forForBinding passing value and environment as the arguments.
14.7.5.4 Runtime Semantics: ForDeclarationBindingInstantiation
With parameter environment.
Assert : environment is adeclarative Environment Record .- For each element name of the
BoundNames ofForBinding , do- If
IsConstantDeclaration ofLetOrConst istrue , then- Perform ! environment.CreateImmutableBinding(name,
true ).
- Perform ! environment.CreateImmutableBinding(name,
- Else,
- Perform ! environment.CreateMutableBinding(name,
false ).
- Perform ! environment.CreateMutableBinding(name,
- If
14.7.5.5 Runtime Semantics: ForInOfLoopEvaluation
With parameter labelSet.
- Let keyResult be ?
ForIn/OfHeadEvaluation (« »,Expression ,enumerate ). - Return ?
ForIn/OfBodyEvaluation (LeftHandSideExpression ,Statement , keyResult,enumerate ,assignment , labelSet).
- Let keyResult be ?
ForIn/OfHeadEvaluation (« »,Expression ,enumerate ). - Return ?
ForIn/OfBodyEvaluation (ForBinding ,Statement , keyResult,enumerate ,varBinding , labelSet).
- Let keyResult be ?
ForIn/OfHeadEvaluation (BoundNames ofForDeclaration ,Expression ,enumerate ). - Return ?
ForIn/OfBodyEvaluation (ForDeclaration ,Statement , keyResult,enumerate ,lexicalBinding , labelSet).
- Let keyResult be ?
ForIn/OfHeadEvaluation (« »,AssignmentExpression ,iterate ). - Return ?
ForIn/OfBodyEvaluation (LeftHandSideExpression ,Statement , keyResult,iterate ,assignment , labelSet).
- Let keyResult be ?
ForIn/OfHeadEvaluation (« »,AssignmentExpression ,iterate ). - Return ?
ForIn/OfBodyEvaluation (ForBinding ,Statement , keyResult,iterate ,varBinding , labelSet).
- Let keyResult be ?
ForIn/OfHeadEvaluation (BoundNames ofForDeclaration ,AssignmentExpression ,iterate ). - Return ?
ForIn/OfBodyEvaluation (ForDeclaration ,Statement , keyResult,iterate ,lexicalBinding , labelSet).
- Let keyResult be ?
ForIn/OfHeadEvaluation (« »,AssignmentExpression ,async-iterate ). - Return ?
ForIn/OfBodyEvaluation (LeftHandSideExpression ,Statement , keyResult,iterate ,assignment , labelSet,async ).
- Let keyResult be ?
ForIn/OfHeadEvaluation (« »,AssignmentExpression ,async-iterate ). - Return ?
ForIn/OfBodyEvaluation (ForBinding ,Statement , keyResult,iterate ,varBinding , labelSet,async ).
- Let keyResult be ?
ForIn/OfHeadEvaluation (BoundNames ofForDeclaration ,AssignmentExpression ,async-iterate ). - Return ?
ForIn/OfBodyEvaluation (ForDeclaration ,Statement , keyResult,iterate ,lexicalBinding , labelSet,async ).
This section is extended by Annex
14.7.5.6 ForIn/OfHeadEvaluation ( uninitializedBoundNames, expr, iterationKind )
The abstract operation ForIn/OfHeadEvaluation takes arguments uninitializedBoundNames, expr, and iterationKind (either
- Let oldEnv be the
running execution context 's LexicalEnvironment. - If uninitializedBoundNames is not an empty
List , thenAssert : uninitializedBoundNames has no duplicate entries.- Let newEnv be
NewDeclarativeEnvironment (oldEnv). - For each String name of uninitializedBoundNames, do
- Perform ! newEnv.CreateMutableBinding(name,
false ).
- Perform ! newEnv.CreateMutableBinding(name,
- Set the
running execution context 's LexicalEnvironment to newEnv.
- Let exprRef be the result of evaluating expr.
- Set the
running execution context 's LexicalEnvironment to oldEnv. - Let exprValue be ?
GetValue (exprRef). - If iterationKind is
enumerate , then- If exprValue is
undefined ornull , then- Return
Completion { [[Type]]:break , [[Value]]:empty , [[Target]]:empty }.
- Return
- Let obj be !
ToObject (exprValue). - Let iterator be ?
EnumerateObjectProperties (obj). - Let nextMethod be !
GetV (iterator,"next" ). - Return the
Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]:false }.
- If exprValue is
- Else,
Assert : iterationKind isiterate orasync-iterate .- If iterationKind is
async-iterate , let iteratorHint beasync . - Else, let iteratorHint be
sync . - Return ?
GetIterator (exprValue, iteratorHint).
14.7.5.7 ForIn/OfBodyEvaluation ( lhs, stmt, iteratorRecord, iterationKind, lhsKind, labelSet [ , iteratorKind ] )
The abstract operation ForIn/OfBodyEvaluation takes arguments lhs, stmt, iteratorRecord, iterationKind, lhsKind (either
- If iteratorKind is not present, set iteratorKind to
sync . - Let oldEnv be the
running execution context 's LexicalEnvironment. - Let V be
undefined . - Let destructuring be
IsDestructuring of lhs. - If destructuring is
true and if lhsKind isassignment , thenAssert : lhs is aLeftHandSideExpression .- Let assignmentPattern be the
AssignmentPattern that iscovered by lhs.
- Repeat,
- Let nextResult be ?
Call (iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]). - If iteratorKind is
async , set nextResult to ?Await (nextResult). - If
Type (nextResult) is not Object, throw aTypeError exception. - Let done be ?
IteratorComplete (nextResult). - If done is
true , returnNormalCompletion (V). - Let nextValue be ?
IteratorValue (nextResult). - If lhsKind is either
assignment orvarBinding , then- If destructuring is
false , then- Let lhsRef be the result of evaluating lhs. (It may be evaluated repeatedly.)
- If destructuring is
- Else,
Assert : lhsKind islexicalBinding .Assert : lhs is aForDeclaration .- Let iterationEnv be
NewDeclarativeEnvironment (oldEnv). - Perform
ForDeclarationBindingInstantiation for lhs passing iterationEnv as the argument. - Set the
running execution context 's LexicalEnvironment to iterationEnv. - If destructuring is
false , thenAssert : lhs binds a single name.- Let lhsName be the sole element of
BoundNames of lhs. - Let lhsRef be !
ResolveBinding (lhsName).
- If destructuring is
false , then- If lhsRef is an
abrupt completion , then- Let status be lhsRef.
- Else if lhsKind is
lexicalBinding , then- Let status be
InitializeReferencedBinding (lhsRef, nextValue).
- Let status be
- Else,
- Let status be
PutValue (lhsRef, nextValue).
- Let status be
- If lhsRef is an
- Else,
- If lhsKind is
assignment , then- Let status be
DestructuringAssignmentEvaluation of assignmentPattern with argument nextValue.
- Let status be
- Else if lhsKind is
varBinding , thenAssert : lhs is aForBinding .- Let status be
BindingInitialization of lhs with arguments nextValue andundefined .
- Else,
Assert : lhsKind islexicalBinding .Assert : lhs is aForDeclaration .- Let status be
ForDeclarationBindingInitialization of lhs with arguments nextValue and iterationEnv.
- If lhsKind is
- If status is an
abrupt completion , then- Set the
running execution context 's LexicalEnvironment to oldEnv. - If iteratorKind is
async , return ?AsyncIteratorClose (iteratorRecord, status). - If iterationKind is
enumerate , then- Return status.
- Else,
Assert : iterationKind isiterate .- Return ?
IteratorClose (iteratorRecord, status).
- Set the
- Let result be the result of evaluating stmt.
- Set the
running execution context 's LexicalEnvironment to oldEnv. - If
LoopContinues (result, labelSet) isfalse , then- If iterationKind is
enumerate , then- Return
Completion (UpdateEmpty (result, V)).
- Return
- Else,
Assert : iterationKind isiterate .- Set status to
UpdateEmpty (result, V). - If iteratorKind is
async , return ?AsyncIteratorClose (iteratorRecord, status). - Return ?
IteratorClose (iteratorRecord, status).
- If iterationKind is
- If result.[[Value]] is not
empty , set V to result.[[Value]].
- Let nextResult be ?
14.7.5.8 Runtime Semantics: Evaluation
- Let bindingId be
StringValue ofBindingIdentifier . - Return ?
ResolveBinding (bindingId).
14.7.5.9 EnumerateObjectProperties ( O )
The abstract operation EnumerateObjectProperties takes argument O. It performs the following steps when called:
Assert :Type (O) is Object.- Return an Iterator object (
27.1.1.2 ) whosenextmethod iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.
The iterator's throw and return methods are next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.
In addition, if neither O nor any object in its prototype chain is a
- the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
- a property is removed from O or an object in its prototype chain,
- a property is added to an object in O's prototype chain, or
- the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.
ECMAScript implementations are not required to implement the algorithm in
The following is an informative definition of an ECMAScript generator function that conforms to these rules:
function* EnumerateObjectProperties(obj) {
const visited = new Set();
for (const key of Reflect.ownKeys(obj)) {
if (typeof key === "symbol") continue;
const desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
const proto = Reflect.getPrototypeOf(obj);
if (proto === null) return;
for (const protoKey of EnumerateObjectProperties(proto)) {
if (!visited.has(protoKey)) yield protoKey;
}
}14.7.5.10 For-In Iterator Objects
A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour of
14.7.5.10.1 CreateForInIterator ( object )
The abstract operation CreateForInIterator takes argument object. It is used to create a For-In Iterator object which iterates over the own and inherited enumerable string properties of object in a specific order. It performs the following steps when called:
Assert :Type (object) is Object.- Let iterator be !
OrdinaryObjectCreate (%ForInIteratorPrototype% , « [[Object]], [[ObjectWasVisited]], [[VisitedKeys]], [[RemainingKeys]] »). - Set iterator.[[Object]] to object.
- Set iterator.[[ObjectWasVisited]] to
false . - Set iterator.[[VisitedKeys]] to a new empty
List . - Set iterator.[[RemainingKeys]] to a new empty
List . - Return iterator.
14.7.5.10.2 The %ForInIteratorPrototype% Object
The %ForInIteratorPrototype% object:
- has properties that are inherited by all For-In Iterator Objects.
- is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%IteratorPrototype% . - is never directly accessible to ECMAScript code.
- has the following properties:
14.7.5.10.2.1 %ForInIteratorPrototype%.next ( )
- Let O be the
this value. Assert :Type (O) is Object.Assert : O has all of the internal slots of a For-In Iterator Instance (14.7.5.10.3 ).- Let object be O.[[Object]].
- Let visited be O.[[VisitedKeys]].
- Let remaining be O.[[RemainingKeys]].
- Repeat,
- If O.[[ObjectWasVisited]] is
false , then- Let keys be ? object.[[OwnPropertyKeys]]().
- For each element key of keys, do
- If
Type (key) is String, then- Append key to remaining.
- If
- Set O.[[ObjectWasVisited]] to
true .
- Repeat, while remaining is not empty,
- Let r be the first element of remaining.
- Remove the first element from remaining.
- If there does not exist an element v of visited such that
SameValue (r, v) istrue , then- Let desc be ? object.[[GetOwnProperty]](r).
- If desc is not
undefined , then- Append r to visited.
- If desc.[[Enumerable]] is
true , returnCreateIterResultObject (r,false ).
- Set object to ? object.[[GetPrototypeOf]]().
- Set O.[[Object]] to object.
- Set O.[[ObjectWasVisited]] to
false . - If object is
null , returnCreateIterResultObject (undefined ,true ).
- If O.[[ObjectWasVisited]] is
14.7.5.10.3 Properties of For-In Iterator Instances
For-In Iterator instances are ordinary objects that inherit properties from the
| Internal Slot | Description |
|---|---|
| [[Object]] | The Object value whose properties are being iterated. |
| [[ObjectWasVisited]] |
|
| [[VisitedKeys]] | A list of String values which have been emitted by this iterator thus far. |
| [[RemainingKeys]] |
A list of String values remaining to be emitted for the current object, before iterating the properties of its prototype (if its prototype is not |
14.8 The continue Statement
Syntax
14.8.1 Static Semantics: Early Errors
-
It is a Syntax Error if this
ContinueStatement is not nested, directly or indirectly (but not crossing function boundaries), within anIterationStatement .
14.8.2 Runtime Semantics: Evaluation
- Return
Completion { [[Type]]:continue , [[Value]]:empty , [[Target]]:empty }.
- Let label be the
StringValue ofLabelIdentifier . - Return
Completion { [[Type]]:continue , [[Value]]:empty , [[Target]]: label }.
14.9 The break Statement
Syntax
14.9.1 Static Semantics: Early Errors
-
It is a Syntax Error if this
BreakStatement is not nested, directly or indirectly (but not crossing function boundaries), within anIterationStatement or aSwitchStatement .
14.9.2 Runtime Semantics: Evaluation
- Return
Completion { [[Type]]:break , [[Value]]:empty , [[Target]]:empty }.
- Let label be the
StringValue ofLabelIdentifier . - Return
Completion { [[Type]]:break , [[Value]]:empty , [[Target]]: label }.
14.10 The return Statement
Syntax
A return statement causes a function to cease execution and, in most cases, returns a value to the caller. If return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's completion record may be replaced with another completion record during evaluation of the finally block.
14.10.1 Runtime Semantics: Evaluation
- Return
Completion { [[Type]]:return , [[Value]]:undefined , [[Target]]:empty }.
- Let exprRef be the result of evaluating
Expression . - Let exprValue be ?
GetValue (exprRef). - If !
GetGeneratorKind () isasync , set exprValue to ?Await (exprValue). - Return
Completion { [[Type]]:return , [[Value]]: exprValue, [[Target]]:empty }.
14.11 The with Statement
Syntax
The with statement adds an
14.11.1 Static Semantics: Early Errors
-
It is a Syntax Error if the code that matches this production is contained in
strict mode code . -
It is a Syntax Error if
IsLabelledFunction (Statement ) istrue .
It is only necessary to apply the second rule if the extension specified in
14.11.2 Runtime Semantics: Evaluation
- Let val be the result of evaluating
Expression . - Let obj be ?
ToObject (?GetValue (val)). - Let oldEnv be the
running execution context 's LexicalEnvironment. - Let newEnv be
NewObjectEnvironment (obj, oldEnv). - Set the withEnvironment flag of newEnv to
true . - Set the
running execution context 's LexicalEnvironment to newEnv. - Let C be the result of evaluating
Statement . - Set the
running execution context 's LexicalEnvironment to oldEnv. - Return
Completion (UpdateEmpty (C,undefined )).
No matter how control leaves the embedded
14.12 The switch Statement
Syntax
14.12.1 Static Semantics: Early Errors
-
It is a Syntax Error if the
LexicallyDeclaredNames ofCaseBlock contains any duplicate entries. -
It is a Syntax Error if any element of the
LexicallyDeclaredNames ofCaseBlock also occurs in theVarDeclaredNames ofCaseBlock .
14.12.2 Runtime Semantics: CaseBlockEvaluation
With parameter input.
- Return
NormalCompletion (undefined ).
- Let V be
undefined . - Let A be the
List ofCaseClause items inCaseClauses , in source text order. - Let found be
false . - For each
CaseClause C of A, do- If found is
false , then- Set found to ?
CaseClauseIsSelected (C, input).
- Set found to ?
- If found is
true , then- Let R be the result of evaluating C.
- If R.[[Value]] is not
empty , set V to R.[[Value]]. - If R is an
abrupt completion , returnCompletion (UpdateEmpty (R, V)).
- If found is
- Return
NormalCompletion (V).
- Let V be
undefined . - If the first
CaseClauses is present, then- Let A be the
List ofCaseClause items in the firstCaseClauses , in source text order.
- Let A be the
- Else,
- Let A be « ».
- Let found be
false . - For each
CaseClause C of A, do- If found is
false , then- Set found to ?
CaseClauseIsSelected (C, input).
- Set found to ?
- If found is
true , then- Let R be the result of evaluating C.
- If R.[[Value]] is not
empty , set V to R.[[Value]]. - If R is an
abrupt completion , returnCompletion (UpdateEmpty (R, V)).
- If found is
- Let foundInB be
false . - If the second
CaseClauses is present, then- Let B be the
List ofCaseClause items in the secondCaseClauses , in source text order.
- Let B be the
- Else,
- Let B be « ».
- If found is
false , then- For each
CaseClause C of B, do- If foundInB is
false , then- Set foundInB to ?
CaseClauseIsSelected (C, input).
- Set foundInB to ?
- If foundInB is
true , then- Let R be the result of evaluating
CaseClause C. - If R.[[Value]] is not
empty , set V to R.[[Value]]. - If R is an
abrupt completion , returnCompletion (UpdateEmpty (R, V)).
- Let R be the result of evaluating
- If foundInB is
- For each
- If foundInB is
true , returnNormalCompletion (V). - Let R be the result of evaluating
DefaultClause . - If R.[[Value]] is not
empty , set V to R.[[Value]]. - If R is an
abrupt completion , returnCompletion (UpdateEmpty (R, V)). - NOTE: The following is another complete iteration of the second
CaseClauses . - For each
CaseClause C of B, do- Let R be the result of evaluating
CaseClause C. - If R.[[Value]] is not
empty , set V to R.[[Value]]. - If R is an
abrupt completion , returnCompletion (UpdateEmpty (R, V)).
- Let R be the result of evaluating
- Return
NormalCompletion (V).
14.12.3 CaseClauseIsSelected ( C, input )
The abstract operation CaseClauseIsSelected takes arguments C (a
Assert : C is an instance of the productionCaseClause : case Expression : StatementList opt - Let exprRef be the result of evaluating the
Expression of C. - Let clauseSelector be ?
GetValue (exprRef). - Return the result of performing
Strict Equality Comparison input === clauseSelector.
This operation does not execute C's
14.12.4 Runtime Semantics: Evaluation
- Let exprRef be the result of evaluating
Expression . - Let switchValue be ?
GetValue (exprRef). - Let oldEnv be the
running execution context 's LexicalEnvironment. - Let blockEnv be
NewDeclarativeEnvironment (oldEnv). - Perform
BlockDeclarationInstantiation (CaseBlock , blockEnv). - Set the
running execution context 's LexicalEnvironment to blockEnv. - Let R be
CaseBlockEvaluation ofCaseBlock with argument switchValue. - Set the
running execution context 's LexicalEnvironment to oldEnv. - Return R.
No matter how control leaves the
- Return
NormalCompletion (empty ).
- Return the result of evaluating
StatementList .
- Return
NormalCompletion (empty ).
- Return the result of evaluating
StatementList .
14.13 Labelled Statements
Syntax
A break and continue statements. ECMAScript has no goto statement. A
14.13.1 Static Semantics: Early Errors
- It is a Syntax Error if any source text matches this rule.
An alternative definition for this rule is provided in
14.13.2 Static Semantics: IsLabelledFunction ( stmt )
The abstract operation IsLabelledFunction takes argument stmt. It performs the following steps when called:
- If stmt is not a
LabelledStatement , returnfalse . - Let item be the
LabelledItem of stmt. - If item is
LabelledItem : FunctionDeclaration true . - Let subStmt be the
Statement of item. - Return
IsLabelledFunction (subStmt).
14.13.3 Runtime Semantics: Evaluation
- Let newLabelSet be a new empty
List . - Return
LabelledEvaluation of thisLabelledStatement with argument newLabelSet.
14.13.4 Runtime Semantics: LabelledEvaluation
With parameter labelSet.
- Let stmtResult be
LoopEvaluation ofIterationStatement with argument labelSet. - If stmtResult.[[Type]] is
break , then- If stmtResult.[[Target]] is
empty , then- If stmtResult.[[Value]] is
empty , set stmtResult toNormalCompletion (undefined ). - Else, set stmtResult to
NormalCompletion (stmtResult.[[Value]]).
- If stmtResult.[[Value]] is
- If stmtResult.[[Target]] is
- Return
Completion (stmtResult).
- Let stmtResult be the result of evaluating
SwitchStatement . - If stmtResult.[[Type]] is
break , then- If stmtResult.[[Target]] is
empty , then- If stmtResult.[[Value]] is
empty , set stmtResult toNormalCompletion (undefined ). - Else, set stmtResult to
NormalCompletion (stmtResult.[[Value]]).
- If stmtResult.[[Value]] is
- If stmtResult.[[Target]] is
- Return
Completion (stmtResult).
A
- Let label be the
StringValue ofLabelIdentifier . - Append label as an element of labelSet.
- Let stmtResult be
LabelledEvaluation ofLabelledItem with argument labelSet. - If stmtResult.[[Type]] is
break andSameValue (stmtResult.[[Target]], label) istrue , then- Set stmtResult to
NormalCompletion (stmtResult.[[Value]]).
- Set stmtResult to
- Return
Completion (stmtResult).
- Return the result of evaluating
FunctionDeclaration .
- Return the result of evaluating
Statement .
The only two productions of
14.14 The throw Statement
Syntax
14.14.1 Runtime Semantics: Evaluation
- Let exprRef be the result of evaluating
Expression . - Let exprValue be ?
GetValue (exprRef). - Return
ThrowCompletion (exprValue).
14.15 The try Statement
Syntax
The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its
14.15.1 Static Semantics: Early Errors
-
It is a Syntax Error if
BoundNames ofCatchParameter contains any duplicate elements. -
It is a Syntax Error if any element of the
BoundNames ofCatchParameter also occurs in theLexicallyDeclaredNames ofBlock . -
It is a Syntax Error if any element of the
BoundNames ofCatchParameter also occurs in theVarDeclaredNames ofBlock .
An alternative
14.15.2 Runtime Semantics: CatchClauseEvaluation
With parameter thrownValue.
- Let oldEnv be the
running execution context 's LexicalEnvironment. - Let catchEnv be
NewDeclarativeEnvironment (oldEnv). - For each element argName of the
BoundNames ofCatchParameter , do- Perform ! catchEnv.CreateMutableBinding(argName,
false ).
- Perform ! catchEnv.CreateMutableBinding(argName,
- Set the
running execution context 's LexicalEnvironment to catchEnv. - Let status be
BindingInitialization ofCatchParameter with arguments thrownValue and catchEnv. - If status is an
abrupt completion , then- Set the
running execution context 's LexicalEnvironment to oldEnv. - Return
Completion (status).
- Set the
- Let B be the result of evaluating
Block . - Set the
running execution context 's LexicalEnvironment to oldEnv. - Return
Completion (B).
- Return the result of evaluating
Block .
No matter how control leaves the
14.15.3 Runtime Semantics: Evaluation
- Let B be the result of evaluating
Block . - If B.[[Type]] is
throw , let C beCatchClauseEvaluation ofCatch with argument B.[[Value]]. - Else, let C be B.
- Return
Completion (UpdateEmpty (C,undefined )).
- Let B be the result of evaluating
Block . - Let F be the result of evaluating
Finally . - If F.[[Type]] is
normal , set F to B. - Return
Completion (UpdateEmpty (F,undefined )).
- Let B be the result of evaluating
Block . - If B.[[Type]] is
throw , let C beCatchClauseEvaluation ofCatch with argument B.[[Value]]. - Else, let C be B.
- Let F be the result of evaluating
Finally . - If F.[[Type]] is
normal , set F to C. - Return
Completion (UpdateEmpty (F,undefined )).
14.16 The debugger Statement
Syntax
14.16.1 Runtime Semantics: Evaluation
Evaluating a
- If an
implementation-defined debugging facility is available and enabled, then- Perform an
implementation-defined debugging action. - Let result be an
implementation-defined Completion value.
- Perform an
- Else,
- Let result be
NormalCompletion (empty ).
- Let result be
- Return result.
15 ECMAScript Language: Functions and Classes
15.1 Parameter Lists
Syntax
15.1.1 Static Semantics: Early Errors
-
It is a Syntax Error if
BoundNames ofFormalParameters contains any duplicate elements.
-
It is a Syntax Error if
IsSimpleParameterList ofFormalParameterList isfalse andBoundNames ofFormalParameterList contains any duplicate elements.
Multiple occurrences of the same
15.1.2 Static Semantics: ContainsExpression
- Return
false .
- Return
ContainsExpression ofBindingPropertyList .
- Return
false .
- Return
ContainsExpression ofBindingRestElement .
- Return
ContainsExpression ofBindingElementList .
- Let has be
ContainsExpression ofBindingElementList . - If has is
true , returntrue . - Return
ContainsExpression ofBindingRestElement .
- Let has be
ContainsExpression ofBindingPropertyList . - If has is
true , returntrue . - Return
ContainsExpression ofBindingProperty .
- Let has be
ContainsExpression ofBindingElementList . - If has is
true , returntrue . - Return
ContainsExpression ofBindingElisionElement .
- Return
ContainsExpression ofBindingElement .
- Let has be
IsComputedPropertyKey ofPropertyName . - If has is
true , returntrue . - Return
ContainsExpression ofBindingElement .
- Return
true .
- Return
false .
- Return
true .
- Return
false .
- Return
ContainsExpression ofBindingPattern .
- Return
false .
- If
ContainsExpression ofFormalParameterList istrue , returntrue . - Return
ContainsExpression ofFunctionRestParameter .
- If
ContainsExpression ofFormalParameterList istrue , returntrue . - Return
ContainsExpression ofFormalParameter .
- Return
false .
- Let formals be
CoveredFormalsList ofCoverParenthesizedExpressionAndArrowParameterList . - Return
ContainsExpression of formals.
- Return
false .
15.1.3 Static Semantics: IsSimpleParameterList
- Return
false .
- Return
false .
- Return
true .
- Return
false .
- Return
true .
- Return
false .
- Return
false .
- If
IsSimpleParameterList ofFormalParameterList isfalse , returnfalse . - Return
IsSimpleParameterList ofFormalParameter .
- Return
IsSimpleParameterList ofBindingElement .
- Return
true .
- Let formals be
CoveredFormalsList ofCoverParenthesizedExpressionAndArrowParameterList . - Return
IsSimpleParameterList of formals.
- Return
true .
- Let head be
CoveredAsyncArrowHead ofCoverCallExpressionAndAsyncArrowHead . - Return
IsSimpleParameterList of head.
15.1.4 Static Semantics: HasInitializer
- Return
false .
- Return
true .
- Return
false .
- Return
true .
- If
HasInitializer ofFormalParameterList istrue , returntrue . - Return
HasInitializer ofFormalParameter .
15.1.5 Static Semantics: ExpectedArgumentCount
- Return 0.
- Return
ExpectedArgumentCount ofFormalParameterList .
The ExpectedArgumentCount of a
- If
HasInitializer ofFormalParameter istrue , return 0. - Return 1.
- Let count be
ExpectedArgumentCount ofFormalParameterList . - If
HasInitializer ofFormalParameterList istrue orHasInitializer ofFormalParameter istrue , return count. - Return count + 1.
- Return 1.
- Let formals be
CoveredFormalsList ofCoverParenthesizedExpressionAndArrowParameterList . - Return
ExpectedArgumentCount of formals.
- If
HasInitializer ofFormalParameter istrue , return 0. - Return 1.
- Return 1.
15.2 Function Definitions
Syntax
15.2.1 Static Semantics: Early Errors
-
If the source code matching
FormalParameters isstrict mode code , the Early Error rules forUniqueFormalParameters : FormalParameters -
If
BindingIdentifier is present and the source code matchingBindingIdentifier isstrict mode code , it is a Syntax Error if theStringValue ofBindingIdentifier is"eval" or"arguments" . -
It is a Syntax Error if
FunctionBodyContainsUseStrict ofFunctionBody istrue andIsSimpleParameterList ofFormalParameters isfalse . -
It is a Syntax Error if any element of the
BoundNames ofFormalParameters also occurs in theLexicallyDeclaredNames ofFunctionBody . -
It is a Syntax Error if
FormalParameters Contains SuperProperty istrue . -
It is a Syntax Error if
FunctionBody Contains SuperProperty istrue . -
It is a Syntax Error if
FormalParameters Contains SuperCall istrue . -
It is a Syntax Error if
FunctionBody Contains SuperCall istrue .
The
-
It is a Syntax Error if the
LexicallyDeclaredNames ofFunctionStatementList contains any duplicate entries. -
It is a Syntax Error if any element of the
LexicallyDeclaredNames ofFunctionStatementList also occurs in theVarDeclaredNames ofFunctionStatementList . -
It is a Syntax Error if
ContainsDuplicateLabels ofFunctionStatementList with argument « » istrue . -
It is a Syntax Error if
ContainsUndefinedBreakTarget ofFunctionStatementList with argument « » istrue . -
It is a Syntax Error if
ContainsUndefinedContinueTarget ofFunctionStatementList with arguments « » and « » istrue .
15.2.2 Static Semantics: FunctionBodyContainsUseStrict
- If the
Directive Prologue ofFunctionBody contains aUse Strict Directive , returntrue ; otherwise, returnfalse .
15.2.3 Runtime Semantics: EvaluateFunctionBody
With parameters functionObject and argumentsList (a
- Perform ?
FunctionDeclarationInstantiation (functionObject, argumentsList). - Return the result of evaluating
FunctionStatementList .
15.2.4 Runtime Semantics: InstantiateOrdinaryFunctionObject
With parameter scope.
- Let name be
StringValue ofBindingIdentifier . - Let sourceText be the source text matched by
FunctionDeclaration . - Let F be
OrdinaryFunctionCreate (%Function.prototype% , sourceText,FormalParameters ,FunctionBody ,non-lexical-this , scope). - Perform
SetFunctionName (F, name). - Perform
MakeConstructor (F). - Return F.
- Let sourceText be the source text matched by
FunctionDeclaration . - Let F be
OrdinaryFunctionCreate (%Function.prototype% , sourceText,FormalParameters ,FunctionBody ,non-lexical-this , scope). - Perform
SetFunctionName (F,"default" ). - Perform
MakeConstructor (F). - Return F.
An anonymous export default declaration, and its function code is therefore always
15.2.5 Runtime Semantics: InstantiateOrdinaryFunctionExpression
With optional parameter name.
- If name is not present, set name to
"" . - Let scope be the LexicalEnvironment of the
running execution context . - Let sourceText be the source text matched by
FunctionExpression . - Let closure be
OrdinaryFunctionCreate (%Function.prototype% , sourceText,FormalParameters ,FunctionBody ,non-lexical-this , scope). - Perform
SetFunctionName (closure, name). - Perform
MakeConstructor (closure). - Return closure.
Assert : name is not present.- Set name to
StringValue ofBindingIdentifier . - Let scope be the
running execution context 's LexicalEnvironment. - Let funcEnv be
NewDeclarativeEnvironment (scope). - Perform funcEnv.CreateImmutableBinding(name,
false ). - Let sourceText be the source text matched by
FunctionExpression . - Let closure be
OrdinaryFunctionCreate (%Function.prototype% , sourceText,FormalParameters ,FunctionBody ,non-lexical-this , funcEnv). - Perform
SetFunctionName (closure, name). - Perform
MakeConstructor (closure). - Perform funcEnv.InitializeBinding(name, closure).
- Return closure.
The
15.2.6 Runtime Semantics: Evaluation
- Return
NormalCompletion (empty ).
An alternative semantics is provided in
- Return
NormalCompletion (empty ).
A
- Return
NormalCompletion (undefined ).
15.3 Arrow Function Definitions
Syntax
Supplemental Syntax
When processing an instance of the production
the interpretation of
15.3.1 Static Semantics: Early Errors
-
It is a Syntax Error if
ArrowParameters Contains YieldExpression istrue . -
It is a Syntax Error if
ArrowParameters Contains AwaitExpression istrue . -
It is a Syntax Error if
ConciseBodyContainsUseStrict ofConciseBody istrue andIsSimpleParameterList ofArrowParameters isfalse . -
It is a Syntax Error if any element of the
BoundNames ofArrowParameters also occurs in theLexicallyDeclaredNames ofConciseBody .
-
It is a Syntax Error if
CoverParenthesizedExpressionAndArrowParameterList is notcovering anArrowFormalParameters . -
All
early error rules forArrowFormalParameters and its derived productions also apply toCoveredFormalsList ofCoverParenthesizedExpressionAndArrowParameterList .
15.3.2 Static Semantics: ConciseBodyContainsUseStrict
- Return
false .
- Return
FunctionBodyContainsUseStrict ofFunctionBody .
15.3.3 Static Semantics: CoveredFormalsList
- Return this
ArrowParameters .
- Return the
ArrowFormalParameters that iscovered byCoverParenthesizedExpressionAndArrowParameterList .
15.3.4 Runtime Semantics: EvaluateConciseBody
With parameters functionObject and argumentsList (a
- Perform ?
FunctionDeclarationInstantiation (functionObject, argumentsList). - Return the result of evaluating
ExpressionBody .
15.3.5 Runtime Semantics: InstantiateArrowFunctionExpression
With optional parameter name.
- If name is not present, set name to
"" . - Let scope be the LexicalEnvironment of the
running execution context . - Let sourceText be the source text matched by
ArrowFunction . - Let parameters be
CoveredFormalsList ofArrowParameters . - Let closure be
OrdinaryFunctionCreate (%Function.prototype% , sourceText, parameters,ConciseBody ,lexical-this , scope). - Perform
SetFunctionName (closure, name). - Return closure.
An arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an super, the super is always contained within a non-super is accessible via the scope that is captured by the
15.3.6 Runtime Semantics: Evaluation
- Return
InstantiateArrowFunctionExpression ofArrowFunction .
- Let exprRef be the result of evaluating
AssignmentExpression . - Let exprValue be ?
GetValue (exprRef). - Return
Completion { [[Type]]:return , [[Value]]: exprValue, [[Target]]:empty }.
15.4 Method Definitions
Syntax
15.4.1 Static Semantics: Early Errors
-
It is a Syntax Error if
FunctionBodyContainsUseStrict ofFunctionBody istrue andIsSimpleParameterList ofUniqueFormalParameters isfalse . -
It is a Syntax Error if any element of the
BoundNames ofUniqueFormalParameters also occurs in theLexicallyDeclaredNames ofFunctionBody .
-
It is a Syntax Error if
BoundNames ofPropertySetParameterList contains any duplicate elements. -
It is a Syntax Error if
FunctionBodyContainsUseStrict ofFunctionBody istrue andIsSimpleParameterList ofPropertySetParameterList isfalse . -
It is a Syntax Error if any element of the
BoundNames ofPropertySetParameterList also occurs in theLexicallyDeclaredNames ofFunctionBody .
15.4.2 Static Semantics: HasDirectSuper
- If
UniqueFormalParameters Contains SuperCall istrue , returntrue . - Return
FunctionBody Contains SuperCall .
- Return
FunctionBody Contains SuperCall .
- If
PropertySetParameterList Contains SuperCall istrue , returntrue . - Return
FunctionBody Contains SuperCall .
- If
UniqueFormalParameters Contains SuperCall istrue , returntrue . - Return
GeneratorBody Contains SuperCall .
- If
UniqueFormalParameters Contains SuperCall istrue , returntrue . - Return
AsyncGeneratorBody Contains SuperCall .
- If
UniqueFormalParameters Contains SuperCall istrue , returntrue . - Return
AsyncFunctionBody Contains SuperCall .
15.4.3 Static Semantics: SpecialMethod
- Return
false .
- Return
true .
15.4.4 Runtime Semantics: DefineMethod
With parameter object and optional parameter functionPrototype.
- Let propKey be the result of evaluating
PropertyName . ReturnIfAbrupt (propKey).- Let scope be the
running execution context 's LexicalEnvironment. - If functionPrototype is present, then
- Let prototype be functionPrototype.
- Else,
- Let prototype be
%Function.prototype% .
- Let prototype be
- Let sourceText be the source text matched by
MethodDefinition . - Let closure be
OrdinaryFunctionCreate (prototype, sourceText,UniqueFormalParameters ,FunctionBody ,non-lexical-this , scope). - Perform
MakeMethod (closure, object). - Return the
Record { [[Key]]: propKey, [[Closure]]: closure }.
15.4.5 Runtime Semantics: MethodDefinitionEvaluation
With parameters object and enumerable.
- Let methodDef be ?
DefineMethod ofMethodDefinition with argument object. - Perform
SetFunctionName (methodDef.[[Closure]], methodDef.[[Key]]). - Let desc be the PropertyDescriptor { [[Value]]: methodDef.[[Closure]], [[Writable]]:
true , [[Enumerable]]: enumerable, [[Configurable]]:true }. - Return ?
DefinePropertyOrThrow (object, methodDef.[[Key]], desc).
- Let propKey be the result of evaluating
PropertyName . ReturnIfAbrupt (propKey).- Let scope be the
running execution context 's LexicalEnvironment. - Let sourceText be the source text matched by
MethodDefinition . - Let formalParameterList be an instance of the production
FormalParameters : [empty] - Let closure be
OrdinaryFunctionCreate (%Function.prototype% , sourceText, formalParameterList,FunctionBody ,non-lexical-this , scope). - Perform
MakeMethod (closure, object). - Perform
SetFunctionName (closure, propKey,"get" ). - Let desc be the PropertyDescriptor { [[Get]]: closure, [[Enumerable]]: enumerable, [[Configurable]]:
true }. - Return ?
DefinePropertyOrThrow (object, propKey, desc).
- Let propKey be the result of evaluating
PropertyName . ReturnIfAbrupt (propKey).- Let scope be the
running execution context 's LexicalEnvironment. - Let sourceText be the source text matched by
MethodDefinition . - Let closure be
OrdinaryFunctionCreate (%Function.prototype% , sourceText,PropertySetParameterList ,FunctionBody ,non-lexical-this , scope). - Perform
MakeMethod (closure, object). - Perform
SetFunctionName (closure, propKey,"set" ). - Let desc be the PropertyDescriptor { [[Set]]: closure, [[Enumerable]]: enumerable, [[Configurable]]:
true }. - Return ?
DefinePropertyOrThrow (object, propKey, desc).
- Let propKey be the result of evaluating
PropertyName . ReturnIfAbrupt (propKey).- Let scope be the
running execution context 's LexicalEnvironment. - Let sourceText be the source text matched by
GeneratorMethod . - Let closure be
OrdinaryFunctionCreate (%GeneratorFunction.prototype% , sourceText,UniqueFormalParameters ,GeneratorBody ,non-lexical-this , scope). - Perform
MakeMethod (closure, object). - Perform
SetFunctionName (closure, propKey). - Let prototype be !
OrdinaryObjectCreate (%GeneratorFunction.prototype.prototype% ). - Perform
DefinePropertyOrThrow (closure,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]:
true , [[Enumerable]]: enumerable, [[Configurable]]:true }. - Return ?
DefinePropertyOrThrow (object, propKey, desc).
- Let propKey be the result of evaluating
PropertyName . ReturnIfAbrupt (propKey).- Let scope be the
running execution context 's LexicalEnvironment. - Let sourceText be the source text matched by
AsyncGeneratorMethod . - Let closure be !
OrdinaryFunctionCreate (%AsyncGeneratorFunction.prototype% , sourceText,UniqueFormalParameters ,AsyncGeneratorBody ,non-lexical-this , scope). - Perform !
MakeMethod (closure, object). - Perform !
SetFunctionName (closure, propKey). - Let prototype be !
OrdinaryObjectCreate (%AsyncGeneratorFunction.prototype.prototype% ). - Perform !
DefinePropertyOrThrow (closure,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Let desc be PropertyDescriptor { [[Value]]: closure, [[Writable]]:
true , [[Enumerable]]: enumerable, [[Configurable]]:true }. - Return ?
DefinePropertyOrThrow (object, propKey, desc).
- Let propKey be the result of evaluating
PropertyName . ReturnIfAbrupt (propKey).- Let scope be the LexicalEnvironment of the
running execution context . - Let sourceText be the source text matched by
AsyncMethod . - Let closure be !
OrdinaryFunctionCreate (%AsyncFunction.prototype% , sourceText,UniqueFormalParameters ,AsyncFunctionBody ,non-lexical-this , scope). - Perform !
MakeMethod (closure, object). - Perform !
SetFunctionName (closure, propKey). - Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]:
true , [[Enumerable]]: enumerable, [[Configurable]]:true }. - Return ?
DefinePropertyOrThrow (object, propKey, desc).
15.5 Generator Function Definitions
Syntax
The syntactic context immediately following yield requires use of the
15.5.1 Static Semantics: Early Errors
-
It is a Syntax Error if
HasDirectSuper ofGeneratorMethod istrue . -
It is a Syntax Error if
UniqueFormalParameters Contains YieldExpression istrue . -
It is a Syntax Error if
FunctionBodyContainsUseStrict ofGeneratorBody istrue andIsSimpleParameterList ofUniqueFormalParameters isfalse . -
It is a Syntax Error if any element of the
BoundNames ofUniqueFormalParameters also occurs in theLexicallyDeclaredNames ofGeneratorBody .
-
If the source code matching
FormalParameters isstrict mode code , the Early Error rules forUniqueFormalParameters : FormalParameters -
If
BindingIdentifier is present and the source code matchingBindingIdentifier isstrict mode code , it is a Syntax Error if theStringValue ofBindingIdentifier is"eval" or"arguments" . -
It is a Syntax Error if
FunctionBodyContainsUseStrict ofGeneratorBody istrue andIsSimpleParameterList ofFormalParameters isfalse . -
It is a Syntax Error if any element of the
BoundNames ofFormalParameters also occurs in theLexicallyDeclaredNames ofGeneratorBody . -
It is a Syntax Error if
FormalParameters Contains YieldExpression istrue . -
It is a Syntax Error if
FormalParameters Contains SuperProperty istrue . -
It is a Syntax Error if
GeneratorBody Contains SuperProperty istrue . -
It is a Syntax Error if
FormalParameters Contains SuperCall istrue . -
It is a Syntax Error if
GeneratorBody Contains SuperCall istrue .
15.5.2 Runtime Semantics: EvaluateGeneratorBody
With parameters functionObject and argumentsList (a
- Perform ?
FunctionDeclarationInstantiation (functionObject, argumentsList). - Let G be ?
OrdinaryCreateFromConstructor (functionObject,"%GeneratorFunction.prototype.prototype%" , « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] »). - Set G.[[GeneratorBrand]] to
empty . - Perform
GeneratorStart (G,FunctionBody ). - Return
Completion { [[Type]]:return , [[Value]]: G, [[Target]]:empty }.
15.5.3 Runtime Semantics: InstantiateGeneratorFunctionObject
With parameter scope.
- Let name be
StringValue ofBindingIdentifier . - Let sourceText be the source text matched by
GeneratorDeclaration . - Let F be
OrdinaryFunctionCreate (%GeneratorFunction.prototype% , sourceText,FormalParameters ,GeneratorBody ,non-lexical-this , scope). - Perform
SetFunctionName (F, name). - Let prototype be !
OrdinaryObjectCreate (%GeneratorFunction.prototype.prototype% ). - Perform
DefinePropertyOrThrow (F,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Return F.
- Let sourceText be the source text matched by
GeneratorDeclaration . - Let F be
OrdinaryFunctionCreate (%GeneratorFunction.prototype% , sourceText,FormalParameters ,GeneratorBody ,non-lexical-this , scope). - Perform
SetFunctionName (F,"default" ). - Let prototype be !
OrdinaryObjectCreate (%GeneratorFunction.prototype.prototype% ). - Perform
DefinePropertyOrThrow (F,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Return F.
An anonymous export default declaration, and its function code is therefore always
15.5.4 Runtime Semantics: InstantiateGeneratorFunctionExpression
With optional parameter name.
- If name is not present, set name to
"" . - Let scope be the LexicalEnvironment of the
running execution context . - Let sourceText be the source text matched by
GeneratorExpression . - Let closure be
OrdinaryFunctionCreate (%GeneratorFunction.prototype% , sourceText,FormalParameters ,GeneratorBody ,non-lexical-this , scope). - Perform
SetFunctionName (closure, name). - Let prototype be !
OrdinaryObjectCreate (%GeneratorFunction.prototype.prototype% ). - Perform
DefinePropertyOrThrow (closure,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Return closure.
Assert : name is not present.- Set name to
StringValue ofBindingIdentifier . - Let scope be the
running execution context 's LexicalEnvironment. - Let funcEnv be
NewDeclarativeEnvironment (scope). - Perform funcEnv.CreateImmutableBinding(name,
false ). - Let sourceText be the source text matched by
GeneratorExpression . - Let closure be
OrdinaryFunctionCreate (%GeneratorFunction.prototype% , sourceText,FormalParameters ,GeneratorBody ,non-lexical-this , funcEnv). - Perform
SetFunctionName (closure, name). - Let prototype be !
OrdinaryObjectCreate (%GeneratorFunction.prototype.prototype% ). - Perform
DefinePropertyOrThrow (closure,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Perform funcEnv.InitializeBinding(name, closure).
- Return closure.
The
15.5.5 Runtime Semantics: Evaluation
- Return ?
Yield (undefined ).
- Let exprRef be the result of evaluating
AssignmentExpression . - Let value be ?
GetValue (exprRef). - Return ?
Yield (value).
- Let generatorKind be !
GetGeneratorKind (). - Let exprRef be the result of evaluating
AssignmentExpression . - Let value be ?
GetValue (exprRef). - Let iteratorRecord be ?
GetIterator (value, generatorKind). - Let iterator be iteratorRecord.[[Iterator]].
- Let received be
NormalCompletion (undefined ). - Repeat,
- If received.[[Type]] is
normal , then- Let innerResult be ?
Call (iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « received.[[Value]] »). - If generatorKind is
async , set innerResult to ?Await (innerResult). - If
Type (innerResult) is not Object, throw aTypeError exception. - Let done be ?
IteratorComplete (innerResult). - If done is
true , then- Return ?
IteratorValue (innerResult).
- Return ?
- If generatorKind is
async , set received toAsyncGeneratorYield (?IteratorValue (innerResult)). - Else, set received to
GeneratorYield (innerResult).
- Let innerResult be ?
- Else if received.[[Type]] is
throw , then- Let throw be ?
GetMethod (iterator,"throw" ). - If throw is not
undefined , then- Let innerResult be ?
Call (throw, iterator, « received.[[Value]] »). - If generatorKind is
async , set innerResult to ?Await (innerResult). - NOTE: Exceptions from the inner iterator
throwmethod are propagated. Normal completions from an innerthrowmethod are processed similarly to an innernext. - If
Type (innerResult) is not Object, throw aTypeError exception. - Let done be ?
IteratorComplete (innerResult). - If done is
true , then- Return ?
IteratorValue (innerResult).
- Return ?
- If generatorKind is
async , set received toAsyncGeneratorYield (?IteratorValue (innerResult)). - Else, set received to
GeneratorYield (innerResult).
- Let innerResult be ?
- Else,
- NOTE: If iterator does not have a
throwmethod, this throw is going to terminate theyield*loop. But first we need to give iterator a chance to clean up. - Let closeCompletion be
Completion { [[Type]]:normal , [[Value]]:empty , [[Target]]:empty }. - If generatorKind is
async , perform ?AsyncIteratorClose (iteratorRecord, closeCompletion). - Else, perform ?
IteratorClose (iteratorRecord, closeCompletion). - NOTE: The next step throws a
TypeError to indicate that there was ayield*protocol violation: iterator does not have athrowmethod. - Throw a
TypeError exception.
- NOTE: If iterator does not have a
- Let throw be ?
- Else,
Assert : received.[[Type]] isreturn .- Let return be ?
GetMethod (iterator,"return" ). - If return is
undefined , then- If generatorKind is
async , set received.[[Value]] to ?Await (received.[[Value]]). - Return
Completion (received).
- If generatorKind is
- Let innerReturnResult be ?
Call (return, iterator, « received.[[Value]] »). - If generatorKind is
async , set innerReturnResult to ?Await (innerReturnResult). - If
Type (innerReturnResult) is not Object, throw aTypeError exception. - Let done be ?
IteratorComplete (innerReturnResult). - If done is
true , then- Let value be ?
IteratorValue (innerReturnResult). - Return
Completion { [[Type]]:return , [[Value]]: value, [[Target]]:empty }.
- Let value be ?
- If generatorKind is
async , set received toAsyncGeneratorYield (?IteratorValue (innerReturnResult)). - Else, set received to
GeneratorYield (innerReturnResult).
- If received.[[Type]] is
15.6 Async Generator Function Definitions
Syntax
15.6.1 Static Semantics: Early Errors
- It is a Syntax Error if
HasDirectSuper ofAsyncGeneratorMethod istrue . - It is a Syntax Error if
UniqueFormalParameters Contains YieldExpression istrue . - It is a Syntax Error if
UniqueFormalParameters Contains AwaitExpression istrue . - It is a Syntax Error if
FunctionBodyContainsUseStrict ofAsyncGeneratorBody istrue andIsSimpleParameterList ofUniqueFormalParameters isfalse . - It is a Syntax Error if any element of the
BoundNames ofUniqueFormalParameters also occurs in theLexicallyDeclaredNames ofAsyncGeneratorBody .
- If the source code matching
FormalParameters isstrict mode code , the Early Error rules forUniqueFormalParameters : FormalParameters - If
BindingIdentifier is present and the source code matchingBindingIdentifier isstrict mode code , it is a Syntax Error if theStringValue ofBindingIdentifier is"eval" or"arguments" . - It is a Syntax Error if
FunctionBodyContainsUseStrict ofAsyncGeneratorBody istrue andIsSimpleParameterList ofFormalParameters isfalse . - It is a Syntax Error if any element of the
BoundNames ofFormalParameters also occurs in theLexicallyDeclaredNames ofAsyncGeneratorBody . - It is a Syntax Error if
FormalParameters Contains YieldExpression istrue . - It is a Syntax Error if
FormalParameters Contains AwaitExpression istrue . - It is a Syntax Error if
FormalParameters Contains SuperProperty istrue . - It is a Syntax Error if
AsyncGeneratorBody Contains SuperProperty istrue . - It is a Syntax Error if
FormalParameters Contains SuperCall istrue . - It is a Syntax Error if
AsyncGeneratorBody Contains SuperCall istrue .
15.6.2 Runtime Semantics: EvaluateAsyncGeneratorBody
With parameters functionObject and argumentsList (a
- Perform ?
FunctionDeclarationInstantiation (functionObject, argumentsList). - Let generator be ?
OrdinaryCreateFromConstructor (functionObject,"%AsyncGeneratorFunction.prototype.prototype%" , « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] »). - Set generator.[[GeneratorBrand]] to
empty . - Perform !
AsyncGeneratorStart (generator,FunctionBody ). - Return
Completion { [[Type]]:return , [[Value]]: generator, [[Target]]:empty }.
15.6.3 Runtime Semantics: InstantiateAsyncGeneratorFunctionObject
With parameter scope.
- Let name be
StringValue ofBindingIdentifier . - Let sourceText be the source text matched by
AsyncGeneratorDeclaration . - Let F be !
OrdinaryFunctionCreate (%AsyncGeneratorFunction.prototype% , sourceText,FormalParameters ,AsyncGeneratorBody ,non-lexical-this , scope). - Perform !
SetFunctionName (F, name). - Let prototype be !
OrdinaryObjectCreate (%AsyncGeneratorFunction.prototype.prototype% ). - Perform !
DefinePropertyOrThrow (F,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Return F.
- Let sourceText be the source text matched by
AsyncGeneratorDeclaration . - Let F be
OrdinaryFunctionCreate (%AsyncGeneratorFunction.prototype% , sourceText,FormalParameters ,AsyncGeneratorBody ,non-lexical-this , scope). - Perform
SetFunctionName (F,"default" ). - Let prototype be !
OrdinaryObjectCreate (%AsyncGeneratorFunction.prototype.prototype% ). - Perform
DefinePropertyOrThrow (F,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Return F.
An anonymous export default declaration.
15.6.4 Runtime Semantics: InstantiateAsyncGeneratorFunctionExpression
With optional parameter name.
- If name is not present, set name to
"" . - Let scope be the LexicalEnvironment of the
running execution context . - Let sourceText be the source text matched by
AsyncGeneratorExpression . - Let closure be !
OrdinaryFunctionCreate (%AsyncGeneratorFunction.prototype% , sourceText,FormalParameters ,AsyncGeneratorBody ,non-lexical-this , scope). - Perform
SetFunctionName (closure, name). - Let prototype be !
OrdinaryObjectCreate (%AsyncGeneratorFunction.prototype.prototype% ). - Perform !
DefinePropertyOrThrow (closure,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Return closure.
Assert : name is not present.- Set name to
StringValue ofBindingIdentifier . - Let scope be the
running execution context 's LexicalEnvironment. - Let funcEnv be !
NewDeclarativeEnvironment (scope). - Perform ! funcEnv.CreateImmutableBinding(name,
false ). - Let sourceText be the source text matched by
AsyncGeneratorExpression . - Let closure be !
OrdinaryFunctionCreate (%AsyncGeneratorFunction.prototype% , sourceText,FormalParameters ,AsyncGeneratorBody ,non-lexical-this , funcEnv). - Perform !
SetFunctionName (closure, name). - Let prototype be !
OrdinaryObjectCreate (%AsyncGeneratorFunction.prototype.prototype% ). - Perform !
DefinePropertyOrThrow (closure,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Perform ! funcEnv.InitializeBinding(name, closure).
- Return closure.
The
15.6.5 Runtime Semantics: Evaluation
15.7 Class Definitions
Syntax
A class definition is always
15.7.1 Static Semantics: Early Errors
-
It is a Syntax Error if
ClassHeritage is not present and the following algorithm evaluates totrue :- Let constructor be
ConstructorMethod ofClassBody . - If constructor is
empty , returnfalse . - Return
HasDirectSuper of constructor.
- Let constructor be
-
It is a Syntax Error if
PrototypePropertyNameList ofClassElementList contains more than one occurrence of"constructor" .
-
It is a Syntax Error if
PropName ofMethodDefinition is not"constructor" andHasDirectSuper ofMethodDefinition istrue . -
It is a Syntax Error if
PropName ofMethodDefinition is"constructor" andSpecialMethod ofMethodDefinition istrue .
-
It is a Syntax Error if
HasDirectSuper ofMethodDefinition istrue . -
It is a Syntax Error if
PropName ofMethodDefinition is"prototype" .
15.7.2 Static Semantics: ClassElementKind
- If
PropName ofMethodDefinition is"constructor" , returnConstructorMethod . - Return
NonConstructorMethod .
- Return
NonConstructorMethod .
- Return
empty .
15.7.3 Static Semantics: ConstructorMethod
- If
ClassElementKind ofClassElement isConstructorMethod , returnClassElement . - Return
empty .
- Let head be
ConstructorMethod ofClassElementList . - If head is not
empty , return head. - If
ClassElementKind ofClassElement isConstructorMethod , returnClassElement . - Return
empty .
Early Error rules ensure that there is only one method definition named
15.7.4 Static Semantics: IsStatic
- Return
false .
- Return
true .
- Return
false .
15.7.5 Static Semantics: NonConstructorMethodDefinitions
- If
ClassElementKind ofClassElement isNonConstructorMethod , then- Return a
List whose sole element isClassElement .
- Return a
- Return a new empty
List .
- Let list be
NonConstructorMethodDefinitions ofClassElementList . - If
ClassElementKind ofClassElement isNonConstructorMethod , then- Append
ClassElement to the end of list.
- Append
- Return list.
15.7.6 Static Semantics: PrototypePropertyNameList
- If
PropName ofClassElement isempty , return a new emptyList . - If
IsStatic ofClassElement istrue , return a new emptyList . - Return a
List whose sole element isPropName ofClassElement .
- Let list be
PrototypePropertyNameList ofClassElementList . - If
PropName ofClassElement isempty , return list. - If
IsStatic ofClassElement istrue , return list. - Append
PropName ofClassElement to the end of list. - Return list.
15.7.7 Runtime Semantics: ClassDefinitionEvaluation
With parameters classBinding and className.
- Let env be the LexicalEnvironment of the
running execution context . - Let classScope be
NewDeclarativeEnvironment (env). - If classBinding is not
undefined , then- Perform classScope.CreateImmutableBinding(classBinding,
true ).
- Perform classScope.CreateImmutableBinding(classBinding,
- If
ClassHeritage is not present, thenopt - Let protoParent be
%Object.prototype% . - Let constructorParent be
%Function.prototype% .
- Let protoParent be
- Else,
- Set the
running execution context 's LexicalEnvironment to classScope. - Let superclassRef be the result of evaluating
ClassHeritage . - Set the
running execution context 's LexicalEnvironment to env. - Let superclass be ?
GetValue (superclassRef). - If superclass is
null , then- Let protoParent be
null . - Let constructorParent be
%Function.prototype% .
- Let protoParent be
- Else if
IsConstructor (superclass) isfalse , throw aTypeError exception. - Else,
- Set the
- Let proto be !
OrdinaryObjectCreate (protoParent). - If
ClassBody is not present, let constructor beopt empty . - Else, let constructor be
ConstructorMethod ofClassBody . - Set the
running execution context 's LexicalEnvironment to classScope. - If constructor is
empty , then- Let steps be the algorithm steps defined in
Default Constructor Functions . - Let F be !
CreateBuiltinFunction (steps, 0, className, « [[ConstructorKind]], [[SourceText]] »,empty , constructorParent).
- Let steps be the algorithm steps defined in
- Else,
- Let constructorInfo be !
DefineMethod of constructor with arguments proto and constructorParent. - Let F be constructorInfo.[[Closure]].
- Perform !
MakeClassConstructor (F). - Perform !
SetFunctionName (F, className).
- Let constructorInfo be !
- Perform !
MakeConstructor (F,false , proto). - If
ClassHeritage is present, set F.[[ConstructorKind]] toopt derived . - Perform !
CreateMethodProperty (proto,"constructor" , F). - If
ClassBody is not present, let methods be a new emptyopt List . - Else, let methods be
NonConstructorMethodDefinitions ofClassBody . - For each
ClassElement m of methods, do- If
IsStatic of m isfalse , then- Let status be
PropertyDefinitionEvaluation of m with arguments proto andfalse .
- Let status be
- Else,
- Let status be
PropertyDefinitionEvaluation of m with arguments F andfalse .
- Let status be
- If status is an
abrupt completion , then- Set the
running execution context 's LexicalEnvironment to env. - Return
Completion (status).
- Set the
- If
- Set the
running execution context 's LexicalEnvironment to env. - If classBinding is not
undefined , then- Perform classScope.InitializeBinding(classBinding, F).
- Return F.
15.7.7.1 Default Constructor Functions
When a Default
- If NewTarget is
undefined , throw aTypeError exception. - Let F be the
active function object . - If F.[[ConstructorKind]] is
derived , then- NOTE: This branch behaves similarly to
constructor(...args) { super(...args); }. The most notable distinction is that while the aforementioned ECMAScript source text observably calls the@@iterator method on%Array.prototype%, a DefaultConstructor Function does not. - Let func be ! F.[[GetPrototypeOf]]().
- If
IsConstructor (func) isfalse , throw aTypeError exception. - Return ?
Construct (func, args, NewTarget).
- NOTE: This branch behaves similarly to
- Else,
- NOTE: This branch behaves similarly to
constructor() {}. - Return ?
OrdinaryCreateFromConstructor (NewTarget,"%Object.prototype%" ).
- NOTE: This branch behaves similarly to
The
15.7.8 Runtime Semantics: BindingClassDeclarationEvaluation
- Let className be
StringValue ofBindingIdentifier . - Let value be ?
ClassDefinitionEvaluation ofClassTail with arguments className and className. - Set value.[[SourceText]] to the source text matched by
ClassDeclaration . - Let env be the
running execution context 's LexicalEnvironment. - Perform ?
InitializeBoundName (className, value, env). - Return value.
- Let value be ?
ClassDefinitionEvaluation ofClassTail with argumentsundefined and"default" . - Set value.[[SourceText]] to the source text matched by
ClassDeclaration . - Return value.
15.7.9 Runtime Semantics: Evaluation
- Perform ?
BindingClassDeclarationEvaluation of thisClassDeclaration . - Return
NormalCompletion (empty ).
- Let value be ?
ClassDefinitionEvaluation ofClassTail with argumentsundefined and"" . - Set value.[[SourceText]] to the source text matched by
ClassExpression . - Return value.
- Let className be
StringValue ofBindingIdentifier . - Let value be ?
ClassDefinitionEvaluation ofClassTail with arguments className and className. - Set value.[[SourceText]] to the source text matched by
ClassExpression . - Return value.
15.8 Async Function Definitions
Syntax
await is parsed as an
- In an
AsyncFunctionBody . - In the
FormalParameters of anAsyncFunctionDeclaration ,AsyncFunctionExpression ,AsyncGeneratorDeclaration , orAsyncGeneratorExpression .AwaitExpression in this position is a Syntax error viastatic semantics .
When await is parsed as a await may be parsed as an identifier when the [Await] parameter is absent. This includes the following contexts:
- Anywhere outside of an
AsyncFunctionBody orFormalParameters of anAsyncFunctionDeclaration ,AsyncFunctionExpression ,AsyncGeneratorDeclaration , orAsyncGeneratorExpression . - In the
BindingIdentifier of aFunctionExpression ,GeneratorExpression , orAsyncGeneratorExpression .
Unlike
15.8.1 Static Semantics: Early Errors
- It is a Syntax Error if
FunctionBodyContainsUseStrict ofAsyncFunctionBody istrue andIsSimpleParameterList ofUniqueFormalParameters isfalse . - It is a Syntax Error if
HasDirectSuper ofAsyncMethod istrue . - It is a Syntax Error if
UniqueFormalParameters Contains AwaitExpression istrue . - It is a Syntax Error if any element of the
BoundNames ofUniqueFormalParameters also occurs in theLexicallyDeclaredNames ofAsyncFunctionBody .
- It is a Syntax Error if
FunctionBodyContainsUseStrict ofAsyncFunctionBody istrue andIsSimpleParameterList ofFormalParameters isfalse . - It is a Syntax Error if
FormalParameters Contains AwaitExpression istrue . - If the source code matching
FormalParameters isstrict mode code , the Early Error rules forUniqueFormalParameters : FormalParameters - If
BindingIdentifier is present and the source code matchingBindingIdentifier isstrict mode code , it is a Syntax Error if theStringValue ofBindingIdentifier is"eval" or"arguments" . - It is a Syntax Error if any element of the
BoundNames ofFormalParameters also occurs in theLexicallyDeclaredNames ofAsyncFunctionBody . - It is a Syntax Error if
FormalParameters Contains SuperProperty istrue . - It is a Syntax Error if
AsyncFunctionBody Contains SuperProperty istrue . - It is a Syntax Error if
FormalParameters Contains SuperCall istrue . - It is a Syntax Error if
AsyncFunctionBody Contains SuperCall istrue .
15.8.2 Runtime Semantics: InstantiateAsyncFunctionObject
With parameter scope.
- Let name be
StringValue ofBindingIdentifier . - Let sourceText be the source text matched by
AsyncFunctionDeclaration . - Let F be !
OrdinaryFunctionCreate (%AsyncFunction.prototype% , sourceText,FormalParameters ,AsyncFunctionBody ,non-lexical-this , scope). - Perform !
SetFunctionName (F, name). - Return F.
- Let sourceText be the source text matched by
AsyncFunctionDeclaration . - Let F be !
OrdinaryFunctionCreate (%AsyncFunction.prototype% , sourceText,FormalParameters ,AsyncFunctionBody ,non-lexical-this , scope). - Perform !
SetFunctionName (F,"default" ). - Return F.
15.8.3 Runtime Semantics: InstantiateAsyncFunctionExpression
With optional parameter name.
- If name is not present, set name to
"" . - Let scope be the LexicalEnvironment of the
running execution context . - Let sourceText be the source text matched by
AsyncFunctionExpression . - Let closure be !
OrdinaryFunctionCreate (%AsyncFunction.prototype% , sourceText,FormalParameters ,AsyncFunctionBody ,non-lexical-this , scope). - Perform
SetFunctionName (closure, name). - Return closure.
Assert : name is not present.- Set name to
StringValue ofBindingIdentifier . - Let scope be the LexicalEnvironment of the
running execution context . - Let funcEnv be !
NewDeclarativeEnvironment (scope). - Perform ! funcEnv.CreateImmutableBinding(name,
false ). - Let sourceText be the source text matched by
AsyncFunctionExpression . - Let closure be !
OrdinaryFunctionCreate (%AsyncFunction.prototype% , sourceText,FormalParameters ,AsyncFunctionBody ,non-lexical-this , funcEnv). - Perform !
SetFunctionName (closure, name). - Perform ! funcEnv.InitializeBinding(name, closure).
- Return closure.
The
15.8.4 Runtime Semantics: EvaluateAsyncFunctionBody
With parameters functionObject and argumentsList (a
- Let promiseCapability be !
NewPromiseCapability (%Promise% ). - Let declResult be
FunctionDeclarationInstantiation (functionObject, argumentsList). - If declResult is not an
abrupt completion , then- Perform !
AsyncFunctionStart (promiseCapability,FunctionBody ).
- Perform !
- Else,
- Perform !
Call (promiseCapability.[[Reject]],undefined , « declResult.[[Value]] »).
- Perform !
- Return
Completion { [[Type]]:return , [[Value]]: promiseCapability.[[Promise]], [[Target]]:empty }.
15.8.5 Runtime Semantics: Evaluation
- Return
NormalCompletion (empty ).
- Return
NormalCompletion (empty ).
- Let exprRef be the result of evaluating
UnaryExpression . - Let value be ?
GetValue (exprRef). - Return ?
Await (value).
15.9 Async Arrow Function Definitions
Syntax
Supplemental Syntax
When processing an instance of the production
the interpretation of
15.9.1 Static Semantics: Early Errors
- It is a Syntax Error if any element of the
BoundNames ofAsyncArrowBindingIdentifier also occurs in theLexicallyDeclaredNames ofAsyncConciseBody .
- It is a Syntax Error if
CoverCallExpressionAndAsyncArrowHead Contains YieldExpression istrue . - It is a Syntax Error if
CoverCallExpressionAndAsyncArrowHead Contains AwaitExpression istrue . - It is a Syntax Error if
CoverCallExpressionAndAsyncArrowHead is notcovering anAsyncArrowHead . - It is a Syntax Error if any element of the
BoundNames ofCoverCallExpressionAndAsyncArrowHead also occurs in theLexicallyDeclaredNames ofAsyncConciseBody . - It is a Syntax Error if
AsyncConciseBodyContainsUseStrict ofAsyncConciseBody istrue andIsSimpleParameterList ofCoverCallExpressionAndAsyncArrowHead isfalse . - All Early Error rules for
AsyncArrowHead and its derived productions apply toCoveredAsyncArrowHead ofCoverCallExpressionAndAsyncArrowHead .
15.9.2 Static Semantics: CoveredAsyncArrowHead
- Return the
AsyncArrowHead that iscovered byCoverCallExpressionAndAsyncArrowHead .
15.9.3 Static Semantics: AsyncConciseBodyContainsUseStrict
- Return
false .
- Return
FunctionBodyContainsUseStrict ofAsyncFunctionBody .
15.9.4 Runtime Semantics: EvaluateAsyncConciseBody
With parameters functionObject and argumentsList (a
- Let promiseCapability be !
NewPromiseCapability (%Promise% ). - Let declResult be
FunctionDeclarationInstantiation (functionObject, argumentsList). - If declResult is not an
abrupt completion , then- Perform !
AsyncFunctionStart (promiseCapability,ExpressionBody ).
- Perform !
- Else,
- Perform !
Call (promiseCapability.[[Reject]],undefined , « declResult.[[Value]] »).
- Perform !
- Return
Completion { [[Type]]:return , [[Value]]: promiseCapability.[[Promise]], [[Target]]:empty }.
15.9.5 Runtime Semantics: InstantiateAsyncArrowFunctionExpression
With optional parameter name.
- If name is not present, set name to
"" . - Let scope be the LexicalEnvironment of the
running execution context . - Let sourceText be the source text matched by
AsyncArrowFunction . - Let parameters be
AsyncArrowBindingIdentifier . - Let closure be !
OrdinaryFunctionCreate (%AsyncFunction.prototype% , sourceText, parameters,AsyncConciseBody ,lexical-this , scope). - Perform
SetFunctionName (closure, name). - Return closure.
- If name is not present, set name to
"" . - Let scope be the LexicalEnvironment of the
running execution context . - Let sourceText be the source text matched by
AsyncArrowFunction . - Let head be
CoveredAsyncArrowHead ofCoverCallExpressionAndAsyncArrowHead . - Let parameters be the
ArrowFormalParameters of head. - Let closure be !
OrdinaryFunctionCreate (%AsyncFunction.prototype% , sourceText, parameters,AsyncConciseBody ,lexical-this , scope). - Perform
SetFunctionName (closure, name). - Return closure.
15.9.6 Runtime Semantics: Evaluation
15.10 Tail Position Calls
15.10.1 Static Semantics: IsInTailPosition ( call )
The abstract operation IsInTailPosition takes argument call. It performs the following steps when called:
Assert : call is aParse Node .- If the source code matching call is
non-strict code , returnfalse . - If call is not contained within a
FunctionBody ,ConciseBody , orAsyncConciseBody , returnfalse . - Let body be the
FunctionBody ,ConciseBody , orAsyncConciseBody that most closely contains call. - If body is the
FunctionBody of aGeneratorBody , returnfalse . - If body is the
FunctionBody of anAsyncFunctionBody , returnfalse . - If body is the
FunctionBody of anAsyncGeneratorBody , returnfalse . - If body is an
AsyncConciseBody , returnfalse . - Return the result of
HasCallInTailPosition of body with argument call.
Tail Position calls are only defined in
15.10.2 Static Semantics: HasCallInTailPosition
With parameter call.
call is a
15.10.2.1 Statement Rules
- Let has be
HasCallInTailPosition ofStatementList with argument call. - If has is
true , returntrue . - Return
HasCallInTailPosition ofStatementListItem with argument call.
- Return
false .
- Let has be
HasCallInTailPosition of the firstStatement with argument call. - If has is
true , returntrue . - Return
HasCallInTailPosition of the secondStatement with argument call.
- Return
HasCallInTailPosition ofStatement with argument call.
- Return
HasCallInTailPosition ofLabelledItem with argument call.
- Return
HasCallInTailPosition ofExpression with argument call.
- Return
HasCallInTailPosition ofCaseBlock with argument call.
- Let has be
false . - If the first
CaseClauses is present, let has beHasCallInTailPosition of the firstCaseClauses with argument call. - If has is
true , returntrue . - Let has be
HasCallInTailPosition ofDefaultClause with argument call. - If has is
true , returntrue . - If the second
CaseClauses is present, let has beHasCallInTailPosition of the secondCaseClauses with argument call. - Return has.
- Let has be
HasCallInTailPosition ofCaseClauses with argument call. - If has is
true , returntrue . - Return
HasCallInTailPosition ofCaseClause with argument call.
- If
StatementList is present, returnHasCallInTailPosition ofStatementList with argument call. - Return
false .
- Return
HasCallInTailPosition ofCatch with argument call.
- Return
HasCallInTailPosition ofFinally with argument call.
- Return
HasCallInTailPosition ofBlock with argument call.
15.10.2.2 Expression Rules
A potential tail position call that is immediately followed by return
- Return
false .
- Return
HasCallInTailPosition ofAssignmentExpression with argument call.
- Let has be
HasCallInTailPosition of the firstAssignmentExpression with argument call. - If has is
true , returntrue . - Return
HasCallInTailPosition of the secondAssignmentExpression with argument call.
- Return
HasCallInTailPosition ofBitwiseORExpression with argument call.
- Return
HasCallInTailPosition ofLogicalANDExpression with argument call.
- Return
HasCallInTailPosition ofBitwiseORExpression with argument call.
- If this
CallExpression is call, returntrue . - Return
false .
- Return
HasCallInTailPosition ofOptionalChain with argument call.
- Return
false .
- If this
OptionalChain is call, returntrue . - Return
false .
- If this
MemberExpression is call, returntrue . - Return
false .
- Let expr be
CoveredParenthesizedExpression ofCoverParenthesizedExpressionAndArrowParameterList . - Return
HasCallInTailPosition of expr with argument call.
- Return
HasCallInTailPosition ofExpression with argument call.
15.10.3 PrepareForTailCall ( )
The abstract operation PrepareForTailCall takes no arguments. It performs the following steps when called:
- Let leafContext be the
running execution context . - Suspend leafContext.
- Pop leafContext from the
execution context stack . Theexecution context now on the top of the stack becomes therunning execution context . Assert : leafContext has no further use. It will never be activated as therunning execution context .
A tail position call must either release any transient internal resources associated with the currently executing function
For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.
16 ECMAScript Language: Scripts and Modules
16.1 Scripts
Syntax
16.1.1 Static Semantics: Early Errors
-
It is a Syntax Error if the
LexicallyDeclaredNames ofScriptBody contains any duplicate entries. -
It is a Syntax Error if any element of the
LexicallyDeclaredNames ofScriptBody also occurs in theVarDeclaredNames ofScriptBody .
-
It is a Syntax Error if
StatementList Contains superunless the source code containingsuperis eval code that is being processed by adirect eval . Additionalearly error rules forsuperwithindirect eval are defined in19.2.1.1 . -
It is a Syntax Error if
StatementList Contains NewTarget unless the source code containingNewTarget is eval code that is being processed by adirect eval . Additionalearly error rules forNewTarget indirect eval are defined in19.2.1.1 . -
It is a Syntax Error if
ContainsDuplicateLabels ofStatementList with argument « » istrue . -
It is a Syntax Error if
ContainsUndefinedBreakTarget ofStatementList with argument « » istrue . -
It is a Syntax Error if
ContainsUndefinedContinueTarget ofStatementList with arguments « » and « » istrue .
16.1.2 Static Semantics: IsStrict
- If
ScriptBody is present and theDirective Prologue ofScriptBody contains aUse Strict Directive , returntrue ; otherwise, returnfalse .
16.1.3 Runtime Semantics: Evaluation
- Return
NormalCompletion (undefined ).
16.1.4 Script Records
A Script Record encapsulates information about a script being evaluated. Each script record contains the fields listed in
| Field Name | Value Type | Meaning |
|---|---|---|
| [[Realm]] |
|
The |
| [[Environment]] |
|
The |
| [[ECMAScriptCode]] |
a |
The result of parsing the source text of this script using |
| [[HostDefined]] |
Any, default value is |
Field reserved for use by |
16.1.5 ParseScript ( sourceText, realm, hostDefined )
The abstract operation ParseScript takes arguments sourceText, realm, and hostDefined. It creates a
An implementation may parse script source text and analyse it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.
16.1.6 ScriptEvaluation ( scriptRecord )
The abstract operation ScriptEvaluation takes argument scriptRecord. It performs the following steps when called:
- Let globalEnv be scriptRecord.[[Realm]].[[GlobalEnv]].
- Let scriptContext be a new ECMAScript code
execution context . - Set the Function of scriptContext to
null . - Set the
Realm of scriptContext to scriptRecord.[[Realm]]. - Set the ScriptOrModule of scriptContext to scriptRecord.
- Set the VariableEnvironment of scriptContext to globalEnv.
- Set the LexicalEnvironment of scriptContext to globalEnv.
- Suspend the currently
running execution context . - Push scriptContext onto the
execution context stack ; scriptContext is now therunning execution context . - Let scriptBody be scriptRecord.[[ECMAScriptCode]].
- Let result be
GlobalDeclarationInstantiation (scriptBody, globalEnv). - If result.[[Type]] is
normal , then- Set result to the result of evaluating scriptBody.
- If result.[[Type]] is
normal and result.[[Value]] isempty , then- Set result to
NormalCompletion (undefined ).
- Set result to
- Suspend scriptContext and remove it from the
execution context stack . Assert : Theexecution context stack is not empty.- Resume the context that is now on the top of the
execution context stack as therunning execution context . - Return
Completion (result).
16.1.7 GlobalDeclarationInstantiation ( script, env )
When an
The abstract operation GlobalDeclarationInstantiation takes arguments script (a
Assert : env is aglobal Environment Record .- Let lexNames be the
LexicallyDeclaredNames of script. - Let varNames be the
VarDeclaredNames of script. - For each element name of lexNames, do
- If env.HasVarDeclaration(name) is
true , throw aSyntaxError exception. - If env.HasLexicalDeclaration(name) is
true , throw aSyntaxError exception. - Let hasRestrictedGlobal be ? env.HasRestrictedGlobalProperty(name).
- If hasRestrictedGlobal is
true , throw aSyntaxError exception.
- If env.HasVarDeclaration(name) is
- For each element name of varNames, do
- If env.HasLexicalDeclaration(name) is
true , throw aSyntaxError exception.
- If env.HasLexicalDeclaration(name) is
- Let varDeclarations be the
VarScopedDeclarations of script. - Let functionsToInitialize be a new empty
List . - Let declaredFunctionNames be a new empty
List . - For each element d of varDeclarations, in reverse
List order, do- If d is neither a
VariableDeclaration nor aForBinding nor aBindingIdentifier , thenAssert : d is either aFunctionDeclaration , aGeneratorDeclaration , anAsyncFunctionDeclaration , or anAsyncGeneratorDeclaration .- NOTE: If there are multiple function declarations for the same name, the last declaration is used.
- Let fn be the sole element of the
BoundNames of d. - If fn is not an element of declaredFunctionNames, then
- Let fnDefinable be ? env.CanDeclareGlobalFunction(fn).
- If fnDefinable is
false , throw aTypeError exception. - Append fn to declaredFunctionNames.
- Insert d as the first element of functionsToInitialize.
- If d is neither a
- Let declaredVarNames be a new empty
List . - For each element d of varDeclarations, do
- If d is a
VariableDeclaration , aForBinding , or aBindingIdentifier , then- For each String vn of the
BoundNames of d, do- If vn is not an element of declaredFunctionNames, then
- Let vnDefinable be ? env.CanDeclareGlobalVar(vn).
- If vnDefinable is
false , throw aTypeError exception. - If vn is not an element of declaredVarNames, then
- Append vn to declaredVarNames.
- If vn is not an element of declaredFunctionNames, then
- For each String vn of the
- If d is a
- NOTE: No abnormal terminations occur after this algorithm step if the
global object is anordinary object . However, if theglobal object is aProxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps. - NOTE: Annex
B.3.3.2 adds additional steps at this point. - Let lexDeclarations be the
LexicallyScopedDeclarations of script. - For each element d of lexDeclarations, do
- NOTE: Lexically declared names are only instantiated here but not initialized.
- For each element dn of the
BoundNames of d, do- If
IsConstantDeclaration of d istrue , then- Perform ? env.CreateImmutableBinding(dn,
true ).
- Perform ? env.CreateImmutableBinding(dn,
- Else,
- Perform ? env.CreateMutableBinding(dn,
false ).
- Perform ? env.CreateMutableBinding(dn,
- If
- For each
Parse Node f of functionsToInitialize, do- Let fn be the sole element of the
BoundNames of f. - Let fo be
InstantiateFunctionObject of f with argument env. - Perform ? env.CreateGlobalFunctionBinding(fn, fo,
false ).
- Let fn be the sole element of the
- For each String vn of declaredVarNames, do
- Perform ? env.CreateGlobalVarBinding(vn,
false ).
- Perform ? env.CreateGlobalVarBinding(vn,
- Return
NormalCompletion (empty ).
Early errors specified in
Unlike explicit var or function declarations, properties that are directly created on the
16.2 Modules
Syntax
16.2.1 Module Semantics
16.2.1.1 Static Semantics: Early Errors
-
It is a Syntax Error if the
LexicallyDeclaredNames ofModuleItemList contains any duplicate entries. -
It is a Syntax Error if any element of the
LexicallyDeclaredNames ofModuleItemList also occurs in theVarDeclaredNames ofModuleItemList . -
It is a Syntax Error if the
ExportedNames ofModuleItemList contains any duplicate entries. -
It is a Syntax Error if any element of the
ExportedBindings ofModuleItemList does not also occur in either theVarDeclaredNames ofModuleItemList , or theLexicallyDeclaredNames ofModuleItemList . -
It is a Syntax Error if
ModuleItemList Contains super. -
It is a Syntax Error if
ModuleItemList Contains NewTarget . -
It is a Syntax Error if
ContainsDuplicateLabels ofModuleItemList with argument « » istrue . -
It is a Syntax Error if
ContainsUndefinedBreakTarget ofModuleItemList with argument « » istrue . -
It is a Syntax Error if
ContainsUndefinedContinueTarget ofModuleItemList with arguments « » and « » istrue .
The duplicate export default
16.2.1.2 Static Semantics: ImportedLocalNames ( importEntries )
The abstract operation ImportedLocalNames takes argument importEntries (a
- Let localNames be a new empty
List . - For each
ImportEntry Record i of importEntries, do- Append i.[[LocalName]] to localNames.
- Return localNames.
16.2.1.3 Static Semantics: ModuleRequests
- Return a new empty
List .
- Return
ModuleRequests ofModuleItem .
- Let moduleNames be
ModuleRequests ofModuleItemList . - Let additionalNames be
ModuleRequests ofModuleItem . - Append to moduleNames each element of additionalNames that is not already an element of moduleNames.
- Return moduleNames.
- Return a new empty
List .
- Return
ModuleRequests ofFromClause .
- Return a
List whose sole element is theSV ofStringLiteral .
- Return the
ModuleRequests ofFromClause .
- Return a new empty
List .
16.2.1.4 Abstract Module Records
A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.
For specification purposes Module Record values are values of the
Module Record defines the fields listed in
| Field Name | Value Type | Meaning |
|---|---|---|
| [[Realm]] |
|
The |
| [[Environment]] |
|
The |
| [[Namespace]] |
Object | |
The Module Namespace Object ( |
| [[HostDefined]] |
Any, default value is |
Field reserved for use by |
| Method | Purpose |
|---|---|
| GetExportedNames([exportStarSet]) | Return a list of all names that are either directly or indirectly exported from this module. |
| ResolveExport(exportName [, resolveSet]) |
Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding Record, of the form { [[Module]]: Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result if it completes normally. |
| Link() |
Prepare the module for evaluation by transitively resolving all module dependencies and creating a |
| Evaluate() |
If this module has already been evaluated successfully, return Link must have completed successfully prior to invoking this method. |
16.2.1.5 Cyclic Module Records
A Cyclic Module Record is used to represent information about a module that can participate in dependency cycles with other modules that are subclasses of the
In addition to the fields defined in
| Field Name | Value Type | Meaning |
|---|---|---|
| [[Status]] |
|
Initially |
| [[EvaluationError]] |
An |
A completion of type |
| [[DFSIndex]] |
|
Auxiliary field used during Link and Evaluate only.
If [[Status]] is |
| [[DFSAncestorIndex]] |
|
Auxiliary field used during Link and Evaluate only. If [[Status]] is |
| [[RequestedModules]] |
|
A |
In addition to the methods defined in
| Method | Purpose |
|---|---|
|
|
Initialize the |
|
|
Evaluate the module's code within its |
16.2.1.5.1 Link ( ) Concrete Method
The Link concrete method of a
Assert : module.[[Status]] is notlinking orevaluating .- Let stack be a new empty
List . - Let result be
InnerModuleLinking (module, stack, 0). - If result is an
abrupt completion , then- For each
Cyclic Module Record m of stack, doAssert : m.[[Status]] islinking .- Set m.[[Status]] to
unlinked . - Set m.[[Environment]] to
undefined . - Set m.[[DFSIndex]] to
undefined . - Set m.[[DFSAncestorIndex]] to
undefined .
Assert : module.[[Status]] isunlinked .- Return result.
- For each
Assert : module.[[Status]] islinked orevaluated .Assert : stack is empty.- Return
undefined .
16.2.1.5.1.1 InnerModuleLinking ( module, stack, index )
The abstract operation InnerModuleLinking takes arguments module (a
- If module is not a
Cyclic Module Record , then- Perform ? module.Link().
- Return index.
- If module.[[Status]] is
linking ,linked , orevaluated , then- Return index.
Assert : module.[[Status]] isunlinked .- Set module.[[Status]] to
linking . - Set module.[[DFSIndex]] to index.
- Set module.[[DFSAncestorIndex]] to index.
- Set index to index + 1.
- Append module to stack.
- For each String required of module.[[RequestedModules]], do
- Let requiredModule be ?
HostResolveImportedModule (module, required). - Set index to ?
InnerModuleLinking (requiredModule, stack, index). - If requiredModule is a
Cyclic Module Record , thenAssert : requiredModule.[[Status]] is eitherlinking ,linked , orevaluated .Assert : requiredModule.[[Status]] islinking if and only if requiredModule is in stack.- If requiredModule.[[Status]] is
linking , then- Set module.[[DFSAncestorIndex]] to
min (module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
- Set module.[[DFSAncestorIndex]] to
- Let requiredModule be ?
- Perform ? module.
InitializeEnvironment (). Assert : module occurs exactly once in stack.Assert : module.[[DFSAncestorIndex]] ≤ module.[[DFSIndex]].- If module.[[DFSAncestorIndex]] = module.[[DFSIndex]], then
- Let done be
false . - Repeat, while done is
false ,- Let requiredModule be the last element in stack.
- Remove the last element of stack.
Assert : requiredModule is aCyclic Module Record .- Set requiredModule.[[Status]] to
linked . - If requiredModule and module are the same
Module Record , set done totrue .
- Let done be
- Return index.
16.2.1.5.2 Evaluate ( ) Concrete Method
The Evaluate concrete method of a
Assert : This call to Evaluate is not happening at the same time as another call to Evaluate within thesurrounding agent .Assert : module.[[Status]] islinked orevaluated .- Let stack be a new empty
List . - Let result be
InnerModuleEvaluation (module, stack, 0). - If result is an
abrupt completion , then- For each
Cyclic Module Record m of stack, doAssert : m.[[Status]] isevaluating .- Set m.[[Status]] to
evaluated . - Set m.[[EvaluationError]] to result.
Assert : module.[[Status]] isevaluated and module.[[EvaluationError]] is result.- Return result.
- For each
Assert : module.[[Status]] isevaluated and module.[[EvaluationError]] isundefined .Assert : stack is empty.- Return
undefined .
16.2.1.5.2.1 InnerModuleEvaluation ( module, stack, index )
The abstract operation InnerModuleEvaluation takes arguments module (a
- If module is not a
Cyclic Module Record , then- Perform ? module.Evaluate().
- Return index.
- If module.[[Status]] is
evaluated , then- If module.[[EvaluationError]] is
undefined , return index. - Otherwise, return module.[[EvaluationError]].
- If module.[[EvaluationError]] is
- If module.[[Status]] is
evaluating , return index. Assert : module.[[Status]] islinked .- Set module.[[Status]] to
evaluating . - Set module.[[DFSIndex]] to index.
- Set module.[[DFSAncestorIndex]] to index.
- Set index to index + 1.
- Append module to stack.
- For each String required of module.[[RequestedModules]], do
- Let requiredModule be !
HostResolveImportedModule (module, required). - NOTE: Link must be completed successfully prior to invoking this method, so every requested module is guaranteed to resolve successfully.
- Set index to ?
InnerModuleEvaluation (requiredModule, stack, index). - If requiredModule is a
Cyclic Module Record , thenAssert : requiredModule.[[Status]] is eitherevaluating orevaluated .Assert : requiredModule.[[Status]] isevaluating if and only if requiredModule is in stack.- If requiredModule.[[Status]] is
evaluating , then- Set module.[[DFSAncestorIndex]] to
min (module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
- Set module.[[DFSAncestorIndex]] to
- Let requiredModule be !
- Perform ? module.
ExecuteModule (). Assert : module occurs exactly once in stack.Assert : module.[[DFSAncestorIndex]] ≤ module.[[DFSIndex]].- If module.[[DFSAncestorIndex]] = module.[[DFSIndex]], then
- Let done be
false . - Repeat, while done is
false ,- Let requiredModule be the last element in stack.
- Remove the last element of stack.
Assert : requiredModule is aCyclic Module Record .- Set requiredModule.[[Status]] to
evaluated . - If requiredModule and module are the same
Module Record , set done totrue .
- Let done be
- Return index.
16.2.1.5.3 Example Cyclic Module Record Graphs
This non-normative section gives a series of examples of the linking and evaluation of a few common module graphs, with a specific focus on how errors can occur.
First consider the following simple module graph:
Let's first assume that there are no error conditions. When a
Consider then cases involving linking errors. If
Finally, consider a case involving evaluation errors. If
The difference here between linking and evaluation errors is due to how evaluation must be only performed once, as it can cause side effects; it is thus important to remember whether evaluation has already been performed, even if unsuccessfully. (In the error case, it makes sense to also remember the exception because otherwise subsequent Evaluate() calls would have to synthesize a new one.) Linking, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a later time with no issues.
Now consider a different type of error condition:
In this scenario, module A declares a dependency on some other module, but no
Lastly, consider a module graph with a cycle:
Here we assume that the entry point is module A, so that the
An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.
Now consider a case where A has an linking error; for example, it tries to import a binding from C that does not exist. In that case, the above steps still occur, including the early return from the second call to
Finally, consider a case where A has an evaluation error; for example, its source code throws an exception. In that case, the evaluation-time analog of the above steps still occurs, including the early return from the second call to
16.2.1.6 Source Text Module Records
A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (
A
In addition to the fields defined in
| Field Name | Value Type | Meaning |
|---|---|---|
| [[ECMAScriptCode]] |
a |
The result of parsing the source text of this module using |
| [[Context]] |
An ECMAScript |
The |
| [[ImportMeta]] | Object |
An object exposed through the import.meta meta property. It is |
| [[ImportEntries]] |
|
A |
| [[LocalExportEntries]] |
|
A |
| [[IndirectExportEntries]] |
|
A export * as namespace declarations.
|
| [[StarExportEntries]] |
|
A export * declarations that occur within the module, not including export * as namespace declarations.
|
An ImportEntry Record is a
| Field Name | Value Type | Meaning |
|---|---|---|
| [[ModuleRequest]] | String |
String value of the |
| [[ImportName]] | String |
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value |
| [[LocalName]] | String | The name that is used to locally access the imported value from within the importing module. |
| Import Statement Form | [[ModuleRequest]] | [[ImportName]] | [[LocalName]] |
|---|---|---|---|
import v from "mod";
|
|
|
|
import * as ns from "mod";
|
|
|
|
import {x} from "mod";
|
|
|
|
import {x as v} from "mod";
|
|
|
|
import "mod";
|
An |
||
An ExportEntry Record is a
| Field Name | Value Type | Meaning |
|---|---|---|
| [[ExportName]] | String | null | The name used to export this binding by this module. |
| [[ModuleRequest]] | String | null |
The String value of the |
| [[ImportName]] | String | null |
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. |
| [[LocalName]] | String | null |
The name that is used to locally access the exported value from within the importing module. |
| Export Statement Form | [[ExportName]] | [[ModuleRequest]] | [[ImportName]] | [[LocalName]] |
|---|---|---|---|---|
export var v;
|
|
|
|
|
export default function f() {}
|
|
|
|
|
export default function () {}
|
|
|
|
|
export default 42;
|
|
|
|
|
export {x};
|
|
|
|
|
export {v as x};
|
|
|
|
|
export {x} from "mod";
|
|
|
|
|
export {v as x} from "mod";
|
|
|
|
|
export * from "mod";
|
|
|
|
|
export * as ns from "mod";
|
|
|
|
|
The following definitions specify the required concrete methods and other
16.2.1.6.1 ParseModule ( sourceText, realm, hostDefined )
The abstract operation ParseModule takes arguments sourceText (ECMAScript source text), realm, and hostDefined. It creates a
Assert : sourceText is an ECMAScript source text (see clause11 ).- Let body be
ParseText (sourceText,Module ). - If body is a
List of errors, return body. - Let requestedModules be the
ModuleRequests of body. - Let importEntries be
ImportEntries of body. - Let importedBoundNames be
ImportedLocalNames (importEntries). - Let indirectExportEntries be a new empty
List . - Let localExportEntries be a new empty
List . - Let starExportEntries be a new empty
List . - Let exportEntries be
ExportEntries of body. - For each
ExportEntry Record ee of exportEntries, do- If ee.[[ModuleRequest]] is
null , then- If ee.[[LocalName]] is not an element of importedBoundNames, then
- Append ee to localExportEntries.
- Else,
- Let ie be the element of importEntries whose [[LocalName]] is the same as ee.[[LocalName]].
- If ie.[[ImportName]] is
"*" , then- NOTE: This is a re-export of an imported module namespace object.
- Append ee to localExportEntries.
- Else,
- NOTE: This is a re-export of a single name.
- Append the
ExportEntry Record { [[ModuleRequest]]: ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]], [[LocalName]]:null , [[ExportName]]: ee.[[ExportName]] } to indirectExportEntries.
- If ee.[[LocalName]] is not an element of importedBoundNames, then
- Else if ee.[[ImportName]] is
"*" and ee.[[ExportName]] isnull , then- Append ee to starExportEntries.
- Else,
- Append ee to indirectExportEntries.
- If ee.[[ModuleRequest]] is
- Return
Source Text Module Record { [[Realm]]: realm, [[Environment]]:undefined , [[Namespace]]:undefined , [[Status]]:unlinked , [[EvaluationError]]:undefined , [[HostDefined]]: hostDefined, [[ECMAScriptCode]]: body, [[Context]]:empty , [[ImportMeta]]:empty , [[RequestedModules]]: requestedModules, [[ImportEntries]]: importEntries, [[LocalExportEntries]]: localExportEntries, [[IndirectExportEntries]]: indirectExportEntries, [[StarExportEntries]]: starExportEntries, [[DFSIndex]]:undefined , [[DFSAncestorIndex]]:undefined }.
An implementation may parse module source text and analyse it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.
16.2.1.6.2 GetExportedNames ( [ exportStarSet ] ) Concrete Method
The GetExportedNames concrete method of a
- If exportStarSet is not present, set exportStarSet to a new empty
List . Assert : exportStarSet is aList of Source Text Module Records.- If exportStarSet contains module, then
- Append module to exportStarSet.
- Let exportedNames be a new empty
List . - For each
ExportEntry Record e of module.[[LocalExportEntries]], doAssert : module provides the direct binding for this export.- Append e.[[ExportName]] to exportedNames.
- For each
ExportEntry Record e of module.[[IndirectExportEntries]], doAssert : module imports a specific binding for this export.- Append e.[[ExportName]] to exportedNames.
- For each
ExportEntry Record e of module.[[StarExportEntries]], do- Let requestedModule be ?
HostResolveImportedModule (module, e.[[ModuleRequest]]). - Let starNames be ? requestedModule.GetExportedNames(exportStarSet).
- For each element n of starNames, do
- If
SameValue (n,"default" ) isfalse , then- If n is not an element of exportedNames, then
- Append n to exportedNames.
- If n is not an element of exportedNames, then
- If
- Let requestedModule be ?
- Return exportedNames.
GetExportedNames does not filter out or throw an exception for names that have ambiguous star export bindings.
16.2.1.6.3 ResolveExport ( exportName [ , resolveSet ] ) Concrete Method
The ResolveExport concrete method of a
ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by the
If a defining module is found, a
This concrete method performs the following steps when called:
- If resolveSet is not present, set resolveSet to a new empty
List . Assert : resolveSet is aList ofRecord { [[Module]], [[ExportName]] }.- For each
Record { [[Module]], [[ExportName]] } r of resolveSet, do- If module and r.[[Module]] are the same
Module Record andSameValue (exportName, r.[[ExportName]]) istrue , thenAssert : This is a circular import request.- Return
null .
- If module and r.[[Module]] are the same
- Append the
Record { [[Module]]: module, [[ExportName]]: exportName } to resolveSet. - For each
ExportEntry Record e of module.[[LocalExportEntries]], do- If
SameValue (exportName, e.[[ExportName]]) istrue , thenAssert : module provides the direct binding for this export.- Return
ResolvedBinding Record { [[Module]]: module, [[BindingName]]: e.[[LocalName]] }.
- If
- For each
ExportEntry Record e of module.[[IndirectExportEntries]], do- If
SameValue (exportName, e.[[ExportName]]) istrue , then- Let importedModule be ?
HostResolveImportedModule (module, e.[[ModuleRequest]]). - If e.[[ImportName]] is
"*" , thenAssert : module does not provide the direct binding for this export.- Return
ResolvedBinding Record { [[Module]]: importedModule, [[BindingName]]:"*namespace*" }.
- Else,
Assert : module imports a specific binding for this export.- Return importedModule.ResolveExport(e.[[ImportName]], resolveSet).
- Let importedModule be ?
- If
- If
SameValue (exportName,"default" ) istrue , thenAssert : Adefaultexport was not explicitly defined by this module.- Return
null . - NOTE: A
defaultexport cannot be provided by anexport *orexport * from "mod"declaration.
- Let starResolution be
null . - For each
ExportEntry Record e of module.[[StarExportEntries]], do- Let importedModule be ?
HostResolveImportedModule (module, e.[[ModuleRequest]]). - Let resolution be ? importedModule.ResolveExport(exportName, resolveSet).
- If resolution is
"ambiguous" , return"ambiguous" . - If resolution is not
null , thenAssert : resolution is aResolvedBinding Record .- If starResolution is
null , set starResolution to resolution. - Else,
Assert : There is more than one*import that includes the requested name.- If resolution.[[Module]] and starResolution.[[Module]] are not the same
Module Record orSameValue (resolution.[[BindingName]], starResolution.[[BindingName]]) isfalse , return"ambiguous" .
- Let importedModule be ?
- Return starResolution.
16.2.1.6.4 InitializeEnvironment ( ) Concrete Method
The InitializeEnvironment concrete method of a
- For each
ExportEntry Record e of module.[[IndirectExportEntries]], do- Let resolution be ? module.ResolveExport(e.[[ExportName]]).
- If resolution is
null or"ambiguous" , throw aSyntaxError exception. Assert : resolution is aResolvedBinding Record .
Assert : All named exports from module are resolvable.- Let realm be module.[[Realm]].
Assert : realm is notundefined .- Let env be
NewModuleEnvironment (realm.[[GlobalEnv]]). - Set module.[[Environment]] to env.
- For each
ImportEntry Record in of module.[[ImportEntries]], do- Let importedModule be !
HostResolveImportedModule (module, in.[[ModuleRequest]]). - NOTE: The above call cannot fail because imported module requests are a subset of module.[[RequestedModules]], and these have been resolved earlier in this algorithm.
- If in.[[ImportName]] is
"*" , then- Let namespace be ?
GetModuleNamespace (importedModule). - Perform ! env.CreateImmutableBinding(in.[[LocalName]],
true ). - Call env.InitializeBinding(in.[[LocalName]], namespace).
- Let namespace be ?
- Else,
- Let resolution be ? importedModule.ResolveExport(in.[[ImportName]]).
- If resolution is
null or"ambiguous" , throw aSyntaxError exception. - If resolution.[[BindingName]] is
"*namespace*" , then- Let namespace be ?
GetModuleNamespace (resolution.[[Module]]). - Perform ! env.CreateImmutableBinding(in.[[LocalName]],
true ). - Call env.InitializeBinding(in.[[LocalName]], namespace).
- Let namespace be ?
- Else,
- Call env.CreateImportBinding(in.[[LocalName]], resolution.[[Module]], resolution.[[BindingName]]).
- Let importedModule be !
- Let moduleContext be a new ECMAScript code
execution context . - Set the Function of moduleContext to
null . Assert : module.[[Realm]] is notundefined .- Set the
Realm of moduleContext to module.[[Realm]]. - Set the ScriptOrModule of moduleContext to module.
- Set the VariableEnvironment of moduleContext to module.[[Environment]].
- Set the LexicalEnvironment of moduleContext to module.[[Environment]].
- Set module.[[Context]] to moduleContext.
- Push moduleContext onto the
execution context stack ; moduleContext is now therunning execution context . - Let code be module.[[ECMAScriptCode]].
- Let varDeclarations be the
VarScopedDeclarations of code. - Let declaredVarNames be a new empty
List . - For each element d of varDeclarations, do
- For each element dn of the
BoundNames of d, do- If dn is not an element of declaredVarNames, then
- Perform ! env.CreateMutableBinding(dn,
false ). - Call env.InitializeBinding(dn,
undefined ). - Append dn to declaredVarNames.
- Perform ! env.CreateMutableBinding(dn,
- If dn is not an element of declaredVarNames, then
- For each element dn of the
- Let lexDeclarations be the
LexicallyScopedDeclarations of code. - For each element d of lexDeclarations, do
- For each element dn of the
BoundNames of d, do- If
IsConstantDeclaration of d istrue , then- Perform ! env.CreateImmutableBinding(dn,
true ).
- Perform ! env.CreateImmutableBinding(dn,
- Else,
- Perform ! env.CreateMutableBinding(dn,
false ).
- Perform ! env.CreateMutableBinding(dn,
- If d is a
FunctionDeclaration , aGeneratorDeclaration , anAsyncFunctionDeclaration , or anAsyncGeneratorDeclaration , then- Let fo be
InstantiateFunctionObject of d with argument env. - Call env.InitializeBinding(dn, fo).
- Let fo be
- If
- For each element dn of the
- Remove moduleContext from the
execution context stack . - Return
NormalCompletion (empty ).
16.2.1.6.5 ExecuteModule ( ) Concrete Method
The ExecuteModule concrete method of a
- Suspend the currently
running execution context . - Let moduleContext be module.[[Context]].
- Push moduleContext onto the
execution context stack ; moduleContext is now therunning execution context . - Let result be the result of evaluating module.[[ECMAScriptCode]].
- Suspend moduleContext and remove it from the
execution context stack . - Resume the context that is now on the top of the
execution context stack as therunning execution context . - Return
Completion (result).
16.2.1.7 HostResolveImportedModule ( referencingScriptOrModule, specifier )
The import()
An example of when referencingScriptOrModule can be
<button type="button" onclick="import('./foo.mjs')">Click me</button>there will be no import()
The implementation of HostResolveImportedModule must conform to the following requirements:
-
The normal return value must be an instance of a concrete subclass of
Module Record . -
If a
Module Record corresponding to the pair referencingScriptOrModule, specifier does not exist or cannot be created, an exception must be thrown. -
Each time this operation is called with a specific referencingScriptOrModule, specifier pair as arguments it must return the same
Module Record instance if it completes normally.
Multiple different referencingScriptOrModule, specifier pairs may map to the same
16.2.1.8 HostImportModuleDynamically ( referencingScriptOrModule, specifier, promiseCapability )
The import()
The implementation of HostImportModuleDynamically must conform to the following requirements:
-
The abstract operation must always complete normally with
undefined . Success or failure must instead be signaled as discussed below. -
The
host environment must conform to one of the two following sets of requirements:- Success path
-
- At some future time, the
host environment must performFinishDynamicImport (referencingScriptOrModule, specifier, promiseCapability,NormalCompletion (undefined )). - Any subsequent call to
HostResolveImportedModule afterFinishDynamicImport has completed, given the arguments referencingScriptOrModule and specifier, must complete normally. - The completion value of any subsequent call to
HostResolveImportedModule afterFinishDynamicImport has completed, given the arguments referencingScriptOrModule and specifier, must be a module which has already been evaluated, i.e. whose Evaluate concrete method has already been called and returned a normal completion.
- At some future time, the
- Failure path
-
- At some future time, the
host environment must performFinishDynamicImport (referencingScriptOrModule, specifier, promiseCapability, anabrupt completion ), with theabrupt completion representing the cause of failure.
- At some future time, the
-
If the
host environment takes the success path once for a given referencingScriptOrModule, specifier pair, it must always do so for subsequent calls. -
The operation must not call promiseCapability.[[Resolve]] or promiseCapability.[[Reject]], but instead must treat promiseCapability as an opaque identifying value to be passed through to
FinishDynamicImport .
The actual process performed is
16.2.1.9 FinishDynamicImport ( referencingScriptOrModule, specifier, promiseCapability, completion )
The abstract operation FinishDynamicImport takes arguments referencingScriptOrModule, specifier, promiseCapability (a import()
- If completion is an
abrupt completion , perform !Call (promiseCapability.[[Reject]],undefined , « completion.[[Value]] »). - Else,
Assert : completion is a normal completion and completion.[[Value]] isundefined .- Let moduleRecord be !
HostResolveImportedModule (referencingScriptOrModule, specifier). Assert : Evaluate has already been invoked on moduleRecord and successfully completed.- Let namespace be
GetModuleNamespace (moduleRecord). - If namespace is an
abrupt completion , perform !Call (promiseCapability.[[Reject]],undefined , « namespace.[[Value]] »). - Else, perform !
Call (promiseCapability.[[Resolve]],undefined , « namespace.[[Value]] »).
16.2.1.10 GetModuleNamespace ( module )
The abstract operation GetModuleNamespace takes argument module. It retrieves the Module Namespace Object representing module's exports, lazily creating it the first time it was requested, and storing it in module.[[Namespace]] for future retrieval. It performs the following steps when called:
Assert : module is an instance of a concrete subclass ofModule Record .Assert : If module is aCyclic Module Record , then module.[[Status]] is notunlinked .- Let namespace be module.[[Namespace]].
- If namespace is
undefined , then- Let exportedNames be ? module.GetExportedNames().
- Let unambiguousNames be a new empty
List . - For each element name of exportedNames, do
- Let resolution be ? module.ResolveExport(name).
- If resolution is a
ResolvedBinding Record , append name to unambiguousNames.
- Set namespace to
ModuleNamespaceCreate (module, unambiguousNames).
- Return namespace.
The only way GetModuleNamespace can throw is via one of the triggered
16.2.1.11 Runtime Semantics: Evaluation
- Return
NormalCompletion (undefined ).
- Let result be the result of evaluating
ModuleItemList . - If result.[[Type]] is
normal and result.[[Value]] isempty , then- Return
NormalCompletion (undefined ).
- Return
- Return
Completion (result).
- Let sl be the result of evaluating
ModuleItemList . ReturnIfAbrupt (sl).- Let s be the result of evaluating
ModuleItem . - Return
Completion (UpdateEmpty (s, sl)).
The value of a
- Return
NormalCompletion (empty ).
16.2.2 Imports
Syntax
16.2.2.1 Static Semantics: Early Errors
-
It is a Syntax Error if the
BoundNames ofImportDeclaration contains any duplicate entries.
16.2.2.2 Static Semantics: ImportEntries
- Return a new empty
List .
- Let entries be
ImportEntries ofModuleItemList . - Append to entries the elements of the
ImportEntries ofModuleItem . - Return entries.
- Return a new empty
List .
- Let module be the sole element of
ModuleRequests ofFromClause . - Return
ImportEntriesForModule ofImportClause with argument module.
- Return a new empty
List .
16.2.2.3 Static Semantics: ImportEntriesForModule
With parameter module.
- Let entries be
ImportEntriesForModule ofImportedDefaultBinding with argument module. - Append to entries the elements of the
ImportEntriesForModule ofNameSpaceImport with argument module. - Return entries.
- Let entries be
ImportEntriesForModule ofImportedDefaultBinding with argument module. - Append to entries the elements of the
ImportEntriesForModule ofNamedImports with argument module. - Return entries.
- Let localName be the sole element of
BoundNames ofImportedBinding . - Let defaultEntry be the
ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]:"default" , [[LocalName]]: localName }. - Return a
List whose sole element is defaultEntry.
- Let localName be the
StringValue ofImportedBinding . - Let entry be the
ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]:"*" , [[LocalName]]: localName }. - Return a
List whose sole element is entry.
- Return a new empty
List .
- Let specs be the
ImportEntriesForModule ofImportsList with argument module. - Append to specs the elements of the
ImportEntriesForModule ofImportSpecifier with argument module. - Return specs.
- Let localName be the sole element of
BoundNames ofImportedBinding . - Let entry be the
ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: localName, [[LocalName]]: localName }. - Return a
List whose sole element is entry.
- Let importName be the
StringValue ofIdentifierName . - Let localName be the
StringValue ofImportedBinding . - Let entry be the
ImportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName }. - Return a
List whose sole element is entry.
16.2.3 Exports
Syntax
16.2.3.1 Static Semantics: Early Errors
-
For each
IdentifierName n inReferencedBindings ofNamedExports : It is a Syntax Error ifStringValue of n is aReservedWord or if theStringValue of n is one of:"implements" ,"interface" ,"let" ,"package" ,"private" ,"protected" ,"public" , or"static" .
The above rule means that each
16.2.3.2 Static Semantics: ExportedBindings
ExportedBindings are the locally bound names that are explicitly associated with a
- Let names be
ExportedBindings ofModuleItemList . - Append to names the elements of the
ExportedBindings ofModuleItem . - Return names.
- Return a new empty
List .
- Return a new empty
List .
- Return the
ExportedBindings ofNamedExports .
- Return the
BoundNames ofVariableStatement .
- Return the
BoundNames ofDeclaration .
- Return the
BoundNames of thisExportDeclaration .
- Return a new empty
List .
- Let names be the
ExportedBindings ofExportsList . - Append to names the elements of the
ExportedBindings ofExportSpecifier . - Return names.
- Return a
List whose sole element is theStringValue ofIdentifierName .
- Return a
List whose sole element is theStringValue of the firstIdentifierName .
16.2.3.3 Static Semantics: ExportedNames
ExportedNames are the externally visible names that a
- Let names be
ExportedNames ofModuleItemList . - Append to names the elements of the
ExportedNames ofModuleItem . - Return names.
- Return the
ExportedNames ofExportDeclaration .
- Return a new empty
List .
- Return the
ExportedNames ofExportFromClause .
- Return a new empty
List .
- Return a
List whose sole element is theStringValue ofIdentifierName .
- Return the
ExportedNames ofNamedExports .
- Return the
BoundNames ofVariableStatement .
- Return the
BoundNames ofDeclaration .
- Return «
"default" ».
- Return a new empty
List .
- Let names be the
ExportedNames ofExportsList . - Append to names the elements of the
ExportedNames ofExportSpecifier . - Return names.
- Return a
List whose sole element is theStringValue ofIdentifierName .
- Return a
List whose sole element is theStringValue of the secondIdentifierName .
16.2.3.4 Static Semantics: ExportEntries
- Return a new empty
List .
- Let entries be
ExportEntries ofModuleItemList . - Append to entries the elements of the
ExportEntries ofModuleItem . - Return entries.
- Return a new empty
List .
- Let module be the sole element of
ModuleRequests ofFromClause . - Return
ExportEntriesForModule ofExportFromClause with argument module.
- Return
ExportEntriesForModule ofNamedExports with argumentnull .
- Let entries be a new empty
List . - Let names be the
BoundNames ofVariableStatement . - For each element name of names, do
- Append the
ExportEntry Record { [[ModuleRequest]]:null , [[ImportName]]:null , [[LocalName]]: name, [[ExportName]]: name } to entries.
- Append the
- Return entries.
- Let entries be a new empty
List . - Let names be the
BoundNames ofDeclaration . - For each element name of names, do
- Append the
ExportEntry Record { [[ModuleRequest]]:null , [[ImportName]]:null , [[LocalName]]: name, [[ExportName]]: name } to entries.
- Append the
- Return entries.
- Let names be
BoundNames ofHoistableDeclaration . - Let localName be the sole element of names.
- Return a
List whose sole element is theExportEntry Record { [[ModuleRequest]]:null , [[ImportName]]:null , [[LocalName]]: localName, [[ExportName]]:"default" }.
- Let names be
BoundNames ofClassDeclaration . - Let localName be the sole element of names.
- Return a
List whose sole element is theExportEntry Record { [[ModuleRequest]]:null , [[ImportName]]:null , [[LocalName]]: localName, [[ExportName]]:"default" }.
- Let entry be the
ExportEntry Record { [[ModuleRequest]]:null , [[ImportName]]:null , [[LocalName]]:"*default*" , [[ExportName]]:"default" }. - Return a
List whose sole element is entry.
16.2.3.5 Static Semantics: ExportEntriesForModule
With parameter module.
- Let entry be the
ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]:"*" , [[LocalName]]:null , [[ExportName]]:null }. - Return a
List whose sole element is entry.
- Let exportName be the
StringValue ofIdentifierName . - Let entry be the
ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]:"*" , [[LocalName]]:null , [[ExportName]]: exportName }. - Return a
List whose sole element is entry.
- Return a new empty
List .
- Let specs be the
ExportEntriesForModule ofExportsList with argument module. - Append to specs the elements of the
ExportEntriesForModule ofExportSpecifier with argument module. - Return specs.
- Let sourceName be the
StringValue ofIdentifierName . - If module is
null , then- Let localName be sourceName.
- Let importName be
null .
- Else,
- Let localName be
null . - Let importName be sourceName.
- Let localName be
- Return a
List whose sole element is theExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.
- Let sourceName be the
StringValue of the firstIdentifierName . - Let exportName be the
StringValue of the secondIdentifierName . - If module is
null , then- Let localName be sourceName.
- Let importName be
null .
- Else,
- Let localName be
null . - Let importName be sourceName.
- Let localName be
- Return a
List whose sole element is theExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.
16.2.3.6 Static Semantics: ReferencedBindings
- Return a new empty
List .
- Let names be the
ReferencedBindings ofExportsList . - Append to names the elements of the
ReferencedBindings ofExportSpecifier . - Return names.
- Return a
List whose sole element is theIdentifierName .
- Return a
List whose sole element is the firstIdentifierName .
16.2.3.7 Runtime Semantics: Evaluation
- Return
NormalCompletion (empty ).
- Return the result of evaluating
VariableStatement .
- Return the result of evaluating
Declaration .
- Return the result of evaluating
HoistableDeclaration .
- Let value be ?
BindingClassDeclarationEvaluation ofClassDeclaration . - Let className be the sole element of
BoundNames ofClassDeclaration . - If className is
"*default*" , then- Let env be the
running execution context 's LexicalEnvironment. - Perform ?
InitializeBoundName ("*default*" , value, env).
- Let env be the
- Return
NormalCompletion (empty ).
- If
IsAnonymousFunctionDefinition (AssignmentExpression ) istrue , then- Let value be ?
NamedEvaluation ofAssignmentExpression with argument"default" .
- Let value be ?
- Else,
- Let rhs be the result of evaluating
AssignmentExpression . - Let value be ?
GetValue (rhs).
- Let rhs be the result of evaluating
- Let env be the
running execution context 's LexicalEnvironment. - Perform ?
InitializeBoundName ("*default*" , value, env). - Return
NormalCompletion (empty ).
17 Error Handling and Language Extensions
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in the eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.
An implementation must report as an
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
-
Except as restricted in
17.1 , ahost or implementation may extendScript syntax,Module syntax, and regular expression pattern or flag syntax. To permit this, all operations (such as callingeval, using a regular expression literal, or using the Function or RegExpconstructor ) that are allowed to throwSyntaxError are permitted to exhibithost-defined behaviour instead of throwingSyntaxError when they encounter ahost-defined extension to the script syntax or regular expression pattern or flag syntax. -
Except as restricted in
17.1 , ahost or implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to havehost-defined behaviour instead of throwing an error (such asReferenceError ).
17.1 Forbidden Extensions
An implementation must not extend this specification in the following ways:
-
ECMAScript function objects defined using syntactic constructors in
strict mode code must not be created with own properties named"caller" or"arguments" . Such own properties also must not be created for function objects defined using anArrowFunction ,MethodDefinition ,GeneratorDeclaration ,GeneratorExpression ,AsyncGeneratorDeclaration ,AsyncGeneratorExpression ,ClassDeclaration ,ClassExpression ,AsyncFunctionDeclaration ,AsyncFunctionExpression , orAsyncArrowFunction regardless of whether the definition is contained instrict mode code . Built-in functions, strict functions created using the Functionconstructor , generator functions created using the Generatorconstructor , async functions created using the AsyncFunctionconstructor , and functions created using thebindmethod also must not be created with such own properties. -
If an implementation extends any
function object with an own property named"caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be astrict function object. If it is anaccessor property , the function that is the value of the property's [[Get]] attribute must never return astrict function when called. -
Neither mapped nor unmapped arguments objects may be created with an own property named
"caller" . -
The behaviour of built-in methods which are specified in ECMA-402, such as those named
toLocaleString, must not be extended except as specified in ECMA-402. -
The RegExp pattern grammars in
22.2.1 andB.1.4 must not be extended to recognize any of the source characters A-Z or a-z asIdentityEscape when the [U] grammar parameter is present.[+U] -
The Syntactic Grammar must not be extended in any manner that allows the token
:to immediately follow source text that matches theBindingIdentifier nonterminal symbol. -
When processing
strict mode code , the syntax ofNumericLiteral must not be extended to includeLegacyOctalIntegerLiteral DecimalIntegerLiteral must not be extended to includeNonOctalDecimalIntegerLiteral B.1.1 . -
TemplateCharacter must not be extended to includeLegacyOctalEscapeSequence NonOctalDecimalEscapeSequence B.1.2 . -
When processing
strict mode code , the extensions defined inB.3.2 ,B.3.3 ,B.3.4 , andB.3.6 must not be supported. -
When parsing for the
Module goal symbol , the lexical grammar extensions defined inB.1.3 must not be supported. -
ImportCall must not be extended.
18 ECMAScript Standard Built-in Objects
There are certain built-in objects available whenever an ECMAScript
Unless specified otherwise, a built-in object that is callable as a function is a built-in
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of that new expression that invokes that
Unless otherwise specified in the description of a particular function, if a built-in function or
Unless otherwise specified in the description of a particular function, if a built-in function or
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in Function.prototype (
Unless otherwise specified every built-in prototype object has the Object.prototype (
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.
Each built-in function defined in this specification is created by calling the
Every built-in
For example, the
Unless otherwise specified, the
Every built-in
The value of the
Unless otherwise specified, the
Every other
Every
19 The Global Object
The global object:
- is created before control enters any
execution context . - does not have a [[Construct]] internal method; it cannot be used as a
constructor with thenewoperator. - does not have a [[Call]] internal method; it cannot be invoked as a function.
- has a [[Prototype]] internal slot whose value is
host-defined . - may have
host defined properties in addition to the properties defined in this specification. This may include a property whose value is the global object itself.
19.1 Value Properties of the Global Object
19.1.1 globalThis
The initial value of the
This property has the attributes { [[Writable]]:
19.1.2 Infinity
The value of Infinity is
19.1.3 NaN
The value of NaN is
19.1.4 undefined
The value of undefined is
19.2 Function Properties of the Global Object
19.2.1 eval ( x )
The eval function is the %eval% intrinsic object. When the eval function is called with one argument x, the following steps are taken:
Assert : Theexecution context stack has at least two elements.- Let callerContext be the second to top element of the
execution context stack . - Let callerRealm be callerContext's
Realm . - Return ?
PerformEval (x, callerRealm,false ,false ).
19.2.1.1 PerformEval ( x, callerRealm, strictCaller, direct )
The abstract operation PerformEval takes arguments x, callerRealm, strictCaller, and direct. It performs the following steps when called:
Assert : If direct isfalse , then strictCaller is alsofalse .- If
Type (x) is not String, return x. - Let evalRealm be
the current Realm Record . - Perform ?
HostEnsureCanCompileStrings (callerRealm, evalRealm). - Let inFunction be
false . - Let inMethod be
false . - Let inDerivedConstructor be
false . - If direct is
true , then- Let thisEnvRec be !
GetThisEnvironment (). - If thisEnvRec is a
function Environment Record , then- Let F be thisEnvRec.[[FunctionObject]].
- Set inFunction to
true . - Set inMethod to thisEnvRec.HasSuperBinding().
- If F.[[ConstructorKind]] is
derived , set inDerivedConstructor totrue .
- Let thisEnvRec be !
- Perform the following substeps in an
implementation-defined order, possibly interleaving parsing and error detection:- Let script be
ParseText (!StringToCodePoints (x),Script ). - If script is a
List of errors, throw aSyntaxError exception. - If script
Contains ScriptBody isfalse , returnundefined . - Let body be the
ScriptBody of script. - If inFunction is
false , and bodyContains NewTarget , throw aSyntaxError exception. - If inMethod is
false , and bodyContains SuperProperty , throw aSyntaxError exception. - If inDerivedConstructor is
false , and bodyContains SuperCall , throw aSyntaxError exception.
- Let script be
- If strictCaller is
true , let strictEval betrue . - Else, let strictEval be
IsStrict of script. - Let runningContext be the
running execution context . - NOTE: If direct is
true , runningContext will be theexecution context that performed thedirect eval . If direct isfalse , runningContext will be theexecution context for the invocation of theevalfunction. - If direct is
true , then- Let lexEnv be
NewDeclarativeEnvironment (runningContext's LexicalEnvironment). - Let varEnv be runningContext's VariableEnvironment.
- Let lexEnv be
- Else,
- Let lexEnv be
NewDeclarativeEnvironment (evalRealm.[[GlobalEnv]]). - Let varEnv be evalRealm.[[GlobalEnv]].
- Let lexEnv be
- If strictEval is
true , set varEnv to lexEnv. - If runningContext is not already suspended, suspend runningContext.
- Let evalContext be a new ECMAScript code
execution context . - Set evalContext's Function to
null . - Set evalContext's
Realm to evalRealm. - Set evalContext's ScriptOrModule to runningContext's ScriptOrModule.
- Set evalContext's VariableEnvironment to varEnv.
- Set evalContext's LexicalEnvironment to lexEnv.
- Push evalContext onto the
execution context stack ; evalContext is now therunning execution context . - Let result be
EvalDeclarationInstantiation (body, varEnv, lexEnv, strictEval). - If result.[[Type]] is
normal , then- Set result to the result of evaluating body.
- If result.[[Type]] is
normal and result.[[Value]] isempty , then- Set result to
NormalCompletion (undefined ).
- Set result to
- Suspend evalContext and remove it from the
execution context stack . - Resume the context that is now on the top of the
execution context stack as therunning execution context . - Return
Completion (result).
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if the calling context is evaluating formal parameter initializers or if either the code of the calling context or the eval code is let, const, or class declarations are always instantiated in a new LexicalEnvironment.
19.2.1.2 HostEnsureCanCompileStrings ( callerRealm, calleeRealm )
The
An implementation of HostEnsureCanCompileStrings may complete normally or abruptly. Any abrupt completions will be propagated to its callers. The default implementation of HostEnsureCanCompileStrings is to unconditionally return an empty normal completion.
19.2.1.3 EvalDeclarationInstantiation ( body, varEnv, lexEnv, strict )
The abstract operation EvalDeclarationInstantiation takes arguments body, varEnv, lexEnv, and strict. It performs the following steps when called:
- Let varNames be the
VarDeclaredNames of body. - Let varDeclarations be the
VarScopedDeclarations of body. - If strict is
false , then- If varEnv is a
global Environment Record , then- For each element name of varNames, do
- If varEnv.HasLexicalDeclaration(name) is
true , throw aSyntaxError exception. - NOTE:
evalwill not create a global var declaration that would be shadowed by a global lexical declaration.
- If varEnv.HasLexicalDeclaration(name) is
- For each element name of varNames, do
- Let thisEnv be lexEnv.
Assert : The following loop will terminate.- Repeat, while thisEnv is not the same as varEnv,
- If thisEnv is not an
object Environment Record , then- NOTE: The environment of with statements cannot contain any lexical declaration so it doesn't need to be checked for var/let hoisting conflicts.
- For each element name of varNames, do
- If thisEnv.HasBinding(name) is
true , then- Throw a
SyntaxError exception. - NOTE: Annex
B.3.5 defines alternate semantics for the above step.
- Throw a
- NOTE: A
direct eval will not hoist var declaration over a like-named lexical declaration.
- If thisEnv.HasBinding(name) is
- Set thisEnv to thisEnv.[[OuterEnv]].
- If thisEnv is not an
- If varEnv is a
- Let functionsToInitialize be a new empty
List . - Let declaredFunctionNames be a new empty
List . - For each element d of varDeclarations, in reverse
List order, do- If d is neither a
VariableDeclaration nor aForBinding nor aBindingIdentifier , thenAssert : d is either aFunctionDeclaration , aGeneratorDeclaration , anAsyncFunctionDeclaration , or anAsyncGeneratorDeclaration .- NOTE: If there are multiple function declarations for the same name, the last declaration is used.
- Let fn be the sole element of the
BoundNames of d. - If fn is not an element of declaredFunctionNames, then
- If varEnv is a
global Environment Record , then- Let fnDefinable be ? varEnv.CanDeclareGlobalFunction(fn).
- If fnDefinable is
false , throw aTypeError exception.
- Append fn to declaredFunctionNames.
- Insert d as the first element of functionsToInitialize.
- If varEnv is a
- If d is neither a
- NOTE: Annex
B.3.3.3 adds additional steps at this point. - Let declaredVarNames be a new empty
List . - For each element d of varDeclarations, do
- If d is a
VariableDeclaration , aForBinding , or aBindingIdentifier , then- For each String vn of the
BoundNames of d, do- If vn is not an element of declaredFunctionNames, then
- If varEnv is a
global Environment Record , then- Let vnDefinable be ? varEnv.CanDeclareGlobalVar(vn).
- If vnDefinable is
false , throw aTypeError exception.
- If vn is not an element of declaredVarNames, then
- Append vn to declaredVarNames.
- If varEnv is a
- If vn is not an element of declaredFunctionNames, then
- For each String vn of the
- If d is a
- NOTE: No abnormal terminations occur after this algorithm step unless varEnv is a
global Environment Record and theglobal object is aProxy exotic object . - Let lexDeclarations be the
LexicallyScopedDeclarations of body. - For each element d of lexDeclarations, do
- NOTE: Lexically declared names are only instantiated here but not initialized.
- For each element dn of the
BoundNames of d, do- If
IsConstantDeclaration of d istrue , then- Perform ? lexEnv.CreateImmutableBinding(dn,
true ).
- Perform ? lexEnv.CreateImmutableBinding(dn,
- Else,
- Perform ? lexEnv.CreateMutableBinding(dn,
false ).
- Perform ? lexEnv.CreateMutableBinding(dn,
- If
- For each
Parse Node f of functionsToInitialize, do- Let fn be the sole element of the
BoundNames of f. - Let fo be
InstantiateFunctionObject of f with argument lexEnv. - If varEnv is a
global Environment Record , then- Perform ? varEnv.CreateGlobalFunctionBinding(fn, fo,
true ).
- Perform ? varEnv.CreateGlobalFunctionBinding(fn, fo,
- Else,
- Let bindingExists be varEnv.HasBinding(fn).
- If bindingExists is
false , then- Let status be ! varEnv.CreateMutableBinding(fn,
true ). Assert : status is not anabrupt completion because of validation preceding step10 .- Perform ! varEnv.InitializeBinding(fn, fo).
- Let status be ! varEnv.CreateMutableBinding(fn,
- Else,
- Perform ! varEnv.SetMutableBinding(fn, fo,
false ).
- Perform ! varEnv.SetMutableBinding(fn, fo,
- Let fn be the sole element of the
- For each String vn of declaredVarNames, do
- If varEnv is a
global Environment Record , then- Perform ? varEnv.CreateGlobalVarBinding(vn,
true ).
- Perform ? varEnv.CreateGlobalVarBinding(vn,
- Else,
- Let bindingExists be varEnv.HasBinding(vn).
- If bindingExists is
false , then- Let status be ! varEnv.CreateMutableBinding(vn,
true ). Assert : status is not anabrupt completion because of validation preceding step10 .- Perform ! varEnv.InitializeBinding(vn,
undefined ).
- Let status be ! varEnv.CreateMutableBinding(vn,
- If varEnv is a
- Return
NormalCompletion (empty ).
An alternative version of this algorithm is described in
19.2.2 isFinite ( number )
The isFinite function is the %isFinite% intrinsic object. When the isFinite function is called with one argument number, the following steps are taken:
- Let num be ?
ToNumber (number). - If num is
NaN ,+∞ 𝔽, or-∞ 𝔽, returnfalse . - Otherwise, return
true .
19.2.3 isNaN ( number )
The isNaN function is the %isNaN% intrinsic object. When the isNaN function is called with one argument number, the following steps are taken:
- Let num be ?
ToNumber (number). - If num is
NaN , returntrue . - Otherwise, return
false .
A reliable way for ECMAScript code to test if a value X is a X !== X. The result will be X is a
19.2.4 parseFloat ( string )
The parseFloat function produces a
The parseFloat function is the %parseFloat% intrinsic object. When the parseFloat function is called with one argument string, the following steps are taken:
- Let inputString be ?
ToString (string). - Let trimmedString be !
TrimString (inputString,start ). - If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a
StrDecimalLiteral (see7.1.4.1 ), returnNaN . - Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies the syntax of a
StrDecimalLiteral . - Let mathFloat be MV of numberString.
- If mathFloat = 0, then
- If the first code unit of trimmedString is the code unit 0x002D (HYPHEN-MINUS), return
-0 𝔽. - Return
+0 𝔽.
- If the first code unit of trimmedString is the code unit 0x002D (HYPHEN-MINUS), return
- Return
𝔽 (mathFloat).
parseFloat may interpret only a leading portion of string as a
19.2.5 parseInt ( string, radix )
The parseInt function produces an 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin with the code unit pairs 0x or 0X.
The parseInt function is the %parseInt% intrinsic object. When the parseInt function is called, the following steps are taken:
- Let inputString be ?
ToString (string). - Let S be !
TrimString (inputString,start ). - Let sign be 1.
- If S is not empty and the first code unit of S is the code unit 0x002D (HYPHEN-MINUS), set sign to -1.
- If S is not empty and the first code unit of S is the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), remove the first code unit from S.
- Let R be
ℝ (?ToInt32 (radix)). - Let stripPrefix be
true . - If R ≠ 0, then
- If R < 2 or R > 36, return
NaN . - If R ≠ 16, set stripPrefix to
false .
- If R < 2 or R > 36, return
- Else,
- Set R to 10.
- If stripPrefix is
true , then- If the length of S is at least 2 and the first two code units of S are either
"0x" or"0X" , then- Remove the first two code units from S.
- Set R to 16.
- If the length of S is at least 2 and the first two code units of S are either
- If S contains a code unit that is not a radix-R digit, let end be the index within S of the first such code unit; otherwise, let end be the length of S.
- Let Z be the
substring of S from 0 to end. - If Z is empty, return
NaN . - Let mathInt be the
integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be animplementation-approximated value representing theinteger value that is represented by Z in radix-R notation.) - If mathInt = 0, then
- If sign = -1, return
-0 𝔽. - Return
+0 𝔽.
- If sign = -1, return
- Return
𝔽 (sign × mathInt).
19.2.6 URI Handling Functions
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.
19.2.6.1 URI Syntax and Semantics
A URI is composed of a sequence of components separated by component separators. The general form is:
: First / Second ; Third ? Fourth
where the italicized names represent components and “:”, “/”, “;” and “?” are reserved for use as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved code units in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved code units represent text and so must be encoded so that they are not interpreted as reserved code units when the component is part of a complete URI.
The following lexical grammar specifies the form of encoded URIs.
Syntax
The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.
Runtime Semantics
When a code unit to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved code units, that code unit must be encoded. The code unit is transformed into its UTF-8 encoding, with
19.2.6.1.1 Encode ( string, unescapedSet )
The abstract operation Encode takes arguments string (a String) and unescapedSet (a String). It performs URI encoding and escaping. It performs the following steps when called:
- Let strLen be the number of code units in string.
- Let R be the empty String.
- Let k be 0.
- Repeat,
- If k = strLen, return R.
- Let C be the code unit at index k within string.
- If C is in unescapedSet, then
- Set k to k + 1.
- Set R to the
string-concatenation of R and C.
- Else,
- Let cp be !
CodePointAt (string, k). - If cp.[[IsUnpairedSurrogate]] is
true , throw aURIError exception. - Set k to k + cp.[[CodeUnitCount]].
- Let Octets be the
List of octets resulting by applying the UTF-8 transformation to cp.[[CodePoint]]. - For each element octet of Octets, do
- Set R to the
string-concatenation of:- R
"%" - the String representation of octet, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
- Set R to the
- Let cp be !
19.2.6.1.2 Decode ( string, reservedSet )
The abstract operation Decode takes arguments string (a String) and reservedSet (a String). It performs URI unescaping and decoding. It performs the following steps when called:
- Let strLen be the length of string.
- Let R be the empty String.
- Let k be 0.
- Repeat,
- If k = strLen, return R.
- Let C be the code unit at index k within string.
- If C is not the code unit 0x0025 (PERCENT SIGN), then
- Let S be the String value containing only the code unit C.
- Else,
- Let start be k.
- If k + 2 ≥ strLen, throw a
URIError exception. - If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a
URIError exception. - Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
- Set k to k + 2.
- Let n be the number of leading 1 bits in B.
- If n = 0, then
- Let C be the code unit whose value is B.
- If C is not in reservedSet, then
- Let S be the String value containing only the code unit C.
- Else,
- Let S be the
substring of string from start to k + 1.
- Let S be the
- Else,
- If n = 1 or n > 4, throw a
URIError exception. - If k + (3 × (n - 1)) ≥ strLen, throw a
URIError exception. - Let Octets be a
List whose sole element is B. - Let j be 1.
- Repeat, while j < n,
- Set k to k + 1.
- If the code unit at index k within string is not the code unit 0x0025 (PERCENT SIGN), throw a
URIError exception. - If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw a
URIError exception. - Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
- Set k to k + 2.
- Append B to Octets.
- Set j to j + 1.
Assert : The length of Octets is n.- If Octets does not contain a valid UTF-8 encoding of a Unicode code point, throw a
URIError exception. - Let V be the code point obtained by applying the UTF-8 transformation to Octets, that is, from a
List of octets into a 21-bit value. - Let S be
UTF16EncodeCodePoint (V).
- If n = 1 or n > 4, throw a
- Set R to the
string-concatenation of R and S. - Set k to k + 1.
This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a sequence of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n > 1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified in
| Code Unit Value | Representation | 1st Octet | 2nd Octet | 3rd Octet | 4th Octet |
|---|---|---|---|---|---|
0x0000 - 0x007F
|
00000000 0zzzzzzz
|
0zzzzzzz
|
|||
0x0080 - 0x07FF
|
00000yyy yyzzzzzz
|
110yyyyy
|
10zzzzzz
|
||
0x0800 - 0xD7FF
|
xxxxyyyy yyzzzzzz
|
1110xxxx
|
10yyyyyy
|
10zzzzzz
|
|
0xD800 - 0xDBFF
followed by 0xDC00 - 0xDFFF
|
110110vv vvwwwwxx
followed by 110111yy yyzzzzzz
|
11110uuu
|
10uuwwww
|
10xxyyyy
|
10zzzzzz
|
0xD800 - 0xDBFF
not followed by 0xDC00 - 0xDFFF
|
causes URIError
|
||||
0xDC00 - 0xDFFF
|
causes URIError
|
||||
0xE000 - 0xFFFF
|
xxxxyyyy yyzzzzzz
|
1110xxxx
|
10yyyyyy
|
10zzzzzz
|
Where
uuuuu = vvvv + 1
to account for the addition of 0x10000 as in section 3.8 of the Unicode Standard (Surrogates).
The above transformation combines each
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw a
19.2.6.2 decodeURI ( encodedURI )
The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code points that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.
The decodeURI function is the %decodeURI% intrinsic object. When the decodeURI function is called with one argument encodedURI, the following steps are taken:
- Let uriString be ?
ToString (encodedURI). - Let reservedURISet be a String containing one instance of each code unit valid in
uriReserved plus"#" . - Return ?
Decode (uriString, reservedURISet).
The code point # is not decoded from escape sequences even though it is not a reserved URI code point.
19.2.6.3 decodeURIComponent ( encodedURIComponent )
The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code points that it represents.
The decodeURIComponent function is the %decodeURIComponent% intrinsic object. When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:
19.2.6.4 encodeURI ( uri )
The encodeURI function computes a new version of a UTF-16 encoded (
The encodeURI function is the %encodeURI% intrinsic object. When the encodeURI function is called with one argument uri, the following steps are taken:
- Let uriString be ?
ToString (uri). - Let unescapedURISet be a String containing one instance of each code unit valid in
uriReserved anduriUnescaped plus"#" . - Return ?
Encode (uriString, unescapedURISet).
The code point # is not encoded to an escape sequence even though it is not a reserved or unescaped URI code point.
19.2.6.5 encodeURIComponent ( uriComponent )
The encodeURIComponent function computes a new version of a UTF-16 encoded (
The encodeURIComponent function is the %encodeURIComponent% intrinsic object. When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:
- Let componentString be ?
ToString (uriComponent). - Let unescapedURIComponentSet be a String containing one instance of each code unit valid in
uriUnescaped . - Return ?
Encode (componentString, unescapedURIComponentSet).
19.3 Constructor Properties of the Global Object
19.3.1 Array ( . . . )
See
19.3.2 ArrayBuffer ( . . . )
See
19.3.3 BigInt ( . . . )
See
19.3.4 BigInt64Array ( . . . )
See
19.3.5 BigUint64Array ( . . . )
See
19.3.6 Boolean ( . . . )
See
19.3.7 DataView ( . . . )
See
19.3.8 Date ( . . . )
See
19.3.9 Error ( . . . )
See
19.3.10 EvalError ( . . . )
See
19.3.11 FinalizationRegistry ( . . . )
See
19.3.12 Float32Array ( . . . )
See
19.3.13 Float64Array ( . . . )
See
19.3.14 Function ( . . . )
See
19.3.15 Int8Array ( . . . )
See
19.3.16 Int16Array ( . . . )
See
19.3.17 Int32Array ( . . . )
See
19.3.18 Map ( . . . )
See
19.3.19 Number ( . . . )
See
19.3.20 Object ( . . . )
See
19.3.21 Promise ( . . . )
See
19.3.22 Proxy ( . . . )
See
19.3.23 RangeError ( . . . )
See
19.3.24 ReferenceError ( . . . )
See
19.3.25 RegExp ( . . . )
See
19.3.26 Set ( . . . )
See
19.3.27 SharedArrayBuffer ( . . . )
See
19.3.28 String ( . . . )
See
19.3.29 Symbol ( . . . )
See
19.3.30 SyntaxError ( . . . )
See
19.3.31 TypeError ( . . . )
See
19.3.32 Uint8Array ( . . . )
See
19.3.33 Uint8ClampedArray ( . . . )
See
19.3.34 Uint16Array ( . . . )
See
19.3.35 Uint32Array ( . . . )
See
19.3.36 URIError ( . . . )
See
19.3.37 WeakMap ( . . . )
See
19.3.38 WeakRef ( . . . )
See
19.3.39 WeakSet ( . . . )
See
19.4 Other Properties of the Global Object
19.4.1 Atomics
See
19.4.2 JSON
See
19.4.3 Math
See
19.4.4 Reflect
See
20 Fundamental Objects
20.1 Object Objects
20.1.1 The Object Constructor
The Object
- is %Object%.
- is the initial value of the
"Object" property of theglobal object . - creates a new
ordinary object when called as aconstructor . - performs a type conversion when called as a function rather than as a
constructor . - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition.
20.1.1.1 Object ( [ value ] )
When the Object function is called with optional argument value, the following steps are taken:
- If NewTarget is neither
undefined nor the active function, then- Return ?
OrdinaryCreateFromConstructor (NewTarget,"%Object.prototype%" ).
- Return ?
- If value is
undefined ornull , return !OrdinaryObjectCreate (%Object.prototype% ). - Return !
ToObject (value).
The Object function is
20.1.2 Properties of the Object Constructor
The Object
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has a
"length" property. - has the following additional properties:
20.1.2.1 Object.assign ( target, ...sources )
The assign function is used to copy the values of all of the enumerable own properties from one or more source objects to a target object. When the assign function is called, the following steps are taken:
The assign function is
20.1.2.2 Object.create ( O, Properties )
The create function creates a new object with a specified prototype. When the create function is called, the following steps are taken:
- If
Type (O) is neither Object nor Null, throw aTypeError exception. - Let obj be !
OrdinaryObjectCreate (O). - If Properties is not
undefined , then- Return ?
ObjectDefineProperties (obj, Properties).
- Return ?
- Return obj.
20.1.2.3 Object.defineProperties ( O, Properties )
The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:
- If
Type (O) is not Object, throw aTypeError exception. - Return ?
ObjectDefineProperties (O, Properties).
20.1.2.3.1 ObjectDefineProperties ( O, Properties )
The abstract operation ObjectDefineProperties takes arguments O and Properties. It performs the following steps when called:
Assert :Type (O) is Object.- Let props be ?
ToObject (Properties). - Let keys be ? props.[[OwnPropertyKeys]]().
- Let descriptors be a new empty
List . - For each element nextKey of keys, do
- Let propDesc be ? props.[[GetOwnProperty]](nextKey).
- If propDesc is not
undefined and propDesc.[[Enumerable]] istrue , then- Let descObj be ?
Get (props, nextKey). - Let desc be ?
ToPropertyDescriptor (descObj). - Append the pair (a two element
List ) consisting of nextKey and desc to the end of descriptors.
- Let descObj be ?
- For each element pair of descriptors, do
- Let P be the first element of pair.
- Let desc be the second element of pair.
- Perform ?
DefinePropertyOrThrow (O, P, desc).
- Return O.
20.1.2.4 Object.defineProperty ( O, P, Attributes )
The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:
- If
Type (O) is not Object, throw aTypeError exception. - Let key be ?
ToPropertyKey (P). - Let desc be ?
ToPropertyDescriptor (Attributes). - Perform ?
DefinePropertyOrThrow (O, key, desc). - Return O.
20.1.2.5 Object.entries ( O )
When the entries function is called with argument O, the following steps are taken:
- Let obj be ?
ToObject (O). - Let nameList be ?
EnumerableOwnPropertyNames (obj,key+value ). - Return
CreateArrayFromList (nameList).
20.1.2.6 Object.freeze ( O )
When the freeze function is called, the following steps are taken:
- If
Type (O) is not Object, return O. - Let status be ?
SetIntegrityLevel (O,frozen ). - If status is
false , throw aTypeError exception. - Return O.
20.1.2.7 Object.fromEntries ( iterable )
When the fromEntries method is called with argument iterable, the following steps are taken:
- Perform ?
RequireObjectCoercible (iterable). - Let obj be !
OrdinaryObjectCreate (%Object.prototype% ). Assert : obj is an extensibleordinary object with no own properties.- Let stepsDefine be the algorithm steps defined in
CreateDataPropertyOnObject Functions . - Let lengthDefine be the number of non-optional parameters of the function definition in
CreateDataPropertyOnObject Functions . - Let adder be !
CreateBuiltinFunction (stepsDefine, lengthDefine,"" , « »). - Return ?
AddEntriesFromIterable (obj, iterable, adder).
20.1.2.7.1 CreateDataPropertyOnObject Functions
A CreateDataPropertyOnObject function is an anonymous built-in function. When a CreateDataPropertyOnObject function is called with arguments key and value, the following steps are taken:
- Let O be the
this value. Assert :Type (O) is Object.Assert : O is an extensibleordinary object .- Let propertyKey be ?
ToPropertyKey (key). - Perform !
CreateDataPropertyOrThrow (O, propertyKey, value). - Return
undefined .
20.1.2.8 Object.getOwnPropertyDescriptor ( O, P )
When the getOwnPropertyDescriptor function is called, the following steps are taken:
- Let obj be ?
ToObject (O). - Let key be ?
ToPropertyKey (P). - Let desc be ? obj.[[GetOwnProperty]](key).
- Return
FromPropertyDescriptor (desc).
20.1.2.9 Object.getOwnPropertyDescriptors ( O )
When the getOwnPropertyDescriptors function is called, the following steps are taken:
- Let obj be ?
ToObject (O). - Let ownKeys be ? obj.[[OwnPropertyKeys]]().
- Let descriptors be !
OrdinaryObjectCreate (%Object.prototype% ). - For each element key of ownKeys, do
- Let desc be ? obj.[[GetOwnProperty]](key).
- Let descriptor be !
FromPropertyDescriptor (desc). - If descriptor is not
undefined , perform !CreateDataPropertyOrThrow (descriptors, key, descriptor).
- Return descriptors.
20.1.2.10 Object.getOwnPropertyNames ( O )
When the getOwnPropertyNames function is called, the following steps are taken:
- Return ?
GetOwnPropertyKeys (O,string ).
20.1.2.11 Object.getOwnPropertySymbols ( O )
When the getOwnPropertySymbols function is called with argument O, the following steps are taken:
- Return ?
GetOwnPropertyKeys (O,symbol ).
20.1.2.11.1 GetOwnPropertyKeys ( O, type )
The abstract operation GetOwnPropertyKeys takes arguments O and type (either
- Let obj be ?
ToObject (O). - Let keys be ? obj.[[OwnPropertyKeys]]().
- Let nameList be a new empty
List . - For each element nextKey of keys, do
- Return
CreateArrayFromList (nameList).
20.1.2.12 Object.getPrototypeOf ( O )
When the getPrototypeOf function is called with argument O, the following steps are taken:
- Let obj be ?
ToObject (O). - Return ? obj.[[GetPrototypeOf]]().
20.1.2.13 Object.is ( value1, value2 )
When the is function is called with arguments value1 and value2, the following steps are taken:
- Return
SameValue (value1, value2).
20.1.2.14 Object.isExtensible ( O )
When the isExtensible function is called with argument O, the following steps are taken:
- If
Type (O) is not Object, returnfalse . - Return ?
IsExtensible (O).
20.1.2.15 Object.isFrozen ( O )
When the isFrozen function is called with argument O, the following steps are taken:
- If
Type (O) is not Object, returntrue . - Return ?
TestIntegrityLevel (O,frozen ).
20.1.2.16 Object.isSealed ( O )
When the isSealed function is called with argument O, the following steps are taken:
- If
Type (O) is not Object, returntrue . - Return ?
TestIntegrityLevel (O,sealed ).
20.1.2.17 Object.keys ( O )
When the keys function is called with argument O, the following steps are taken:
- Let obj be ?
ToObject (O). - Let nameList be ?
EnumerableOwnPropertyNames (obj,key ). - Return
CreateArrayFromList (nameList).
20.1.2.18 Object.preventExtensions ( O )
When the preventExtensions function is called, the following steps are taken:
- If
Type (O) is not Object, return O. - Let status be ? O.[[PreventExtensions]]().
- If status is
false , throw aTypeError exception. - Return O.
20.1.2.19 Object.prototype
The initial value of Object.prototype is the
This property has the attributes { [[Writable]]:
20.1.2.20 Object.seal ( O )
When the seal function is called, the following steps are taken:
- If
Type (O) is not Object, return O. - Let status be ?
SetIntegrityLevel (O,sealed ). - If status is
false , throw aTypeError exception. - Return O.
20.1.2.21 Object.setPrototypeOf ( O, proto )
When the setPrototypeOf function is called with arguments O and proto, the following steps are taken:
- Set O to ?
RequireObjectCoercible (O). - If
Type (proto) is neither Object nor Null, throw aTypeError exception. - If
Type (O) is not Object, return O. - Let status be ? O.[[SetPrototypeOf]](proto).
- If status is
false , throw aTypeError exception. - Return O.
20.1.2.22 Object.values ( O )
When the values function is called with argument O, the following steps are taken:
- Let obj be ?
ToObject (O). - Let nameList be ?
EnumerableOwnPropertyNames (obj,value ). - Return
CreateArrayFromList (nameList).
20.1.3 Properties of the Object Prototype Object
The Object prototype object:
- is %Object.prototype%.
- has an [[Extensible]] internal slot whose value is
true . - has the internal methods defined for ordinary objects, except for the [[SetPrototypeOf]] method, which is as defined in
10.4.7.1 . (Thus, it is animmutable prototype exotic object .) - has a [[Prototype]] internal slot whose value is
null .
20.1.3.1 Object.prototype.constructor
The initial value of Object.prototype.constructor is
20.1.3.2 Object.prototype.hasOwnProperty ( V )
When the hasOwnProperty method is called with argument V, the following steps are taken:
- Let P be ?
ToPropertyKey (V). - Let O be ?
ToObject (this value). - Return ?
HasOwnProperty (O, P).
20.1.3.3 Object.prototype.isPrototypeOf ( V )
When the isPrototypeOf method is called with argument V, the following steps are taken:
20.1.3.4 Object.prototype.propertyIsEnumerable ( V )
When the propertyIsEnumerable method is called with argument V, the following steps are taken:
- Let P be ?
ToPropertyKey (V). - Let O be ?
ToObject (this value). - Let desc be ? O.[[GetOwnProperty]](P).
- If desc is
undefined , returnfalse . - Return desc.[[Enumerable]].
This method does not consider objects in the prototype chain.
20.1.3.5 Object.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )
When the toLocaleString method is called, the following steps are taken:
- Let O be the
this value. - Return ?
Invoke (O,"toString" ).
The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocaleString functions. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.
This function provides a generic toLocaleString implementation for objects that have no locale-specific toString behaviour. Array, Number, Date, and toLocaleString methods.
ECMA-402 intentionally does not provide an alternative to this default implementation.
20.1.3.6 Object.prototype.toString ( )
When the toString method is called, the following steps are taken:
- If the
this value isundefined , return"[object Undefined]" . - If the
this value isnull , return"[object Null]" . - Let O be !
ToObject (this value). - Let isArray be ?
IsArray (O). - If isArray is
true , let builtinTag be"Array" . - Else if O has a [[ParameterMap]] internal slot, let builtinTag be
"Arguments" . - Else if O has a [[Call]] internal method, let builtinTag be
"Function" . - Else if O has an [[ErrorData]] internal slot, let builtinTag be
"Error" . - Else if O has a [[BooleanData]] internal slot, let builtinTag be
"Boolean" . - Else if O has a [[NumberData]] internal slot, let builtinTag be
"Number" . - Else if O has a [[StringData]] internal slot, let builtinTag be
"String" . - Else if O has a [[DateValue]] internal slot, let builtinTag be
"Date" . - Else if O has a [[RegExpMatcher]] internal slot, let builtinTag be
"RegExp" . - Else, let builtinTag be
"Object" . - Let tag be ?
Get (O,@@toStringTag ). - If
Type (tag) is not String, set tag to builtinTag. - Return the
string-concatenation of"[object " , tag, and"]" .
Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use
20.1.3.7 Object.prototype.valueOf ( )
When the valueOf method is called, the following steps are taken:
- Return ?
ToObject (this value).
20.1.4 Properties of Object Instances
Object instances have no special properties beyond those inherited from the
20.2 Function Objects
20.2.1 The Function Constructor
The Function
- is %Function%.
- is the initial value of the
"Function" property of theglobal object . - creates and initializes a new
function object when called as a function rather than as aconstructor . Thus the function callFunction(…)is equivalent to the object creation expressionnew Function(…)with the same arguments. - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include asupercall to the Functionconstructor to create and initialize a subclass instance with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction, AsyncFunction, and AsyncGeneratorFunction subclasses.
20.2.1.1 Function ( p1, p2, … , pn, body )
The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.
When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “ p ” arguments, and where body might also not be provided), the following steps are taken:
- Let C be the
active function object . - Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
- Return ?
CreateDynamicFunction (C, NewTarget,normal , args).
It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:
new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")20.2.1.1.1 CreateDynamicFunction ( constructor, newTarget, kind, args )
The abstract operation CreateDynamicFunction takes arguments constructor (a new was initially applied to. args is the argument values that were passed to constructor. It performs the following steps when called:
Assert : Theexecution context stack has at least two elements.- Let callerContext be the second to top element of the
execution context stack . - Let callerRealm be callerContext's
Realm . - Let calleeRealm be
the current Realm Record . - Perform ?
HostEnsureCanCompileStrings (callerRealm, calleeRealm). - If newTarget is
undefined , set newTarget to constructor. - If kind is
normal , then- Let goal be the grammar symbol
FunctionBody .[~Yield, ~Await] - Let parameterGoal be the grammar symbol
FormalParameters .[~Yield, ~Await] - Let fallbackProto be
"%Function.prototype%" .
- Let goal be the grammar symbol
- Else if kind is
generator , then- Let goal be the grammar symbol
GeneratorBody . - Let parameterGoal be the grammar symbol
FormalParameters .[+Yield, ~Await] - Let fallbackProto be
"%GeneratorFunction.prototype%" .
- Let goal be the grammar symbol
- Else if kind is
async , then- Let goal be the grammar symbol
AsyncFunctionBody . - Let parameterGoal be the grammar symbol
FormalParameters .[~Yield, +Await] - Let fallbackProto be
"%AsyncFunction.prototype%" .
- Let goal be the grammar symbol
- Else,
Assert : kind isasyncGenerator .- Let goal be the grammar symbol
AsyncGeneratorBody . - Let parameterGoal be the grammar symbol
FormalParameters .[+Yield, +Await] - Let fallbackProto be
"%AsyncGeneratorFunction.prototype%" .
- Let argCount be the number of elements in args.
- Let P be the empty String.
- If argCount = 0, let bodyArg be the empty String.
- Else if argCount = 1, let bodyArg be args[0].
- Else,
Assert : argCount > 1.- Let firstArg be args[0].
- Set P to ?
ToString (firstArg). - Let k be 1.
- Repeat, while k < argCount - 1,
- Let nextArg be args[k].
- Let nextArgString be ?
ToString (nextArg). - Set P to the
string-concatenation of P,"," (a comma), and nextArgString. - Set k to k + 1.
- Let bodyArg be args[k].
- Let bodyString be the
string-concatenation of 0x000A (LINE FEED), ?ToString (bodyArg), and 0x000A (LINE FEED). - Let prefix be the prefix associated with kind in
Table 50 . - Let sourceString be the
string-concatenation of prefix," anonymous(" , P, 0x000A (LINE FEED),") {" , bodyString, and"}" . - Let sourceText be !
StringToCodePoints (sourceString). - Perform the following substeps in an
implementation-defined order, possibly interleaving parsing and error detection:- Let parameters be
ParseText (!StringToCodePoints (P), parameterGoal). - If parameters is a
List of errors, throw aSyntaxError exception. - Let body be
ParseText (!StringToCodePoints (bodyString), goal). - If body is a
List of errors, throw aSyntaxError exception. - Let strict be
FunctionBodyContainsUseStrict of body. - If strict is
true , apply theearly error rules forUniqueFormalParameters : FormalParameters - If strict is
true andIsSimpleParameterList of parameters isfalse , throw aSyntaxError exception. - If any element of the
BoundNames of parameters also occurs in theLexicallyDeclaredNames of body, throw aSyntaxError exception. - If body
Contains SuperCall istrue , throw aSyntaxError exception. - If parameters
Contains SuperCall istrue , throw aSyntaxError exception. - If body
Contains SuperProperty istrue , throw aSyntaxError exception. - If parameters
Contains SuperProperty istrue , throw aSyntaxError exception. - If kind is
generator orasyncGenerator , then- If parameters
Contains YieldExpression istrue , throw aSyntaxError exception.
- If parameters
- If kind is
async orasyncGenerator , then- If parameters
Contains AwaitExpression istrue , throw aSyntaxError exception.
- If parameters
- If strict is
true , then- If
BoundNames of parameters contains any duplicate elements, throw aSyntaxError exception.
- If
- Let parameters be
- Let proto be ?
GetPrototypeFromConstructor (newTarget, fallbackProto). - Let realmF be
the current Realm Record . - Let scope be realmF.[[GlobalEnv]].
- Let F be !
OrdinaryFunctionCreate (proto, sourceText, parameters, body,non-lexical-this , scope). - Perform
SetFunctionName (F,"anonymous" ). - If kind is
generator , then- Let prototype be !
OrdinaryObjectCreate (%GeneratorFunction.prototype.prototype% ). - Perform
DefinePropertyOrThrow (F,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }).
- Let prototype be !
- Else if kind is
asyncGenerator , then- Let prototype be !
OrdinaryObjectCreate (%AsyncGeneratorFunction.prototype.prototype% ). - Perform
DefinePropertyOrThrow (F,"prototype" , PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }).
- Let prototype be !
- Else if kind is
normal , performMakeConstructor (F). - NOTE: Functions whose kind is
async are not constructible and do not have a [[Construct]] internal method or a"prototype" property. - Return F.
CreateDynamicFunction defines a
| Kind | Prefix |
|---|---|
20.2.2 Properties of the Function Constructor
The Function
- is itself a built-in
function object . - has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
20.2.2.1 Function.length
This is a
20.2.2.2 Function.prototype
The value of Function.prototype is the
This property has the attributes { [[Writable]]:
20.2.3 Properties of the Function Prototype Object
The Function prototype object:
- is %Function.prototype%.
- is itself a built-in
function object . - accepts any arguments and returns
undefined when invoked. - does not have a [[Construct]] internal method; it cannot be used as a
constructor with thenewoperator. - has a [[Prototype]] internal slot whose value is
%Object.prototype% . - does not have a
"prototype" property. - has a
"length" property whose value is+0 𝔽. - has a
"name" property whose value is the empty String.
The Function prototype object is specified to be a
20.2.3.1 Function.prototype.apply ( thisArg, argArray )
When the apply method is called with arguments thisArg and argArray, the following steps are taken:
- Let func be the
this value. - If
IsCallable (func) isfalse , throw aTypeError exception. - If argArray is
undefined ornull , then- Perform
PrepareForTailCall (). - Return ?
Call (func, thisArg).
- Perform
- Let argList be ?
CreateListFromArrayLike (argArray). - Perform
PrepareForTailCall (). - Return ?
Call (func, thisArg, argList).
The thisArg value is passed without modification as the
If func is an arrow function or a
20.2.3.2 Function.prototype.bind ( thisArg, ...args )
When the bind method is called with argument thisArg and zero or more args, it performs the following steps:
- Let Target be the
this value. - If
IsCallable (Target) isfalse , throw aTypeError exception. - Let F be ?
BoundFunctionCreate (Target, thisArg, args). - Let L be 0.
- Let targetHasLength be ?
HasOwnProperty (Target,"length" ). - If targetHasLength is
true , then- Let targetLen be ?
Get (Target,"length" ). - If
Type (targetLen) is Number, then- If targetLen is
+∞ 𝔽, set L to +∞. - Else if targetLen is
-∞ 𝔽, set L to 0. - Else,
- Let targetLenAsInt be !
ToIntegerOrInfinity (targetLen). Assert : targetLenAsInt is finite.- Let argCount be the number of elements in args.
- Set L to
max (targetLenAsInt - argCount, 0).
- Let targetLenAsInt be !
- If targetLen is
- Let targetLen be ?
- Perform !
SetFunctionLength (F, L). - Let targetName be ?
Get (Target,"name" ). - If
Type (targetName) is not String, set targetName to the empty String. - Perform
SetFunctionName (F, targetName,"bound" ). - Return F.
Function objects created using Function.prototype.bind are exotic objects. They also do not have a
If Target is an arrow function or a
20.2.3.3 Function.prototype.call ( thisArg, ...args )
When the call method is called with argument thisArg and zero or more args, the following steps are taken:
- Let func be the
this value. - If
IsCallable (func) isfalse , throw aTypeError exception. - Perform
PrepareForTailCall (). - Return ?
Call (func, thisArg, args).
The thisArg value is passed without modification as the
If func is an arrow function or a
20.2.3.4 Function.prototype.constructor
The initial value of Function.prototype.constructor is
20.2.3.5 Function.prototype.toString ( )
When the toString method is called, the following steps are taken:
- Let func be the
this value. - If
Type (func) is Object and func has a [[SourceText]] internal slot and func.[[SourceText]] is a sequence of Unicode code points and !HostHasSourceTextAvailable (func) istrue , then- Return !
CodePointsToString (func.[[SourceText]]).
- Return !
- If func is a
built-in function object , return animplementation-defined String source code representation of func. The representation must have the syntax of aNativeFunction . Additionally, if func has an [[InitialName]] internal slot and func.[[InitialName]] is a String, the portion of the returned String that would be matched byNativeFunctionAccessor opt PropertyName must be the value of func.[[InitialName]]. - If
Type (func) is Object andIsCallable (func) istrue , return animplementation-defined String source code representation of func. The representation must have the syntax of aNativeFunction . - Throw a
TypeError exception.
20.2.3.6 Function.prototype [ @@hasInstance ] ( V )
When the @@hasInstance method of an object F is called with value V, the following steps are taken:
- Let F be the
this value. - Return ?
OrdinaryHasInstance (F, V).
The value of the
This property has the attributes { [[Writable]]:
This is the default implementation of @@hasInstance that most functions inherit. @@hasInstance is called by the instanceof operator to determine whether a value is an instance of a specific
v instanceof Fevaluates as
F[@@hasInstance](v)A instanceof by exposing a different @@hasInstance method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.
20.2.4 Function Instances
Every Function instance is an ECMAScript Function.prototype.bind method (
Function instances have the following properties:
20.2.4.1 length
The value of the
20.2.4.2 name
The value of the
Anonymous functions objects that do not have a contextual name associated with them by this specification use the empty String as the value of the
20.2.4.3 prototype
Function instances that can be used as a
This property has the attributes { [[Writable]]:
Function objects created using Function.prototype.bind, or by evaluating a
20.2.5 HostHasSourceTextAvailable ( func )
The
An implementation of HostHasSourceTextAvailable must complete normally in all cases. This operation must be deterministic with respect to its parameters. Each time it is called with a specific func as its argument, it must return the same completion record. The default implementation of HostHasSourceTextAvailable is to unconditionally return a normal completion with a value of
20.3 Boolean Objects
20.3.1 The Boolean Constructor
The Boolean
- is %Boolean%.
- is the initial value of the
"Boolean" property of theglobal object . - creates and initializes a new Boolean object when called as a
constructor . - performs a type conversion when called as a function rather than as a
constructor . - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include asupercall to the Booleanconstructor to create and initialize the subclass instance with a [[BooleanData]] internal slot.
20.3.1.1 Boolean ( value )
When Boolean is called with argument value, the following steps are taken:
- Let b be !
ToBoolean (value). - If NewTarget is
undefined , return b. - Let O be ?
OrdinaryCreateFromConstructor (NewTarget,"%Boolean.prototype%" , « [[BooleanData]] »). - Set O.[[BooleanData]] to b.
- Return O.
20.3.2 Properties of the Boolean Constructor
The Boolean
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
20.3.2.1 Boolean.prototype
The initial value of Boolean.prototype is the
This property has the attributes { [[Writable]]:
20.3.3 Properties of the Boolean Prototype Object
The Boolean prototype object:
- is %Boolean.prototype%.
- is an
ordinary object . - is itself a Boolean object; it has a [[BooleanData]] internal slot with the value
false . - has a [[Prototype]] internal slot whose value is
%Object.prototype% .
The abstract operation thisBooleanValue takes argument value. It performs the following steps when called:
20.3.3.1 Boolean.prototype.constructor
The initial value of Boolean.prototype.constructor is
20.3.3.2 Boolean.prototype.toString ( )
The following steps are taken:
- Let b be ?
thisBooleanValue (this value). - If b is
true , return"true" ; else return"false" .
20.3.3.3 Boolean.prototype.valueOf ( )
The following steps are taken:
- Return ?
thisBooleanValue (this value).
20.3.4 Properties of Boolean Instances
Boolean instances are ordinary objects that inherit properties from the
20.4 Symbol Objects
20.4.1 The Symbol Constructor
The Symbol
- is %Symbol%.
- is the initial value of the
"Symbol" property of theglobal object . - returns a new Symbol value when called as a function.
- is not intended to be used with the
newoperator. - is not intended to be subclassed.
- may be used as the value of an
extendsclause of a class definition but asupercall to it will cause an exception.
20.4.1.1 Symbol ( [ description ] )
When Symbol is called with optional argument description, the following steps are taken:
- If NewTarget is not
undefined , throw aTypeError exception. - If description is
undefined , let descString beundefined . - Else, let descString be ?
ToString (description). - Return a new unique Symbol value whose [[Description]] value is descString.
20.4.2 Properties of the Symbol Constructor
The Symbol
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
20.4.2.1 Symbol.asyncIterator
The initial value of Symbol.asyncIterator is the well known symbol
This property has the attributes { [[Writable]]:
20.4.2.2 Symbol.for ( key )
When Symbol.for is called with argument key it performs the following steps:
- Let stringKey be ?
ToString (key). - For each element e of the GlobalSymbolRegistry
List , do- If
SameValue (e.[[Key]], stringKey) istrue , return e.[[Symbol]].
- If
Assert : GlobalSymbolRegistry does not currently contain an entry for stringKey.- Let newSymbol be a new unique Symbol value whose [[Description]] value is stringKey.
- Append the
Record { [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistryList . - Return newSymbol.
The GlobalSymbolRegistry is a
20.4.2.3 Symbol.hasInstance
The initial value of Symbol.hasInstance is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.4 Symbol.isConcatSpreadable
The initial value of Symbol.isConcatSpreadable is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.5 Symbol.iterator
The initial value of Symbol.iterator is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.6 Symbol.keyFor ( sym )
When Symbol.keyFor is called with argument sym it performs the following steps:
20.4.2.7 Symbol.match
The initial value of Symbol.match is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.8 Symbol.matchAll
The initial value of Symbol.matchAll is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.9 Symbol.prototype
The initial value of Symbol.prototype is the
This property has the attributes { [[Writable]]:
20.4.2.10 Symbol.replace
The initial value of Symbol.replace is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.11 Symbol.search
The initial value of Symbol.search is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.12 Symbol.species
The initial value of Symbol.species is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.13 Symbol.split
The initial value of Symbol.split is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.14 Symbol.toPrimitive
The initial value of Symbol.toPrimitive is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.15 Symbol.toStringTag
The initial value of Symbol.toStringTag is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.2.16 Symbol.unscopables
The initial value of Symbol.unscopables is the well-known symbol
This property has the attributes { [[Writable]]:
20.4.3 Properties of the Symbol Prototype Object
The Symbol prototype object:
- is %Symbol.prototype%.
- is an
ordinary object . - is not a Symbol instance and does not have a [[SymbolData]] internal slot.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% .
The abstract operation thisSymbolValue takes argument value. It performs the following steps when called:
20.4.3.1 Symbol.prototype.constructor
The initial value of Symbol.prototype.constructor is
20.4.3.2 get Symbol.prototype.description
Symbol.prototype.description is an
- Let s be the
this value. - Let sym be ?
thisSymbolValue (s). - Return sym.[[Description]].
20.4.3.3 Symbol.prototype.toString ( )
The following steps are taken:
- Let sym be ?
thisSymbolValue (this value). - Return
SymbolDescriptiveString (sym).
20.4.3.3.1 SymbolDescriptiveString ( sym )
The abstract operation SymbolDescriptiveString takes argument sym. It performs the following steps when called:
Assert :Type (sym) is Symbol.- Let desc be sym's [[Description]] value.
- If desc is
undefined , set desc to the empty String. Assert :Type (desc) is String.- Return the
string-concatenation of"Symbol(" , desc, and")" .
20.4.3.4 Symbol.prototype.valueOf ( )
The following steps are taken:
- Return ?
thisSymbolValue (this value).
20.4.3.5 Symbol.prototype [ @@toPrimitive ] ( hint )
This function is called by ECMAScript language operators to convert a Symbol object to a primitive value.
When the @@toPrimitive method is called with argument hint, the following steps are taken:
- Return ?
thisSymbolValue (this value).
The value of the
This property has the attributes { [[Writable]]:
The argument is ignored.
20.4.3.6 Symbol.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
20.4.4 Properties of Symbol Instances
Symbol instances are ordinary objects that inherit properties from the
20.5 Error Objects
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in name property of the prototype object, in the message property of the prototype object, and in the presence of the errors property.
20.5.1 The Error Constructor
The Error
- is %Error%.
- is the initial value of the
"Error" property of theglobal object . - creates and initializes a new Error object when called as a function rather than as a
constructor . Thus the function callError(…)is equivalent to the object creation expressionnew Error(…)with the same arguments. - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include asupercall to the Errorconstructor to create and initialize subclass instances with an [[ErrorData]] internal slot.
20.5.1.1 Error ( message )
When the Error function is called with argument message, the following steps are taken:
- If NewTarget is
undefined , let newTarget be theactive function object ; else let newTarget be NewTarget. - Let O be ?
OrdinaryCreateFromConstructor (newTarget,"%Error.prototype%" , « [[ErrorData]] »). - If message is not
undefined , then- Let msg be ?
ToString (message). - Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]:
true , [[Enumerable]]:false , [[Configurable]]:true }. - Perform !
DefinePropertyOrThrow (O,"message" , msgDesc).
- Let msg be ?
- Return O.
20.5.2 Properties of the Error Constructor
The Error
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
20.5.2.1 Error.prototype
The initial value of Error.prototype is the
This property has the attributes { [[Writable]]:
20.5.3 Properties of the Error Prototype Object
The Error prototype object:
- is %Error.prototype%.
- is an
ordinary object . - is not an Error instance and does not have an [[ErrorData]] internal slot.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% .
20.5.3.1 Error.prototype.constructor
The initial value of Error.prototype.constructor is
20.5.3.2 Error.prototype.message
The initial value of Error.prototype.message is the empty String.
20.5.3.3 Error.prototype.name
The initial value of Error.prototype.name is
20.5.3.4 Error.prototype.toString ( )
The following steps are taken:
- Let O be the
this value. - If
Type (O) is not Object, throw aTypeError exception. - Let name be ?
Get (O,"name" ). - If name is
undefined , set name to"Error" ; otherwise set name to ?ToString (name). - Let msg be ?
Get (O,"message" ). - If msg is
undefined , set msg to the empty String; otherwise set msg to ?ToString (msg). - If name is the empty String, return msg.
- If msg is the empty String, return name.
- Return the
string-concatenation of name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and msg.
20.5.4 Properties of Error Instances
Error instances are ordinary objects that inherit properties from the Object.prototype.toString.
20.5.5 Native Error Types Used in This Standard
A new instance of one of the NativeError objects below or of the AggregateError object is thrown when a runtime error is detected. All NativeError objects share the same structure, as described in
20.5.5.1 EvalError
The EvalError
This exception is not currently used within this specification. This object remains for compatibility with previous editions of this specification.
20.5.5.2 RangeError
The RangeError
Indicates a value that is not in the set or range of allowable values.
20.5.5.3 ReferenceError
The ReferenceError
Indicate that an invalid reference has been detected.
20.5.5.4 SyntaxError
The SyntaxError
Indicates that a parsing error has occurred.
20.5.5.5 TypeError
The TypeError
TypeError is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause.
20.5.5.6 URIError
The URIError
Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.
20.5.6 NativeError Object Structure
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in
For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from
20.5.6.1 The NativeError Constructors
Each NativeError
- creates and initializes a new NativeError object when called as a function rather than as a
constructor . A call of the object as a function is equivalent to calling it as aconstructor with the same arguments. Thus the function callNativeError(…)is equivalent to the object creation expressionnew NativeError(…)with the same arguments. - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include asupercall to the NativeErrorconstructor to create and initialize subclass instances with an [[ErrorData]] internal slot.
20.5.6.1.1 NativeError ( message )
When a NativeError function is called with argument message, the following steps are taken:
- If NewTarget is
undefined , let newTarget be theactive function object ; else let newTarget be NewTarget. - Let O be ?
OrdinaryCreateFromConstructor (newTarget,"%NativeError.prototype%", « [[ErrorData]] »). - If message is not
undefined , then- Let msg be ?
ToString (message). - Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]:
true , [[Enumerable]]:false , [[Configurable]]:true }. - Perform !
DefinePropertyOrThrow (O,"message" , msgDesc).
- Let msg be ?
- Return O.
The actual value of the string passed in step
20.5.6.2 Properties of the NativeError Constructors
Each NativeError
- has a [[Prototype]] internal slot whose value is
%Error% . - has a
"name" property whose value is the String value"NativeError" . - has the following properties:
20.5.6.2.1 NativeError.prototype
The initial value of NativeError.prototype is a NativeError prototype object (
This property has the attributes { [[Writable]]:
20.5.6.3 Properties of the NativeError Prototype Objects
Each NativeError prototype object:
- is an
ordinary object . - is not an Error instance and does not have an [[ErrorData]] internal slot.
- has a [[Prototype]] internal slot whose value is
%Error.prototype% .
20.5.6.3.1 NativeError.prototype.constructor
The initial value of the
20.5.6.3.2 NativeError.prototype.message
The initial value of the
20.5.6.3.3 NativeError.prototype.name
The initial value of the
20.5.6.4 Properties of NativeError Instances
NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value is Object.prototype.toString (
20.5.7 AggregateError Objects
20.5.7.1 The AggregateError Constructor
The AggregateError
- is %AggregateError%.
- is the initial value of the
"AggregateError" property of theglobal object . - creates and initializes a new AggregateError object when called as a function rather than as a
constructor . Thus the function callAggregateError(…)is equivalent to the object creation expressionnew AggregateError(…)with the same arguments. - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified AggregateError behaviour must include asupercall to the AggregateErrorconstructor to create and initialize subclass instances with an [[ErrorData]] internal slot.
20.5.7.1.1 AggregateError ( errors, message )
When the
- If NewTarget is
undefined , let newTarget be theactive function object ; else let newTarget be NewTarget. - Let O be ?
OrdinaryCreateFromConstructor (newTarget,"%AggregateError.prototype%" , « [[ErrorData]] »). - If message is not
undefined , then- Let msg be ?
ToString (message). - Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]:
true , [[Enumerable]]:false , [[Configurable]]:true }. - Perform !
DefinePropertyOrThrow (O,"message" , msgDesc).
- Let msg be ?
- Let errorsList be ?
IterableToList (errors). - Perform !
DefinePropertyOrThrow (O,"errors" , PropertyDescriptor { [[Configurable]]:true , [[Enumerable]]:false , [[Writable]]:true , [[Value]]: !CreateArrayFromList (errorsList) }). - Return O.
20.5.7.2 Properties of the AggregateError Constructor
The AggregateError
- has a [[Prototype]] internal slot whose value is
%Error% . - has the following properties:
20.5.7.2.1 AggregateError.prototype
The initial value of AggregateError.prototype is
This property has the attributes { [[Writable]]:
20.5.7.3 Properties of the AggregateError Prototype Object
The AggregateError prototype object:
- is %AggregateError.prototype%.
- is an
ordinary object . - is not an Error instance or an AggregateError instance and does not have an [[ErrorData]] internal slot.
- has a [[Prototype]] internal slot whose value is
%Error.prototype% .
20.5.7.3.1 AggregateError.prototype.constructor
The initial value of AggregateError.prototype.constructor is
20.5.7.3.2 AggregateError.prototype.message
The initial value of AggregateError.prototype.message is the empty String.
20.5.7.3.3 AggregateError.prototype.name
The initial value of AggregateError.prototype.name is
20.5.7.4 Properties of AggregateError Instances
AggregateError instances are ordinary objects that inherit properties from their Object.prototype.toString (
21 Numbers and Dates
21.1 Number Objects
21.1.1 The Number Constructor
The Number
- is %Number%.
- is the initial value of the
"Number" property of theglobal object . - creates and initializes a new Number object when called as a
constructor . - performs a type conversion when called as a function rather than as a
constructor . - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include asupercall to the Numberconstructor to create and initialize the subclass instance with a [[NumberData]] internal slot.
21.1.1.1 Number ( value )
When Number is called with argument value, the following steps are taken:
- If value is present, then
- Else,
- Let n be
+0 𝔽.
- Let n be
- If NewTarget is
undefined , return n. - Let O be ?
OrdinaryCreateFromConstructor (NewTarget,"%Number.prototype%" , « [[NumberData]] »). - Set O.[[NumberData]] to n.
- Return O.
21.1.2 Properties of the Number Constructor
The Number
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
21.1.2.1 Number.EPSILON
The value of Number.EPSILON is the
This property has the attributes { [[Writable]]:
21.1.2.2 Number.isFinite ( number )
When Number.isFinite is called with one argument number, the following steps are taken:
- If
Type (number) is not Number, returnfalse . - If number is
NaN ,+∞ 𝔽, or-∞ 𝔽, returnfalse . - Otherwise, return
true .
21.1.2.3 Number.isInteger ( number )
When Number.isInteger is called with one argument number, the following steps are taken:
- Return !
IsIntegralNumber (number).
21.1.2.4 Number.isNaN ( number )
When Number.isNaN is called with one argument number, the following steps are taken:
- If
Type (number) is not Number, returnfalse . - If number is
NaN , returntrue . - Otherwise, return
false .
This function differs from the global isNaN function (
21.1.2.5 Number.isSafeInteger ( number )
When Number.isSafeInteger is called with one argument number, the following steps are taken:
- If !
IsIntegralNumber (number) istrue , then - Return
false .
21.1.2.6 Number.MAX_SAFE_INTEGER
The value of Number.MAX_SAFE_INTEGER is the largest
The value of Number.MAX_SAFE_INTEGER is
This property has the attributes { [[Writable]]:
21.1.2.7 Number.MAX_VALUE
The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately
This property has the attributes { [[Writable]]:
21.1.2.8 Number.MIN_SAFE_INTEGER
The value of Number.MIN_SAFE_INTEGER is the smallest
The value of Number.MIN_SAFE_INTEGER is
This property has the attributes { [[Writable]]:
21.1.2.9 Number.MIN_VALUE
The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately
In the Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]:
21.1.2.10 Number.NaN
The value of Number.NaN is
This property has the attributes { [[Writable]]:
21.1.2.11 Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is
This property has the attributes { [[Writable]]:
21.1.2.12 Number.parseFloat ( string )
The value of the Number.parseFloat
21.1.2.13 Number.parseInt ( string, radix )
The value of the Number.parseInt
21.1.2.14 Number.POSITIVE_INFINITY
The value of Number.POSITIVE_INFINITY is
This property has the attributes { [[Writable]]:
21.1.2.15 Number.prototype
The initial value of Number.prototype is the
This property has the attributes { [[Writable]]:
21.1.3 Properties of the Number Prototype Object
The Number prototype object:
- is %Number.prototype%.
- is an
ordinary object . - is itself a Number object; it has a [[NumberData]] internal slot with the value
+0 𝔽. - has a [[Prototype]] internal slot whose value is
%Object.prototype% .
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the
The abstract operation thisNumberValue takes argument value. It performs the following steps when called:
The phrase “this
21.1.3.1 Number.prototype.constructor
The initial value of Number.prototype.constructor is
21.1.3.2 Number.prototype.toExponential ( fractionDigits )
Return a String containing this
- Let x be ?
thisNumberValue (this value). - Let f be ?
ToIntegerOrInfinity (fractionDigits). Assert : If fractionDigits isundefined , then f is 0.- If x is not finite, return !
Number::toString (x). - If f < 0 or f > 100, throw a
RangeError exception. - Set x to
ℝ (x). - Let s be the empty String.
- If x < 0, then
- Set s to
"-" . - Set x to -x.
- Set s to
- If x = 0, then
- Let m be the String value consisting of f + 1 occurrences of the code unit 0x0030 (DIGIT ZERO).
- Let e be 0.
- Else,
- If fractionDigits is not
undefined , then- Let e and n be integers such that 10f ≤ n < 10f + 1 and for which n × 10e - n - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - f is larger.
- Else,
- Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f + 1, n × 10e - f is x, and f is as small as possible. Note that the decimal representation of n has f + 1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
- Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
- If fractionDigits is not
- If f ≠ 0, then
- Let a be the first code unit of m.
- Let b be the other f code units of m.
- Set m to the
string-concatenation of a,"." , and b.
- If e = 0, then
- Let c be
"+" . - Let d be
"0" .
- Let c be
- Else,
- If e > 0, let c be
"+" . - Else,
Assert : e < 0.- Let c be
"-" . - Set e to -e.
- Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
- If e > 0, let c be
- Set m to the
string-concatenation of m,"e" , c, and d. - Return the
string-concatenation of s and m.
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step
- Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f + 1, n × 10e - f is x, and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which n × 10e - f is closest in value to x. If there are two such possible values of n, choose the one that is even.
21.1.3.3 Number.prototype.toFixed ( fractionDigits )
toFixed returns a String containing this
The following steps are performed:
- Let x be ?
thisNumberValue (this value). - Let f be ?
ToIntegerOrInfinity (fractionDigits). Assert : If fractionDigits isundefined , then f is 0.- If f is not finite, throw a
RangeError exception. - If f < 0 or f > 100, throw a
RangeError exception. - If x is not finite, return !
Number::toString (x). - Set x to
ℝ (x). - Let s be the empty String.
- If x < 0, then
- Set s to
"-" . - Set x to -x.
- Set s to
- If x ≥ 1021, then
- Else,
- Let n be an
integer for which n / 10f - x is as close to zero as possible. If there are two such n, pick the larger n. - If n = 0, let m be the String
"0" . Otherwise, let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes). - If f ≠ 0, then
- Let k be the length of m.
- If k ≤ f, then
- Let z be the String value consisting of f + 1 - k occurrences of the code unit 0x0030 (DIGIT ZERO).
- Set m to the
string-concatenation of z and m. - Set k to f + 1.
- Let a be the first k - f code units of m.
- Let b be the other f code units of m.
- Set m to the
string-concatenation of a,"." , and b.
- Let n be an
- Return the
string-concatenation of s and m.
The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent Number values. For example,
(1000000000000000128).toString() returns
(1000000000000000128).toFixed(0) returns
21.1.3.4 Number.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Number.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Produces a String value that represents this toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
21.1.3.5 Number.prototype.toPrecision ( precision )
Return a String containing this
- Let x be ?
thisNumberValue (this value). - If precision is
undefined , return !ToString (x). - Let p be ?
ToIntegerOrInfinity (precision). - If x is not finite, return !
Number::toString (x). - If p < 1 or p > 100, throw a
RangeError exception. - Set x to
ℝ (x). - Let s be the empty String.
- If x < 0, then
- Set s to the code unit 0x002D (HYPHEN-MINUS).
- Set x to -x.
- If x = 0, then
- Let m be the String value consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
- Let e be 0.
- Else,
- Let e and n be integers such that 10p - 1 ≤ n < 10p and for which n × 10e - p + 1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - p + 1 is larger.
- Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
- If e < -6 or e ≥ p, then
Assert : e ≠ 0.- If p ≠ 1, then
- Let a be the first code unit of m.
- Let b be the other p - 1 code units of m.
- Set m to the
string-concatenation of a,"." , and b.
- If e > 0, then
- Let c be the code unit 0x002B (PLUS SIGN).
- Else,
Assert : e < 0.- Let c be the code unit 0x002D (HYPHEN-MINUS).
- Set e to -e.
- Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
- Return the
string-concatenation of s, m, the code unit 0x0065 (LATIN SMALL LETTER E), c, and d.
- If e = p - 1, return the
string-concatenation of s and m. - If e ≥ 0, then
- Set m to the
string-concatenation of the first e + 1 code units of m, the code unit 0x002E (FULL STOP), and the remaining p - (e + 1) code units of m.
- Set m to the
- Else,
- Set m to the
string-concatenation of the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e + 1) occurrences of the code unit 0x0030 (DIGIT ZERO), and the String m.
- Set m to the
- Return the
string-concatenation of s and m.
21.1.3.6 Number.prototype.toString ( [ radix ] )
The optional radix should be an
The following steps are performed:
- Let x be ?
thisNumberValue (this value). - If radix is
undefined , let radixMV be 10. - Else, let radixMV be ?
ToIntegerOrInfinity (radix). - If radixMV < 2 or radixMV > 36, throw a
RangeError exception. - If radixMV = 10, return !
ToString (x). - Return the String representation of this
Number value using the radix specified by radixMV. Lettersa-zare used for digits with values 10 through 35. The precise algorithm isimplementation-defined , however the algorithm should be a generalization of that specified in6.1.6.1.20 .
The toString function is not generic; it throws a
The toString method is
21.1.3.7 Number.prototype.valueOf ( )
- Return ?
thisNumberValue (this value).
21.1.4 Properties of Number Instances
Number instances are ordinary objects that inherit properties from the
21.2 BigInt Objects
21.2.1 The BigInt Constructor
The BigInt
- is %BigInt%.
- is the initial value of the
"BigInt" property of theglobal object . - performs a type conversion when called as a function rather than as a
constructor . - is not intended to be used with the
newoperator or to be subclassed. It may be used as the value of anextendsclause of a class definition but asupercall to the BigIntconstructor will cause an exception.
21.2.1.1 BigInt ( value )
When BigInt is called with argument value, the following steps are taken:
- If NewTarget is not
undefined , throw aTypeError exception. - Let prim be ?
ToPrimitive (value,number ). - If
Type (prim) is Number, return ?NumberToBigInt (prim). - Otherwise, return ?
ToBigInt (value).
21.2.1.1.1 NumberToBigInt ( number )
The abstract operation NumberToBigInt takes argument number (a Number). It performs the following steps when called:
- If
IsIntegralNumber (number) isfalse , throw aRangeError exception. - Return the BigInt value that represents
ℝ (number).
21.2.2 Properties of the BigInt Constructor
The value of the [[Prototype]] internal slot of the BigInt
The BigInt
21.2.2.1 BigInt.asIntN ( bits, bigint )
When the BigInt.asIntN function is called with two arguments bits and bigint, the following steps are taken:
21.2.2.2 BigInt.asUintN ( bits, bigint )
When the BigInt.asUintN function is called with two arguments bits and bigint, the following steps are taken:
21.2.2.3 BigInt.prototype
The initial value of BigInt.prototype is the
This property has the attributes { [[Writable]]:
21.2.3 Properties of the BigInt Prototype Object
The BigInt prototype object:
- is %BigInt.prototype%.
- is an
ordinary object . - is not a BigInt object; it does not have a [[BigIntData]] internal slot.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% .
The abstract operation thisBigIntValue takes argument value. It performs the following steps when called:
The phrase “this BigInt value” within the specification of a method refers to the result returned by calling the abstract operation
21.2.3.1 BigInt.prototype.constructor
The initial value of BigInt.prototype.constructor is
21.2.3.2 BigInt.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the BigInt.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
Produces a String value that represents this BigInt value formatted according to the conventions of the toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
21.2.3.3 BigInt.prototype.toString ( [ radix ] )
The optional radix should be an
The following steps are performed:
- Let x be ?
thisBigIntValue (this value). - If radix is
undefined , let radixMV be 10. - Else, let radixMV be ?
ToIntegerOrInfinity (radix). - If radixMV < 2 or radixMV > 36, throw a
RangeError exception. - If radixMV = 10, return !
ToString (x). - Return the String representation of this
Number value using the radix specified by radixMV. Lettersa-zare used for digits with values 10 through 35. The precise algorithm isimplementation-defined , however the algorithm should be a generalization of that specified in6.1.6.2.23 .
The toString function is not generic; it throws a
21.2.3.4 BigInt.prototype.valueOf ( )
- Return ?
thisBigIntValue (this value).
21.2.3.5 BigInt.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
21.3 The Math Object
The Math object:
- is %Math%.
- is the initial value of the
"Math" property of theglobal object . - is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is not a
function object . - does not have a [[Construct]] internal method; it cannot be used as a
constructor with thenewoperator. - does not have a [[Call]] internal method; it cannot be invoked as a function.
In this specification, the phrase “the
21.3.1 Value Properties of the Math Object
21.3.1.1 Math.E
The
This property has the attributes { [[Writable]]:
21.3.1.2 Math.LN10
The
This property has the attributes { [[Writable]]:
21.3.1.3 Math.LN2
The
This property has the attributes { [[Writable]]:
21.3.1.4 Math.LOG10E
The
This property has the attributes { [[Writable]]:
The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.
21.3.1.5 Math.LOG2E
The
This property has the attributes { [[Writable]]:
The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.
21.3.1.6 Math.PI
The
This property has the attributes { [[Writable]]:
21.3.1.7 Math.SQRT1_2
The
This property has the attributes { [[Writable]]:
The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.
21.3.1.8 Math.SQRT2
The
This property has the attributes { [[Writable]]:
21.3.1.9 Math [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
21.3.2 Function Properties of the Math Object
The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log, log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
21.3.2.1 Math.abs ( x )
Returns the absolute value of x; the result has the same magnitude as x but has positive sign.
When the Math.abs method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , returnNaN . - If n is
-0 𝔽, return+0 𝔽. - If n is
-∞ 𝔽, return+∞ 𝔽. - If n <
+0 𝔽, return -n. - Return n.
21.3.2.2 Math.acos ( x )
Returns the inverse cosine of x. The result is expressed in radians and ranges from
When the Math.acos method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n >1 𝔽, or n <-1 𝔽, returnNaN . - If n is
1 𝔽, return+0 𝔽. - Return an
implementation-approximated value representing the result of the inverse cosine ofℝ (n).
21.3.2.3 Math.acosh ( x )
Returns the inverse hyperbolic cosine of x.
When the Math.acosh method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN or n is+∞ 𝔽, return n. - If n is
1 𝔽, return+0 𝔽. - If n <
1 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the inverse hyperbolic cosine ofℝ (n).
21.3.2.4 Math.asin ( x )
Returns the inverse sine of x. The result is expressed in radians and ranges from
When the Math.asin method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, or n is-0 𝔽, return n. - If n >
1 𝔽 or n <-1 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the inverse sine ofℝ (n).
21.3.2.5 Math.asinh ( x )
Returns the inverse hyperbolic sine of x.
When the Math.asinh method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, n is+∞ 𝔽, or n is-∞ 𝔽, return n. - Return an
implementation-approximated value representing the result of the inverse hyperbolic sine ofℝ (n).
21.3.2.6 Math.atan ( x )
Returns the inverse tangent of x. The result is expressed in radians and ranges from
When the Math.atan method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, or n is-0 𝔽, return n. - If n is
+∞ 𝔽, return animplementation-approximated value representing π / 2. - If n is
-∞ 𝔽, return animplementation-approximated value representing -π / 2. - Return an
implementation-approximated value representing the result of the inverse tangent ofℝ (n).
21.3.2.7 Math.atanh ( x )
Returns the inverse hyperbolic tangent of x.
When the Math.atanh method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, or n is-0 𝔽, return n. - If n >
1 𝔽 or n <-1 𝔽, returnNaN . - If n is
1 𝔽, return+∞ 𝔽. - If n is
-1 𝔽, return-∞ 𝔽. - Return an
implementation-approximated value representing the result of the inverse hyperbolic tangent ofℝ (n).
21.3.2.8 Math.atan2 ( y, x )
Returns the inverse tangent of the quotient
When the Math.atan2 method is called with arguments y and x, the following steps are taken:
- Let ny be ?
ToNumber (y). - Let nx be ?
ToNumber (x). - If ny is
NaN or nx isNaN , returnNaN . - If ny is
+∞ 𝔽, then- If nx is
+∞ 𝔽, return animplementation-approximated value representing π / 4. - If nx is
-∞ 𝔽, return animplementation-approximated value representing 3π / 4. - Return an
implementation-approximated value representing π / 2.
- If nx is
- If ny is
-∞ 𝔽, then- If nx is
+∞ 𝔽, return animplementation-approximated value representing -π / 4. - If nx is
-∞ 𝔽, return animplementation-approximated value representing -3π / 4. - Return an
implementation-approximated value representing -π / 2.
- If nx is
- If ny is
+0 𝔽, then- If nx >
+0 𝔽 or nx is+0 𝔽, return+0 𝔽. - Return an
implementation-approximated value representing π.
- If nx >
- If ny is
-0 𝔽, then- If nx >
+0 𝔽 or nx is+0 𝔽, return-0 𝔽. - Return an
implementation-approximated value representing -π.
- If nx >
Assert : ny is finite and is neither+0 𝔽 nor-0 𝔽.- If ny >
+0 𝔽, then- If nx is
+∞ 𝔽, return+0 𝔽. - If nx is
-∞ 𝔽, return animplementation-approximated value representing π. - If nx is
+0 𝔽 or nx is-0 𝔽, return animplementation-approximated value representing π / 2.
- If nx is
- If ny <
+0 𝔽, then- If nx is
+∞ 𝔽, return-0 𝔽. - If nx is
-∞ 𝔽, return animplementation-approximated value representing -π. - If nx is
+0 𝔽 or nx is-0 𝔽, return animplementation-approximated value representing -π / 2.
- If nx is
Assert : nx is finite and is neither+0 𝔽 nor-0 𝔽.- Return an
implementation-approximated value representing the result of the inverse tangent of the quotientℝ (ny) /ℝ (nx).
21.3.2.9 Math.cbrt ( x )
Returns the cube root of x.
When the Math.cbrt method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, n is+∞ 𝔽, or n is-∞ 𝔽, return n. - Return an
implementation-approximated value representing the result of the cube root ofℝ (n).
21.3.2.10 Math.ceil ( x )
Returns the smallest (closest to -∞)
When the Math.ceil method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, n is+∞ 𝔽, or n is-∞ 𝔽, return n. - If n <
+0 𝔽 and n >-1 𝔽, return-0 𝔽. - If n is an
integral Number , return n. - Return the smallest (closest to -∞)
integral Number value that is not less than n.
The value of Math.ceil(x) is the same as the value of -Math.floor(-x).
21.3.2.11 Math.clz32 ( x )
When the Math.clz32 method is called with argument x, the following steps are taken:
If n is
21.3.2.12 Math.cos ( x )
Returns the cosine of x. The argument is expressed in radians.
When the Math.cos method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, or n is-0 𝔽, return n. - If n is
+∞ 𝔽 or n is-∞ 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the cosine ofℝ (n).
21.3.2.13 Math.cosh ( x )
Returns the hyperbolic cosine of x.
When the Math.cosh method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+∞ 𝔽, or n is-∞ 𝔽, return n. - If n is
+0 𝔽 or n is-0 𝔽, return1 𝔽. - Return an
implementation-approximated value representing the result of the hyperbolic cosine ofℝ (n).
The value of Math.cosh(x) is the same as the value of (Math.exp(x) + Math.exp(-x)) / 2.
21.3.2.14 Math.exp ( x )
Returns the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).
When the Math.exp method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN or n is+∞ 𝔽, return n. - If n is
+0 𝔽 or n is-0 𝔽, return1 𝔽. - If n is
-∞ 𝔽, return+0 𝔽. - Return an
implementation-approximated value representing the result of the exponential function ofℝ (n).
21.3.2.15 Math.expm1 ( x )
Returns the result of subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close to 0.
When the Math.expm1 method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, or n is+∞ 𝔽, return n. - If n is
-∞ 𝔽, return-1 𝔽. - Return an
implementation-approximated value representing the result of subtracting 1 from the exponential function ofℝ (n).
21.3.2.16 Math.floor ( x )
Returns the greatest (closest to +∞)
When the Math.floor method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, n is+∞ 𝔽, or n is-∞ 𝔽, return n. - If n <
1 𝔽 and n >+0 𝔽, return+0 𝔽. - If n is an
integral Number , return n. - Return the greatest (closest to +∞)
integral Number value that is not greater than n.
The value of Math.floor(x) is the same as the value of -Math.ceil(-x).
21.3.2.17 Math.fround ( x )
When the Math.fround method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , returnNaN . - If n is one of
+0 𝔽,-0 𝔽,+∞ 𝔽, or-∞ 𝔽, return n. - Let n32 be the result of converting n to a value in
IEEE 754-2019 binary32 format using roundTiesToEven mode. - Let n64 be the result of converting n32 to a value in
IEEE 754-2019 binary64 format. - Return the ECMAScript
Number value corresponding to n64.
21.3.2.18 Math.hypot ( ...args )
Returns the square root of the sum of squares of its arguments.
When the Math.hypot method is called with zero or more arguments which form the rest parameter ...args, the following steps are taken:
- Let coerced be a new empty
List . - For each element arg of args, do
- Let n be ?
ToNumber (arg). - Append n to coerced.
- Let n be ?
- For each element number of coerced, do
- If number is
+∞ 𝔽 or number is-∞ 𝔽, return+∞ 𝔽.
- If number is
- Let onlyZero be
true . - For each element number of coerced, do
- If number is
NaN , returnNaN . - If number is neither
+0 𝔽 nor-0 𝔽, set onlyZero tofalse .
- If number is
- If onlyZero is
true , return+0 𝔽. - Return an
implementation-approximated value representing the square root of the sum of squares of the mathematical values of the elements of coerced.
The hypot method is
Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.
21.3.2.19 Math.imul ( x, y )
When Math.imul is called with arguments x and y, the following steps are taken:
21.3.2.20 Math.log ( x )
Returns the natural logarithm of x.
When the Math.log method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN or n is+∞ 𝔽, return n. - If n is
1 𝔽, return+0 𝔽. - If n is
+0 𝔽 or n is-0 𝔽, return-∞ 𝔽. - If n <
+0 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the natural logarithm ofℝ (n).
21.3.2.21 Math.log1p ( x )
Returns the natural logarithm of 1 + x. The result is computed in a way that is accurate even when the value of x is close to zero.
When the Math.log1p method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, or n is+∞ 𝔽, return n. - If n is
-1 𝔽, return-∞ 𝔽. - If n <
-1 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the natural logarithm of 1 +ℝ (n).
21.3.2.22 Math.log10 ( x )
Returns the base 10 logarithm of x.
When the Math.log10 method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN or n is+∞ 𝔽, return n. - If n is
1 𝔽, return+0 𝔽. - If n is
+0 𝔽 or n is-0 𝔽, return-∞ 𝔽. - If n <
+0 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the base 10 logarithm ofℝ (n).
21.3.2.23 Math.log2 ( x )
Returns the base 2 logarithm of x.
When the Math.log2 method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN or n is+∞ 𝔽, return n. - If n is
1 𝔽, return+0 𝔽. - If n is
+0 𝔽 or n is-0 𝔽, return-∞ 𝔽. - If n <
+0 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the base 2 logarithm ofℝ (n).
21.3.2.24 Math.max ( ...args )
Given zero or more arguments, calls
When the Math.max method is called with zero or more arguments which form the rest parameter ...args, the following steps are taken:
- Let coerced be a new empty
List . - For each element arg of args, do
- Let n be ?
ToNumber (arg). - Append n to coerced.
- Let n be ?
- Let highest be
-∞ 𝔽. - For each element number of coerced, do
- If number is
NaN , returnNaN . - If number is
+0 𝔽 and highest is-0 𝔽, set highest to+0 𝔽. - If number > highest, set highest to number.
- If number is
- Return highest.
The comparison of values to determine the largest value is done using the
The max method is
21.3.2.25 Math.min ( ...args )
Given zero or more arguments, calls
When the Math.min method is called with zero or more arguments which form the rest parameter ...args, the following steps are taken:
- Let coerced be a new empty
List . - For each element arg of args, do
- Let n be ?
ToNumber (arg). - Append n to coerced.
- Let n be ?
- Let lowest be
+∞ 𝔽. - For each element number of coerced, do
- If number is
NaN , returnNaN . - If number is
-0 𝔽 and lowest is+0 𝔽, set lowest to-0 𝔽. - If number < lowest, set lowest to number.
- If number is
- Return lowest.
The comparison of values to determine the largest value is done using the
The min method is
21.3.2.26 Math.pow ( base, exponent )
When the Math.pow method is called with arguments base and exponent, the following steps are taken:
21.3.2.27 Math.random ( )
Returns a
Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.
21.3.2.28 Math.round ( x )
Returns the
When the Math.round method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN ,+∞ 𝔽,-∞ 𝔽, or anintegral Number , return n. - If n <
0.5 𝔽 and n >+0 𝔽, return+0 𝔽. - If n <
+0 𝔽 and n ≥-0.5 𝔽, return-0 𝔽. - Return the
integral Number closest to n, preferring the Number closer to +∞ in the case of a tie.
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
The value of Math.round(x) is not always the same as the value of Math.floor(x + 0.5). When x is Math.round(x) returns Math.floor(x + 0.5) returns Math.round(x) may also differ from the value of Math.floor(x + 0.5)because of internal rounding when computing x + 0.5.
21.3.2.29 Math.sign ( x )
Returns the sign of x, indicating whether x is positive, negative, or zero.
When the Math.sign method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, or n is-0 𝔽, return n. - If n <
+0 𝔽, return-1 𝔽. - Return
1 𝔽.
21.3.2.30 Math.sin ( x )
Returns the sine of x. The argument is expressed in radians.
When the Math.sin method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, or n is-0 𝔽, return n. - If n is
+∞ 𝔽 or n is-∞ 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the sine ofℝ (n).
21.3.2.31 Math.sinh ( x )
Returns the hyperbolic sine of x.
When the Math.sinh method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, n is+∞ 𝔽, or n is-∞ 𝔽, return n. - Return an
implementation-approximated value representing the result of the hyperbolic sine ofℝ (n).
The value of Math.sinh(x) is the same as the value of (Math.exp(x) - Math.exp(-x)) / 2.
21.3.2.32 Math.sqrt ( x )
Returns the square root of x.
When the Math.sqrt method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, or n is+∞ 𝔽, return n. - If n <
+0 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the square root ofℝ (n).
21.3.2.33 Math.tan ( x )
Returns the tangent of x. The argument is expressed in radians.
When the Math.tan method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, or n is-0 𝔽, return n. - If n is
+∞ 𝔽, or n is-∞ 𝔽, returnNaN . - Return an
implementation-approximated value representing the result of the tangent ofℝ (n).
21.3.2.34 Math.tanh ( x )
Returns the hyperbolic tangent of x.
When the Math.tanh method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, or n is-0 𝔽, return n. - If n is
+∞ 𝔽, return1 𝔽. - If n is
-∞ 𝔽, return-1 𝔽. - Return an
implementation-approximated value representing the result of the hyperbolic tangent ofℝ (n).
The value of Math.tanh(x) is the same as the value of (Math.exp(x) - Math.exp(-x)) / (Math.exp(x) + Math.exp(-x)).
21.3.2.35 Math.trunc ( x )
Returns the integral part of the number x, removing any fractional digits. If x is already integral, the result is x.
When the Math.trunc method is called with argument x, the following steps are taken:
- Let n be ?
ToNumber (x). - If n is
NaN , n is+0 𝔽, n is-0 𝔽, n is+∞ 𝔽, or n is-∞ 𝔽, return n. - If n <
1 𝔽 and n >+0 𝔽, return+0 𝔽. - If n <
+0 𝔽 and n >-1 𝔽, return-0 𝔽. - Return the
integral Number nearest n in the direction of+0 𝔽.
21.4 Date Objects
21.4.1 Overview of Date Objects and Definitions of Abstract Operations
The following functions are
21.4.1.1 Time Values and Time Range
Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular sharing definition in terms of the proleptic Gregorian calendar, an epoch of midnight at the beginning of 1 January 1970 UTC, and an accounting of every day as comprising exactly 86,400 seconds (each of which is 1000 milliseconds long).
An ECMAScript time value is a Number, either a finite
Time values do not account for UTC leap seconds—there are no time values representing instants within positive leap seconds, and there are time values representing instants removed from the UTC timeline by negative leap seconds. However, the definition of time values nonetheless yields piecewise alignment with UTC, with discontinuities only at leap second boundaries and zero difference outside of leap seconds.
A Number can exactly represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992 (
The exact moment of midnight at the beginning of 1 January 1970 UTC is represented by the time value
The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This yields an average of 365.2425 days per year, which is 31,556,952,000 milliseconds. Therefore, the maximum range a Number could represent exactly with millisecond precision is approximately -285,426 to 285,426 years relative to 1970. The smaller range supported by a time value as specified in this section is approximately -273,790 to 273,790 years relative to 1970.
21.4.1.2 Day Number and Time within Day
A given
where the number of milliseconds per day is
The remainder is called the time within the day:
21.4.1.3 Year Number
ECMAScript uses a proleptic Gregorian calendar to map a day number to a year number and to determine the month and date within that year. In this calendar, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by
All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:
The
A
The leap-year function is
21.4.1.4 Month Number
Months are identified by an
where
A month value of
21.4.1.5 Date Number
A date number is identified by an
21.4.1.6 Week Day
The weekday for a particular
A weekday value of
21.4.1.7 LocalTZA ( t, isUTC )
LocalTZA( t, isUTC ) is an
When isUTC is true,
When isUTC is false,
Input t is nominally a
When
If an implementation does not support a conversion described above or if political rules for time t are not available within the implementation, the result must be
It is recommended that implementations use the time zone information of the IANA Time Zone Database https://www.iana.org/time-zones/.
1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall backward), but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05. LocalTZA(
2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04). LocalTZA(
Local time zone offset values may be positive or negative.
21.4.1.8 LocalTime ( t )
The abstract operation LocalTime takes argument t. It converts t from UTC to local time. It performs the following steps when called:
- Return t +
LocalTZA (t,true ).
Two different input time values
21.4.1.9 UTC ( t )
The abstract operation UTC takes argument t. It converts t from local time to UTC. It performs the following steps when called:
- Return t -
LocalTZA (t,false ).
21.4.1.10 Hours, Minutes, Second, and Milliseconds
The following
where
21.4.1.11 MakeTime ( hour, min, sec, ms )
The abstract operation MakeTime takes arguments hour (a Number), min (a Number), sec (a Number), and ms (a Number). It calculates a number of milliseconds. It performs the following steps when called:
- If hour is not finite or min is not finite or sec is not finite or ms is not finite, return
NaN . - Let h be
𝔽 (!ToIntegerOrInfinity (hour)). - Let m be
𝔽 (!ToIntegerOrInfinity (min)). - Let s be
𝔽 (!ToIntegerOrInfinity (sec)). - Let milli be
𝔽 (!ToIntegerOrInfinity (ms)). - Let t be ((h
*msPerHour +m*msPerMinute )+s*msPerSecond )+milli, performing the arithmetic according toIEEE 754-2019 rules (that is, as if using the ECMAScript operators*and+). - Return t.
21.4.1.12 MakeDay ( year, month, date )
The abstract operation MakeDay takes arguments year (a Number), month (a Number), and date (a Number). It calculates a number of days. It performs the following steps when called:
- If year is not finite or month is not finite or date is not finite, return
NaN . - Let y be
𝔽 (!ToIntegerOrInfinity (year)). - Let m be
𝔽 (!ToIntegerOrInfinity (month)). - Let dt be
𝔽 (!ToIntegerOrInfinity (date)). - Let ym be y +
𝔽 (floor (ℝ (m) / 12)). - If ym is not finite, return
NaN . - Let mn be
𝔽 (ℝ (m)modulo 12). - Find a finite
time value t such thatYearFromTime (t) is ym andMonthFromTime (t) is mn andDateFromTime (t) is1 𝔽; but if this is not possible (because some argument is out of range), returnNaN . - Return
Day (t) + dt -1 𝔽.
21.4.1.13 MakeDate ( day, time )
The abstract operation MakeDate takes arguments day (a Number) and time (a Number). It calculates a number of milliseconds. It performs the following steps when called:
- If day is not finite or time is not finite, return
NaN . - Let tv be day ×
msPerDay + time. - If tv is not finite, return
NaN . - Return tv.
21.4.1.14 TimeClip ( time )
The abstract operation TimeClip takes argument time (a Number). It calculates a number of milliseconds. It performs the following steps when called:
- If time is not finite, return
NaN . - If
abs (ℝ (time)) > 8.64 × 1015, returnNaN . - Return
𝔽 (!ToIntegerOrInfinity (time)).
21.4.1.15 Date Time String Format
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 calendar date extended format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the elements are as follows:
YYYY
|
is the year in the proleptic Gregorian calendar as four decimal digits from 0000 to 9999, or as an |
-
|
|
MM
|
is the month of the year as two decimal digits from 01 (January) to 12 (December). |
DD
|
is the day of the month as two decimal digits from 01 to 31. |
T
|
|
HH
|
is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24. |
:
|
|
mm
|
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59. |
ss
|
is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59. |
.
|
|
sss
|
is the number of complete milliseconds since the start of the second as three decimal digits. |
Z
|
is the UTC offset representation specified as HH:mm (indicating local time ahead of or behind UTC, respectively)
|
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional UTC offset representation appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
A string containing out-of-bounds or nonconforming elements is not a valid instance of this format.
As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the latter form as "end of a calendar day" is consistent with ISO 8601, even though that specification reserves it for describing time intervals and does not permit it within representations of single points in time.
There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, both ISO 8601 and this format specify numeric representations of time zone offsets.
21.4.1.15.1 Expanded Years
Date.parse
Examples of date-time values with expanded years:
| -271821-04-20T00:00:00Z | 271822 B.C. |
| -000001-01-01T00:00:00Z | 2 B.C. |
| +000000-01-01T00:00:00Z | 1 B.C. |
| +000001-01-01T00:00:00Z | 1 A.D. |
| +001970-01-01T00:00:00Z | 1970 A.D. |
| +002009-12-15T00:00:00Z | 2009 A.D. |
| +275760-09-13T00:00:00Z | 275760 A.D. |
21.4.2 The Date Constructor
The Date
- is %Date%.
- is the initial value of the
"Date" property of theglobal object . - creates and initializes a new Date object when called as a
constructor . - returns a String representing the current time (UTC) when called as a function rather than as a
constructor . - is a function whose behaviour differs based upon the number and types of its arguments.
- is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include asupercall to the Dateconstructor to create and initialize the subclass instance with a [[DateValue]] internal slot. - has a
"length" property whose value is7 𝔽.
21.4.2.1 Date ( ...values )
When the Date function is called, the following steps are taken:
- If NewTarget is
undefined , then- Let now be the
time value (UTC) identifying the current time. - Return
ToDateString (now).
- Let now be the
- Let numberOfArgs be the number of elements in values.
- If numberOfArgs = 0, then
- Let dv be the
time value (UTC) identifying the current time.
- Let dv be the
- Else if numberOfArgs = 1, then
- Let value be values[0].
- If
Type (value) is Object and value has a [[DateValue]] internal slot, then- Let tv be !
thisTimeValue (value).
- Let tv be !
- Else,
- Let v be ?
ToPrimitive (value). - If
Type (v) is String, thenAssert : The next step never returns anabrupt completion becauseType (v) is String.- Let tv be the result of parsing v as a date, in exactly the same manner as for the
parsemethod (21.4.3.2 ).
- Else,
- Let tv be ?
ToNumber (v).
- Let tv be ?
- Let v be ?
- Let dv be
TimeClip (tv).
- Else,
Assert : numberOfArgs ≥ 2.- Let y be ?
ToNumber (values[0]). - Let m be ?
ToNumber (values[1]). - If numberOfArgs > 2, let dt be ?
ToNumber (values[2]); else let dt be1 𝔽. - If numberOfArgs > 3, let h be ?
ToNumber (values[3]); else let h be+0 𝔽. - If numberOfArgs > 4, let min be ?
ToNumber (values[4]); else let min be+0 𝔽. - If numberOfArgs > 5, let s be ?
ToNumber (values[5]); else let s be+0 𝔽. - If numberOfArgs > 6, let milli be ?
ToNumber (values[6]); else let milli be+0 𝔽. - If y is
NaN , let yr beNaN . - Else,
- Let yi be !
ToIntegerOrInfinity (y). - If 0 ≤ yi ≤ 99, let yr be
1900 𝔽 +𝔽 (yi); otherwise, let yr be y.
- Let yi be !
- Let finalDate be
MakeDate (MakeDay (yr, m, dt),MakeTime (h, min, s, milli)). - Let dv be
TimeClip (UTC (finalDate)).
- Let O be ?
OrdinaryCreateFromConstructor (NewTarget,"%Date.prototype%" , « [[DateValue]] »). - Set O.[[DateValue]] to dv.
- Return O.
21.4.3 Properties of the Date Constructor
The Date
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
21.4.3.1 Date.now ( )
The now function returns the now.
21.4.3.2 Date.parse ( string )
The parse function applies the parse interprets the resulting String as a date and time; it returns a Number, the UTC Date.parse to return
If the String conforms to the MM or DD elements are absent, HH, mm, or ss elements are absent, sss element is absent,
If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:
x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())However, the expression
Date.parse(x.toLocaleString())is not required to produce the same Date.parse is toString or toUTCString method.
21.4.3.3 Date.prototype
The initial value of Date.prototype is the
This property has the attributes { [[Writable]]:
21.4.3.4 Date.UTC ( year [ , month [ , date [ , hours [ , minutes [ , seconds [ , ms ] ] ] ] ] ] )
When the UTC function is called, the following steps are taken:
- Let y be ?
ToNumber (year). - If month is present, let m be ?
ToNumber (month); else let m be+0 𝔽. - If date is present, let dt be ?
ToNumber (date); else let dt be1 𝔽. - If hours is present, let h be ?
ToNumber (hours); else let h be+0 𝔽. - If minutes is present, let min be ?
ToNumber (minutes); else let min be+0 𝔽. - If seconds is present, let s be ?
ToNumber (seconds); else let s be+0 𝔽. - If ms is present, let milli be ?
ToNumber (ms); else let milli be+0 𝔽. - If y is
NaN , let yr beNaN . - Else,
- Let yi be !
ToIntegerOrInfinity (y). - If 0 ≤ yi ≤ 99, let yr be
1900 𝔽 +𝔽 (yi); otherwise, let yr be y.
- Let yi be !
- Return
TimeClip (MakeDate (MakeDay (yr, m, dt),MakeTime (h, min, s, milli))).
The UTC function is
The UTC function differs from the Date
21.4.4 Properties of the Date Prototype Object
The Date prototype object:
- is %Date.prototype%.
- is itself an
ordinary object . - is not a Date instance and does not have a [[DateValue]] internal slot.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% .
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the
The abstract operation thisTimeValue takes argument value. It performs the following steps when called:
- If
Type (value) is Object and value has a [[DateValue]] internal slot, then- Return value.[[DateValue]].
- Throw a
TypeError exception.
In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is the
21.4.4.1 Date.prototype.constructor
The initial value of Date.prototype.constructor is
21.4.4.2 Date.prototype.getDate ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
DateFromTime (LocalTime (t)).
21.4.4.3 Date.prototype.getDay ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
WeekDay (LocalTime (t)).
21.4.4.4 Date.prototype.getFullYear ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
YearFromTime (LocalTime (t)).
21.4.4.5 Date.prototype.getHours ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
HourFromTime (LocalTime (t)).
21.4.4.6 Date.prototype.getMilliseconds ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
msFromTime (LocalTime (t)).
21.4.4.7 Date.prototype.getMinutes ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
MinFromTime (LocalTime (t)).
21.4.4.8 Date.prototype.getMonth ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
MonthFromTime (LocalTime (t)).
21.4.4.9 Date.prototype.getSeconds ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
SecFromTime (LocalTime (t)).
21.4.4.10 Date.prototype.getTime ( )
The following steps are performed:
- Return ?
thisTimeValue (this value).
21.4.4.11 Date.prototype.getTimezoneOffset ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return (t -
LocalTime (t)) /msPerMinute .
21.4.4.12 Date.prototype.getUTCDate ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
DateFromTime (t).
21.4.4.13 Date.prototype.getUTCDay ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
WeekDay (t).
21.4.4.14 Date.prototype.getUTCFullYear ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
YearFromTime (t).
21.4.4.15 Date.prototype.getUTCHours ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
HourFromTime (t).
21.4.4.16 Date.prototype.getUTCMilliseconds ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
msFromTime (t).
21.4.4.17 Date.prototype.getUTCMinutes ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
MinFromTime (t).
21.4.4.18 Date.prototype.getUTCMonth ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
MonthFromTime (t).
21.4.4.19 Date.prototype.getUTCSeconds ( )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
SecFromTime (t).
21.4.4.20 Date.prototype.setDate ( date )
The following steps are performed:
- Let t be
LocalTime (?thisTimeValue (this value)). - Let dt be ?
ToNumber (date). - Let newDate be
MakeDate (MakeDay (YearFromTime (t),MonthFromTime (t), dt),TimeWithinDay (t)). - Let u be
TimeClip (UTC (newDate)). - Set the [[DateValue]] internal slot of
this Date object to u. - Return u.
21.4.4.21 Date.prototype.setFullYear ( year [ , month [ , date ] ] )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , set t to+0 𝔽; otherwise, set t toLocalTime (t). - Let y be ?
ToNumber (year). - If month is not present, let m be
MonthFromTime (t); otherwise, let m be ?ToNumber (month). - If date is not present, let dt be
DateFromTime (t); otherwise, let dt be ?ToNumber (date). - Let newDate be
MakeDate (MakeDay (y, m, dt),TimeWithinDay (t)). - Let u be
TimeClip (UTC (newDate)). - Set the [[DateValue]] internal slot of
this Date object to u. - Return u.
The setFullYear method is
If month is not present, this method behaves as if month was present with the value getMonth(). If date is not present, it behaves as if date was present with the value getDate().
21.4.4.22 Date.prototype.setHours ( hour [ , min [ , sec [ , ms ] ] ] )
The following steps are performed:
- Let t be
LocalTime (?thisTimeValue (this value)). - Let h be ?
ToNumber (hour). - If min is not present, let m be
MinFromTime (t); otherwise, let m be ?ToNumber (min). - If sec is not present, let s be
SecFromTime (t); otherwise, let s be ?ToNumber (sec). - If ms is not present, let milli be
msFromTime (t); otherwise, let milli be ?ToNumber (ms). - Let date be
MakeDate (Day (t),MakeTime (h, m, s, milli)). - Let u be
TimeClip (UTC (date)). - Set the [[DateValue]] internal slot of
this Date object to u. - Return u.
The setHours method is
If min is not present, this method behaves as if min was present with the value getMinutes(). If sec is not present, it behaves as if sec was present with the value getSeconds(). If ms is not present, it behaves as if ms was present with the value getMilliseconds().
21.4.4.23 Date.prototype.setMilliseconds ( ms )
The following steps are performed:
- Let t be
LocalTime (?thisTimeValue (this value)). - Set ms to ?
ToNumber (ms). - Let time be
MakeTime (HourFromTime (t),MinFromTime (t),SecFromTime (t), ms). - Let u be
TimeClip (UTC (MakeDate (Day (t), time))). - Set the [[DateValue]] internal slot of
this Date object to u. - Return u.
21.4.4.24 Date.prototype.setMinutes ( min [ , sec [ , ms ] ] )
The following steps are performed:
- Let t be
LocalTime (?thisTimeValue (this value)). - Let m be ?
ToNumber (min). - If sec is not present, let s be
SecFromTime (t); otherwise, let s be ?ToNumber (sec). - If ms is not present, let milli be
msFromTime (t); otherwise, let milli be ?ToNumber (ms). - Let date be
MakeDate (Day (t),MakeTime (HourFromTime (t), m, s, milli)). - Let u be
TimeClip (UTC (date)). - Set the [[DateValue]] internal slot of
this Date object to u. - Return u.
The setMinutes method is
If sec is not present, this method behaves as if sec was present with the value getSeconds(). If ms is not present, this behaves as if ms was present with the value getMilliseconds().
21.4.4.25 Date.prototype.setMonth ( month [ , date ] )
The following steps are performed:
- Let t be
LocalTime (?thisTimeValue (this value)). - Let m be ?
ToNumber (month). - If date is not present, let dt be
DateFromTime (t); otherwise, let dt be ?ToNumber (date). - Let newDate be
MakeDate (MakeDay (YearFromTime (t), m, dt),TimeWithinDay (t)). - Let u be
TimeClip (UTC (newDate)). - Set the [[DateValue]] internal slot of
this Date object to u. - Return u.
The setMonth method is
If date is not present, this method behaves as if date was present with the value getDate().
21.4.4.26 Date.prototype.setSeconds ( sec [ , ms ] )
The following steps are performed:
- Let t be
LocalTime (?thisTimeValue (this value)). - Let s be ?
ToNumber (sec). - If ms is not present, let milli be
msFromTime (t); otherwise, let milli be ?ToNumber (ms). - Let date be
MakeDate (Day (t),MakeTime (HourFromTime (t),MinFromTime (t), s, milli)). - Let u be
TimeClip (UTC (date)). - Set the [[DateValue]] internal slot of
this Date object to u. - Return u.
The setSeconds method is
If ms is not present, this method behaves as if ms was present with the value getMilliseconds().
21.4.4.27 Date.prototype.setTime ( time )
The following steps are performed:
- Perform ?
thisTimeValue (this value). - Let t be ?
ToNumber (time). - Let v be
TimeClip (t). - Set the [[DateValue]] internal slot of
this Date object to v. - Return v.
21.4.4.28 Date.prototype.setUTCDate ( date )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - Let dt be ?
ToNumber (date). - Let newDate be
MakeDate (MakeDay (YearFromTime (t),MonthFromTime (t), dt),TimeWithinDay (t)). - Let v be
TimeClip (newDate). - Set the [[DateValue]] internal slot of
this Date object to v. - Return v.
21.4.4.29 Date.prototype.setUTCFullYear ( year [ , month [ , date ] ] )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , set t to+0 𝔽. - Let y be ?
ToNumber (year). - If month is not present, let m be
MonthFromTime (t); otherwise, let m be ?ToNumber (month). - If date is not present, let dt be
DateFromTime (t); otherwise, let dt be ?ToNumber (date). - Let newDate be
MakeDate (MakeDay (y, m, dt),TimeWithinDay (t)). - Let v be
TimeClip (newDate). - Set the [[DateValue]] internal slot of
this Date object to v. - Return v.
The setUTCFullYear method is
If month is not present, this method behaves as if month was present with the value getUTCMonth(). If date is not present, it behaves as if date was present with the value getUTCDate().
21.4.4.30 Date.prototype.setUTCHours ( hour [ , min [ , sec [ , ms ] ] ] )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - Let h be ?
ToNumber (hour). - If min is not present, let m be
MinFromTime (t); otherwise, let m be ?ToNumber (min). - If sec is not present, let s be
SecFromTime (t); otherwise, let s be ?ToNumber (sec). - If ms is not present, let milli be
msFromTime (t); otherwise, let milli be ?ToNumber (ms). - Let newDate be
MakeDate (Day (t),MakeTime (h, m, s, milli)). - Let v be
TimeClip (newDate). - Set the [[DateValue]] internal slot of
this Date object to v. - Return v.
The setUTCHours method is
If min is not present, this method behaves as if min was present with the value getUTCMinutes(). If sec is not present, it behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value getUTCMilliseconds().
21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - Let milli be ?
ToNumber (ms). - Let time be
MakeTime (HourFromTime (t),MinFromTime (t),SecFromTime (t), milli). - Let v be
TimeClip (MakeDate (Day (t), time)). - Set the [[DateValue]] internal slot of
this Date object to v. - Return v.
21.4.4.32 Date.prototype.setUTCMinutes ( min [ , sec [ , ms ] ] )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - Let m be ?
ToNumber (min). - If sec is not present, let s be
SecFromTime (t). - Else,
- Let s be ?
ToNumber (sec).
- Let s be ?
- If ms is not present, let milli be
msFromTime (t). - Else,
- Let milli be ?
ToNumber (ms).
- Let milli be ?
- Let date be
MakeDate (Day (t),MakeTime (HourFromTime (t), m, s, milli)). - Let v be
TimeClip (date). - Set the [[DateValue]] internal slot of
this Date object to v. - Return v.
The setUTCMinutes method is
If sec is not present, this method behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it function behaves as if ms was present with the value return by getUTCMilliseconds().
21.4.4.33 Date.prototype.setUTCMonth ( month [ , date ] )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - Let m be ?
ToNumber (month). - If date is not present, let dt be
DateFromTime (t). - Else,
- Let dt be ?
ToNumber (date).
- Let dt be ?
- Let newDate be
MakeDate (MakeDay (YearFromTime (t), m, dt),TimeWithinDay (t)). - Let v be
TimeClip (newDate). - Set the [[DateValue]] internal slot of
this Date object to v. - Return v.
The setUTCMonth method is
If date is not present, this method behaves as if date was present with the value getUTCDate().
21.4.4.34 Date.prototype.setUTCSeconds ( sec [ , ms ] )
The following steps are performed:
- Let t be ?
thisTimeValue (this value). - Let s be ?
ToNumber (sec). - If ms is not present, let milli be
msFromTime (t). - Else,
- Let milli be ?
ToNumber (ms).
- Let milli be ?
- Let date be
MakeDate (Day (t),MakeTime (HourFromTime (t),MinFromTime (t), s, milli)). - Let v be
TimeClip (date). - Set the [[DateValue]] internal slot of
this Date object to v. - Return v.
The setUTCSeconds method is
If ms is not present, this method behaves as if ms was present with the value getUTCMilliseconds().
21.4.4.35 Date.prototype.toDateString ( )
The following steps are performed:
- Let O be
this Date object . - Let tv be ?
thisTimeValue (O). - If tv is
NaN , return"Invalid Date" . - Let t be
LocalTime (tv). - Return
DateString (t).
21.4.4.36 Date.prototype.toISOString ( )
If
21.4.4.37 Date.prototype.toJSON ( key )
This function provides a String representation of a Date object for use by JSON.stringify (
When the toJSON method is called with argument key, the following steps are taken:
- Let O be ?
ToObject (this value). - Let tv be ?
ToPrimitive (O,number ). - If
Type (tv) is Number and tv is not finite, returnnull . - Return ?
Invoke (O,"toISOString" ).
The argument is ignored.
The toJSON function is intentionally generic; it does not require that its toISOString method.
21.4.4.38 Date.prototype.toLocaleDateString ( [ reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleDateString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleDateString method is used.
This function returns a String value. The contents of the String are
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
21.4.4.39 Date.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
This function returns a String value. The contents of the String are
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
21.4.4.40 Date.prototype.toLocaleTimeString ( [ reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleTimeString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleTimeString method is used.
This function returns a String value. The contents of the String are
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
21.4.4.41 Date.prototype.toString ( )
The following steps are performed:
- Let tv be ?
thisTimeValue (this value). - Return
ToDateString (tv).
For any Date object d such that d.[[DateValue]] is evenly divisible by 1000, the result of Date.parse(d.toString()) = d.valueOf(). See
The toString function is not generic; it throws a
21.4.4.41.1 TimeString ( tv )
The abstract operation TimeString takes argument tv. It performs the following steps when called:
Assert :Type (tv) is Number.Assert : tv is notNaN .- Let hour be the String representation of
HourFromTime (tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary. - Let minute be the String representation of
MinFromTime (tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary. - Let second be the String representation of
SecFromTime (tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary. - Return the
string-concatenation of hour,":" , minute,":" , second, the code unit 0x0020 (SPACE), and"GMT" .
21.4.4.41.2 DateString ( tv )
The abstract operation DateString takes argument tv. It performs the following steps when called:
Assert :Type (tv) is Number.Assert : tv is notNaN .- Let weekday be the Name of the entry in
Table 52 with the NumberWeekDay (tv). - Let month be the Name of the entry in
Table 53 with the NumberMonthFromTime (tv). - Let day be the String representation of
DateFromTime (tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary. - Let yv be
YearFromTime (tv). - If yv ≥
+0 𝔽, let yearSign be the empty String; otherwise, let yearSign be"-" . - Let year be the String representation of
abs (ℝ (yv)), formatted as a decimal number. - Let paddedYear be !
StringPad (year,4 𝔽,"0" ,start ). - Return the
string-concatenation of weekday, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), yearSign, and paddedYear.
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21.4.4.41.3 TimeZoneString ( tv )
The abstract operation TimeZoneString takes argument tv. It performs the following steps when called:
Assert :Type (tv) is Number.Assert : tv is notNaN .- Let offset be
LocalTZA (tv,true ). - If offset ≥
+0 𝔽, then- Let offsetSign be
"+" . - Let absOffset be offset.
- Let offsetSign be
- Else,
- Let offsetSign be
"-" . - Let absOffset be -offset.
- Let offsetSign be
- Let offsetMin be the String representation of
MinFromTime (absOffset), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary. - Let offsetHour be the String representation of
HourFromTime (absOffset), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary. - Let tzName be an
implementation-defined string that is either the empty String or thestring-concatenation of the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), animplementation-defined timezone name, and the code unit 0x0029 (RIGHT PARENTHESIS). - Return the
string-concatenation of offsetSign, offsetHour, offsetMin, and tzName.
21.4.4.41.4 ToDateString ( tv )
The abstract operation ToDateString takes argument tv. It performs the following steps when called:
Assert :Type (tv) is Number.- If tv is
NaN , return"Invalid Date" . - Let t be
LocalTime (tv). - Return the
string-concatenation ofDateString (t), the code unit 0x0020 (SPACE),TimeString (t), andTimeZoneString (tv).
21.4.4.42 Date.prototype.toTimeString ( )
The following steps are performed:
- Let O be
this Date object . - Let tv be ?
thisTimeValue (O). - If tv is
NaN , return"Invalid Date" . - Let t be
LocalTime (tv). - Return the
string-concatenation ofTimeString (t) andTimeZoneString (tv).
21.4.4.43 Date.prototype.toUTCString ( )
The toUTCString method returns a String value representing the instance in time corresponding to
- Let O be
this Date object . - Let tv be ?
thisTimeValue (O). - If tv is
NaN , return"Invalid Date" . - Let weekday be the Name of the entry in
Table 52 with the NumberWeekDay (tv). - Let month be the Name of the entry in
Table 53 with the NumberMonthFromTime (tv). - Let day be the String representation of
DateFromTime (tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary. - Let yv be
YearFromTime (tv). - If yv ≥
+0 𝔽, let yearSign be the empty String; otherwise, let yearSign be"-" . - Let year be the String representation of
abs (ℝ (yv)), formatted as a decimal number. - Let paddedYear be !
StringPad (year,4 𝔽,"0" ,start ). - Return the
string-concatenation of weekday,"," , the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), yearSign, paddedYear, the code unit 0x0020 (SPACE), andTimeString (tv).
21.4.4.44 Date.prototype.valueOf ( )
The following steps are performed:
- Return ?
thisTimeValue (this value).
21.4.4.45 Date.prototype [ @@toPrimitive ] ( hint )
This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed values for hint are
When the @@toPrimitive method is called with argument hint, the following steps are taken:
- Let O be the
this value. - If
Type (O) is not Object, throw aTypeError exception. - If hint is
"string" or"default" , then- Let tryFirst be
string .
- Let tryFirst be
- Else if hint is
"number" , then- Let tryFirst be
number .
- Let tryFirst be
- Else, throw a
TypeError exception. - Return ?
OrdinaryToPrimitive (O, tryFirst).
The value of the
This property has the attributes { [[Writable]]:
21.4.5 Properties of Date Instances
Date instances are ordinary objects that inherit properties from the
22 Text Processing
22.1 String Objects
22.1.1 The String Constructor
The String
- is %String%.
- is the initial value of the
"String" property of theglobal object . - creates and initializes a new String object when called as a
constructor . - performs a type conversion when called as a function rather than as a
constructor . - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include asupercall to the Stringconstructor to create and initialize the subclass instance with a [[StringData]] internal slot.
22.1.1.1 String ( value )
When String is called with argument value, the following steps are taken:
- If value is not present, let s be the empty String.
- Else,
- If NewTarget is
undefined andType (value) is Symbol, returnSymbolDescriptiveString (value). - Let s be ?
ToString (value).
- If NewTarget is
- If NewTarget is
undefined , return s. - Return !
StringCreate (s, ?GetPrototypeFromConstructor (NewTarget,"%String.prototype%" )).
22.1.2 Properties of the String Constructor
The String
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
22.1.2.1 String.fromCharCode ( ...codeUnits )
The String.fromCharCode function may be called with any number of arguments which form the rest parameter codeUnits. The following steps are taken:
The fromCharCode function is
22.1.2.2 String.fromCodePoint ( ...codePoints )
The String.fromCodePoint function may be called with any number of arguments which form the rest parameter codePoints. The following steps are taken:
- Let result be the empty String.
- For each element next of codePoints, do
- Let nextCP be ?
ToNumber (next). - If !
IsIntegralNumber (nextCP) isfalse , throw aRangeError exception. - If
ℝ (nextCP) < 0 orℝ (nextCP) > 0x10FFFF, throw aRangeError exception. - Set result to the
string-concatenation of result and !UTF16EncodeCodePoint (ℝ (nextCP)).
- Let nextCP be ?
Assert : If codePoints is empty, then result is the empty String.- Return result.
The fromCodePoint function is
22.1.2.3 String.prototype
The initial value of String.prototype is the
This property has the attributes { [[Writable]]:
22.1.2.4 String.raw ( template, ...substitutions )
The String.raw function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form the
- Let numberOfSubstitutions be the number of elements in substitutions.
- Let cooked be ?
ToObject (template). - Let raw be ?
ToObject (?Get (cooked,"raw" )). - Let literalSegments be ?
LengthOfArrayLike (raw). - If literalSegments ≤ 0, return the empty String.
- Let stringElements be a new empty
List . - Let nextIndex be 0.
- Repeat,
- Let nextKey be !
ToString (𝔽 (nextIndex)). - Let nextSeg be ?
ToString (?Get (raw, nextKey)). - Append the code unit elements of nextSeg to the end of stringElements.
- If nextIndex + 1 = literalSegments, then
- Return the String value whose code units are the elements in the
List stringElements. If stringElements has no elements, the empty String is returned.
- Return the String value whose code units are the elements in the
- If nextIndex < numberOfSubstitutions, let next be substitutions[nextIndex].
- Else, let next be the empty String.
- Let nextSub be ?
ToString (next). - Append the code unit elements of nextSub to the end of stringElements.
- Set nextIndex to nextIndex + 1.
- Let nextKey be !
The raw function is intended for use as a tag function of a Tagged Template (
22.1.3 Properties of the String Prototype Object
The String prototype object:
- is %String.prototype%.
- is a
String exotic object and has the internal methods specified for such objects. - has a [[StringData]] internal slot whose value is the empty String.
- has a
"length" property whose initial value is+0 𝔽 and whose attributes are { [[Writable]]:false , [[Enumerable]]:false , [[Configurable]]:false }. - has a [[Prototype]] internal slot whose value is
%Object.prototype% .
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the
The abstract operation thisStringValue takes argument value. It performs the following steps when called:
22.1.3.1 String.prototype.charAt ( pos )
Returns a single element String containing the code unit at index pos within the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.
If pos is an x.charAt(pos) is equivalent to the result of x.substring(pos, pos + 1).
When the charAt method is called with one argument pos, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let position be ?
ToIntegerOrInfinity (pos). - Let size be the length of S.
- If position < 0 or position ≥ size, return the empty String.
- Return the String value of length 1, containing one code unit from S, namely the code unit at index position.
The charAt function is intentionally generic; it does not require that its
22.1.3.2 String.prototype.charCodeAt ( pos )
Returns a Number (a non-negative
When the charCodeAt method is called with one argument pos, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let position be ?
ToIntegerOrInfinity (pos). - Let size be the length of S.
- If position < 0 or position ≥ size, return
NaN . - Return the
Number value for the numeric value of the code unit at index position within the String S.
The charCodeAt function is intentionally generic; it does not require that its
22.1.3.3 String.prototype.codePointAt ( pos )
Returns a non-negative
When the codePointAt method is called with one argument pos, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let position be ?
ToIntegerOrInfinity (pos). - Let size be the length of S.
- If position < 0 or position ≥ size, return
undefined . - Let cp be !
CodePointAt (S, position). - Return
𝔽 (cp.[[CodePoint]]).
The codePointAt function is intentionally generic; it does not require that its
22.1.3.4 String.prototype.concat ( ...args )
When the concat method is called it returns the String value consisting of the code units of the
When the concat method is called with zero or more arguments, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let R be S.
- For each element next of args, do
- Let nextString be ?
ToString (next). - Set R to the
string-concatenation of R and nextString.
- Let nextString be ?
- Return R.
The concat method is
The concat function is intentionally generic; it does not require that its
22.1.3.5 String.prototype.constructor
The initial value of String.prototype.constructor is
22.1.3.6 String.prototype.endsWith ( searchString [ , endPosition ] )
The following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let isRegExp be ?
IsRegExp (searchString). - If isRegExp is
true , throw aTypeError exception. - Let searchStr be ?
ToString (searchString). - Let len be the length of S.
- If endPosition is
undefined , let pos be len; else let pos be ?ToIntegerOrInfinity (endPosition). - Let end be the result of
clamping pos between 0 and len. - Let searchLength be the length of searchStr.
- If searchLength = 0, return
true . - Let start be end - searchLength.
- If start < 0, return
false . - Let substring be the
substring of S from start to end. - Return !
SameValueNonNumeric (substring, searchStr).
Returns
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
The endsWith function is intentionally generic; it does not require that its
22.1.3.7 String.prototype.includes ( searchString [ , position ] )
The includes method takes two arguments, searchString and position, and performs the following steps:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let isRegExp be ?
IsRegExp (searchString). - If isRegExp is
true , throw aTypeError exception. - Let searchStr be ?
ToString (searchString). - Let pos be ?
ToIntegerOrInfinity (position). Assert : If position isundefined , then pos is 0.- Let len be the length of S.
- Let start be the result of
clamping pos between 0 and len. - Let index be !
StringIndexOf (S, searchStr, start). - If index is not -1, return
true . - Return
false .
If searchString appears as a
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
The includes function is intentionally generic; it does not require that its
22.1.3.8 String.prototype.indexOf ( searchString [ , position ] )
If searchString appears as a
The indexOf method takes two arguments, searchString and position, and performs the following steps:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let searchStr be ?
ToString (searchString). - Let pos be ?
ToIntegerOrInfinity (position). Assert : If position isundefined , then pos is 0.- Let len be the length of S.
- Let start be the result of
clamping pos between 0 and len. - Return
𝔽 (!StringIndexOf (S, searchStr, start)).
The indexOf function is intentionally generic; it does not require that its
22.1.3.9 String.prototype.lastIndexOf ( searchString [ , position ] )
If searchString appears as a
The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let searchStr be ?
ToString (searchString). - Let numPos be ?
ToNumber (position). Assert : If position isundefined , then numPos isNaN .- If numPos is
NaN , let pos be +∞; otherwise, let pos be !ToIntegerOrInfinity (numPos). - Let len be the length of S.
- Let start be the result of
clamping pos between 0 and len. - Let searchLen be the length of searchStr.
- Let k be the largest possible non-negative
integer not larger than start such that k + searchLen ≤ len, and for all non-negative integers j such that j < searchLen, the code unit at index k + j within S is the same as the code unit at index j within searchStr; but if there is no suchinteger , let k be -1. - Return
𝔽 (k).
The lastIndexOf function is intentionally generic; it does not require that its
22.1.3.10 String.prototype.localeCompare ( that [ , reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the localeCompare method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the localeCompare method is used.
When the localeCompare method is called with argument that, it returns a Number other than
Before performing the comparisons, the following steps are performed to prepare the Strings:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let That be ?
ToString (that).
The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.
The localeCompare method, if considered as a function of two arguments
The actual return values are 0 when comparing Strings that are considered canonically equivalent.
The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.
This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the
The localeCompare function is intentionally generic; it does not require that its
22.1.3.11 String.prototype.match ( regexp )
When the match method is called with argument regexp, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - If regexp is neither
undefined nornull , then - Let S be ?
ToString (O). - Let rx be ?
RegExpCreate (regexp,undefined ). - Return ?
Invoke (rx,@@match , « S »).
The match function is intentionally generic; it does not require that its
22.1.3.12 String.prototype.matchAll ( regexp )
Performs a regular expression match of the String representing the
When the matchAll method is called, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - If regexp is neither
undefined nornull , then- Let isRegExp be ?
IsRegExp (regexp). - If isRegExp is
true , then- Let flags be ?
Get (regexp,"flags" ). - Perform ?
RequireObjectCoercible (flags). - If ?
ToString (flags) does not contain"g" , throw aTypeError exception.
- Let flags be ?
- Let matcher be ?
GetMethod (regexp,@@matchAll ). - If matcher is not
undefined , then- Return ?
Call (matcher, regexp, « O »).
- Return ?
- Let isRegExp be ?
- Let S be ?
ToString (O). - Let rx be ?
RegExpCreate (regexp,"g" ). - Return ?
Invoke (rx,@@matchAll , « S »).
matchAll function is intentionally generic, it does not require that its String.prototype.split, String.prototype.matchAll is designed to typically act without mutating its inputs.22.1.3.13 String.prototype.normalize ( [ form ] )
When the normalize method is called with one argument form, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - If form is
undefined , let f be"NFC" . - Else, let f be ?
ToString (form). - If f is not one of
"NFC" ,"NFD" ,"NFKC" , or"NFKD" , throw aRangeError exception. - Let ns be the String value that is the result of normalizing S into the normalization form named by f as specified in https://unicode.org/reports/tr15/.
- Return ns.
The normalize function is intentionally generic; it does not require that its
22.1.3.14 String.prototype.padEnd ( maxLength [ , fillString ] )
When the padEnd method is called, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Return ?
StringPad (O, maxLength, fillString,end ).
22.1.3.15 String.prototype.padStart ( maxLength [ , fillString ] )
When the padStart method is called, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Return ?
StringPad (O, maxLength, fillString,start ).
22.1.3.15.1 StringPad ( O, maxLength, fillString, placement )
The abstract operation StringPad takes arguments O, maxLength, fillString, and placement. It performs the following steps when called:
Assert : placement isstart orend .- Let S be ?
ToString (O). - Let intMaxLength be
ℝ (?ToLength (maxLength)). - Let stringLength be the length of S.
- If intMaxLength ≤ stringLength, return S.
- If fillString is
undefined , let filler be the String value consisting solely of the code unit 0x0020 (SPACE). - Else, let filler be ?
ToString (fillString). - If filler is the empty String, return S.
- Let fillLen be intMaxLength - stringLength.
- Let truncatedStringFiller be the String value consisting of repeated concatenations of filler truncated to length fillLen.
- If placement is
start , return thestring-concatenation of truncatedStringFiller and S. - Else, return the
string-concatenation of S and truncatedStringFiller.
The argument maxLength will be clamped such that it can be no smaller than the length of S.
The argument fillString defaults to
22.1.3.16 String.prototype.repeat ( count )
The following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let n be ?
ToIntegerOrInfinity (count). - If n < 0 or n is +∞, throw a
RangeError exception. - If n is 0, return the empty String.
- Return the String value that is made from n copies of S appended together.
This method creates the String value consisting of the code units of the
The repeat function is intentionally generic; it does not require that its
22.1.3.17 String.prototype.replace ( searchValue, replaceValue )
When the replace method is called with arguments searchValue and replaceValue, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - If searchValue is neither
undefined nornull , then - Let string be ?
ToString (O). - Let searchString be ?
ToString (searchValue). - Let functionalReplace be
IsCallable (replaceValue). - If functionalReplace is
false , then- Set replaceValue to ?
ToString (replaceValue).
- Set replaceValue to ?
- Let searchLength be the length of searchString.
- Let position be !
StringIndexOf (string, searchString, 0). - If position is -1, return string.
- Let preserved be the
substring of string from 0 to position. - If functionalReplace is
true , then - Else,
Assert :Type (replaceValue) is String.- Let captures be a new empty
List . - Let replacement be !
GetSubstitution (searchString, string, position, captures,undefined , replaceValue).
- Return the
string-concatenation of preserved, replacement, and thesubstring of string from position + searchLength.
The replace function is intentionally generic; it does not require that its
22.1.3.17.1 GetSubstitution ( matched, str, position, captures, namedCaptures, replacement )
The abstract operation GetSubstitution takes arguments matched, str, position (a non-negative
Assert :Type (matched) is String.- Let matchLength be the number of code units in matched.
Assert :Type (str) is String.- Let stringLength be the number of code units in str.
Assert : position ≤ stringLength.Assert : captures is a possibly emptyList of Strings.Assert :Type (replacement) is String.- Let tailPos be position + matchLength.
- Let m be the number of elements in captures.
- Let result be the String value derived from replacement by copying code unit elements from replacement to result while performing replacements as specified in
Table 54 . These$replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements. - Return result.
| Code units | Unicode Characters | Replacement text |
|---|---|---|
| 0x0024, 0x0024 |
$$
|
$
|
| 0x0024, 0x0026 |
$&
|
matched |
| 0x0024, 0x0060 |
$`
|
The replacement is the |
| 0x0024, 0x0027 |
$'
|
If tailPos ≥ stringLength, the replacement is the empty String. Otherwise the replacement is the |
|
0x0024, N
Where 0x0031 ≤ N ≤ 0x0039 |
$n where
n is one of 1 2 3 4 5 6 7 8 9 and $n is not followed by a decimal digit
|
The nth element of captures, where n is a single digit in the range 1 to 9. If n ≤ m and the nth element of captures is |
|
0x0024, N, N
Where 0x0030 ≤ N ≤ 0x0039 |
$nn where
n is one of 0 1 2 3 4 5 6 7 8 9
|
The nnth element of captures, where nn is a two-digit decimal number in the range 01 to 99. If nn ≤ m and the nnth element of captures is |
| 0x0024, 0x003C |
$<
|
|
| 0x0024 |
$ in any context that does not match any of the above.
|
$
|
22.1.3.18 String.prototype.replaceAll ( searchValue, replaceValue )
When the replaceAll method is called with arguments searchValue and replaceValue, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - If searchValue is neither
undefined nornull , then- Let isRegExp be ?
IsRegExp (searchValue). - If isRegExp is
true , then- Let flags be ?
Get (searchValue,"flags" ). - Perform ?
RequireObjectCoercible (flags). - If ?
ToString (flags) does not contain"g" , throw aTypeError exception.
- Let flags be ?
- Let replacer be ?
GetMethod (searchValue,@@replace ). - If replacer is not
undefined , then- Return ?
Call (replacer, searchValue, « O, replaceValue »).
- Return ?
- Let isRegExp be ?
- Let string be ?
ToString (O). - Let searchString be ?
ToString (searchValue). - Let functionalReplace be
IsCallable (replaceValue). - If functionalReplace is
false , then- Set replaceValue to ?
ToString (replaceValue).
- Set replaceValue to ?
- Let searchLength be the length of searchString.
- Let advanceBy be
max (1, searchLength). - Let matchPositions be a new empty
List . - Let position be !
StringIndexOf (string, searchString, 0). - Repeat, while position is not -1,
- Append position to the end of matchPositions.
- Set position to !
StringIndexOf (string, searchString, position + advanceBy).
- Let endOfLastMatch be 0.
- Let result be the empty String.
- For each element p of matchPositions, do
- Let preserved be the
substring of string from endOfLastMatch to p. - If functionalReplace is
true , then - Else,
Assert :Type (replaceValue) is String.- Let captures be a new empty
List . - Let replacement be !
GetSubstitution (searchString, string, p, captures,undefined , replaceValue).
- Set result to the
string-concatenation of result, preserved, and replacement. - Set endOfLastMatch to p + searchLength.
- Let preserved be the
- If endOfLastMatch < the length of string, then
- Set result to the
string-concatenation of result and thesubstring of string from endOfLastMatch.
- Set result to the
- Return result.
22.1.3.19 String.prototype.search ( regexp )
When the search method is called with argument regexp, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - If regexp is neither
undefined nornull , then - Let string be ?
ToString (O). - Let rx be ?
RegExpCreate (regexp,undefined ). - Return ?
Invoke (rx,@@search , « string »).
The search function is intentionally generic; it does not require that its
22.1.3.20 String.prototype.slice ( start, end )
The slice method takes two arguments, start and end, and returns a
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let len be the length of S.
- Let intStart be ?
ToIntegerOrInfinity (start). - If intStart is -∞, let from be 0.
- Else if intStart < 0, let from be
max (len + intStart, 0). - Else, let from be
min (intStart, len). - If end is
undefined , let intEnd be len; else let intEnd be ?ToIntegerOrInfinity (end). - If intEnd is -∞, let to be 0.
- Else if intEnd < 0, let to be
max (len + intEnd, 0). - Else, let to be
min (intEnd, len). - If from ≥ to, return the empty String.
- Return the
substring of S from from to to.
The slice function is intentionally generic; it does not require that its
22.1.3.21 String.prototype.split ( separator, limit )
Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any String in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as a RegExp, that has a
When the split method is called, the following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - If separator is neither
undefined nornull , then - Let S be ?
ToString (O). - Let A be !
ArrayCreate (0). - Let lengthA be 0.
- If limit is
undefined , let lim be 232 - 1; else let lim beℝ (?ToUint32 (limit)). - Let R be ?
ToString (separator). - If lim = 0, return A.
- If separator is
undefined , then- Perform !
CreateDataPropertyOrThrow (A,"0" , S). - Return A.
- Perform !
- Let s be the length of S.
- If s = 0, then
- If R is not the empty String, then
- Perform !
CreateDataPropertyOrThrow (A,"0" , S).
- Perform !
- Return A.
- If R is not the empty String, then
- Let p be 0.
- Let q be p.
- Repeat, while q ≠ s,
- Let e be
SplitMatch (S, q, R). - If e is
not-matched , set q to q + 1. - Else,
- Let e be
- Let T be the
substring of S from p to s. - Perform !
CreateDataPropertyOrThrow (A, !ToString (𝔽 (lengthA)), T). - Return A.
The value of separator may be an empty String. In this case, separator does not match the empty
If the
If separator is
The split function is intentionally generic; it does not require that its
22.1.3.21.1 SplitMatch ( S, q, R )
The abstract operation SplitMatch takes arguments S (a String), q (a non-negative
- Let r be the number of code units in R.
- Let s be the number of code units in S.
- If q + r > s, return
not-matched . - If there exists an
integer i between 0 (inclusive) and r (exclusive) such that the code unit at index q + i within S is different from the code unit at index i within R, returnnot-matched . - Return q + r.
22.1.3.22 String.prototype.startsWith ( searchString [ , position ] )
The following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let isRegExp be ?
IsRegExp (searchString). - If isRegExp is
true , throw aTypeError exception. - Let searchStr be ?
ToString (searchString). - Let len be the length of S.
- If position is
undefined , let pos be 0; else let pos be ?ToIntegerOrInfinity (position). - Let start be the result of
clamping pos between 0 and len. - Let searchLength be the length of searchStr.
- If searchLength = 0, return
true . - Let end be start + searchLength.
- If end > len, return
false . - Let substring be the
substring of S from start to end. - Return !
SameValueNonNumeric (substring, searchStr).
This method returns
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
The startsWith function is intentionally generic; it does not require that its
22.1.3.23 String.prototype.substring ( start, end )
The substring method takes two arguments, start and end, and returns a
If either argument is
If start is larger than end, they are swapped.
The following steps are taken:
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let len be the length of S.
- Let intStart be ?
ToIntegerOrInfinity (start). - If end is
undefined , let intEnd be len; else let intEnd be ?ToIntegerOrInfinity (end). - Let finalStart be the result of
clamping intStart between 0 and len. - Let finalEnd be the result of
clamping intEnd between 0 and len. - Let from be
min (finalStart, finalEnd). - Let to be
max (finalStart, finalEnd). - Return the
substring of S from from to to.
The substring function is intentionally generic; it does not require that its
22.1.3.24 String.prototype.toLocaleLowerCase ( [ reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleLowerCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleLowerCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for the
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
The toLocaleLowerCase function is intentionally generic; it does not require that its
22.1.3.25 String.prototype.toLocaleUpperCase ( [ reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleUpperCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleUpperCase method is used.
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for the
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
The toLocaleUpperCase function is intentionally generic; it does not require that its
22.1.3.26 String.prototype.toLowerCase ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let sText be !
StringToCodePoints (S). - Let lowerText be the result of toLowercase(sText), according to the Unicode Default Case Conversion algorithm.
- Let L be !
CodePointsToString (lowerText). - Return L.
The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the UnicodeData.txt file, but also all locale-insensitive mappings in the SpecialCasings.txt file that accompanies it).
The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().
The toLowerCase function is intentionally generic; it does not require that its
22.1.3.27 String.prototype.toString ( )
When the toString method is called, the following steps are taken:
- Return ?
thisStringValue (this value).
For a String object, the toString method happens to return the same thing as the valueOf method.
22.1.3.28 String.prototype.toUpperCase ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
This function behaves in exactly the same way as String.prototype.toLowerCase, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.
The toUpperCase function is intentionally generic; it does not require that its
22.1.3.29 String.prototype.trim ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
The following steps are taken:
- Let S be the
this value. - Return ?
TrimString (S,start+end ).
The trim function is intentionally generic; it does not require that its
22.1.3.29.1 TrimString ( string, where )
The abstract operation TrimString takes arguments string and where. It interprets string as a sequence of UTF-16 encoded code points, as described in
- Let str be ?
RequireObjectCoercible (string). - Let S be ?
ToString (str). - If where is
start , let T be the String value that is a copy of S with leading white space removed. - Else if where is
end , let T be the String value that is a copy of S with trailing white space removed. - Else,
Assert : where isstart+end .- Let T be the String value that is a copy of S with both leading and trailing white space removed.
- Return T.
The definition of white space is the union of
22.1.3.30 String.prototype.trimEnd ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
The following steps are taken:
- Let S be the
this value. - Return ?
TrimString (S,end ).
The trimEnd function is intentionally generic; it does not require that its
22.1.3.31 String.prototype.trimStart ( )
This function interprets a String value as a sequence of UTF-16 encoded code points, as described in
The following steps are taken:
- Let S be the
this value. - Return ?
TrimString (S,start ).
The trimStart function is intentionally generic; it does not require that its
22.1.3.32 String.prototype.valueOf ( )
When the valueOf method is called, the following steps are taken:
- Return ?
thisStringValue (this value).
22.1.3.33 String.prototype [ @@iterator ] ( )
When the @@iterator method is called it returns an Iterator object (
- Let O be ?
RequireObjectCoercible (this value). - Let s be ?
ToString (O). - Let closure be a new
Abstract Closure with no parameters that captures s and performs the following steps when called:- Let position be 0.
- Let len be the length of s.
- Repeat, while position < len,
- Let cp be !
CodePointAt (s, position). - Let nextIndex be position + cp.[[CodeUnitCount]].
- Let resultString be the
substring of s from position to nextIndex. - Set position to nextIndex.
- Perform ?
Yield (resultString).
- Let cp be !
- Return
undefined .
- Return !
CreateIteratorFromClosure (closure,"%StringIteratorPrototype%" ,%StringIteratorPrototype% ).
The value of the
22.1.4 Properties of String Instances
String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from the
String instances have a
22.1.4.1 length
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]:
22.1.5 String Iterator Objects
A String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a named
22.1.5.1 The %StringIteratorPrototype% Object
The %StringIteratorPrototype% object:
- has properties that are inherited by all String Iterator Objects.
- is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%IteratorPrototype% . - has the following properties:
22.1.5.1.1 %StringIteratorPrototype%.next ( )
- Return ?
GeneratorResume (this value,empty ,"%StringIteratorPrototype%" ).
22.1.5.1.2 %StringIteratorPrototype% [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
22.2 RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated flags.
The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.
22.2.1 Patterns
The RegExp
Syntax
Each \u u u \u
A number of productions in this section are given alternative definitions in section
22.2.1.1 Static Semantics: Early Errors
This section is amended in
- It is a Syntax Error if NcapturingParens ≥ 232 - 1.
-
It is a Syntax Error if
Pattern contains multipleGroupSpecifier s whose enclosedRegExpIdentifierName s have the sameCapturingGroupName .
-
It is a Syntax Error if the MV of the first
DecimalDigits is larger than the MV of the secondDecimalDigits .
-
It is a Syntax Error if the enclosing
Pattern does not contain aGroupSpecifier with an enclosedRegExpIdentifierName whoseCapturingGroupName equals theCapturingGroupName of theRegExpIdentifierName of this production'sGroupName .
-
It is a Syntax Error if the
CapturingGroupNumber ofDecimalEscape is larger than NcapturingParens (22.2.2.1 ).
-
It is a Syntax Error if
IsCharacterClass of the firstClassAtom istrue orIsCharacterClass of the secondClassAtom istrue . -
It is a Syntax Error if
IsCharacterClass of the firstClassAtom isfalse andIsCharacterClass of the secondClassAtom isfalse and theCharacterValue of the firstClassAtom is larger than theCharacterValue of the secondClassAtom .
-
It is a Syntax Error if
IsCharacterClass ofClassAtomNoDash istrue orIsCharacterClass ofClassAtom istrue . -
It is a Syntax Error if
IsCharacterClass ofClassAtomNoDash isfalse andIsCharacterClass ofClassAtom isfalse and theCharacterValue ofClassAtomNoDash is larger than theCharacterValue ofClassAtom .
-
It is a Syntax Error if the
CharacterValue ofRegExpUnicodeEscapeSequence is not the code point value of"$" ,"_" , or some code point matched by theUnicodeIDStart lexical grammar production.
-
It is a Syntax Error if the result of performing
UTF16SurrogatePairToCodePoint on the two code points matched byUnicodeLeadSurrogate andUnicodeTrailSurrogate respectively is not matched by theUnicodeIDStart lexical grammar production.
-
It is a Syntax Error if the
CharacterValue ofRegExpUnicodeEscapeSequence is not the code point value of"$" ,"_" , <ZWNJ>, <ZWJ>, or some code point matched by theUnicodeIDContinue lexical grammar production.
-
It is a Syntax Error if the result of performing
UTF16SurrogatePairToCodePoint on the two code points matched byUnicodeLeadSurrogate andUnicodeTrailSurrogate respectively is not matched by theUnicodeIDContinue lexical grammar production.
-
It is a Syntax Error if the
List of Unicode code points that isSourceText ofUnicodePropertyName is not identical to aList of Unicode code points that is a Unicodeproperty name or property alias listed in the “Property name and aliases” column ofTable 56 . -
It is a Syntax Error if the
List of Unicode code points that isSourceText ofUnicodePropertyValue is not identical to aList of Unicode code points that is a value or value alias for the Unicode property or property alias given bySourceText ofUnicodePropertyName listed in the “Property value and aliases” column of the corresponding tablesTable 58 orTable 59 .
-
It is a Syntax Error if the
List of Unicode code points that isSourceText ofLoneUnicodePropertyNameOrValue is not identical to aList of Unicode code points that is a Unicode general category or general category alias listed in the “Property value and aliases” column ofTable 58 , nor a binary property or binary property alias listed in the “Property name and aliases” column ofTable 57 .
22.2.1.2 Static Semantics: CapturingGroupNumber
This section is amended in
- Return the MV of
NonZeroDigit .
- Let n be the number of code points in
DecimalDigits . - Return (the MV of
NonZeroDigit × 10n plus the MV ofDecimalDigits ).
The definitions of “the MV of
22.2.1.3 Static Semantics: IsCharacterClass
This section is amended in
- Return
false .
- Return
true .
22.2.1.4 Static Semantics: CharacterValue
This section is amended in
- Return the code point value of U+002D (HYPHEN-MINUS).
- Let ch be the code point matched by
SourceCharacter . - Return the code point value of ch.
- Return the code point value of U+0008 (BACKSPACE).
- Return the code point value of U+002D (HYPHEN-MINUS).
- Return the code point value according to
Table 55 .
| ControlEscape | Code Point Value | Code Point | Unicode Name | Symbol |
|---|---|---|---|---|
t
|
9 |
U+0009
|
CHARACTER TABULATION | <HT> |
n
|
10 |
U+000A
|
LINE FEED (LF) | <LF> |
v
|
11 |
U+000B
|
LINE TABULATION | <VT> |
f
|
12 |
U+000C
|
FORM FEED (FF) | <FF> |
r
|
13 |
U+000D
|
CARRIAGE RETURN (CR) | <CR> |
- Let ch be the code point matched by
ControlLetter . - Let i be ch's code point value.
- Return the remainder of dividing i by 32.
- Return the code point value of U+0000 (NULL).
\0 represents the <NUL> character and cannot be followed by a decimal digit.
- Return the MV of
HexEscapeSequence .
- Let lead be the
CharacterValue ofHexLeadSurrogate . - Let trail be the
CharacterValue ofHexTrailSurrogate . - Let cp be
UTF16SurrogatePairToCodePoint (lead, trail). - Return the code point value of cp.
- Return the MV of
Hex4Digits .
- Return the MV of
CodePoint .
- Return the MV of
HexDigits .
- Let ch be the code point matched by
IdentityEscape . - Return the code point value of ch.
22.2.1.5 Static Semantics: SourceText
- Return the
List , in source text order, of Unicode code points in the source text matched by this production.
22.2.1.6 Static Semantics: CapturingGroupName
- Let idText be the source text matched by
RegExpIdentifierName . - Let idTextUnescaped be the result of replacing any occurrences of
\RegExpUnicodeEscapeSequence in idText with the code point represented by theRegExpUnicodeEscapeSequence . - Return !
CodePointsToString (idTextUnescaped).
22.2.2 Pattern Semantics
This section is amended in
A regular expression pattern is converted into an
A u. A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (
The syntax and semantics of
For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character)
Patterns are passed to the RegExp
An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.
22.2.2.1 Notation
The descriptions below use the following aliases:
-
Input is a
List whose elements are the characters of the String being matched by the regular expression pattern. Each character is either a code unit or a code point, depending upon the kind of pattern involved. The notation Input[n] means the nth character of Input, where n can range between 0 (inclusive) and InputLength (exclusive). - InputLength is the number of characters in Input.
-
NcapturingParens is the total number of left-capturing parentheses (i.e. the total number of
Atom :: ( GroupSpecifier Disjunction ) (pattern character that is matched by the(terminal of theAtom :: ( GroupSpecifier Disjunction ) -
DotAll is
true if the RegExp object's [[OriginalFlags]] internal slot contains"s" and otherwise isfalse . -
IgnoreCase is
true if the RegExp object's [[OriginalFlags]] internal slot contains"i" and otherwise isfalse . -
Multiline is
true if the RegExp object's [[OriginalFlags]] internal slot contains"m" and otherwise isfalse . -
Unicode is
true if the RegExp object's [[OriginalFlags]] internal slot contains"u" and otherwise isfalse . -
WordCharacters is the mathematical set that is the union of all sixty-three characters in
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_" (letters, numbers, and U+005F (LOW LINE) in the Unicode Basic Latin block) and all characters c for which c is not in that set butCanonicalize (c) is. WordCharacters cannot contain more than sixty-three characters unless Unicode and IgnoreCase are bothtrue .
Furthermore, the descriptions below use the following internal data structures:
-
A CharSet is a mathematical set of characters. When the Unicode flag is
true , “all characters” means the CharSet containing all code point values; otherwise “all characters” means the CharSet containing all code unit values. -
A State is an ordered pair (endIndex, captures) where endIndex is an
integer and captures is aList of NcapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The nth element of captures is either aList of characters that represents the value obtained by the nth set of capturing parentheses orundefined if the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process. -
A MatchResult is either a State or the special token
failure that indicates that the match failed. -
A Continuation is an
Abstract Closure that takes one State argument and returns a MatchResult result. The Continuation attempts to match the remaining portion (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation returns the final State that it reached; if the match fails, the Continuation returnsfailure . -
A Matcher is an
Abstract Closure that takes two arguments—a State and a Continuation—and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can, the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.
22.2.2.2 Pattern
The production
- Evaluate
Disjunction with 1 as its direction argument to obtain a Matcher m. - Return a new
Abstract Closure with parameters (str, index) that captures m and performs the following steps when called:Assert :Type (str) is String.Assert : index is a non-negativeinteger which is ≤ the length of str.- If Unicode is
true , let Input be !StringToCodePoints (str). Otherwise, let Input be aList whose elements are the code units that are the elements of str. Input will be used throughout the algorithms in22.2.2 . Each element of Input is considered to be a character. - Let InputLength be the number of characters contained in Input. This alias will be used throughout the algorithms in
22.2.2 . - Let listIndex be the index into Input of the character that was obtained from element index of str.
- Let c be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
Assert : y is a State.- Return y.
- Let cap be a
List of NcapturingParensundefined values, indexed 1 through NcapturingParens. - Let x be the State (listIndex, cap).
- Return m(x, c).
A Pattern evaluates (“compiles”) to an
22.2.2.3 Disjunction
With parameter direction.
The production
- Evaluate
Alternative with argument direction to obtain a Matcher m. - Return m.
The production
- Evaluate
Alternative with argument direction to obtain a Matcher m1. - Evaluate
Disjunction with argument direction to obtain a Matcher m2. - Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
The | regular expression operator separates two alternatives. The pattern first tries to match the left | produce
/a|ab/.exec("abc")returns the result
/((a)|(ab))((c)|(bc))/.exec("abc")returns the array
["abc", "a", "a", undefined, "bc", undefined, "bc"]and not
["abc", "ab", undefined, "ab", "c", "c", undefined]The order in which the two alternatives are tried is independent of the value of direction.
22.2.2.4 Alternative
With parameter direction.
The production
The production
- Evaluate
Alternative with argument direction to obtain a Matcher m1. - Evaluate
Term with argument direction to obtain a Matcher m2. - If direction = 1, then
- Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
- Else,
Assert : direction is -1.- Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
Consecutive
22.2.2.5 Term
With parameter direction.
The production
- Return the Matcher that is the result of evaluating
Assertion .
The resulting Matcher is independent of direction.
The production
- Return the Matcher that is the result of evaluating
Atom with argument direction.
The production
- Evaluate
Atom with argument direction to obtain a Matcher m. - Evaluate
Quantifier to obtain the three results: a non-negativeinteger min, a non-negativeinteger (or +∞) max, and Boolean greedy. Assert : min ≤ max.- Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of this
Term . This is the total number ofAtom :: ( GroupSpecifier Disjunction ) Term . - Let parenCount be the number of left-capturing parentheses in
Atom . This is the total number ofAtom :: ( GroupSpecifier Disjunction ) Atom . - Return a new Matcher with parameters (x, c) that captures m, min, max, greedy, parenIndex, and parenCount and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Return !
RepeatMatcher (m, min, max, greedy, x, c, parenIndex, parenCount).
22.2.2.5.1 RepeatMatcher ( m, min, max, greedy, x, c, parenIndex, parenCount )
The abstract operation RepeatMatcher takes arguments m (a Matcher), min (a non-negative
- If max = 0, return c(x).
- Let d be a new Continuation with parameters (y) that captures m, min, max, greedy, x, c, parenIndex, and parenCount and performs the following steps when called:
Assert : y is a State.- If min = 0 and y's endIndex = x's endIndex, return
failure . - If min = 0, let min2 be 0; otherwise let min2 be min - 1.
- If max is +∞, let max2 be +∞; otherwise let max2 be max - 1.
- Return !
RepeatMatcher (m, min2, max2, greedy, y, c, parenIndex, parenCount).
- Let cap be a copy of x's captures
List . - For each
integer k such that parenIndex < k and k ≤ parenIndex + parenCount, set cap[k] toundefined . - Let e be x's endIndex.
- Let xr be the State (e, cap).
- If min ≠ 0, return m(xr, d).
- If greedy is
false , then- Let z be c(x).
- If z is not
failure , return z. - Return m(xr, d).
- Let z be m(xr, d).
- If z is not
failure , return z. - Return c(x).
An
If the
Compare
/a[a-z]{2,4}/.exec("abcdefghi")which returns
/a[a-z]{2,4}?/.exec("abcdefghi")which returns
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")which, by the choice point ordering above, returns the array
["aaba", "ba"]and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:
"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/, "$1")which returns the gcd in unary notation
Step
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]because each iteration of the outermost * clears all captured Strings contained in the quantified
Step
/(a*)*/.exec("b")or the slightly more complicated:
/(a*)b\1+/.exec("baaaac")which returns the array
["b", ""]22.2.2.6 Assertion
The production
- Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let e be x's endIndex.
- If e = 0, or if Multiline is
true and the character Input[e - 1] is one ofLineTerminator , then- Return c(x).
- Return
failure .
Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if Multiline is
The production
- Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let e be x's endIndex.
- If e = InputLength, or if Multiline is
true and the character Input[e] is one ofLineTerminator , then- Return c(x).
- Return
failure .
The production
- Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let e be x's endIndex.
- Let a be !
IsWordChar (e - 1). - Let b be !
IsWordChar (e). - If a is
true and b isfalse , or if a isfalse and b istrue , return c(x). - Return
failure .
The production
- Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let e be x's endIndex.
- Let a be !
IsWordChar (e - 1). - Let b be !
IsWordChar (e). - If a is
true and b istrue , or if a isfalse and b isfalse , return c(x). - Return
failure .
The production
- Evaluate
Disjunction with 1 as its direction argument to obtain a Matcher m. - Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
Assert : y is a State.- Return y.
- Let r be m(x, d).
- If r is
failure , returnfailure . - Let y be r's State.
- Let cap be y's captures
List . - Let xe be x's endIndex.
- Let z be the State (xe, cap).
- Return c(z).
The production
- Evaluate
Disjunction with 1 as its direction argument to obtain a Matcher m. - Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
The production
- Evaluate
Disjunction with -1 as its direction argument to obtain a Matcher m. - Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
Assert : y is a State.- Return y.
- Let r be m(x, d).
- If r is
failure , returnfailure . - Let y be r's State.
- Let cap be y's captures
List . - Let xe be x's endIndex.
- Let z be the State (xe, cap).
- Return c(z).
The production
- Evaluate
Disjunction with -1 as its direction argument to obtain a Matcher m. - Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
22.2.2.6.1 IsWordChar ( e )
The abstract operation IsWordChar takes argument e (an
- If e = -1 or e is InputLength, return
false . - Let c be the character Input[e].
- If c is in WordCharacters, return
true . - Return
false .
22.2.2.7 Quantifier
The production
- Evaluate
QuantifierPrefix to obtain the two results: aninteger min and aninteger (or +∞) max. - Return the three results min, max, and
true .
The production
- Evaluate
QuantifierPrefix to obtain the two results: aninteger min and aninteger (or +∞) max. - Return the three results min, max, and
false .
The production
- Return the two results 0 and +∞.
The production
- Return the two results 1 and +∞.
The production
- Return the two results 0 and 1.
The production
- Let i be the MV of
DecimalDigits (see12.8.3 ). - Return the two results i and i.
The production
- Let i be the MV of
DecimalDigits . - Return the two results i and +∞.
The production
- Let i be the MV of the first
DecimalDigits . - Let j be the MV of the second
DecimalDigits . - Return the two results i and j.
22.2.2.8 Atom
With parameter direction.
The production
- Let ch be the character matched by
PatternCharacter . - Let A be a one-element CharSet containing the character ch.
- Return !
CharacterSetMatcher (A,false , direction).
The production
- Let A be the CharSet of all characters.
- If DotAll is not
true , then- Remove from A all characters corresponding to a code point on the right-hand side of the
LineTerminator production.
- Remove from A all characters corresponding to a code point on the right-hand side of the
- Return !
CharacterSetMatcher (A,false , direction).
The production
- Return the Matcher that is the result of evaluating
AtomEscape with argument direction.
The production
- Evaluate
CharacterClass to obtain a CharSet A and a Boolean invert. - Return !
CharacterSetMatcher (A, invert, direction).
The production
- Evaluate
Disjunction with argument direction to obtain a Matcher m. - Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of this
Atom . This is the total number ofAtom :: ( GroupSpecifier Disjunction ) Atom . - Return a new Matcher with parameters (x, c) that captures direction, m, and parenIndex and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let d be a new Continuation with parameters (y) that captures x, c, direction, and parenIndex and performs the following steps when called:
- Return m(x, d).
The production
- Return the Matcher that is the result of evaluating
Disjunction with argument direction.
22.2.2.8.1 CharacterSetMatcher ( A, invert, direction )
The abstract operation CharacterSetMatcher takes arguments A (a CharSet), invert (a Boolean), and direction (1 or -1). It performs the following steps when called:
- Return a new Matcher with parameters (x, c) that captures A, invert, and direction and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let e be x's endIndex.
- Let f be e + direction.
- If f < 0 or f > InputLength, return
failure . - Let index be
min (e, f). - Let ch be the character Input[index].
- Let cc be
Canonicalize (ch). - If there exists a member a of A such that
Canonicalize (a) is cc, let found betrue . Otherwise, let found befalse . - If invert is
false and found isfalse , returnfailure . - If invert is
true and found istrue , returnfailure . - Let cap be x's captures
List . - Let y be the State (f, cap).
- Return c(y).
22.2.2.8.2 Canonicalize ( ch )
The abstract operation Canonicalize takes argument ch (a character). It performs the following steps when called:
- If Unicode is
true and IgnoreCase istrue , then- If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
- Return ch.
- If IgnoreCase is
false , return ch. Assert : ch is a UTF-16 code unit.- Let cp be the code point whose numeric value is that of ch.
- Let u be the result of toUppercase(« cp »), according to the Unicode Default Case Conversion algorithm.
- Let uStr be !
CodePointsToString (u). - If uStr does not consist of a single code unit, return ch.
- Let cu be uStr's single code unit element.
- If the numeric value of ch ≥ 128 and the numeric value of cu < 128, return ch.
- Return cu.
Parentheses of the form ( ) serve both to group the components of the \ followed by a non-zero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching (?: ) instead.
The form (?= ) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside (?= form (this unusual behaviour is inherited from Perl). This only matters when the
For example,
/(?=(a+))/.exec("baaabac")matches the empty String immediately after the first b and therefore returns the array:
["", "aaa"]To illustrate the lack of backtracking into the lookahead, consider:
/(?=(a+))a*b\1/.exec("baaabac")This expression returns
["aba", "a"]and not:
["aaaba", "a"]The form (?! ) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches against
["baaabaac", "ba", undefined, "abaac"]In case-insignificant matches when Unicode is ß (U+00DF) to SS. It may however map a code point outside the Basic Latin range to a character within, for example, ſ (U+017F) to s. Such characters are not mapped if Unicode is /[a-z]/i, but they will match /[a-z]/ui.
22.2.2.8.3 UnicodeMatchProperty ( p )
The abstract operation UnicodeMatchProperty takes argument p (a
Assert : p is aList of Unicode code points that is identical to aList of Unicode code points that is a Unicodeproperty name or property alias listed in the “Property name and aliases” column ofTable 56 orTable 57 .- Let c be the canonical
property name of p as given in the “Canonicalproperty name ” column of the corresponding row. - Return the
List of Unicode code points of c.
Implementations must support the Unicode property names and aliases listed in
For example, Script_Extensions (scx (property alias) are valid, but script_extensions or Scx aren't.
The listed properties form a superset of what UTS18 RL1.2 requires.
| Canonical |
|
|---|---|
General_Category |
General_Category |
gc |
|
Script |
Script |
sc |
|
Script_Extensions |
Script_Extensions |
scx |
| Canonical |
|
|---|---|
ASCII |
ASCII |
ASCII_Hex_Digit |
ASCII_Hex_Digit |
AHex |
|
Alphabetic |
Alphabetic |
Alpha |
|
Any |
Any |
Assigned |
Assigned |
Bidi_Control |
Bidi_Control |
Bidi_C |
|
Bidi_Mirrored |
Bidi_Mirrored |
Bidi_M |
|
Case_Ignorable |
Case_Ignorable |
CI |
|
Cased |
Cased |
Changes_When_Casefolded |
Changes_When_Casefolded |
CWCF |
|
Changes_When_Casemapped |
Changes_When_Casemapped |
CWCM |
|
Changes_When_Lowercased |
Changes_When_Lowercased |
CWL |
|
Changes_When_NFKC_Casefolded |
Changes_When_NFKC_Casefolded |
CWKCF |
|
Changes_When_Titlecased |
Changes_When_Titlecased |
CWT |
|
Changes_When_Uppercased |
Changes_When_Uppercased |
CWU |
|
Dash |
Dash |
Default_Ignorable_Code_Point |
Default_Ignorable_Code_Point |
DI |
|
Deprecated |
Deprecated |
Dep |
|
Diacritic |
Diacritic |
Dia |
|
Emoji |
Emoji |
Emoji_Component |
Emoji_Component |
EComp |
|
Emoji_Modifier |
Emoji_Modifier |
EMod |
|
Emoji_Modifier_Base |
Emoji_Modifier_Base |
EBase |
|
Emoji_Presentation |
Emoji_Presentation |
EPres |
|
Extended_Pictographic |
Extended_Pictographic |
ExtPict |
|
Extender |
Extender |
Ext |
|
Grapheme_Base |
Grapheme_Base |
Gr_Base |
|
Grapheme_Extend |
Grapheme_Extend |
Gr_Ext |
|
Hex_Digit |
Hex_Digit |
Hex |
|
IDS_Binary_Operator |
IDS_Binary_Operator |
IDSB |
|
IDS_Trinary_Operator |
IDS_Trinary_Operator |
IDST |
|
ID_Continue |
ID_Continue |
IDC |
|
ID_Start |
ID_Start |
IDS |
|
Ideographic |
Ideographic |
Ideo |
|
Join_Control |
Join_Control |
Join_C |
|
Logical_Order_Exception |
Logical_Order_Exception |
LOE |
|
Lowercase |
Lowercase |
Lower |
|
Math |
Math |
Noncharacter_Code_Point |
Noncharacter_Code_Point |
NChar |
|
Pattern_Syntax |
Pattern_Syntax |
Pat_Syn |
|
Pattern_White_Space |
Pattern_White_Space |
Pat_WS |
|
Quotation_Mark |
Quotation_Mark |
QMark |
|
Radical |
Radical |
Regional_Indicator |
Regional_Indicator |
RI |
|
Sentence_Terminal |
Sentence_Terminal |
STerm |
|
Soft_Dotted |
Soft_Dotted |
SD |
|
Terminal_Punctuation |
Terminal_Punctuation |
Term |
|
Unified_Ideograph |
Unified_Ideograph |
UIdeo |
|
Uppercase |
Uppercase |
Upper |
|
Variation_Selector |
Variation_Selector |
VS |
|
White_Space |
White_Space |
space |
|
XID_Continue |
XID_Continue |
XIDC |
|
XID_Start |
XID_Start |
XIDS |
22.2.2.8.4 UnicodeMatchPropertyValue ( p, v )
The abstract operation UnicodeMatchPropertyValue takes arguments p (a
Assert : p is aList of Unicode code points that is identical to aList of Unicode code points that is a canonical, unaliased Unicodeproperty name listed in the “Canonicalproperty name ” column ofTable 56 .Assert : v is aList of Unicode code points that is identical to aList of Unicode code points that is a property value or property value alias for Unicode property p listed in the “Property value and aliases” column ofTable 58 orTable 59 .- Let value be the canonical property value of v as given in the “Canonical property value” column of the corresponding row.
- Return the
List of Unicode code points of value.
Implementations must support the Unicode property value names and aliases listed in
For example, Xpeo and Old_Persian are valid Script_Extensions values, but xpeo and Old Persian aren't.
This algorithm differs from the matching rules for symbolic values listed in UAX44: case, Is prefix is not supported.
General_Category| Property value and aliases | Canonical property value |
|---|---|
Cased_Letter |
Cased_Letter |
LC |
|
Close_Punctuation |
Close_Punctuation |
Pe |
|
Connector_Punctuation |
Connector_Punctuation |
Pc |
|
Control |
Control |
Cc |
|
cntrl |
|
Currency_Symbol |
Currency_Symbol |
Sc |
|
Dash_Punctuation |
Dash_Punctuation |
Pd |
|
Decimal_Number |
Decimal_Number |
Nd |
|
digit |
|
Enclosing_Mark |
Enclosing_Mark |
Me |
|
Final_Punctuation |
Final_Punctuation |
Pf |
|
Format |
Format |
Cf |
|
Initial_Punctuation |
Initial_Punctuation |
Pi |
|
Letter |
Letter |
L |
|
Letter_Number |
Letter_Number |
Nl |
|
Line_Separator |
Line_Separator |
Zl |
|
Lowercase_Letter |
Lowercase_Letter |
Ll |
|
Mark |
Mark |
M |
|
Combining_Mark |
|
Math_Symbol |
Math_Symbol |
Sm |
|
Modifier_Letter |
Modifier_Letter |
Lm |
|
Modifier_Symbol |
Modifier_Symbol |
Sk |
|
Nonspacing_Mark |
Nonspacing_Mark |
Mn |
|
Number |
Number |
N |
|
Open_Punctuation |
Open_Punctuation |
Ps |
|
Other |
Other |
C |
|
Other_Letter |
Other_Letter |
Lo |
|
Other_Number |
Other_Number |
No |
|
Other_Punctuation |
Other_Punctuation |
Po |
|
Other_Symbol |
Other_Symbol |
So |
|
Paragraph_Separator |
Paragraph_Separator |
Zp |
|
Private_Use |
Private_Use |
Co |
|
Punctuation |
Punctuation |
P |
|
punct |
|
Separator |
Separator |
Z |
|
Space_Separator |
Space_Separator |
Zs |
|
Spacing_Mark |
Spacing_Mark |
Mc |
|
Surrogate |
Surrogate |
Cs |
|
Symbol |
Symbol |
S |
|
Titlecase_Letter |
Titlecase_Letter |
Lt |
|
Unassigned |
Unassigned |
Cn |
|
Uppercase_Letter |
Uppercase_Letter |
Lu |
Script and Script_Extensions| Property value and aliases | Canonical property value |
|---|---|
Adlam |
Adlam |
Adlm |
|
Ahom |
Ahom |
Anatolian_Hieroglyphs |
Anatolian_Hieroglyphs |
Hluw |
|
Arabic |
Arabic |
Arab |
|
Armenian |
Armenian |
Armn |
|
Avestan |
Avestan |
Avst |
|
Balinese |
Balinese |
Bali |
|
Bamum |
Bamum |
Bamu |
|
Bassa_Vah |
Bassa_Vah |
Bass |
|
Batak |
Batak |
Batk |
|
Bengali |
Bengali |
Beng |
|
Bhaiksuki |
Bhaiksuki |
Bhks |
|
Bopomofo |
Bopomofo |
Bopo |
|
Brahmi |
Brahmi |
Brah |
|
Braille |
Braille |
Brai |
|
Buginese |
Buginese |
Bugi |
|
Buhid |
Buhid |
Buhd |
|
Canadian_Aboriginal |
Canadian_Aboriginal |
Cans |
|
Carian |
Carian |
Cari |
|
Caucasian_Albanian |
Caucasian_Albanian |
Aghb |
|
Chakma |
Chakma |
Cakm |
|
Cham |
Cham |
Chorasmian |
Chorasmian |
Chrs |
|
Cherokee |
Cherokee |
Cher |
|
Common |
Common |
Zyyy |
|
Coptic |
Coptic |
Copt |
|
Qaac |
|
Cuneiform |
Cuneiform |
Xsux |
|
Cypriot |
Cypriot |
Cprt |
|
Cyrillic |
Cyrillic |
Cyrl |
|
Deseret |
Deseret |
Dsrt |
|
Devanagari |
Devanagari |
Deva |
|
Dives_Akuru |
Dives_Akuru |
Diak |
|
Dogra |
Dogra |
Dogr |
|
Duployan |
Duployan |
Dupl |
|
Egyptian_Hieroglyphs |
Egyptian_Hieroglyphs |
Egyp |
|
Elbasan |
Elbasan |
Elba |
|
Elymaic |
Elymaic |
Elym |
|
Ethiopic |
Ethiopic |
Ethi |
|
Georgian |
Georgian |
Geor |
|
Glagolitic |
Glagolitic |
Glag |
|
Gothic |
Gothic |
Goth |
|
Grantha |
Grantha |
Gran |
|
Greek |
Greek |
Grek |
|
Gujarati |
Gujarati |
Gujr |
|
Gunjala_Gondi |
Gunjala_Gondi |
Gong |
|
Gurmukhi |
Gurmukhi |
Guru |
|
Han |
Han |
Hani |
|
Hangul |
Hangul |
Hang |
|
Hanifi_Rohingya |
Hanifi_Rohingya |
Rohg |
|
Hanunoo |
Hanunoo |
Hano |
|
Hatran |
Hatran |
Hatr |
|
Hebrew |
Hebrew |
Hebr |
|
Hiragana |
Hiragana |
Hira |
|
Imperial_Aramaic |
Imperial_Aramaic |
Armi |
|
Inherited |
Inherited |
Zinh |
|
Qaai |
|
Inscriptional_Pahlavi |
Inscriptional_Pahlavi |
Phli |
|
Inscriptional_Parthian |
Inscriptional_Parthian |
Prti |
|
Javanese |
Javanese |
Java |
|
Kaithi |
Kaithi |
Kthi |
|
Kannada |
Kannada |
Knda |
|
Katakana |
Katakana |
Kana |
|
Kayah_Li |
Kayah_Li |
Kali |
|
Kharoshthi |
Kharoshthi |
Khar |
|
Khitan_Small_Script |
Khitan_Small_Script |
Kits |
|
Khmer |
Khmer |
Khmr |
|
Khojki |
Khojki |
Khoj |
|
Khudawadi |
Khudawadi |
Sind |
|
Lao |
Lao |
Laoo |
|
Latin |
Latin |
Latn |
|
Lepcha |
Lepcha |
Lepc |
|
Limbu |
Limbu |
Limb |
|
Linear_A |
Linear_A |
Lina |
|
Linear_B |
Linear_B |
Linb |
|
Lisu |
Lisu |
Lycian |
Lycian |
Lyci |
|
Lydian |
Lydian |
Lydi |
|
Mahajani |
Mahajani |
Mahj |
|
Makasar |
Makasar |
Maka |
|
Malayalam |
Malayalam |
Mlym |
|
Mandaic |
Mandaic |
Mand |
|
Manichaean |
Manichaean |
Mani |
|
Marchen |
Marchen |
Marc |
|
Medefaidrin |
Medefaidrin |
Medf |
|
Masaram_Gondi |
Masaram_Gondi |
Gonm |
|
Meetei_Mayek |
Meetei_Mayek |
Mtei |
|
Mende_Kikakui |
Mende_Kikakui |
Mend |
|
Meroitic_Cursive |
Meroitic_Cursive |
Merc |
|
Meroitic_Hieroglyphs |
Meroitic_Hieroglyphs |
Mero |
|
Miao |
Miao |
Plrd |
|
Modi |
Modi |
Mongolian |
Mongolian |
Mong |
|
Mro |
Mro |
Mroo |
|
Multani |
Multani |
Mult |
|
Myanmar |
Myanmar |
Mymr |
|
Nabataean |
Nabataean |
Nbat |
|
Nandinagari |
Nandinagari |
Nand |
|
New_Tai_Lue |
New_Tai_Lue |
Talu |
|
Newa |
Newa |
Nko |
Nko |
Nkoo |
|
Nushu |
Nushu |
Nshu |
|
Nyiakeng_Puachue_Hmong |
Nyiakeng_Puachue_Hmong |
Hmnp |
|
Ogham |
Ogham |
Ogam |
|
Ol_Chiki |
Ol_Chiki |
Olck |
|
Old_Hungarian |
Old_Hungarian |
Hung |
|
Old_Italic |
Old_Italic |
Ital |
|
Old_North_Arabian |
Old_North_Arabian |
Narb |
|
Old_Permic |
Old_Permic |
Perm |
|
Old_Persian |
Old_Persian |
Xpeo |
|
Old_Sogdian |
Old_Sogdian |
Sogo |
|
Old_South_Arabian |
Old_South_Arabian |
Sarb |
|
Old_Turkic |
Old_Turkic |
Orkh |
|
Oriya |
Oriya |
Orya |
|
Osage |
Osage |
Osge |
|
Osmanya |
Osmanya |
Osma |
|
Pahawh_Hmong |
Pahawh_Hmong |
Hmng |
|
Palmyrene |
Palmyrene |
Palm |
|
Pau_Cin_Hau |
Pau_Cin_Hau |
Pauc |
|
Phags_Pa |
Phags_Pa |
Phag |
|
Phoenician |
Phoenician |
Phnx |
|
Psalter_Pahlavi |
Psalter_Pahlavi |
Phlp |
|
Rejang |
Rejang |
Rjng |
|
Runic |
Runic |
Runr |
|
Samaritan |
Samaritan |
Samr |
|
Saurashtra |
Saurashtra |
Saur |
|
Sharada |
Sharada |
Shrd |
|
Shavian |
Shavian |
Shaw |
|
Siddham |
Siddham |
Sidd |
|
SignWriting |
SignWriting |
Sgnw |
|
Sinhala |
Sinhala |
Sinh |
|
Sogdian |
Sogdian |
Sogd |
|
Sora_Sompeng |
Sora_Sompeng |
Sora |
|
Soyombo |
Soyombo |
Soyo |
|
Sundanese |
Sundanese |
Sund |
|
Syloti_Nagri |
Syloti_Nagri |
Sylo |
|
Syriac |
Syriac |
Syrc |
|
Tagalog |
Tagalog |
Tglg |
|
Tagbanwa |
Tagbanwa |
Tagb |
|
Tai_Le |
Tai_Le |
Tale |
|
Tai_Tham |
Tai_Tham |
Lana |
|
Tai_Viet |
Tai_Viet |
Tavt |
|
Takri |
Takri |
Takr |
|
Tamil |
Tamil |
Taml |
|
Tangut |
Tangut |
Tang |
|
Telugu |
Telugu |
Telu |
|
Thaana |
Thaana |
Thaa |
|
Thai |
Thai |
Tibetan |
Tibetan |
Tibt |
|
Tifinagh |
Tifinagh |
Tfng |
|
Tirhuta |
Tirhuta |
Tirh |
|
Ugaritic |
Ugaritic |
Ugar |
|
Vai |
Vai |
Vaii |
|
Wancho |
Wancho |
Wcho |
|
Warang_Citi |
Warang_Citi |
Wara |
|
Yezidi |
Yezidi |
Yezi |
|
Yi |
Yi |
Yiii |
|
Zanabazar_Square |
Zanabazar_Square |
Zanb |
22.2.2.9 AtomEscape
With parameter direction.
The production
- Evaluate
DecimalEscape to obtain aninteger n. Assert : n ≤ NcapturingParens.- Return !
BackreferenceMatcher (n, direction).
The production
- Evaluate
CharacterEscape to obtain a character ch. - Let A be a one-element CharSet containing the character ch.
- Return !
CharacterSetMatcher (A,false , direction).
The production
- Evaluate
CharacterClassEscape to obtain a CharSet A. - Return !
CharacterSetMatcher (A,false , direction).
An escape sequence of the form \ followed by a non-zero decimal number n matches the result of the nth set of capturing parentheses (
The production
- Search the enclosing
Pattern for an instance of aGroupSpecifier containing aRegExpIdentifierName which has aCapturingGroupName equal to theCapturingGroupName of theRegExpIdentifierName contained inGroupName . Assert : A unique suchGroupSpecifier is found.- Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of the located
GroupSpecifier . This is the total number ofAtom :: ( GroupSpecifier Disjunction ) GroupSpecifier , including its immediately enclosingAtom . - Return !
BackreferenceMatcher (parenIndex, direction).
22.2.2.9.1 BackreferenceMatcher ( n, direction )
The abstract operation BackreferenceMatcher takes arguments n (a positive
Assert : n ≥ 1.- Return a new Matcher with parameters (x, c) that captures n and direction and performs the following steps when called:
Assert : x is a State.Assert : c is a Continuation.- Let cap be x's captures
List . - Let s be cap[n].
- If s is
undefined , return c(x). - Let e be x's endIndex.
- Let len be the number of elements in s.
- Let f be e + direction × len.
- If f < 0 or f > InputLength, return
failure . - Let g be
min (e, f). - If there exists an
integer i between 0 (inclusive) and len (exclusive) such thatCanonicalize (s[i]) is not the same character value asCanonicalize (Input[g + i]), returnfailure . - Let y be the State (f, cap).
- Return c(y).
22.2.2.10 CharacterEscape
The
- Let cv be the
CharacterValue of thisCharacterEscape . - Return the character whose character value is cv.
22.2.2.11 DecimalEscape
The
- Return the
CapturingGroupNumber of thisDecimalEscape .
If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to be a backreference. It is an error if n is greater than the total number of left-capturing parentheses in the entire regular expression.
22.2.2.12 CharacterClassEscape
The production
- Return the ten-element CharSet containing the characters
0through9inclusive.
The production
- Return the CharSet containing all characters not in the CharSet returned by
CharacterClassEscape :: d
The production
- Return the CharSet containing all characters corresponding to a code point on the right-hand side of the
WhiteSpace orLineTerminator productions.
The production
- Return the CharSet containing all characters not in the CharSet returned by
CharacterClassEscape :: s
The production
- Return WordCharacters.
The production
- Return the CharSet containing all characters not in the CharSet returned by
CharacterClassEscape :: w
The production
- Return the CharSet containing all Unicode code points included in the CharSet returned by
UnicodePropertyValueExpression .
The production
- Return the CharSet containing all Unicode code points not included in the CharSet returned by
UnicodePropertyValueExpression .
The production
- Let ps be
SourceText ofUnicodePropertyName . - Let p be !
UnicodeMatchProperty (ps). Assert : p is a Unicodeproperty name or property alias listed in the “Property name and aliases” column ofTable 56 .- Let vs be
SourceText ofUnicodePropertyValue . - Let v be !
UnicodeMatchPropertyValue (p, vs). - Return the CharSet containing all Unicode code points whose character database definition includes the property p with value v.
The production
- Let s be
SourceText ofLoneUnicodePropertyNameOrValue . - If !
UnicodeMatchPropertyValue (General_Category, s) is identical to aList of Unicode code points that is the name of a Unicode general category or general category alias listed in the “Property value and aliases” column ofTable 58 , then- Return the CharSet containing all Unicode code points whose character database definition includes the property “General_Category” with value s.
- Let p be !
UnicodeMatchProperty (s). Assert : p is a binary Unicode property or binary property alias listed in the “Property name and aliases” column ofTable 57 .- Return the CharSet containing all Unicode code points whose character database definition includes the property p with value “True”.
22.2.2.13 CharacterClass
The production
- Evaluate
ClassRanges to obtain a CharSet A. - Return the two results A and
false .
The production
- Evaluate
ClassRanges to obtain a CharSet A. - Return the two results A and
true .
22.2.2.14 ClassRanges
The production
- Return the empty CharSet.
The production
- Return the CharSet that is the result of evaluating
NonemptyClassRanges .
22.2.2.15 NonemptyClassRanges
The production
- Return the CharSet that is the result of evaluating
ClassAtom .
The production
- Evaluate
ClassAtom to obtain a CharSet A. - Evaluate
NonemptyClassRangesNoDash to obtain a CharSet B. - Return the union of CharSets A and B.
The production
- Evaluate the first
ClassAtom to obtain a CharSet A. - Evaluate the second
ClassAtom to obtain a CharSet B. - Evaluate
ClassRanges to obtain a CharSet C. - Let D be !
CharacterRange (A, B). - Return the union of D and C.
22.2.2.15.1 CharacterRange ( A, B )
The abstract operation CharacterRange takes arguments A (a CharSet) and B (a CharSet). It performs the following steps when called:
Assert : A and B each contain exactly one character.- Let a be the one character in CharSet A.
- Let b be the one character in CharSet B.
- Let i be the character value of character a.
- Let j be the character value of character b.
Assert : i ≤ j.- Return the CharSet containing all characters with a character value greater than or equal to i and less than or equal to j.
22.2.2.16 NonemptyClassRangesNoDash
The production
- Return the CharSet that is the result of evaluating
ClassAtom .
The production
- Evaluate
ClassAtomNoDash to obtain a CharSet A. - Evaluate
NonemptyClassRangesNoDash to obtain a CharSet B. - Return the union of CharSets A and B.
The production
- Evaluate
ClassAtomNoDash to obtain a CharSet A. - Evaluate
ClassAtom to obtain a CharSet B. - Evaluate
ClassRanges to obtain a CharSet C. - Let D be !
CharacterRange (A, B). - Return the union of D and C.
Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.
A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of
22.2.2.17 ClassAtom
The production
- Return the CharSet containing the single character
-U+002D (HYPHEN-MINUS).
The production
- Return the CharSet that is the result of evaluating
ClassAtomNoDash .
22.2.2.18 ClassAtomNoDash
The production
- Return the CharSet containing the character matched by
SourceCharacter .
The production
- Return the CharSet that is the result of evaluating
ClassEscape .
22.2.2.19 ClassEscape
The
- Let cv be the
CharacterValue of thisClassEscape . - Let c be the character whose character value is cv.
- Return the CharSet containing the single character c.
- Return the CharSet that is the result of evaluating
CharacterClassEscape .
A \b, \B, and backreferences. Inside a \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside a
22.2.3 The RegExp Constructor
The RegExp
- is %RegExp%.
- is the initial value of the
"RegExp" property of theglobal object . - creates and initializes a new RegExp object when called as a function rather than as a
constructor . Thus the function callRegExp(…)is equivalent to the object creation expressionnew RegExp(…)with the same arguments. - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include asupercall to the RegExpconstructor to create and initialize subclass instances with the necessary internal slots.
22.2.3.1 RegExp ( pattern, flags )
The following steps are taken:
- Let patternIsRegExp be ?
IsRegExp (pattern). - If NewTarget is
undefined , then- Let newTarget be the
active function object . - If patternIsRegExp is
true and flags isundefined , then
- Let newTarget be the
- Else, let newTarget be NewTarget.
- If
Type (pattern) is Object and pattern has a [[RegExpMatcher]] internal slot, then- Let P be pattern.[[OriginalSource]].
- If flags is
undefined , let F be pattern.[[OriginalFlags]]. - Else, let F be flags.
- Else if patternIsRegExp is
true , then - Else,
- Let P be pattern.
- Let F be flags.
- Let O be ?
RegExpAlloc (newTarget). - Return ?
RegExpInitialize (O, P, F).
If pattern is supplied using a
22.2.3.2 Abstract Operations for the RegExp Constructor
22.2.3.2.1 RegExpAlloc ( newTarget )
The abstract operation RegExpAlloc takes argument newTarget. It performs the following steps when called:
- Let obj be ?
OrdinaryCreateFromConstructor (newTarget,"%RegExp.prototype%" , « [[RegExpMatcher]], [[OriginalSource]], [[OriginalFlags]] »). - Perform !
DefinePropertyOrThrow (obj,"lastIndex" , PropertyDescriptor { [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }). - Return obj.
22.2.3.2.2 RegExpInitialize ( obj, pattern, flags )
The abstract operation RegExpInitialize takes arguments obj, pattern, and flags. It performs the following steps when called:
- If pattern is
undefined , let P be the empty String. - Else, let P be ?
ToString (pattern). - If flags is
undefined , let F be the empty String. - Else, let F be ?
ToString (flags). - If F contains any code unit other than
"g" ,"i" ,"m" ,"s" ,"u" , or"y" or if it contains the same code unit more than once, throw aSyntaxError exception. - If F contains
"u" , let u betrue ; else let u befalse . - If u is
true , then- Let patternText be !
StringToCodePoints (P). - Let patternCharacters be a
List whose elements are the code points of patternText.
- Let patternText be !
- Else,
- Let patternText be the result of interpreting each of P's 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
- Let patternCharacters be a
List whose elements are the code unit elements of P.
- Let parseResult be
ParsePattern (patternText, u). - If parseResult is a non-empty
List ofSyntaxError objects, throw aSyntaxError exception. Assert : parseResult is aParse Node forPattern .- Set obj.[[OriginalSource]] to P.
- Set obj.[[OriginalFlags]] to F.
- Set obj.[[RegExpMatcher]] to the
Abstract Closure that evaluates parseResult by applying the semantics provided in22.2.2 using patternCharacters as the pattern'sList ofSourceCharacter values and F as the flag parameters. - Perform ?
Set (obj,"lastIndex" ,+0 𝔽,true ). - Return obj.
22.2.3.2.3 Static Semantics: ParsePattern ( patternText, u )
The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code points) and u (a Boolean). It performs the following steps when called:
22.2.3.2.4 RegExpCreate ( P, F )
The abstract operation RegExpCreate takes arguments P and F. It performs the following steps when called:
- Let obj be ?
RegExpAlloc (%RegExp% ). - Return ?
RegExpInitialize (obj, P, F).
22.2.3.2.5 EscapeRegExpPattern ( P, F )
The abstract operation EscapeRegExpPattern takes arguments P and F. It performs the following steps when called:
- Let S be a String in the form of a
Pattern ([~U] Pattern if F contains[+U] "u" ) equivalent to P interpreted as UTF-16 encoded Unicode code points (6.1.4 ), in which certain code points are escaped as described below. S may or may not be identical to P; however, theAbstract Closure that would result from evaluating S as aPattern ([~U] Pattern if F contains[+U] "u" ) must behave identically to theAbstract Closure given by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation using the same values for P and F must produce identical results. - The code points
/or anyLineTerminator occurring in the pattern shall be escaped in S as necessary to ensure that thestring-concatenation of"/" , S,"/" , and F can be parsed (in an appropriate lexical context) as aRegularExpressionLiteral that behaves identically to the constructed regular expression. For example, if P is"/" , then S could be"\/" or"\u002F" , among other possibilities, but not"/" , because///followed by F would be parsed as aSingleLineComment rather than aRegularExpressionLiteral . If P is the empty String, this specification can be met by letting S be"(?:)" . - Return S.
22.2.4 Properties of the RegExp Constructor
The RegExp
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
22.2.4.1 RegExp.prototype
The initial value of RegExp.prototype is the
This property has the attributes { [[Writable]]:
22.2.4.2 get RegExp [ @@species ]
RegExp[@@species] is an
- Return the
this value.
The value of the
RegExp prototype methods normally use their
22.2.5 Properties of the RegExp Prototype Object
The RegExp prototype object:
- is %RegExp.prototype%.
- is an
ordinary object . - is not a RegExp instance and does not have a [[RegExpMatcher]] internal slot or any of the other internal slots of RegExp instance objects.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% .
The RegExp prototype object does not have a
22.2.5.1 RegExp.prototype.constructor
The initial value of RegExp.prototype.constructor is
22.2.5.2 RegExp.prototype.exec ( string )
Performs a regular expression match of string against the regular expression and returns an Array object containing the results of the match, or
The String
- Let R be the
this value. - Perform ?
RequireInternalSlot (R, [[RegExpMatcher]]). - Let S be ?
ToString (string). - Return ?
RegExpBuiltinExec (R, S).
22.2.5.2.1 RegExpExec ( R, S )
The abstract operation RegExpExec takes arguments R and S. It performs the following steps when called:
Assert :Type (R) is Object.Assert :Type (S) is String.- Let exec be ?
Get (R,"exec" ). - If
IsCallable (exec) istrue , then - Perform ?
RequireInternalSlot (R, [[RegExpMatcher]]). - Return ?
RegExpBuiltinExec (R, S).
If a callable
22.2.5.2.2 RegExpBuiltinExec ( R, S )
The abstract operation RegExpBuiltinExec takes arguments R and S. It performs the following steps when called:
Assert : R is an initialized RegExp instance.Assert :Type (S) is String.- Let length be the number of code units in S.
- Let lastIndex be
ℝ (?ToLength (?Get (R,"lastIndex" ))). - Let flags be R.[[OriginalFlags]].
- If flags contains
"g" , let global betrue ; else let global befalse . - If flags contains
"y" , let sticky betrue ; else let sticky befalse . - If global is
false and sticky isfalse , set lastIndex to 0. - Let matcher be R.[[RegExpMatcher]].
- If flags contains
"u" , let fullUnicode betrue ; else let fullUnicode befalse . - Let matchSucceeded be
false . - Repeat, while matchSucceeded is
false ,- If lastIndex > length, then
- If global is
true or sticky istrue , then- Perform ?
Set (R,"lastIndex" ,+0 𝔽,true ).
- Perform ?
- Return
null .
- If global is
- Let r be matcher(S, lastIndex).
- If r is
failure , then- If sticky is
true , then- Perform ?
Set (R,"lastIndex" ,+0 𝔽,true ). - Return
null .
- Perform ?
- Set lastIndex to
AdvanceStringIndex (S, lastIndex, fullUnicode).
- If sticky is
- Else,
Assert : r is a State.- Set matchSucceeded to
true .
- If lastIndex > length, then
- Let e be r's endIndex value.
- If fullUnicode is
true , then- e is an index into the Input character list, derived from S, matched by matcher. Let eUTF be the smallest index into S that corresponds to the character at element e of Input. If e is greater than or equal to the number of elements in Input, then eUTF is the number of code units in S.
- Set e to eUTF.
- If global is
true or sticky istrue , then - Let n be the number of elements in r's captures
List . (This is the same value as22.2.2.1 's NcapturingParens.) Assert : n < 232 - 1.- Let A be !
ArrayCreate (n + 1). Assert : Themathematical value of A's"length" property is n + 1.- Perform !
CreateDataPropertyOrThrow (A,"index" ,𝔽 (lastIndex)). - Perform !
CreateDataPropertyOrThrow (A,"input" , S). - Let matchedSubstr be the
substring of S from lastIndex to e. - Perform !
CreateDataPropertyOrThrow (A,"0" , matchedSubstr). - If R contains any
GroupName , then- Let groups be !
OrdinaryObjectCreate (null ).
- Let groups be !
- Else,
- Let groups be
undefined .
- Let groups be
- Perform !
CreateDataPropertyOrThrow (A,"groups" , groups). - For each
integer i such that i ≥ 1 and i ≤ n, do- Let captureI be ith element of r's captures
List . - If captureI is
undefined , let capturedValue beundefined . - Else if fullUnicode is
true , thenAssert : captureI is aList of code points.- Let capturedValue be !
CodePointsToString (captureI).
- Else,
- Perform !
CreateDataPropertyOrThrow (A, !ToString (𝔽 (i)), capturedValue). - If the ith capture of R was defined with a
GroupName , then- Let s be the
CapturingGroupName of the correspondingRegExpIdentifierName . - Perform !
CreateDataPropertyOrThrow (groups, s, capturedValue).
- Let s be the
- Let captureI be ith element of r's captures
- Return A.
22.2.5.2.3 AdvanceStringIndex ( S, index, unicode )
The abstract operation AdvanceStringIndex takes arguments S (a String), index (a non-negative
Assert : index ≤ 253 - 1.- If unicode is
false , return index + 1. - Let length be the number of code units in S.
- If index + 1 ≥ length, return index + 1.
- Let cp be !
CodePointAt (S, index). - Return index + cp.[[CodeUnitCount]].
22.2.5.3 get RegExp.prototype.dotAll
RegExp.prototype.dotAll is an
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - If R does not have an [[OriginalFlags]] internal slot, then
- If
SameValue (R,%RegExp.prototype% ) istrue , returnundefined . - Otherwise, throw a
TypeError exception.
- If
- Let flags be R.[[OriginalFlags]].
- If flags contains the code unit 0x0073 (LATIN SMALL LETTER S), return
true . - Return
false .
22.2.5.4 get RegExp.prototype.flags
RegExp.prototype.flags is an
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - Let result be the empty String.
- Let global be !
ToBoolean (?Get (R,"global" )). - If global is
true , append the code unit 0x0067 (LATIN SMALL LETTER G) as the last code unit of result. - Let ignoreCase be !
ToBoolean (?Get (R,"ignoreCase" )). - If ignoreCase is
true , append the code unit 0x0069 (LATIN SMALL LETTER I) as the last code unit of result. - Let multiline be !
ToBoolean (?Get (R,"multiline" )). - If multiline is
true , append the code unit 0x006D (LATIN SMALL LETTER M) as the last code unit of result. - Let dotAll be !
ToBoolean (?Get (R,"dotAll" )). - If dotAll is
true , append the code unit 0x0073 (LATIN SMALL LETTER S) as the last code unit of result. - Let unicode be !
ToBoolean (?Get (R,"unicode" )). - If unicode is
true , append the code unit 0x0075 (LATIN SMALL LETTER U) as the last code unit of result. - Let sticky be !
ToBoolean (?Get (R,"sticky" )). - If sticky is
true , append the code unit 0x0079 (LATIN SMALL LETTER Y) as the last code unit of result. - Return result.
22.2.5.5 get RegExp.prototype.global
RegExp.prototype.global is an
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - If R does not have an [[OriginalFlags]] internal slot, then
- If
SameValue (R,%RegExp.prototype% ) istrue , returnundefined . - Otherwise, throw a
TypeError exception.
- If
- Let flags be R.[[OriginalFlags]].
- If flags contains the code unit 0x0067 (LATIN SMALL LETTER G), return
true . - Return
false .
22.2.5.6 get RegExp.prototype.ignoreCase
RegExp.prototype.ignoreCase is an
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - If R does not have an [[OriginalFlags]] internal slot, then
- If
SameValue (R,%RegExp.prototype% ) istrue , returnundefined . - Otherwise, throw a
TypeError exception.
- If
- Let flags be R.[[OriginalFlags]].
- If flags contains the code unit 0x0069 (LATIN SMALL LETTER I), return
true . - Return
false .
22.2.5.7 RegExp.prototype [ @@match ] ( string )
When the @@match method is called with argument string, the following steps are taken:
- Let rx be the
this value. - If
Type (rx) is not Object, throw aTypeError exception. - Let S be ?
ToString (string). - Let global be !
ToBoolean (?Get (rx,"global" )). - If global is
false , then- Return ?
RegExpExec (rx, S).
- Return ?
- Else,
Assert : global istrue .- Let fullUnicode be !
ToBoolean (?Get (rx,"unicode" )). - Perform ?
Set (rx,"lastIndex" ,+0 𝔽,true ). - Let A be !
ArrayCreate (0). - Let n be 0.
- Repeat,
- Let result be ?
RegExpExec (rx, S). - If result is
null , then- If n = 0, return
null . - Return A.
- If n = 0, return
- Else,
- Let matchStr be ?
ToString (?Get (result,"0" )). - Perform !
CreateDataPropertyOrThrow (A, !ToString (𝔽 (n)), matchStr). - If matchStr is the empty String, then
- Set n to n + 1.
- Let matchStr be ?
- Let result be ?
The value of the
The
22.2.5.8 RegExp.prototype [ @@matchAll ] ( string )
When the @@matchAll method is called with argument string, the following steps are taken:
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - Let S be ?
ToString (string). - Let C be ?
SpeciesConstructor (R,%RegExp% ). - Let flags be ?
ToString (?Get (R,"flags" )). - Let matcher be ?
Construct (C, « R, flags »). - Let lastIndex be ?
ToLength (?Get (R,"lastIndex" )). - Perform ?
Set (matcher,"lastIndex" , lastIndex,true ). - If flags contains
"g" , let global betrue . - Else, let global be
false . - If flags contains
"u" , let fullUnicode betrue . - Else, let fullUnicode be
false . - Return !
CreateRegExpStringIterator (matcher, S, global, fullUnicode).
The value of the
22.2.5.9 get RegExp.prototype.multiline
RegExp.prototype.multiline is an
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - If R does not have an [[OriginalFlags]] internal slot, then
- If
SameValue (R,%RegExp.prototype% ) istrue , returnundefined . - Otherwise, throw a
TypeError exception.
- If
- Let flags be R.[[OriginalFlags]].
- If flags contains the code unit 0x006D (LATIN SMALL LETTER M), return
true . - Return
false .
22.2.5.10 RegExp.prototype [ @@replace ] ( string, replaceValue )
When the @@replace method is called with arguments string and replaceValue, the following steps are taken:
- Let rx be the
this value. - If
Type (rx) is not Object, throw aTypeError exception. - Let S be ?
ToString (string). - Let lengthS be the number of code unit elements in S.
- Let functionalReplace be
IsCallable (replaceValue). - If functionalReplace is
false , then- Set replaceValue to ?
ToString (replaceValue).
- Set replaceValue to ?
- Let global be !
ToBoolean (?Get (rx,"global" )). - If global is
true , then - Let results be a new empty
List . - Let done be
false . - Repeat, while done is
false ,- Let result be ?
RegExpExec (rx, S). - If result is
null , set done totrue . - Else,
- Append result to the end of results.
- If global is
false , set done totrue . - Else,
- Let result be ?
- Let accumulatedResult be the empty String.
- Let nextSourcePosition be 0.
- For each element result of results, do
- Let resultLength be ?
LengthOfArrayLike (result). - Let nCaptures be
max (resultLength - 1, 0). - Let matched be ?
ToString (?Get (result,"0" )). - Let matchLength be the number of code units in matched.
- Let position be ?
ToIntegerOrInfinity (?Get (result,"index" )). - Set position to the result of
clamping position between 0 and lengthS. - Let n be 1.
- Let captures be a new empty
List . - Repeat, while n ≤ nCaptures,
- Let namedCaptures be ?
Get (result,"groups" ). - If functionalReplace is
true , then- Let replacerArgs be « matched ».
- Append in
List order the elements of captures to the end of theList replacerArgs. - Append
𝔽 (position) and S to replacerArgs. - If namedCaptures is not
undefined , then- Append namedCaptures as the last element of replacerArgs.
- Let replValue be ?
Call (replaceValue,undefined , replacerArgs). - Let replacement be ?
ToString (replValue).
- Else,
- If namedCaptures is not
undefined , then- Set namedCaptures to ?
ToObject (namedCaptures).
- Set namedCaptures to ?
- Let replacement be ?
GetSubstitution (matched, S, position, captures, namedCaptures, replaceValue).
- If namedCaptures is not
- If position ≥ nextSourcePosition, then
- NOTE: position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to change the global flag or other characteristics of rx. In such cases, the corresponding substitution is ignored.
- Set accumulatedResult to the
string-concatenation of accumulatedResult, thesubstring of S from nextSourcePosition to position, and replacement. - Set nextSourcePosition to position + matchLength.
- Let resultLength be ?
- If nextSourcePosition ≥ lengthS, return accumulatedResult.
- Return the
string-concatenation of accumulatedResult and thesubstring of S from nextSourcePosition.
The value of the
22.2.5.11 RegExp.prototype [ @@search ] ( string )
When the @@search method is called with argument string, the following steps are taken:
- Let rx be the
this value. - If
Type (rx) is not Object, throw aTypeError exception. - Let S be ?
ToString (string). - Let previousLastIndex be ?
Get (rx,"lastIndex" ). - If
SameValue (previousLastIndex,+0 𝔽) isfalse , then- Perform ?
Set (rx,"lastIndex" ,+0 𝔽,true ).
- Perform ?
- Let result be ?
RegExpExec (rx, S). - Let currentLastIndex be ?
Get (rx,"lastIndex" ). - If
SameValue (currentLastIndex, previousLastIndex) isfalse , then- Perform ?
Set (rx,"lastIndex" , previousLastIndex,true ).
- Perform ?
- If result is
null , return-1 𝔽. - Return ?
Get (result,"index" ).
The value of the
The
22.2.5.12 get RegExp.prototype.source
RegExp.prototype.source is an
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - If R does not have an [[OriginalSource]] internal slot, then
- If
SameValue (R,%RegExp.prototype% ) istrue , return"(?:)" . - Otherwise, throw a
TypeError exception.
- If
Assert : R has an [[OriginalFlags]] internal slot.- Let src be R.[[OriginalSource]].
- Let flags be R.[[OriginalFlags]].
- Return
EscapeRegExpPattern (src, flags).
22.2.5.13 RegExp.prototype [ @@split ] ( string, limit )
Returns an Array object into which substrings of the result of converting string to a String have been stored. The substrings are determined by searching from left to right for matches of the
The /a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while /a*/[Symbol.split]("ab") evaluates to the array ["","b"].)
If string is (or converts to) the empty String, the result depends on whether the regular expression can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If the regular expression contains capturing parentheses, then each time separator is matched the results (including any
/<(\/)?([^<>]+)>/[Symbol.split]("A<B>bold</B>and<CODE>coded</CODE>")evaluates to the array
["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""]If limit is not
When the @@split method is called, the following steps are taken:
- Let rx be the
this value. - If
Type (rx) is not Object, throw aTypeError exception. - Let S be ?
ToString (string). - Let C be ?
SpeciesConstructor (rx,%RegExp% ). - Let flags be ?
ToString (?Get (rx,"flags" )). - If flags contains
"u" , let unicodeMatching betrue . - Else, let unicodeMatching be
false . - If flags contains
"y" , let newFlags be flags. - Else, let newFlags be the
string-concatenation of flags and"y" . - Let splitter be ?
Construct (C, « rx, newFlags »). - Let A be !
ArrayCreate (0). - Let lengthA be 0.
- If limit is
undefined , let lim be 232 - 1; else let lim beℝ (?ToUint32 (limit)). - If lim is 0, return A.
- Let size be the length of S.
- If size is 0, then
- Let z be ?
RegExpExec (splitter, S). - If z is not
null , return A. - Perform !
CreateDataPropertyOrThrow (A,"0" , S). - Return A.
- Let z be ?
- Let p be 0.
- Let q be p.
- Repeat, while q < size,
- Perform ?
Set (splitter,"lastIndex" ,𝔽 (q),true ). - Let z be ?
RegExpExec (splitter, S). - If z is
null , set q toAdvanceStringIndex (S, q, unicodeMatching). - Else,
- Let e be
ℝ (?ToLength (?Get (splitter,"lastIndex" ))). - Set e to
min (e, size). - If e = p, set q to
AdvanceStringIndex (S, q, unicodeMatching). - Else,
- Let T be the
substring of S from p to q. - Perform !
CreateDataPropertyOrThrow (A, !ToString (𝔽 (lengthA)), T). - Set lengthA to lengthA + 1.
- If lengthA = lim, return A.
- Set p to e.
- Let numberOfCaptures be ?
LengthOfArrayLike (z). - Set numberOfCaptures to
max (numberOfCaptures - 1, 0). - Let i be 1.
- Repeat, while i ≤ numberOfCaptures,
- Set q to p.
- Let T be the
- Let e be
- Perform ?
- Let T be the
substring of S from p to size. - Perform !
CreateDataPropertyOrThrow (A, !ToString (𝔽 (lengthA)), T). - Return A.
The value of the
The @@split method ignores the value of the
22.2.5.14 get RegExp.prototype.sticky
RegExp.prototype.sticky is an
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - If R does not have an [[OriginalFlags]] internal slot, then
- If
SameValue (R,%RegExp.prototype% ) istrue , returnundefined . - Otherwise, throw a
TypeError exception.
- If
- Let flags be R.[[OriginalFlags]].
- If flags contains the code unit 0x0079 (LATIN SMALL LETTER Y), return
true . - Return
false .
22.2.5.15 RegExp.prototype.test ( S )
The following steps are taken:
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - Let string be ?
ToString (S). - Let match be ?
RegExpExec (R, string). - If match is not
null , returntrue ; else returnfalse .
22.2.5.16 RegExp.prototype.toString ( )
The returned String has the form of a
22.2.5.17 get RegExp.prototype.unicode
RegExp.prototype.unicode is an
- Let R be the
this value. - If
Type (R) is not Object, throw aTypeError exception. - If R does not have an [[OriginalFlags]] internal slot, then
- If
SameValue (R,%RegExp.prototype% ) istrue , returnundefined . - Otherwise, throw a
TypeError exception.
- If
- Let flags be R.[[OriginalFlags]].
- If flags contains the code unit 0x0075 (LATIN SMALL LETTER U), return
true . - Return
false .
22.2.6 Properties of RegExp Instances
RegExp instances are ordinary objects that inherit properties from the
Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties RegExp.prototype.
RegExp instances also have the following property:
22.2.6.1 lastIndex
The value of the
22.2.7 RegExp String Iterator Objects
A RegExp String Iterator is an object, that represents a specific iteration over some specific String instance object, matching against some specific RegExp instance object. There is not a named
22.2.7.1 CreateRegExpStringIterator ( R, S, global, fullUnicode )
The abstract operation CreateRegExpStringIterator takes arguments R, S, global, and fullUnicode. It performs the following steps when called:
Assert :Type (S) is String.Assert :Type (global) is Boolean.Assert :Type (fullUnicode) is Boolean.- Let closure be a new
Abstract Closure with no parameters that captures R, S, global, and fullUnicode and performs the following steps when called:- Repeat,
- Let match be ?
RegExpExec (R, S). - If match is
null , returnundefined . - If global is
false , then- Perform ?
Yield (match). - Return
undefined .
- Perform ?
- Let matchStr be ?
ToString (?Get (match,"0" )). - If matchStr is the empty String, then
- Perform ?
Yield (match).
- Let match be ?
- Repeat,
- Return !
CreateIteratorFromClosure (closure,"%RegExpStringIteratorPrototype%" ,%RegExpStringIteratorPrototype% ).
22.2.7.2 The %RegExpStringIteratorPrototype% Object
The %RegExpStringIteratorPrototype% object:
- has properties that are inherited by all RegExp String Iterator Objects.
- is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%IteratorPrototype% . - has the following properties:
22.2.7.2.1 %RegExpStringIteratorPrototype%.next ( )
- Return ?
GeneratorResume (this value,empty ,"%RegExpStringIteratorPrototype%" ).
22.2.7.2.2 %RegExpStringIteratorPrototype% [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
23 Indexed Collections
23.1 Array Objects
Array objects are exotic objects that give special treatment to a certain class of property names. See
23.1.1 The Array Constructor
The Array
- is %Array%.
- is the initial value of the
"Array" property of theglobal object . - creates and initializes a new
Array exotic object when called as aconstructor . - also creates and initializes a new Array object when called as a function rather than as a
constructor . Thus the function callArray(…)is equivalent to the object creation expressionnew Array(…)with the same arguments. - is a function whose behaviour differs based upon the number and types of its arguments.
- is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include asupercall to the Arrayconstructor to initialize subclass instances that are Array exotic objects. However, most of theArray.prototypemethods are generic methods that are not dependent upon theirthis value being anArray exotic object . - has a
"length" property whose value is1 𝔽.
23.1.1.1 Array ( ...values )
When the Array function is called, the following steps are taken:
- If NewTarget is
undefined , let newTarget be theactive function object ; else let newTarget be NewTarget. - Let proto be ?
GetPrototypeFromConstructor (newTarget,"%Array.prototype%" ). - Let numberOfArgs be the number of elements in values.
- If numberOfArgs = 0, then
- Return !
ArrayCreate (0, proto).
- Return !
- Else if numberOfArgs = 1, then
- Let len be values[0].
- Let array be !
ArrayCreate (0, proto). - If
Type (len) is not Number, then- Perform !
CreateDataPropertyOrThrow (array,"0" , len). - Let intLen be
1 𝔽.
- Perform !
- Else,
- Let intLen be !
ToUint32 (len). - If intLen is not the same value as len, throw a
RangeError exception.
- Let intLen be !
- Perform !
Set (array,"length" , intLen,true ). - Return array.
- Else,
Assert : numberOfArgs ≥ 2.- Let array be ?
ArrayCreate (numberOfArgs, proto). - Let k be 0.
- Repeat, while k < numberOfArgs,
- Let Pk be !
ToString (𝔽 (k)). - Let itemK be values[k].
- Perform !
CreateDataPropertyOrThrow (array, Pk, itemK). - Set k to k + 1.
- Let Pk be !
Assert : Themathematical value of array's"length" property is numberOfArgs.- Return array.
23.1.2 Properties of the Array Constructor
The Array
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
23.1.2.1 Array.from ( items [ , mapfn [ , thisArg ] ] )
When the from method is called with argument items and optional arguments mapfn and thisArg, the following steps are taken:
- Let C be the
this value. - If mapfn is
undefined , let mapping befalse . - Else,
- If
IsCallable (mapfn) isfalse , throw aTypeError exception. - Let mapping be
true .
- If
- Let usingIterator be ?
GetMethod (items,@@iterator ). - If usingIterator is not
undefined , then- If
IsConstructor (C) istrue , then- Let A be ?
Construct (C).
- Let A be ?
- Else,
- Let A be !
ArrayCreate (0).
- Let A be !
- Let iteratorRecord be ?
GetIterator (items,sync , usingIterator). - Let k be 0.
- Repeat,
- If k ≥ 253 - 1, then
- Let error be
ThrowCompletion (a newly createdTypeError object). - Return ?
IteratorClose (iteratorRecord, error).
- Let error be
- Let Pk be !
ToString (𝔽 (k)). - Let next be ?
IteratorStep (iteratorRecord). - If next is
false , then - Let nextValue be ?
IteratorValue (next). - If mapping is
true , then- Let mappedValue be
Call (mapfn, thisArg, « nextValue,𝔽 (k) »). - If mappedValue is an
abrupt completion , return ?IteratorClose (iteratorRecord, mappedValue). - Set mappedValue to mappedValue.[[Value]].
- Let mappedValue be
- Else, let mappedValue be nextValue.
- Let defineStatus be
CreateDataPropertyOrThrow (A, Pk, mappedValue). - If defineStatus is an
abrupt completion , return ?IteratorClose (iteratorRecord, defineStatus). - Set k to k + 1.
- If k ≥ 253 - 1, then
- If
- NOTE: items is not an Iterable so assume it is an
array-like object . - Let arrayLike be !
ToObject (items). - Let len be ?
LengthOfArrayLike (arrayLike). - If
IsConstructor (C) istrue , then - Else,
- Let A be ?
ArrayCreate (len).
- Let A be ?
- Let k be 0.
- Repeat, while k < len,
- Perform ?
Set (A,"length" ,𝔽 (len),true ). - Return A.
The from function is an intentionally generic factory method; it does not require that its
23.1.2.2 Array.isArray ( arg )
The isArray function takes one argument arg, and performs the following steps:
- Return ?
IsArray (arg).
23.1.2.3 Array.of ( ...items )
When the of method is called with any number of arguments, the following steps are taken:
- Let len be the number of elements in items.
- Let lenNumber be
𝔽 (len). - Let C be the
this value. - If
IsConstructor (C) istrue , then- Let A be ?
Construct (C, « lenNumber »).
- Let A be ?
- Else,
- Let A be ?
ArrayCreate (len).
- Let A be ?
- Let k be 0.
- Repeat, while k < len,
- Let kValue be items[k].
- Let Pk be !
ToString (𝔽 (k)). - Perform ?
CreateDataPropertyOrThrow (A, Pk, kValue). - Set k to k + 1.
- Perform ?
Set (A,"length" , lenNumber,true ). - Return A.
The of function is an intentionally generic factory method; it does not require that its
23.1.2.4 Array.prototype
The value of Array.prototype is the
This property has the attributes { [[Writable]]:
23.1.2.5 get Array [ @@species ]
Array[@@species] is an
- Return the
this value.
The value of the
Array prototype methods normally use their
23.1.3 Properties of the Array Prototype Object
The Array prototype object:
- is %Array.prototype%.
- is an
Array exotic object and has the internal methods specified for such objects. - has a
"length" property whose initial value is+0 𝔽 and whose attributes are { [[Writable]]:true , [[Enumerable]]:false , [[Configurable]]:false }. - has a [[Prototype]] internal slot whose value is
%Object.prototype% .
The Array prototype object is specified to be an
23.1.3.1 Array.prototype.concat ( ...items )
When the concat method is called with zero or more arguments, it returns an array containing the array elements of the object followed by the array elements of each argument.
The following steps are taken:
- Let O be ?
ToObject (this value). - Let A be ?
ArraySpeciesCreate (O, 0). - Let n be 0.
- Prepend O to items.
- For each element E of items, do
- Let spreadable be ?
IsConcatSpreadable (E). - If spreadable is
true , then- Let k be 0.
- Let len be ?
LengthOfArrayLike (E). - If n + len > 253 - 1, throw a
TypeError exception. - Repeat, while k < len,
- Let P be !
ToString (𝔽 (k)). - Let exists be ?
HasProperty (E, P). - If exists is
true , then- Let subElement be ?
Get (E, P). - Perform ?
CreateDataPropertyOrThrow (A, !ToString (𝔽 (n)), subElement).
- Let subElement be ?
- Set n to n + 1.
- Set k to k + 1.
- Let P be !
- Else,
- NOTE: E is added as a single item rather than spread.
- If n ≥ 253 - 1, throw a
TypeError exception. - Perform ?
CreateDataPropertyOrThrow (A, !ToString (𝔽 (n)), E). - Set n to n + 1.
- Let spreadable be ?
- Perform ?
Set (A,"length" ,𝔽 (n),true ). - Return A.
The concat method is
The explicit setting of the
The concat function is intentionally generic; it does not require that its
23.1.3.1.1 IsConcatSpreadable ( O )
The abstract operation IsConcatSpreadable takes argument O. It performs the following steps when called:
- If
Type (O) is not Object, returnfalse . - Let spreadable be ?
Get (O,@@isConcatSpreadable ). - If spreadable is not
undefined , return !ToBoolean (spreadable). - Return ?
IsArray (O).
23.1.3.2 Array.prototype.constructor
The initial value of Array.prototype.constructor is
23.1.3.3 Array.prototype.copyWithin ( target, start [ , end ] )
The copyWithin method takes up to three arguments target, start and end.
The end argument is optional with the length of the
The following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - Let relativeTarget be ?
ToIntegerOrInfinity (target). - If relativeTarget is -∞, let to be 0.
- Else if relativeTarget < 0, let to be
max (len + relativeTarget, 0). - Else, let to be
min (relativeTarget, len). - Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let from be 0.
- Else if relativeStart < 0, let from be
max (len + relativeStart, 0). - Else, let from be
min (relativeStart, len). - If end is
undefined , let relativeEnd be len; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let final be 0.
- Else if relativeEnd < 0, let final be
max (len + relativeEnd, 0). - Else, let final be
min (relativeEnd, len). - Let count be
min (final - from, len - to). - If from < to and to < from + count, then
- Let direction be -1.
- Set from to from + count - 1.
- Set to to to + count - 1.
- Else,
- Let direction be 1.
- Repeat, while count > 0,
- Let fromKey be !
ToString (𝔽 (from)). - Let toKey be !
ToString (𝔽 (to)). - Let fromPresent be ?
HasProperty (O, fromKey). - If fromPresent is
true , then - Else,
Assert : fromPresent isfalse .- Perform ?
DeletePropertyOrThrow (O, toKey).
- Set from to from + direction.
- Set to to to + direction.
- Set count to count - 1.
- Let fromKey be !
- Return O.
The copyWithin function is intentionally generic; it does not require that its
23.1.3.4 Array.prototype.entries ( )
The following steps are taken:
- Let O be ?
ToObject (this value). - Return
CreateArrayIterator (O,key+value ).
23.1.3.5 Array.prototype.every ( callbackfn [ , thisArg ] )
callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns every immediately returns every will return
If a thisArg parameter is provided, it will be used as the
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns
When the every method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
true .
The every function is intentionally generic; it does not require that its
23.1.3.6 Array.prototype.fill ( value [ , start [ , end ] ] )
The fill method takes up to three arguments value, start and end.
The start and end arguments are optional with default values of 0 and the length of the
The following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let k be 0.
- Else if relativeStart < 0, let k be
max (len + relativeStart, 0). - Else, let k be
min (relativeStart, len). - If end is
undefined , let relativeEnd be len; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let final be 0.
- Else if relativeEnd < 0, let final be
max (len + relativeEnd, 0). - Else, let final be
min (relativeEnd, len). - Repeat, while k < final,
- Return O.
The fill function is intentionally generic; it does not require that its
23.1.3.7 Array.prototype.filter ( callbackfn [ , thisArg ] )
callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returns
If a thisArg parameter is provided, it will be used as the
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.
When the filter method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let A be ?
ArraySpeciesCreate (O, 0). - Let k be 0.
- Let to be 0.
- Repeat, while k < len,
- Let Pk be !
ToString (𝔽 (k)). - Let kPresent be ?
HasProperty (O, Pk). - If kPresent is
true , then - Set k to k + 1.
- Let Pk be !
- Return A.
The filter function is intentionally generic; it does not require that its
23.1.3.8 Array.prototype.find ( predicate [ , thisArg ] )
The find method is called with one or two arguments, predicate and thisArg.
predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. find calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns find immediately returns that element value. Otherwise, find returns
If a thisArg parameter is provided, it will be used as the
predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.
find does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.
The range of elements processed by find is set before the first call to predicate. Elements that are appended to the array after the call to find begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that find visits them; elements that are deleted after the call to find begins and before being visited are not visited.
When the find method is called, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (predicate) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
undefined .
The find function is intentionally generic; it does not require that its
23.1.3.9 Array.prototype.findIndex ( predicate [ , thisArg ] )
predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. findIndex calls predicate once for each element of the array, in ascending order, until it finds one where predicate returns findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.
If a thisArg parameter is provided, it will be used as the
predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.
findIndex does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.
The range of elements processed by findIndex is set before the first call to predicate. Elements that are appended to the array after the call to findIndex begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that findIndex visits them; elements that are deleted after the call to findIndex begins and before being visited are not visited.
When the findIndex method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (predicate) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
-1 𝔽.
The findIndex function is intentionally generic; it does not require that its
23.1.3.10 Array.prototype.flat ( [ depth ] )
When the flat method is called with zero or one arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let sourceLen be ?
LengthOfArrayLike (O). - Let depthNum be 1.
- If depth is not
undefined , then- Set depthNum to ?
ToIntegerOrInfinity (depth). - If depthNum < 0, set depthNum to 0.
- Set depthNum to ?
- Let A be ?
ArraySpeciesCreate (O, 0). - Perform ?
FlattenIntoArray (A, O, sourceLen, 0, depthNum). - Return A.
23.1.3.10.1 FlattenIntoArray ( target, source, sourceLen, start, depth [ , mapperFunction, thisArg ] )
The abstract operation FlattenIntoArray takes arguments target, source, sourceLen (a non-negative
Assert :Type (target) is Object.Assert :Type (source) is Object.Assert : If mapperFunction is present, then !IsCallable (mapperFunction) istrue , thisArg is present, and depth is 1.- Let targetIndex be start.
- Let sourceIndex be
+0 𝔽. - Repeat, while
ℝ (sourceIndex) < sourceLen,- Let P be !
ToString (sourceIndex). - Let exists be ?
HasProperty (source, P). - If exists is
true , then- Let element be ?
Get (source, P). - If mapperFunction is present, then
- Set element to ?
Call (mapperFunction, thisArg, « element, sourceIndex, source »).
- Set element to ?
- Let shouldFlatten be
false . - If depth > 0, then
- Set shouldFlatten to ?
IsArray (element).
- Set shouldFlatten to ?
- If shouldFlatten is
true , then- If depth is +∞, let newDepth be +∞.
- Else, let newDepth be depth - 1.
- Let elementLen be ?
LengthOfArrayLike (element). - Set targetIndex to ?
FlattenIntoArray (target, element, elementLen, targetIndex, newDepth).
- Else,
- If targetIndex ≥ 253 - 1, throw a
TypeError exception. - Perform ?
CreateDataPropertyOrThrow (target, !ToString (𝔽 (targetIndex)), element). - Set targetIndex to targetIndex + 1.
- If targetIndex ≥ 253 - 1, throw a
- Let element be ?
- Set sourceIndex to sourceIndex +
1 𝔽.
- Let P be !
- Return targetIndex.
23.1.3.11 Array.prototype.flatMap ( mapperFunction [ , thisArg ] )
When the flatMap method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let sourceLen be ?
LengthOfArrayLike (O). - If !
IsCallable (mapperFunction) isfalse , throw aTypeError exception. - Let A be ?
ArraySpeciesCreate (O, 0). - Perform ?
FlattenIntoArray (A, O, sourceLen, 0, 1, mapperFunction, thisArg). - Return A.
23.1.3.12 Array.prototype.forEach ( callbackfn [ , thisArg ] )
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by forEach is set before the first call to callbackfn. Elements which are appended to the array after the call to forEach begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time forEach visits them; elements that are deleted after the call to forEach begins and before being visited are not visited.
When the forEach method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
undefined .
The forEach function is intentionally generic; it does not require that its
23.1.3.13 Array.prototype.includes ( searchElement [ , fromIndex ] )
includes compares searchElement to the elements of the array, in ascending order, using the
The optional second argument fromIndex defaults to
When the includes method is called, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If len is 0, return
false . - Let n be ?
ToIntegerOrInfinity (fromIndex). Assert : If fromIndex isundefined , then n is 0.- If n is +∞, return
false . - Else if n is -∞, set n to 0.
- If n ≥ 0, then
- Let k be n.
- Else,
- Let k be len + n.
- If k < 0, set k to 0.
- Repeat, while k < len,
- Let elementK be ?
Get (O, !ToString (𝔽 (k))). - If
SameValueZero (searchElement, elementK) istrue , returntrue . - Set k to k + 1.
- Let elementK be ?
- Return
false .
The includes function is intentionally generic; it does not require that its
The includes method intentionally differs from the similar indexOf method in two ways. First, it uses the
23.1.3.14 Array.prototype.indexOf ( searchElement [ , fromIndex ] )
indexOf compares searchElement to the elements of the array, in ascending order, using the
The optional second argument fromIndex defaults to
When the indexOf method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If len is 0, return
-1 𝔽. - Let n be ?
ToIntegerOrInfinity (fromIndex). Assert : If fromIndex isundefined , then n is 0.- If n is +∞, return
-1 𝔽. - Else if n is -∞, set n to 0.
- If n ≥ 0, then
- Let k be n.
- Else,
- Let k be len + n.
- If k < 0, set k to 0.
- Repeat, while k < len,
- Let kPresent be ?
HasProperty (O, !ToString (𝔽 (k))). - If kPresent is
true , then- Let elementK be ?
Get (O, !ToString (𝔽 (k))). - Let same be the result of performing
Strict Equality Comparison searchElement === elementK. - If same is
true , return𝔽 (k).
- Let elementK be ?
- Set k to k + 1.
- Let kPresent be ?
- Return
-1 𝔽.
The indexOf function is intentionally generic; it does not require that its
23.1.3.15 Array.prototype.join ( separator )
The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.
The join method takes one argument, separator, and performs the following steps:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If separator is
undefined , let sep be the single-element String"," . - Else, let sep be ?
ToString (separator). - Let R be the empty String.
- Let k be 0.
- Repeat, while k < len,
- If k > 0, set R to the
string-concatenation of R and sep. - Let element be ?
Get (O, !ToString (𝔽 (k))). - If element is
undefined ornull , let next be the empty String; otherwise, let next be ?ToString (element). - Set R to the
string-concatenation of R and next. - Set k to k + 1.
- If k > 0, set R to the
- Return R.
The join function is intentionally generic; it does not require that its
23.1.3.16 Array.prototype.keys ( )
The following steps are taken:
- Let O be ?
ToObject (this value). - Return
CreateArrayIterator (O,key ).
23.1.3.17 Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )
lastIndexOf compares searchElement to the elements of the array in descending order using the
The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is less than
When the lastIndexOf method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If len is 0, return
-1 𝔽. - If fromIndex is present, let n be ?
ToIntegerOrInfinity (fromIndex); else let n be len - 1. - If n is -∞, return
-1 𝔽. - If n ≥ 0, then
- Let k be
min (n, len - 1).
- Let k be
- Else,
- Let k be len + n.
- Repeat, while k ≥ 0,
- Let kPresent be ?
HasProperty (O, !ToString (𝔽 (k))). - If kPresent is
true , then- Let elementK be ?
Get (O, !ToString (𝔽 (k))). - Let same be the result of performing
Strict Equality Comparison searchElement === elementK. - If same is
true , return𝔽 (k).
- Let elementK be ?
- Set k to k - 1.
- Let kPresent be ?
- Return
-1 𝔽.
The lastIndexOf function is intentionally generic; it does not require that its
23.1.3.18 Array.prototype.map ( callbackfn [ , thisArg ] )
callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.
When the map method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let A be ?
ArraySpeciesCreate (O, len). - Let k be 0.
- Repeat, while k < len,
- Let Pk be !
ToString (𝔽 (k)). - Let kPresent be ?
HasProperty (O, Pk). - If kPresent is
true , then- Let kValue be ?
Get (O, Pk). - Let mappedValue be ?
Call (callbackfn, thisArg, « kValue,𝔽 (k), O »). - Perform ?
CreateDataPropertyOrThrow (A, Pk, mappedValue).
- Let kValue be ?
- Set k to k + 1.
- Let Pk be !
- Return A.
The map function is intentionally generic; it does not require that its
23.1.3.19 Array.prototype.pop ( )
The last element of the array is removed from the array and returned.
When the pop method is called, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If len = 0, then
- Perform ?
Set (O,"length" ,+0 𝔽,true ). - Return
undefined .
- Perform ?
- Else,
The pop function is intentionally generic; it does not require that its
23.1.3.20 Array.prototype.push ( ...items )
The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.
When the push method is called with zero or more arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - Let argCount be the number of elements in items.
- If len + argCount > 253 - 1, throw a
TypeError exception. - For each element E of items, do
- Perform ?
Set (O,"length" ,𝔽 (len),true ). - Return
𝔽 (len).
The push method is
The push function is intentionally generic; it does not require that its
23.1.3.21 Array.prototype.reduce ( callbackfn [ , initialValue ] )
callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element after the first element present in the array, in ascending order.
callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was supplied, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is a
reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.
When the reduce method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - If len = 0 and initialValue is not present, throw a
TypeError exception. - Let k be 0.
- Let accumulator be
undefined . - If initialValue is present, then
- Set accumulator to initialValue.
- Else,
- Let kPresent be
false . - Repeat, while kPresent is
false and k < len,- Let Pk be !
ToString (𝔽 (k)). - Set kPresent to ?
HasProperty (O, Pk). - If kPresent is
true , then- Set accumulator to ?
Get (O, Pk).
- Set accumulator to ?
- Set k to k + 1.
- Let Pk be !
- If kPresent is
false , throw aTypeError exception.
- Let kPresent be
- Repeat, while k < len,
- Return accumulator.
The reduce function is intentionally generic; it does not require that its
23.1.3.22 Array.prototype.reduceRight ( callbackfn [ , initialValue ] )
callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element after the first element present in the array, in descending order.
callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was supplied, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value. It is a
reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.
When the reduceRight method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - If len is 0 and initialValue is not present, throw a
TypeError exception. - Let k be len - 1.
- Let accumulator be
undefined . - If initialValue is present, then
- Set accumulator to initialValue.
- Else,
- Let kPresent be
false . - Repeat, while kPresent is
false and k ≥ 0,- Let Pk be !
ToString (𝔽 (k)). - Set kPresent to ?
HasProperty (O, Pk). - If kPresent is
true , then- Set accumulator to ?
Get (O, Pk).
- Set accumulator to ?
- Set k to k - 1.
- Let Pk be !
- If kPresent is
false , throw aTypeError exception.
- Let kPresent be
- Repeat, while k ≥ 0,
- Return accumulator.
The reduceRight function is intentionally generic; it does not require that its
23.1.3.23 Array.prototype.reverse ( )
The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.
When the reverse method is called, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - Let middle be
floor (len / 2). - Let lower be 0.
- Repeat, while lower ≠ middle,
- Let upper be len - lower - 1.
- Let upperP be !
ToString (𝔽 (upper)). - Let lowerP be !
ToString (𝔽 (lower)). - Let lowerExists be ?
HasProperty (O, lowerP). - If lowerExists is
true , then- Let lowerValue be ?
Get (O, lowerP).
- Let lowerValue be ?
- Let upperExists be ?
HasProperty (O, upperP). - If upperExists is
true , then- Let upperValue be ?
Get (O, upperP).
- Let upperValue be ?
- If lowerExists is
true and upperExists istrue , then - Else if lowerExists is
false and upperExists istrue , then- Perform ?
Set (O, lowerP, upperValue,true ). - Perform ?
DeletePropertyOrThrow (O, upperP).
- Perform ?
- Else if lowerExists is
true and upperExists isfalse , then- Perform ?
DeletePropertyOrThrow (O, lowerP). - Perform ?
Set (O, upperP, lowerValue,true ).
- Perform ?
- Else,
Assert : lowerExists and upperExists are bothfalse .- No action is required.
- Set lower to lower + 1.
- Return O.
The reverse function is intentionally generic; it does not require that its
23.1.3.24 Array.prototype.shift ( )
The first element of the array is removed from the array and returned.
When the shift method is called, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If len = 0, then
- Perform ?
Set (O,"length" ,+0 𝔽,true ). - Return
undefined .
- Perform ?
- Let first be ?
Get (O,"0" ). - Let k be 1.
- Repeat, while k < len,
- Let from be !
ToString (𝔽 (k)). - Let to be !
ToString (𝔽 (k - 1)). - Let fromPresent be ?
HasProperty (O, from). - If fromPresent is
true , then - Else,
Assert : fromPresent isfalse .- Perform ?
DeletePropertyOrThrow (O, to).
- Set k to k + 1.
- Let from be !
- Perform ?
DeletePropertyOrThrow (O, !ToString (𝔽 (len - 1))). - Perform ?
Set (O,"length" ,𝔽 (len - 1),true ). - Return first.
The shift function is intentionally generic; it does not require that its
23.1.3.25 Array.prototype.slice ( start, end )
The slice method takes two arguments, start and end, and returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is
The following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let k be 0.
- Else if relativeStart < 0, let k be
max (len + relativeStart, 0). - Else, let k be
min (relativeStart, len). - If end is
undefined , let relativeEnd be len; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let final be 0.
- Else if relativeEnd < 0, let final be
max (len + relativeEnd, 0). - Else, let final be
min (relativeEnd, len). - Let count be
max (final - k, 0). - Let A be ?
ArraySpeciesCreate (O, count). - Let n be 0.
- Repeat, while k < final,
- Let Pk be !
ToString (𝔽 (k)). - Let kPresent be ?
HasProperty (O, Pk). - If kPresent is
true , then- Let kValue be ?
Get (O, Pk). - Perform ?
CreateDataPropertyOrThrow (A, !ToString (𝔽 (n)), kValue).
- Let kValue be ?
- Set k to k + 1.
- Set n to n + 1.
- Let Pk be !
- Perform ?
Set (A,"length" ,𝔽 (n),true ). - Return A.
The explicit setting of the
The slice function is intentionally generic; it does not require that its
23.1.3.26 Array.prototype.some ( callbackfn [ , thisArg ] )
callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns some immediately returns some returns
If a thisArg parameter is provided, it will be used as the
callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.
some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns
When the some method is called with one or two arguments, the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
false .
The some function is intentionally generic; it does not require that its
23.1.3.27 Array.prototype.sort ( comparefn )
The elements of this array are sorted. The sort must be stable (that is, elements that compare equal must remain in their original order). If comparefn is not
The following steps are taken:
- If comparefn is not
undefined andIsCallable (comparefn) isfalse , throw aTypeError exception. - Let obj be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (obj). - Let items be a new empty
List . - Let k be 0.
- Repeat, while k < len,
- Let Pk be !
ToString (𝔽 (k)). - Let kPresent be ?
HasProperty (obj, Pk). - If kPresent is
true , then- Let kValue be ?
Get (obj, Pk). - Append kValue to items.
- Let kValue be ?
- Set k to k + 1.
- Let Pk be !
- Let itemCount be the number of elements in items.
- Sort items using an
implementation-defined sequence of calls toSortCompare . If any such call returns anabrupt completion , stop before performing any further calls toSortCompare or steps in this algorithm and return that completion. - Let j be 0.
- Repeat, while j < itemCount,
- Repeat, while j < len,
- Perform ?
DeletePropertyOrThrow (obj, !ToString (𝔽 (j))). - Set j to j + 1.
- Perform ?
- Return obj.
The sort order is the ordering, after completion of this function, of the sort function is then determined as follows:
The sort order is
-
If comparefn is not
undefined and is not a consistent comparison function for the elements of items (see below). -
If comparefn is
undefined andSortCompare does not act as a consistent comparison function. -
If comparefn is
undefined and all applications ofToString , to any specific value passed as an argument toSortCompare , do not produce the same result.
Unless the sort order is specified above to be
-
There must be some mathematical permutation π of the non-negative integers less than itemCount, such that for every non-negative
integer j less than itemCount, the elementold[j] is exactly the same asnew[π(j)] . -
Then for all non-negative integers j and k, each less than itemCount, if
SortCompare (old[j], old[k]) < 0SortCompare below), thenπ(j) < π(k) .
Here the notation
A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation
-
Calling comparefn(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore,
Type (v) is Number, and v is notNaN . Note that this implies that exactly one of a <CF b, a =CF b, and a >CF b will be true for a given pair of a and b. - Calling comparefn(a, b) does not modify obj or any object on obj's prototype chain.
- a =CF a (reflexivity)
- If a =CF b, then b =CF a (symmetry)
- If a =CF b and b =CF c, then a =CF c (transitivity of =CF)
- If a <CF b and b <CF c, then a <CF c (transitivity of <CF)
- If a >CF b and b >CF c, then a >CF c (transitivity of >CF)
The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.
The sort function is intentionally generic; it does not require that its
23.1.3.27.1 SortCompare ( x, y )
The abstract operation SortCompare takes arguments x and y. It also has access to the comparefn argument passed to the current invocation of the sort method. It performs the following steps when called:
- If x and y are both
undefined , return+0 𝔽. - If x is
undefined , return1 𝔽. - If y is
undefined , return-1 𝔽. - If comparefn is not
undefined , then - Let xString be ?
ToString (x). - Let yString be ?
ToString (y). - Let xSmaller be the result of performing
Abstract Relational Comparison xString < yString. - If xSmaller is
true , return-1 𝔽. - Let ySmaller be the result of performing
Abstract Relational Comparison yString < xString. - If ySmaller is
true , return1 𝔽. - Return
+0 𝔽.
Because non-existent property values always compare greater than
Method calls performed by the
23.1.3.28 Array.prototype.splice ( start, deleteCount, ...items )
When the splice method is called with two or more arguments start, deleteCount and zero or more items, the deleteCount elements of the array starting at
The following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let actualStart be 0.
- Else if relativeStart < 0, let actualStart be
max (len + relativeStart, 0). - Else, let actualStart be
min (relativeStart, len). - If start is not present, then
- Let insertCount be 0.
- Let actualDeleteCount be 0.
- Else if deleteCount is not present, then
- Let insertCount be 0.
- Let actualDeleteCount be len - actualStart.
- Else,
- Let insertCount be the number of elements in items.
- Let dc be ?
ToIntegerOrInfinity (deleteCount). - Let actualDeleteCount be the result of
clamping dc between 0 and len - actualStart.
- If len + insertCount - actualDeleteCount > 253 - 1, throw a
TypeError exception. - Let A be ?
ArraySpeciesCreate (O, actualDeleteCount). - Let k be 0.
- Repeat, while k < actualDeleteCount,
- Let from be !
ToString (𝔽 (actualStart + k)). - Let fromPresent be ?
HasProperty (O, from). - If fromPresent is
true , then- Let fromValue be ?
Get (O, from). - Perform ?
CreateDataPropertyOrThrow (A, !ToString (𝔽 (k)), fromValue).
- Let fromValue be ?
- Set k to k + 1.
- Let from be !
- Perform ?
Set (A,"length" ,𝔽 (actualDeleteCount),true ). - Let itemCount be the number of elements in items.
- If itemCount < actualDeleteCount, then
- Set k to actualStart.
- Repeat, while k < (len - actualDeleteCount),
- Let from be !
ToString (𝔽 (k + actualDeleteCount)). - Let to be !
ToString (𝔽 (k + itemCount)). - Let fromPresent be ?
HasProperty (O, from). - If fromPresent is
true , then - Else,
Assert : fromPresent isfalse .- Perform ?
DeletePropertyOrThrow (O, to).
- Set k to k + 1.
- Let from be !
- Set k to len.
- Repeat, while k > (len - actualDeleteCount + itemCount),
- Perform ?
DeletePropertyOrThrow (O, !ToString (𝔽 (k - 1))). - Set k to k - 1.
- Perform ?
- Else if itemCount > actualDeleteCount, then
- Set k to (len - actualDeleteCount).
- Repeat, while k > actualStart,
- Let from be !
ToString (𝔽 (k + actualDeleteCount - 1)). - Let to be !
ToString (𝔽 (k + itemCount - 1)). - Let fromPresent be ?
HasProperty (O, from). - If fromPresent is
true , then - Else,
Assert : fromPresent isfalse .- Perform ?
DeletePropertyOrThrow (O, to).
- Set k to k - 1.
- Let from be !
- Set k to actualStart.
- For each element E of items, do
- Perform ?
Set (O,"length" ,𝔽 (len - actualDeleteCount + itemCount),true ). - Return A.
The explicit setting of the
The splice function is intentionally generic; it does not require that its
23.1.3.29 Array.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Array.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.
The first edition of ECMA-402 did not include a replacement specification for the Array.prototype.toLocaleString method.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
The following steps are taken:
- Let array be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (array). - Let separator be the String value for the list-separator String appropriate for the
host environment 's current locale (this is derived in animplementation-defined way). - Let R be the empty String.
- Let k be 0.
- Repeat, while k < len,
- If k > 0, then
- Set R to the
string-concatenation of R and separator.
- Set R to the
- Let nextElement be ?
Get (array, !ToString (𝔽 (k))). - If nextElement is not
undefined ornull , then- Let S be ?
ToString (?Invoke (nextElement,"toLocaleString" )). - Set R to the
string-concatenation of R and S.
- Let S be ?
- Set k to k + 1.
- If k > 0, then
- Return R.
The elements of the array are converted to Strings using their toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in an toString, except that the result of this function is intended to be locale-specific.
The toLocaleString function is intentionally generic; it does not require that its
23.1.3.30 Array.prototype.toString ( )
When the toString method is called, the following steps are taken:
- Let array be ?
ToObject (this value). - Let func be ?
Get (array,"join" ). - If
IsCallable (func) isfalse , set func to the intrinsic function %Object.prototype.toString%. - Return ?
Call (func, array).
The toString function is intentionally generic; it does not require that its
23.1.3.31 Array.prototype.unshift ( ...items )
The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.
When the unshift method is called with zero or more arguments item1, item2, etc., the following steps are taken:
- Let O be ?
ToObject (this value). - Let len be ?
LengthOfArrayLike (O). - Let argCount be the number of elements in items.
- If argCount > 0, then
- If len + argCount > 253 - 1, throw a
TypeError exception. - Let k be len.
- Repeat, while k > 0,
- Let from be !
ToString (𝔽 (k - 1)). - Let to be !
ToString (𝔽 (k + argCount - 1)). - Let fromPresent be ?
HasProperty (O, from). - If fromPresent is
true , then - Else,
Assert : fromPresent isfalse .- Perform ?
DeletePropertyOrThrow (O, to).
- Set k to k - 1.
- Let from be !
- Let j be
+0 𝔽. - For each element E of items, do
- If len + argCount > 253 - 1, throw a
- Perform ?
Set (O,"length" ,𝔽 (len + argCount),true ). - Return
𝔽 (len + argCount).
The unshift method is
The unshift function is intentionally generic; it does not require that its
23.1.3.32 Array.prototype.values ( )
The following steps are taken:
- Let O be ?
ToObject (this value). - Return
CreateArrayIterator (O,value ).
23.1.3.33 Array.prototype [ @@iterator ] ( )
The initial value of the Array.prototype.values property.
23.1.3.34 Array.prototype [ @@unscopables ]
The initial value of the
- Let unscopableList be !
OrdinaryObjectCreate (null ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"copyWithin" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"entries" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"fill" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"find" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"findIndex" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"flat" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"flatMap" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"includes" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"keys" ,true ). - Perform !
CreateDataPropertyOrThrow (unscopableList,"values" ,true ). - Return unscopableList.
This property has the attributes { [[Writable]]:
The own property names of this object are property names that were not included as standard properties of Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array object.
23.1.4 Properties of Array Instances
Array instances are Array exotic objects and have the internal methods specified for such objects. Array instances inherit properties from the
Array instances have a
23.1.4.1 length
The
The
Reducing the value of the
23.1.5 Array Iterator Objects
An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There is not a named
23.1.5.1 CreateArrayIterator ( array, kind )
The abstract operation CreateArrayIterator takes arguments array and kind. This operation is used to create iterator objects for Array methods that return such iterators. It performs the following steps when called:
Assert :Type (array) is Object.Assert : kind iskey+value ,key , orvalue .- Let closure be a new
Abstract Closure with no parameters that captures kind and array and performs the following steps when called:- Let index be 0.
- Repeat,
- If array has a [[TypedArrayName]] internal slot, then
- If
IsDetachedBuffer (array.[[ViewedArrayBuffer]]) istrue , throw aTypeError exception. - Let len be array.[[ArrayLength]].
- If
- Else,
- Let len be ?
LengthOfArrayLike (array).
- Let len be ?
- If index ≥ len, return
undefined . - If kind is
key , perform ?Yield (𝔽 (index)). - Else,
- Set index to index + 1.
- If array has a [[TypedArrayName]] internal slot, then
- Return !
CreateIteratorFromClosure (closure,"%ArrayIteratorPrototype%" ,%ArrayIteratorPrototype% ).
23.1.5.2 The %ArrayIteratorPrototype% Object
The %ArrayIteratorPrototype% object:
- has properties that are inherited by all Array Iterator Objects.
- is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%IteratorPrototype% . - has the following properties:
23.1.5.2.1 %ArrayIteratorPrototype%.next ( )
- Return ?
GeneratorResume (this value,empty ,"%ArrayIteratorPrototype%" ).
23.1.5.2.2 %ArrayIteratorPrototype% [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
23.2 TypedArray Objects
TypedArray objects present an array-like view of an underlying binary data buffer (
|
|
Element Type | Element Size | Conversion Operation | Description |
|---|---|---|---|---|
|
Int8Array
%Int8Array% |
|
1 |
|
8-bit two's complement signed |
|
Uint8Array
%Uint8Array% |
|
1 |
|
8-bit unsigned |
|
Uint8ClampedArray
%Uint8ClampedArray% |
|
1 |
|
8-bit unsigned |
|
Int16Array
%Int16Array% |
|
2 |
|
16-bit two's complement signed |
|
Uint16Array
%Uint16Array% |
|
2 |
|
16-bit unsigned |
|
Int32Array
%Int32Array% |
|
4 |
|
32-bit two's complement signed |
|
Uint32Array
%Uint32Array% |
|
4 |
|
32-bit unsigned |
|
BigInt64Array
%BigInt64Array% |
|
8 |
|
64-bit two's complement signed |
|
BigUint64Array
%BigUint64Array% |
|
8 |
|
64-bit unsigned |
|
Float32Array
%Float32Array% |
|
4 | 32-bit IEEE floating point | |
|
Float64Array
%Float64Array% |
|
8 | 64-bit IEEE floating point |
In the definitions below, references to TypedArray should be replaced with the appropriate
23.2.1 The %TypedArray% Intrinsic Object
The %TypedArray% intrinsic object:
- is a
constructor function object that all of the TypedArrayconstructor objects inherit from. - along with its corresponding prototype object, provides common properties that are inherited by all TypedArray constructors and their instances.
- does not have a global name or appear as a property of the
global object . - acts as the abstract superclass of the various TypedArray constructors.
- will throw an error when invoked, because it is an abstract class
constructor . The TypedArray constructors do not perform asupercall to it.
23.2.1.1 %TypedArray% ( )
The
- Throw a
TypeError exception.
The
23.2.2 Properties of the %TypedArray% Intrinsic Object
The
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has a
"name" property whose value is"TypedArray" . - has the following properties:
23.2.2.1 %TypedArray%.from ( source [ , mapfn [ , thisArg ] ] )
When the from method is called with argument source, and optional arguments mapfn and thisArg, the following steps are taken:
- Let C be the
this value. - If
IsConstructor (C) isfalse , throw aTypeError exception. - If mapfn is
undefined , let mapping befalse . - Else,
- If
IsCallable (mapfn) isfalse , throw aTypeError exception. - Let mapping be
true .
- If
- Let usingIterator be ?
GetMethod (source,@@iterator ). - If usingIterator is not
undefined , then- Let values be ?
IterableToList (source, usingIterator). - Let len be the number of elements in values.
- Let targetObj be ?
TypedArrayCreate (C, «𝔽 (len) »). - Let k be 0.
- Repeat, while k < len,
Assert : values is now an emptyList .- Return targetObj.
- Let values be ?
- NOTE: source is not an Iterable so assume it is already an
array-like object . - Let arrayLike be !
ToObject (source). - Let len be ?
LengthOfArrayLike (arrayLike). - Let targetObj be ?
TypedArrayCreate (C, «𝔽 (len) »). - Let k be 0.
- Repeat, while k < len,
- Return targetObj.
23.2.2.2 %TypedArray%.of ( ...items )
When the of method is called with any number of arguments, the following steps are taken:
- Let len be the number of elements in items.
- Let C be the
this value. - If
IsConstructor (C) isfalse , throw aTypeError exception. - Let newObj be ?
TypedArrayCreate (C, «𝔽 (len) »). - Let k be 0.
- Repeat, while k < len,
- Return newObj.
23.2.2.3 %TypedArray%.prototype
The initial value of .prototype is the
This property has the attributes { [[Writable]]:
23.2.2.4 get %TypedArray% [ @@species ]
[@@species] is an
- Return the
this value.
The value of the
23.2.3 Properties of the %TypedArray% Prototype Object
The %TypedArray% prototype object:
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is %TypedArray.prototype%.
- is an
ordinary object . - does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
23.2.3.1 get %TypedArray%.prototype.buffer
.prototype.buffer is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[TypedArrayName]]). Assert : O has a [[ViewedArrayBuffer]] internal slot.- Let buffer be O.[[ViewedArrayBuffer]].
- Return buffer.
23.2.3.2 get %TypedArray%.prototype.byteLength
.prototype.byteLength is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[TypedArrayName]]). Assert : O has a [[ViewedArrayBuffer]] internal slot.- Let buffer be O.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , return+0 𝔽. - Let size be O.[[ByteLength]].
- Return
𝔽 (size).
23.2.3.3 get %TypedArray%.prototype.byteOffset
.prototype.byteOffset is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[TypedArrayName]]). Assert : O has a [[ViewedArrayBuffer]] internal slot.- Let buffer be O.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , return+0 𝔽. - Let offset be O.[[ByteOffset]].
- Return
𝔽 (offset).
23.2.3.4 %TypedArray%.prototype.constructor
The initial value of .prototype.constructor is the
23.2.3.5 %TypedArray%.prototype.copyWithin ( target, start [ , end ] )
The interpretation and use of the arguments of .prototype.copyWithin are the same as for Array.prototype.copyWithin as defined in
The following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- Let relativeTarget be ?
ToIntegerOrInfinity (target). - If relativeTarget is -∞, let to be 0.
- Else if relativeTarget < 0, let to be
max (len + relativeTarget, 0). - Else, let to be
min (relativeTarget, len). - Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let from be 0.
- Else if relativeStart < 0, let from be
max (len + relativeStart, 0). - Else, let from be
min (relativeStart, len). - If end is
undefined , let relativeEnd be len; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let final be 0.
- Else if relativeEnd < 0, let final be
max (len + relativeEnd, 0). - Else, let final be
min (relativeEnd, len). - Let count be
min (final - from, len - to). - If count > 0, then
- NOTE: The copying must be performed in a manner that preserves the bit-level encoding of the source data.
- Let buffer be O.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Let typedArrayName be the String value of O.[[TypedArrayName]].
- Let elementSize be the Element Size value specified in
Table 60 for typedArrayName. - Let byteOffset be O.[[ByteOffset]].
- Let toByteIndex be to × elementSize + byteOffset.
- Let fromByteIndex be from × elementSize + byteOffset.
- Let countBytes be count × elementSize.
- If fromByteIndex < toByteIndex and toByteIndex < fromByteIndex + countBytes, then
- Let direction be -1.
- Set fromByteIndex to fromByteIndex + countBytes - 1.
- Set toByteIndex to toByteIndex + countBytes - 1.
- Else,
- Let direction be 1.
- Repeat, while countBytes > 0,
- Let value be
GetValueFromBuffer (buffer, fromByteIndex,Uint8 ,true ,Unordered ). - Perform
SetValueInBuffer (buffer, toByteIndex,Uint8 , value,true ,Unordered ). - Set fromByteIndex to fromByteIndex + direction.
- Set toByteIndex to toByteIndex + direction.
- Set countBytes to countBytes - 1.
- Let value be
- Return O.
23.2.3.6 %TypedArray%.prototype.entries ( )
The following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Return
CreateArrayIterator (O,key+value ).
23.2.3.7 %TypedArray%.prototype.every ( callbackfn [ , thisArg ] )
The interpretation and use of the arguments of .prototype.every are the same as for Array.prototype.every as defined in
When the every method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
true .
This function is not generic. The
23.2.3.8 %TypedArray%.prototype.fill ( value [ , start [ , end ] ] )
The interpretation and use of the arguments of .prototype.fill are the same as for Array.prototype.fill as defined in
The following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If O.[[ContentType]] is
BigInt , set value to ?ToBigInt (value). - Otherwise, set value to ?
ToNumber (value). - Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let k be 0.
- Else if relativeStart < 0, let k be
max (len + relativeStart, 0). - Else, let k be
min (relativeStart, len). - If end is
undefined , let relativeEnd be len; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let final be 0.
- Else if relativeEnd < 0, let final be
max (len + relativeEnd, 0). - Else, let final be
min (relativeEnd, len). - If
IsDetachedBuffer (O.[[ViewedArrayBuffer]]) istrue , throw aTypeError exception. - Repeat, while k < final,
- Return O.
23.2.3.9 %TypedArray%.prototype.filter ( callbackfn [ , thisArg ] )
The interpretation and use of the arguments of .prototype.filter are the same as for Array.prototype.filter as defined in
When the filter method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let kept be a new empty
List . - Let k be 0.
- Let captured be 0.
- Repeat, while k < len,
- Let A be ?
TypedArraySpeciesCreate (O, «𝔽 (captured) »). - Let n be 0.
- For each element e of kept, do
- Return A.
This function is not generic. The
23.2.3.10 %TypedArray%.prototype.find ( predicate [ , thisArg ] )
The interpretation and use of the arguments of .prototype.find are the same as for Array.prototype.find as defined in
When the find method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (predicate) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
undefined .
This function is not generic. The
23.2.3.11 %TypedArray%.prototype.findIndex ( predicate [ , thisArg ] )
The interpretation and use of the arguments of .prototype.findIndex are the same as for Array.prototype.findIndex as defined in
When the findIndex method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (predicate) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
-1 𝔽.
This function is not generic. The
23.2.3.12 %TypedArray%.prototype.forEach ( callbackfn [ , thisArg ] )
The interpretation and use of the arguments of .prototype.forEach are the same as for Array.prototype.forEach as defined in
When the forEach method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
undefined .
This function is not generic. The
23.2.3.13 %TypedArray%.prototype.includes ( searchElement [ , fromIndex ] )
The interpretation and use of the arguments of .prototype.includes are the same as for Array.prototype.includes as defined in
When the includes method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If len is 0, return
false . - Let n be ?
ToIntegerOrInfinity (fromIndex). Assert : If fromIndex isundefined , then n is 0.- If n is +∞, return
false . - Else if n is -∞, set n to 0.
- If n ≥ 0, then
- Let k be n.
- Else,
- Let k be len + n.
- If k < 0, set k to 0.
- Repeat, while k < len,
- Let elementK be !
Get (O, !ToString (𝔽 (k))). - If
SameValueZero (searchElement, elementK) istrue , returntrue . - Set k to k + 1.
- Let elementK be !
- Return
false .
This function is not generic. The
23.2.3.14 %TypedArray%.prototype.indexOf ( searchElement [ , fromIndex ] )
The interpretation and use of the arguments of .prototype.indexOf are the same as for Array.prototype.indexOf as defined in
When the indexOf method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If len is 0, return
-1 𝔽. - Let n be ?
ToIntegerOrInfinity (fromIndex). Assert : If fromIndex isundefined , then n is 0.- If n is +∞, return
-1 𝔽. - Else if n is -∞, set n to 0.
- If n ≥ 0, then
- Let k be n.
- Else,
- Let k be len + n.
- If k < 0, set k to 0.
- Repeat, while k < len,
- Let kPresent be !
HasProperty (O, !ToString (𝔽 (k))). - If kPresent is
true , then- Let elementK be !
Get (O, !ToString (𝔽 (k))). - Let same be the result of performing
Strict Equality Comparison searchElement === elementK. - If same is
true , return𝔽 (k).
- Let elementK be !
- Set k to k + 1.
- Let kPresent be !
- Return
-1 𝔽.
This function is not generic. The
23.2.3.15 %TypedArray%.prototype.join ( separator )
The interpretation and use of the arguments of .prototype.join are the same as for Array.prototype.join as defined in
When the join method is called with one argument separator, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If separator is
undefined , let sep be the single-element String"," . - Else, let sep be ?
ToString (separator). - Let R be the empty String.
- Let k be 0.
- Repeat, while k < len,
- If k > 0, set R to the
string-concatenation of R and sep. - Let element be !
Get (O, !ToString (𝔽 (k))). - If element is
undefined , let next be the empty String; otherwise, let next be !ToString (element). - Set R to the
string-concatenation of R and next. - Set k to k + 1.
- If k > 0, set R to the
- Return R.
This function is not generic. The
23.2.3.16 %TypedArray%.prototype.keys ( )
The following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Return
CreateArrayIterator (O,key ).
23.2.3.17 %TypedArray%.prototype.lastIndexOf ( searchElement [ , fromIndex ] )
The interpretation and use of the arguments of .prototype.lastIndexOf are the same as for Array.prototype.lastIndexOf as defined in
When the lastIndexOf method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If len is 0, return
-1 𝔽. - If fromIndex is present, let n be ?
ToIntegerOrInfinity (fromIndex); else let n be len - 1. - If n is -∞, return
-1 𝔽. - If n ≥ 0, then
- Let k be
min (n, len - 1).
- Let k be
- Else,
- Let k be len + n.
- Repeat, while k ≥ 0,
- Let kPresent be !
HasProperty (O, !ToString (𝔽 (k))). - If kPresent is
true , then- Let elementK be !
Get (O, !ToString (𝔽 (k))). - Let same be the result of performing
Strict Equality Comparison searchElement === elementK. - If same is
true , return𝔽 (k).
- Let elementK be !
- Set k to k - 1.
- Let kPresent be !
- Return
-1 𝔽.
This function is not generic. The
23.2.3.18 get %TypedArray%.prototype.length
.prototype.length is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[TypedArrayName]]). Assert : O has [[ViewedArrayBuffer]] and [[ArrayLength]] internal slots.- Let buffer be O.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , return+0 𝔽. - Let length be O.[[ArrayLength]].
- Return
𝔽 (length).
This function is not generic. The
23.2.3.19 %TypedArray%.prototype.map ( callbackfn [ , thisArg ] )
The interpretation and use of the arguments of .prototype.map are the same as for Array.prototype.map as defined in
When the map method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let A be ?
TypedArraySpeciesCreate (O, «𝔽 (len) »). - Let k be 0.
- Repeat, while k < len,
- Return A.
This function is not generic. The
23.2.3.20 %TypedArray%.prototype.reduce ( callbackfn [ , initialValue ] )
The interpretation and use of the arguments of .prototype.reduce are the same as for Array.prototype.reduce as defined in
When the reduce method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - If len = 0 and initialValue is not present, throw a
TypeError exception. - Let k be 0.
- Let accumulator be
undefined . - If initialValue is present, then
- Set accumulator to initialValue.
- Else,
- Repeat, while k < len,
- Return accumulator.
This function is not generic. The
23.2.3.21 %TypedArray%.prototype.reduceRight ( callbackfn [ , initialValue ] )
The interpretation and use of the arguments of .prototype.reduceRight are the same as for Array.prototype.reduceRight as defined in
When the reduceRight method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - If len is 0 and initialValue is not present, throw a
TypeError exception. - Let k be len - 1.
- Let accumulator be
undefined . - If initialValue is present, then
- Set accumulator to initialValue.
- Else,
- Repeat, while k ≥ 0,
- Return accumulator.
This function is not generic. The
23.2.3.22 %TypedArray%.prototype.reverse ( )
The interpretation and use of the arguments of .prototype.reverse are the same as for Array.prototype.reverse as defined in
When the reverse method is called, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- Let middle be
floor (len / 2). - Let lower be 0.
- Repeat, while lower ≠ middle,
- Return O.
This function is not generic. The
23.2.3.23 %TypedArray%.prototype.set ( source [ , offset ] )
.prototype.set is a function whose behaviour differs based upon the type of its first argument.
This function is not generic. The
Sets multiple values in this TypedArray, reading the values from source. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.
- Let target be the
this value. - Perform ?
RequireInternalSlot (target, [[TypedArrayName]]). Assert : target has a [[ViewedArrayBuffer]] internal slot.- Let targetOffset be ?
ToIntegerOrInfinity (offset). - If targetOffset < 0, throw a
RangeError exception. - If source is an Object that has a [[TypedArrayName]] internal slot, then
- Perform ?
SetTypedArrayFromTypedArray (target, targetOffset, source).
- Perform ?
- Else,
- Perform ?
SetTypedArrayFromArrayLike (target, targetOffset, source).
- Perform ?
- Return
undefined .
23.2.3.23.1 SetTypedArrayFromTypedArray ( target, targetOffset, source )
The abstract operation SetTypedArrayFromTypedArray takes arguments target (a TypedArray object), targetOffset (a non-negative
Assert : source is an Object that has a [[TypedArrayName]] internal slot.- Let targetBuffer be target.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (targetBuffer) istrue , throw aTypeError exception. - Let targetLength be target.[[ArrayLength]].
- Let srcBuffer be source.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (srcBuffer) istrue , throw aTypeError exception. - Let targetName be the String value of target.[[TypedArrayName]].
- Let targetType be the Element Type value in
Table 60 for targetName. - Let targetElementSize be the Element Size value specified in
Table 60 for targetName. - Let targetByteOffset be target.[[ByteOffset]].
- Let srcName be the String value of source.[[TypedArrayName]].
- Let srcType be the Element Type value in
Table 60 for srcName. - Let srcElementSize be the Element Size value specified in
Table 60 for srcName. - Let srcLength be source.[[ArrayLength]].
- Let srcByteOffset be source.[[ByteOffset]].
- If targetOffset is +∞, throw a
RangeError exception. - If srcLength + targetOffset > targetLength, throw a
RangeError exception. - If target.[[ContentType]] ≠ source.[[ContentType]], throw a
TypeError exception. - If both
IsSharedArrayBuffer (srcBuffer) andIsSharedArrayBuffer (targetBuffer) aretrue , then- If srcBuffer.[[ArrayBufferData]] and targetBuffer.[[ArrayBufferData]] are the same
Shared Data Block values, let same betrue ; else let same befalse .
- If srcBuffer.[[ArrayBufferData]] and targetBuffer.[[ArrayBufferData]] are the same
- Else, let same be
SameValue (srcBuffer, targetBuffer). - If same is
true , then- Let srcByteLength be source.[[ByteLength]].
- Set srcBuffer to ?
CloneArrayBuffer (srcBuffer, srcByteOffset, srcByteLength,%ArrayBuffer% ). - NOTE:
%ArrayBuffer% is used to clone srcBuffer because is it known to not have any observable side-effects. - Let srcByteIndex be 0.
- Else, let srcByteIndex be srcByteOffset.
- Let targetByteIndex be targetOffset × targetElementSize + targetByteOffset.
- Let limit be targetByteIndex + targetElementSize × srcLength.
- If srcType is the same as targetType, then
- NOTE: If srcType and targetType are the same, the transfer must be performed in a manner that preserves the bit-level encoding of the source data.
- Repeat, while targetByteIndex < limit,
- Let value be
GetValueFromBuffer (srcBuffer, srcByteIndex,Uint8 ,true ,Unordered ). - Perform
SetValueInBuffer (targetBuffer, targetByteIndex,Uint8 , value,true ,Unordered ). - Set srcByteIndex to srcByteIndex + 1.
- Set targetByteIndex to targetByteIndex + 1.
- Let value be
- Else,
- Repeat, while targetByteIndex < limit,
- Let value be
GetValueFromBuffer (srcBuffer, srcByteIndex, srcType,true ,Unordered ). - Perform
SetValueInBuffer (targetBuffer, targetByteIndex, targetType, value,true ,Unordered ). - Set srcByteIndex to srcByteIndex + srcElementSize.
- Set targetByteIndex to targetByteIndex + targetElementSize.
- Let value be
- Repeat, while targetByteIndex < limit,
23.2.3.23.2 SetTypedArrayFromArrayLike ( target, targetOffset, source )
The abstract operation SetTypedArrayFromArrayLike takes arguments target (a TypedArray object), targetOffset (a non-negative
Assert : source is anyECMAScript language value other than an Object with a [[TypedArrayName]] internal slot.- Let targetBuffer be target.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (targetBuffer) istrue , throw aTypeError exception. - Let targetLength be target.[[ArrayLength]].
- Let targetName be the String value of target.[[TypedArrayName]].
- Let targetElementSize be the Element Size value specified in
Table 60 for targetName. - Let targetType be the Element Type value in
Table 60 for targetName. - Let targetByteOffset be target.[[ByteOffset]].
- Let src be ?
ToObject (source). - Let srcLength be ?
LengthOfArrayLike (src). - If targetOffset is +∞, throw a
RangeError exception. - If srcLength + targetOffset > targetLength, throw a
RangeError exception. - Let targetByteIndex be targetOffset × targetElementSize + targetByteOffset.
- Let k be 0.
- Let limit be targetByteIndex + targetElementSize × srcLength.
- Repeat, while targetByteIndex < limit,
- Let Pk be !
ToString (𝔽 (k)). - Let value be ?
Get (src, Pk). - If target.[[ContentType]] is
BigInt , set value to ?ToBigInt (value). - Otherwise, set value to ?
ToNumber (value). - If
IsDetachedBuffer (targetBuffer) istrue , throw aTypeError exception. - Perform
SetValueInBuffer (targetBuffer, targetByteIndex, targetType, value,true ,Unordered ). - Set k to k + 1.
- Set targetByteIndex to targetByteIndex + targetElementSize.
- Let Pk be !
23.2.3.24 %TypedArray%.prototype.slice ( start, end )
The interpretation and use of the arguments of .prototype.slice are the same as for Array.prototype.slice as defined in
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let k be 0.
- Else if relativeStart < 0, let k be
max (len + relativeStart, 0). - Else, let k be
min (relativeStart, len). - If end is
undefined , let relativeEnd be len; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let final be 0.
- Else if relativeEnd < 0, let final be
max (len + relativeEnd, 0). - Else, let final be
min (relativeEnd, len). - Let count be
max (final - k, 0). - Let A be ?
TypedArraySpeciesCreate (O, «𝔽 (count) »). - If count > 0, then
- If
IsDetachedBuffer (O.[[ViewedArrayBuffer]]) istrue , throw aTypeError exception. - Let srcName be the String value of O.[[TypedArrayName]].
- Let srcType be the Element Type value in
Table 60 for srcName. - Let targetName be the String value of A.[[TypedArrayName]].
- Let targetType be the Element Type value in
Table 60 for targetName. - If srcType is different from targetType, then
- Else,
- Let srcBuffer be O.[[ViewedArrayBuffer]].
- Let targetBuffer be A.[[ViewedArrayBuffer]].
- Let elementSize be the Element Size value specified in
Table 60 for Element Type srcType. - NOTE: If srcType and targetType are the same, the transfer must be performed in a manner that preserves the bit-level encoding of the source data.
- Let srcByteOffset be O.[[ByteOffset]].
- Let targetByteIndex be A.[[ByteOffset]].
- Let srcByteIndex be (k × elementSize) + srcByteOffset.
- Let limit be targetByteIndex + count × elementSize.
- Repeat, while targetByteIndex < limit,
- Let value be
GetValueFromBuffer (srcBuffer, srcByteIndex,Uint8 ,true ,Unordered ). - Perform
SetValueInBuffer (targetBuffer, targetByteIndex,Uint8 , value,true ,Unordered ). - Set srcByteIndex to srcByteIndex + 1.
- Set targetByteIndex to targetByteIndex + 1.
- Let value be
- If
- Return A.
This function is not generic. The
23.2.3.25 %TypedArray%.prototype.some ( callbackfn [ , thisArg ] )
The interpretation and use of the arguments of .prototype.some are the same as for Array.prototype.some as defined in
When the some method is called with one or two arguments, the following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Let len be O.[[ArrayLength]].
- If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let k be 0.
- Repeat, while k < len,
- Return
false .
This function is not generic. The
23.2.3.26 %TypedArray%.prototype.sort ( comparefn )
.prototype.sort is a distinct function that, except as described below, implements the same requirements as those of Array.prototype.sort as defined in .prototype.sort specification may be optimized with the knowledge that the
This function is not generic. The
Upon entry, the following steps are performed to initialize evaluation of the sort function. These steps are used instead of steps
- If comparefn is not
undefined andIsCallable (comparefn) isfalse , throw aTypeError exception. - Let obj be the
this value. - Let buffer be ?
ValidateTypedArray (obj). - Let len be obj.[[ArrayLength]].
The following version of .prototype.sort. It performs a numeric comparison rather than the string comparison used in
The abstract operation TypedArraySortCompare takes arguments x and y. It also has access to the comparefn and buffer values of the current invocation of the sort method. It performs the following steps when called:
Assert : BothType (x) andType (y) are Number or both are BigInt.- If comparefn is not
undefined , then- Let v be ?
ToNumber (?Call (comparefn,undefined , « x, y »)). - If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - If v is
NaN , return+0 𝔽. - Return v.
- Let v be ?
- If x and y are both
NaN , return+0 𝔽. - If x is
NaN , return1 𝔽. - If y is
NaN , return-1 𝔽. - If x < y, return
-1 𝔽. - If x > y, return
1 𝔽. - If x is
-0 𝔽 and y is+0 𝔽, return-1 𝔽. - If x is
+0 𝔽 and y is-0 𝔽, return1 𝔽. - Return
+0 𝔽.
Because
23.2.3.27 %TypedArray%.prototype.subarray ( begin, end )
Returns a new TypedArray object whose element type is the same as this TypedArray and whose ArrayBuffer is the same as the ArrayBuffer of this TypedArray, referencing the elements at begin, inclusive, up to end, exclusive. If either begin or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[TypedArrayName]]). Assert : O has a [[ViewedArrayBuffer]] internal slot.- Let buffer be O.[[ViewedArrayBuffer]].
- Let srcLength be O.[[ArrayLength]].
- Let relativeBegin be ?
ToIntegerOrInfinity (begin). - If relativeBegin is -∞, let beginIndex be 0.
- Else if relativeBegin < 0, let beginIndex be
max (srcLength + relativeBegin, 0). - Else, let beginIndex be
min (relativeBegin, srcLength). - If end is
undefined , let relativeEnd be srcLength; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let endIndex be 0.
- Else if relativeEnd < 0, let endIndex be
max (srcLength + relativeEnd, 0). - Else, let endIndex be
min (relativeEnd, srcLength). - Let newLength be
max (endIndex - beginIndex, 0). - Let constructorName be the String value of O.[[TypedArrayName]].
- Let elementSize be the Element Size value specified in
Table 60 for constructorName. - Let srcByteOffset be O.[[ByteOffset]].
- Let beginByteOffset be srcByteOffset + beginIndex × elementSize.
- Let argumentsList be « buffer,
𝔽 (beginByteOffset),𝔽 (newLength) ». - Return ?
TypedArraySpeciesCreate (O, argumentsList).
This function is not generic. The
23.2.3.28 %TypedArray%.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )
.prototype.toLocaleString is a distinct function that implements the same algorithm as Array.prototype.toLocaleString as defined in
This function is not generic.
If the ECMAScript implementation includes the ECMA-402 Internationalization API this function is based upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402 specification.
23.2.3.29 %TypedArray%.prototype.toString ( )
The initial value of the .prototype.toString Array.prototype.toString method defined in
23.2.3.30 %TypedArray%.prototype.values ( )
The following steps are taken:
- Let O be the
this value. - Perform ?
ValidateTypedArray (O). - Return
CreateArrayIterator (O,value ).
23.2.3.31 %TypedArray%.prototype [ @@iterator ] ( )
The initial value of the .prototype.values property.
23.2.3.32 get %TypedArray%.prototype [ @@toStringTag ]
.prototype[@@toStringTag] is an
This property has the attributes { [[Enumerable]]:
The initial value of the
23.2.4 Abstract Operations for TypedArray Objects
23.2.4.1 TypedArraySpeciesCreate ( exemplar, argumentList )
The abstract operation TypedArraySpeciesCreate takes arguments exemplar and argumentList. It is used to specify the creation of a new TypedArray object using a
Assert : exemplar is an Object that has [[TypedArrayName]] and [[ContentType]] internal slots.- Let defaultConstructor be the intrinsic object listed in column one of
Table 60 for exemplar.[[TypedArrayName]]. - Let constructor be ?
SpeciesConstructor (exemplar, defaultConstructor). - Let result be ?
TypedArrayCreate (constructor, argumentList). Assert : result has [[TypedArrayName]] and [[ContentType]] internal slots.- If result.[[ContentType]] ≠ exemplar.[[ContentType]], throw a
TypeError exception. - Return result.
23.2.4.2 TypedArrayCreate ( constructor, argumentList )
The abstract operation TypedArrayCreate takes arguments constructor and argumentList. It is used to specify the creation of a new TypedArray object using a
- Let newTypedArray be ?
Construct (constructor, argumentList). - Perform ?
ValidateTypedArray (newTypedArray). - If argumentList is a
List of a single Number, then- If newTypedArray.[[ArrayLength]] <
ℝ (argumentList[0]), throw aTypeError exception.
- If newTypedArray.[[ArrayLength]] <
- Return newTypedArray.
23.2.4.3 ValidateTypedArray ( O )
The abstract operation ValidateTypedArray takes argument O. It performs the following steps when called:
- Perform ?
RequireInternalSlot (O, [[TypedArrayName]]). Assert : O has a [[ViewedArrayBuffer]] internal slot.- Let buffer be O.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Return buffer.
23.2.5 The TypedArray Constructors
Each TypedArray
- is an intrinsic object that has the structure described below, differing only in the name used as the
constructor name instead of TypedArray, inTable 60 . - is a function whose behaviour differs based upon the number and types of its arguments. The actual behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed to it.
- is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified TypedArray behaviour must include asupercall to the TypedArrayconstructor to create and initialize the subclass instance with the internal state necessary to support the%TypedArray% .prototypebuilt-in methods. - has a
"length" property whose value is3 𝔽.
23.2.5.1 TypedArray ( ...args )
Each TypedArray
- If NewTarget is
undefined , throw aTypeError exception. - Let constructorName be the String value of the
Constructor Name value specified inTable 60 for this TypedArrayconstructor . - Let proto be
"%TypedArray.prototype%". - Let numberOfArgs be the number of elements in args.
- If numberOfArgs = 0, then
- Return ?
AllocateTypedArray (constructorName, NewTarget, proto, 0).
- Return ?
- Else,
- Let firstArgument be args[0].
- If
Type (firstArgument) is Object, then- Let O be ?
AllocateTypedArray (constructorName, NewTarget, proto). - If firstArgument has a [[TypedArrayName]] internal slot, then
- Perform ?
InitializeTypedArrayFromTypedArray (O, firstArgument).
- Perform ?
- Else if firstArgument has an [[ArrayBufferData]] internal slot, then
- If numberOfArgs > 1, let byteOffset be args[1]; else let byteOffset be
undefined . - If numberOfArgs > 2, let length be args[2]; else let length be
undefined . - Perform ?
InitializeTypedArrayFromArrayBuffer (O, firstArgument, byteOffset, length).
- If numberOfArgs > 1, let byteOffset be args[1]; else let byteOffset be
- Else,
Assert :Type (firstArgument) is Object and firstArgument does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.- Let usingIterator be ?
GetMethod (firstArgument,@@iterator ). - If usingIterator is not
undefined , then- Let values be ?
IterableToList (firstArgument, usingIterator). - Perform ?
InitializeTypedArrayFromList (O, values).
- Let values be ?
- Else,
- NOTE: firstArgument is not an Iterable so assume it is already an
array-like object . - Perform ?
InitializeTypedArrayFromArrayLike (O, firstArgument).
- NOTE: firstArgument is not an Iterable so assume it is already an
- Return O.
- Let O be ?
- Else,
Assert : firstArgument is not an Object.- Let elementLength be ?
ToIndex (firstArgument). - Return ?
AllocateTypedArray (constructorName, NewTarget, proto, elementLength).
23.2.5.1.1 AllocateTypedArray ( constructorName, newTarget, defaultProto [ , length ] )
The abstract operation AllocateTypedArray takes arguments constructorName (a String which is the name of a TypedArray
- Let proto be ?
GetPrototypeFromConstructor (newTarget, defaultProto). - Let obj be !
IntegerIndexedObjectCreate (proto). Assert : obj.[[ViewedArrayBuffer]] isundefined .- Set obj.[[TypedArrayName]] to constructorName.
- If constructorName is
"BigInt64Array" or"BigUint64Array" , set obj.[[ContentType]] toBigInt . - Otherwise, set obj.[[ContentType]] to
Number . - If length is not present, then
- Set obj.[[ByteLength]] to 0.
- Set obj.[[ByteOffset]] to 0.
- Set obj.[[ArrayLength]] to 0.
- Else,
- Perform ?
AllocateTypedArrayBuffer (obj, length).
- Perform ?
- Return obj.
23.2.5.1.2 InitializeTypedArrayFromTypedArray ( O, srcArray )
The abstract operation InitializeTypedArrayFromTypedArray takes arguments O (a TypedArray object) and srcArray (a TypedArray object). It performs the following steps when called:
Assert : O is an Object that has a [[TypedArrayName]] internal slot.Assert : srcArray is an Object that has a [[TypedArrayName]] internal slot.- Let srcData be srcArray.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (srcData) istrue , throw aTypeError exception. - Let constructorName be the String value of O.[[TypedArrayName]].
- Let elementType be the Element Type value in
Table 60 for constructorName. - Let elementLength be srcArray.[[ArrayLength]].
- Let srcName be the String value of srcArray.[[TypedArrayName]].
- Let srcType be the Element Type value in
Table 60 for srcName. - Let srcElementSize be the Element Size value specified in
Table 60 for srcName. - Let srcByteOffset be srcArray.[[ByteOffset]].
- Let elementSize be the Element Size value specified in
Table 60 for constructorName. - Let byteLength be elementSize × elementLength.
- If
IsSharedArrayBuffer (srcData) isfalse , then- Let bufferConstructor be ?
SpeciesConstructor (srcData,%ArrayBuffer% ).
- Let bufferConstructor be ?
- Else,
- Let bufferConstructor be
%ArrayBuffer% .
- Let bufferConstructor be
- If elementType is the same as srcType, then
- Let data be ?
CloneArrayBuffer (srcData, srcByteOffset, byteLength, bufferConstructor).
- Let data be ?
- Else,
- Let data be ?
AllocateArrayBuffer (bufferConstructor, byteLength). - If
IsDetachedBuffer (srcData) istrue , throw aTypeError exception. - If srcArray.[[ContentType]] ≠ O.[[ContentType]], throw a
TypeError exception. - Let srcByteIndex be srcByteOffset.
- Let targetByteIndex be 0.
- Let count be elementLength.
- Repeat, while count > 0,
- Let value be
GetValueFromBuffer (srcData, srcByteIndex, srcType,true ,Unordered ). - Perform
SetValueInBuffer (data, targetByteIndex, elementType, value,true ,Unordered ). - Set srcByteIndex to srcByteIndex + srcElementSize.
- Set targetByteIndex to targetByteIndex + elementSize.
- Set count to count - 1.
- Let value be
- Let data be ?
- Set O.[[ViewedArrayBuffer]] to data.
- Set O.[[ByteLength]] to byteLength.
- Set O.[[ByteOffset]] to 0.
- Set O.[[ArrayLength]] to elementLength.
23.2.5.1.3 InitializeTypedArrayFromArrayBuffer ( O, buffer, byteOffset, length )
The abstract operation InitializeTypedArrayFromArrayBuffer takes arguments O (a TypedArray object), buffer (an ArrayBuffer object), byteOffset (an
Assert : O is an Object that has a [[TypedArrayName]] internal slot.Assert : buffer is an Object that has an [[ArrayBufferData]] internal slot.- Let constructorName be the String value of O.[[TypedArrayName]].
- Let elementSize be the Element Size value specified in
Table 60 for constructorName. - Let offset be ?
ToIndex (byteOffset). - If offset
modulo elementSize ≠ 0, throw aRangeError exception. - If length is not
undefined , then- Let newLength be ?
ToIndex (length).
- Let newLength be ?
- If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
- If length is
undefined , then- If bufferByteLength
modulo elementSize ≠ 0, throw aRangeError exception. - Let newByteLength be bufferByteLength - offset.
- If newByteLength < 0, throw a
RangeError exception.
- If bufferByteLength
- Else,
- Let newByteLength be newLength × elementSize.
- If offset + newByteLength > bufferByteLength, throw a
RangeError exception.
- Set O.[[ViewedArrayBuffer]] to buffer.
- Set O.[[ByteLength]] to newByteLength.
- Set O.[[ByteOffset]] to offset.
- Set O.[[ArrayLength]] to newByteLength / elementSize.
23.2.5.1.4 InitializeTypedArrayFromList ( O, values )
The abstract operation InitializeTypedArrayFromList takes arguments O (a TypedArray object) and values (a
Assert : O is an Object that has a [[TypedArrayName]] internal slot.- Let len be the number of elements in values.
- Perform ?
AllocateTypedArrayBuffer (O, len). - Let k be 0.
- Repeat, while k < len,
Assert : values is now an emptyList .
23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O, arrayLike )
The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a TypedArray object) and arrayLike (an Object that is neither a TypedArray object nor an ArrayBuffer object). It performs the following steps when called:
Assert : O is an Object that has a [[TypedArrayName]] internal slot.- Let len be ?
LengthOfArrayLike (arrayLike). - Perform ?
AllocateTypedArrayBuffer (O, len). - Let k be 0.
- Repeat, while k < len,
23.2.5.1.6 AllocateTypedArrayBuffer ( O, length )
The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray object) and length (a non-negative
Assert : O is an Object that has a [[ViewedArrayBuffer]] internal slot.Assert : O.[[ViewedArrayBuffer]] isundefined .- Let constructorName be the String value of O.[[TypedArrayName]].
- Let elementSize be the Element Size value specified in
Table 60 for constructorName. - Let byteLength be elementSize × length.
- Let data be ?
AllocateArrayBuffer (%ArrayBuffer% , byteLength). - Set O.[[ViewedArrayBuffer]] to data.
- Set O.[[ByteLength]] to byteLength.
- Set O.[[ByteOffset]] to 0.
- Set O.[[ArrayLength]] to length.
- Return O.
23.2.6 Properties of the TypedArray Constructors
Each TypedArray
- has a [[Prototype]] internal slot whose value is
%TypedArray% . - has a
"name" property whose value is the String value of theconstructor name specified for it inTable 60 . - has the following properties:
23.2.6.1 TypedArray.BYTES_PER_ELEMENT
The value of TypedArray.BYTES_PER_ELEMENT is the Element Size value specified in
This property has the attributes { [[Writable]]:
23.2.6.2 TypedArray.prototype
The initial value of TypedArray.prototype is the corresponding TypedArray prototype intrinsic object (
This property has the attributes { [[Writable]]:
23.2.7 Properties of the TypedArray Prototype Objects
Each TypedArray prototype object:
- has a [[Prototype]] internal slot whose value is
%TypedArray.prototype% . - is an
ordinary object . - does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
23.2.7.1 TypedArray.prototype.BYTES_PER_ELEMENT
The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value specified in
This property has the attributes { [[Writable]]:
23.2.7.2 TypedArray.prototype.constructor
The initial value of a TypedArray.prototype.constructor is the corresponding
23.2.8 Properties of TypedArray Instances
TypedArray instances are
24 Keyed Collections
24.1 Map Objects
Map objects are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated using the
Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Map objects specification is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation model.
24.1.1 The Map Constructor
The Map
- is %Map%.
- is the initial value of the
"Map" property of theglobal object . - creates and initializes a new Map object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value in an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified Map behaviour must include asupercall to the Mapconstructor to create and initialize the subclass instance with the internal state necessary to support theMap.prototypebuilt-in methods.
24.1.1.1 Map ( [ iterable ] )
When the Map function is called with optional argument iterable, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - Let map be ?
OrdinaryCreateFromConstructor (NewTarget,"%Map.prototype%" , « [[MapData]] »). - Set map.[[MapData]] to a new empty
List . - If iterable is either
undefined ornull , return map. - Let adder be ?
Get (map,"set" ). - Return ?
AddEntriesFromIterable (map, iterable, adder).
If the parameter iterable is present, it is expected to be an object that implements an
24.1.1.2 AddEntriesFromIterable ( target, iterable, adder )
The abstract operation AddEntriesFromIterable takes arguments target, iterable, and adder (a
- If
IsCallable (adder) isfalse , throw aTypeError exception. Assert : iterable is present, and is neitherundefined nornull .- Let iteratorRecord be ?
GetIterator (iterable). - Repeat,
- Let next be ?
IteratorStep (iteratorRecord). - If next is
false , return target. - Let nextItem be ?
IteratorValue (next). - If
Type (nextItem) is not Object, then- Let error be
ThrowCompletion (a newly createdTypeError object). - Return ?
IteratorClose (iteratorRecord, error).
- Let error be
- Let k be
Get (nextItem,"0" ). - If k is an
abrupt completion , return ?IteratorClose (iteratorRecord, k). - Let v be
Get (nextItem,"1" ). - If v is an
abrupt completion , return ?IteratorClose (iteratorRecord, v). - Let status be
Call (adder, target, « k.[[Value]], v.[[Value]] »). - If status is an
abrupt completion , return ?IteratorClose (iteratorRecord, status).
- Let next be ?
The parameter iterable is expected to be an object that implements an
24.1.2 Properties of the Map Constructor
The Map
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
24.1.2.1 Map.prototype
The initial value of Map.prototype is the
This property has the attributes { [[Writable]]:
24.1.2.2 get Map [ @@species ]
Map[@@species] is an
- Return the
this value.
The value of the
Methods that create derived collection objects should call
24.1.3 Properties of the Map Prototype Object
The Map prototype object:
- is %Map.prototype%.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have a [[MapData]] internal slot.
24.1.3.1 Map.prototype.clear ( )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[MapData]]). - Let entries be the
List that is M.[[MapData]]. - For each
Record { [[Key]], [[Value]] } p of entries, do- Set p.[[Key]] to
empty . - Set p.[[Value]] to
empty .
- Set p.[[Key]] to
- Return
undefined .
24.1.3.2 Map.prototype.constructor
The initial value of Map.prototype.constructor is
24.1.3.3 Map.prototype.delete ( key )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[MapData]]). - Let entries be the
List that is M.[[MapData]]. - For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty andSameValueZero (p.[[Key]], key) istrue , then- Set p.[[Key]] to
empty . - Set p.[[Value]] to
empty . - Return
true .
- Set p.[[Key]] to
- If p.[[Key]] is not
- Return
false .
The value
24.1.3.4 Map.prototype.entries ( )
The following steps are taken:
- Let M be the
this value. - Return ?
CreateMapIterator (M,key+value ).
24.1.3.5 Map.prototype.forEach ( callbackfn [ , thisArg ] )
When the forEach method is called with one or two arguments, the following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[MapData]]). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let entries be the
List that is M.[[MapData]]. - For each
Record { [[Key]], [[Value]] } e of entries, do- If e.[[Key]] is not
empty , then- Perform ?
Call (callbackfn, thisArg, « e.[[Value]], e.[[Key]], M »).
- Perform ?
- If e.[[Key]] is not
- Return
undefined .
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each key/value pair present in the map object, in key insertion order. callbackfn is called only for keys of the map which actually exist; it is not called for keys that have been deleted from the map.
If a thisArg parameter is provided, it will be used as the
callbackfn is called with three arguments: the value of the item, the key of the item, and the Map object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Keys that are deleted after the call to forEach begins and before being visited are not visited unless the key is added again before the forEach call completes.
24.1.3.6 Map.prototype.get ( key )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[MapData]]). - Let entries be the
List that is M.[[MapData]]. - For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty andSameValueZero (p.[[Key]], key) istrue , return p.[[Value]].
- If p.[[Key]] is not
- Return
undefined .
24.1.3.7 Map.prototype.has ( key )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[MapData]]). - Let entries be the
List that is M.[[MapData]]. - For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty andSameValueZero (p.[[Key]], key) istrue , returntrue .
- If p.[[Key]] is not
- Return
false .
24.1.3.8 Map.prototype.keys ( )
The following steps are taken:
- Let M be the
this value. - Return ?
CreateMapIterator (M,key ).
24.1.3.9 Map.prototype.set ( key, value )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[MapData]]). - Let entries be the
List that is M.[[MapData]]. - For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty andSameValueZero (p.[[Key]], key) istrue , then- Set p.[[Value]] to value.
- Return M.
- If p.[[Key]] is not
- If key is
-0 𝔽, set key to+0 𝔽. - Let p be the
Record { [[Key]]: key, [[Value]]: value }. - Append p as the last element of entries.
- Return M.
24.1.3.10 get Map.prototype.size
Map.prototype.size is an
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[MapData]]). - Let entries be the
List that is M.[[MapData]]. - Let count be 0.
- For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty , set count to count + 1.
- If p.[[Key]] is not
- Return
𝔽 (count).
24.1.3.11 Map.prototype.values ( )
The following steps are taken:
- Let M be the
this value. - Return ?
CreateMapIterator (M,value ).
24.1.3.12 Map.prototype [ @@iterator ] ( )
The initial value of the
24.1.3.13 Map.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
24.1.4 Properties of Map Instances
Map instances are ordinary objects that inherit properties from the Map prototype. Map instances also have a [[MapData]] internal slot.
24.1.5 Map Iterator Objects
A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a named
24.1.5.1 CreateMapIterator ( map, kind )
The abstract operation CreateMapIterator takes arguments map and kind. This operation is used to create iterator objects for Map methods that return such iterators. It performs the following steps when called:
Assert : kind iskey+value ,key , orvalue .- Perform ?
RequireInternalSlot (map, [[MapData]]). - Let closure be a new
Abstract Closure with no parameters that captures map and kind and performs the following steps when called:- Let entries be the
List that is map.[[MapData]]. - Let index be 0.
- Let numEntries be the number of elements of entries.
- Repeat, while index < numEntries,
- Let e be the
Record { [[Key]], [[Value]] } that is the value of entries[index]. - Set index to index + 1.
- If e.[[Key]] is not
empty , then- If kind is
key , let result be e.[[Key]]. - Else if kind is
value , let result be e.[[Value]]. - Else,
Assert : kind iskey+value .- Let result be !
CreateArrayFromList (« e.[[Key]], e.[[Value]] »).
- Perform ?
Yield (result). - NOTE: the number of elements in entries may have changed while execution of this abstract operation was paused by
Yield . - Set numEntries to the number of elements of entries.
- If kind is
- Let e be the
- Return
undefined .
- Let entries be the
- Return !
CreateIteratorFromClosure (closure,"%MapIteratorPrototype%" ,%MapIteratorPrototype% ).
24.1.5.2 The %MapIteratorPrototype% Object
The %MapIteratorPrototype% object:
- has properties that are inherited by all Map Iterator Objects.
- is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%IteratorPrototype% . - has the following properties:
24.1.5.2.1 %MapIteratorPrototype%.next ( )
- Return ?
GeneratorResume (this value,empty ,"%MapIteratorPrototype%" ).
24.1.5.2.2 %MapIteratorPrototype% [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
24.2 Set Objects
Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's collection. Distinct values are discriminated using the
Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Set objects specification is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation model.
24.2.1 The Set Constructor
The Set
- is %Set%.
- is the initial value of the
"Set" property of theglobal object . - creates and initializes a new Set object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value in an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified Set behaviour must include asupercall to the Setconstructor to create and initialize the subclass instance with the internal state necessary to support theSet.prototypebuilt-in methods.
24.2.1.1 Set ( [ iterable ] )
When the Set function is called with optional argument iterable, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - Let set be ?
OrdinaryCreateFromConstructor (NewTarget,"%Set.prototype%" , « [[SetData]] »). - Set set.[[SetData]] to a new empty
List . - If iterable is either
undefined ornull , return set. - Let adder be ?
Get (set,"add" ). - If
IsCallable (adder) isfalse , throw aTypeError exception. - Let iteratorRecord be ?
GetIterator (iterable). - Repeat,
- Let next be ?
IteratorStep (iteratorRecord). - If next is
false , return set. - Let nextValue be ?
IteratorValue (next). - Let status be
Call (adder, set, « nextValue »). - If status is an
abrupt completion , return ?IteratorClose (iteratorRecord, status).
- Let next be ?
24.2.2 Properties of the Set Constructor
The Set
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
24.2.2.1 Set.prototype
The initial value of Set.prototype is the
This property has the attributes { [[Writable]]:
24.2.2.2 get Set [ @@species ]
Set[@@species] is an
- Return the
this value.
The value of the
Methods that create derived collection objects should call
24.2.3 Properties of the Set Prototype Object
The Set prototype object:
- is %Set.prototype%.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have a [[SetData]] internal slot.
24.2.3.1 Set.prototype.add ( value )
The following steps are taken:
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[SetData]]). - Let entries be the
List that is S.[[SetData]]. - For each element e of entries, do
- If e is not
empty andSameValueZero (e, value) istrue , then- Return S.
- If e is not
- If value is
-0 𝔽, set value to+0 𝔽. - Append value as the last element of entries.
- Return S.
24.2.3.2 Set.prototype.clear ( )
The following steps are taken:
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[SetData]]). - Let entries be the
List that is S.[[SetData]]. - For each element e of entries, do
- Replace the element of entries whose value is e with an element whose value is
empty .
- Replace the element of entries whose value is e with an element whose value is
- Return
undefined .
24.2.3.3 Set.prototype.constructor
The initial value of Set.prototype.constructor is
24.2.3.4 Set.prototype.delete ( value )
The following steps are taken:
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[SetData]]). - Let entries be the
List that is S.[[SetData]]. - For each element e of entries, do
- If e is not
empty andSameValueZero (e, value) istrue , then- Replace the element of entries whose value is e with an element whose value is
empty . - Return
true .
- Replace the element of entries whose value is e with an element whose value is
- If e is not
- Return
false .
The value
24.2.3.5 Set.prototype.entries ( )
The following steps are taken:
- Let S be the
this value. - Return ?
CreateSetIterator (S,key+value ).
For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.
24.2.3.6 Set.prototype.forEach ( callbackfn [ , thisArg ] )
When the forEach method is called with one or two arguments, the following steps are taken:
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[SetData]]). - If
IsCallable (callbackfn) isfalse , throw aTypeError exception. - Let entries be the
List that is S.[[SetData]]. - For each element e of entries, do
- If e is not
empty , then- Perform ?
Call (callbackfn, thisArg, « e, e, S »).
- Perform ?
- If e is not
- Return
undefined .
callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each value present in the set object, in value insertion order. callbackfn is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used as the
callbackfn is called with three arguments: the first two arguments are a value contained in the Set. The same value is passed for both arguments. The Set object being traversed is passed as the third argument.
The callbackfn is called with three arguments to be consistent with the call back functions used by forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.
Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Values that are deleted after the call to forEach begins and before being visited are not visited unless the value is added again before the forEach call completes. New values added after the call to forEach begins are visited.
24.2.3.7 Set.prototype.has ( value )
The following steps are taken:
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[SetData]]). - Let entries be the
List that is S.[[SetData]]. - For each element e of entries, do
- If e is not
empty andSameValueZero (e, value) istrue , returntrue .
- If e is not
- Return
false .
24.2.3.8 Set.prototype.keys ( )
The initial value of the
For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.
24.2.3.9 get Set.prototype.size
Set.prototype.size is an
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[SetData]]). - Let entries be the
List that is S.[[SetData]]. - Let count be 0.
- For each element e of entries, do
- If e is not
empty , set count to count + 1.
- If e is not
- Return
𝔽 (count).
24.2.3.10 Set.prototype.values ( )
The following steps are taken:
- Let S be the
this value. - Return ?
CreateSetIterator (S,value ).
24.2.3.11 Set.prototype [ @@iterator ] ( )
The initial value of the
24.2.3.12 Set.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
24.2.4 Properties of Set Instances
Set instances are ordinary objects that inherit properties from the Set prototype. Set instances also have a [[SetData]] internal slot.
24.2.5 Set Iterator Objects
A Set Iterator is an
24.2.5.1 CreateSetIterator ( set, kind )
The abstract operation CreateSetIterator takes arguments set and kind. This operation is used to create iterator objects for Set methods that return such iterators. It performs the following steps when called:
Assert : kind iskey+value orvalue .- Perform ?
RequireInternalSlot (set, [[SetData]]). - Let closure be a new
Abstract Closure with no parameters that captures set and kind and performs the following steps when called:- Let index be 0.
- Let entries be the
List that is set.[[SetData]]. - Let numEntries be the number of elements of entries.
- Repeat, while index < numEntries,
- Let e be entries[index].
- Set index to index + 1.
- If e is not
empty , then- If kind is
key+value , then- Perform ?
Yield (!CreateArrayFromList (« e, e »)).
- Perform ?
- Else,
- NOTE: the number of elements in entries may have changed while execution of this abstract operation was paused by
Yield . - Set numEntries to the number of elements of entries.
- If kind is
- Return
undefined .
- Return !
CreateIteratorFromClosure (closure,"%SetIteratorPrototype%" ,%SetIteratorPrototype% ).
24.2.5.2 The %SetIteratorPrototype% Object
The %SetIteratorPrototype% object:
- has properties that are inherited by all Set Iterator Objects.
- is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%IteratorPrototype% . - has the following properties:
24.2.5.2.1 %SetIteratorPrototype%.next ( )
- Return ?
GeneratorResume (this value,empty ,"%SetIteratorPrototype%" ).
24.2.5.2.2 %SetIteratorPrototype% [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
24.3 WeakMap Objects
WeakMap objects are collections of key/value pairs where the keys are objects and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the objects it holds as keys. In certain conditions, objects which are not
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.
WeakMap objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this WeakMap objects specification are only intended to describe the required observable semantics of WeakMap objects. It is not intended to be a viable implementation model.
WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet, the object otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object mapping of weak map instances to keys. Alternatively each weak map may internally store its key to value mappings but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSets:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.
Alexandra Barros, Roberto Ierusalimschy, Eliminating Cycles in Weak Tables. Journal of Universal Computer Science - J.UCS, vol. 14, no. 21, pp. 3481-3497, 2008, http://www.jucs.org/jucs_14_21/eliminating_cycles_in_weak
24.3.1 The WeakMap Constructor
The WeakMap
- is %WeakMap%.
- is the initial value of the
"WeakMap" property of theglobal object . - creates and initializes a new WeakMap object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value in an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified WeakMap behaviour must include asupercall to the WeakMapconstructor to create and initialize the subclass instance with the internal state necessary to support theWeakMap.prototypebuilt-in methods.
24.3.1.1 WeakMap ( [ iterable ] )
When the WeakMap function is called with optional argument iterable, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - Let map be ?
OrdinaryCreateFromConstructor (NewTarget,"%WeakMap.prototype%" , « [[WeakMapData]] »). - Set map.[[WeakMapData]] to a new empty
List . - If iterable is either
undefined ornull , return map. - Let adder be ?
Get (map,"set" ). - Return ?
AddEntriesFromIterable (map, iterable, adder).
If the parameter iterable is present, it is expected to be an object that implements an
24.3.2 Properties of the WeakMap Constructor
The WeakMap
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
24.3.2.1 WeakMap.prototype
The initial value of WeakMap.prototype is the
This property has the attributes { [[Writable]]:
24.3.3 Properties of the WeakMap Prototype Object
The WeakMap prototype object:
- is %WeakMap.prototype%.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have a [[WeakMapData]] internal slot.
24.3.3.1 WeakMap.prototype.constructor
The initial value of WeakMap.prototype.constructor is
24.3.3.2 WeakMap.prototype.delete ( key )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[WeakMapData]]). - Let entries be the
List that is M.[[WeakMapData]]. - If
Type (key) is not Object, returnfalse . - For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty andSameValue (p.[[Key]], key) istrue , then- Set p.[[Key]] to
empty . - Set p.[[Value]] to
empty . - Return
true .
- Set p.[[Key]] to
- If p.[[Key]] is not
- Return
false .
The value
24.3.3.3 WeakMap.prototype.get ( key )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[WeakMapData]]). - Let entries be the
List that is M.[[WeakMapData]]. - If
Type (key) is not Object, returnundefined . - For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty andSameValue (p.[[Key]], key) istrue , return p.[[Value]].
- If p.[[Key]] is not
- Return
undefined .
24.3.3.4 WeakMap.prototype.has ( key )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[WeakMapData]]). - Let entries be the
List that is M.[[WeakMapData]]. - If
Type (key) is not Object, returnfalse . - For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty andSameValue (p.[[Key]], key) istrue , returntrue .
- If p.[[Key]] is not
- Return
false .
24.3.3.5 WeakMap.prototype.set ( key, value )
The following steps are taken:
- Let M be the
this value. - Perform ?
RequireInternalSlot (M, [[WeakMapData]]). - Let entries be the
List that is M.[[WeakMapData]]. - If
Type (key) is not Object, throw aTypeError exception. - For each
Record { [[Key]], [[Value]] } p of entries, do- If p.[[Key]] is not
empty andSameValue (p.[[Key]], key) istrue , then- Set p.[[Value]] to value.
- Return M.
- If p.[[Key]] is not
- Let p be the
Record { [[Key]]: key, [[Value]]: value }. - Append p as the last element of entries.
- Return M.
24.3.3.6 WeakMap.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
24.3.4 Properties of WeakMap Instances
WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.
24.4 WeakSet Objects
WeakSet objects are collections of objects. A distinct object may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for enumerating the objects it holds. In certain conditions, objects which are not
An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet becomes inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular object that does not require the observer to present the observed object.
WeakSet objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this WeakSet objects specification is only intended to describe the required observable semantics of WeakSet objects. It is not intended to be a viable implementation model.
See the NOTE in
24.4.1 The WeakSet Constructor
The WeakSet
- is %WeakSet%.
- is the initial value of the
"WeakSet" property of theglobal object . - creates and initializes a new WeakSet object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value in an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified WeakSet behaviour must include asupercall to the WeakSetconstructor to create and initialize the subclass instance with the internal state necessary to support theWeakSet.prototypebuilt-in methods.
24.4.1.1 WeakSet ( [ iterable ] )
When the WeakSet function is called with optional argument iterable, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - Let set be ?
OrdinaryCreateFromConstructor (NewTarget,"%WeakSet.prototype%" , « [[WeakSetData]] »). - Set set.[[WeakSetData]] to a new empty
List . - If iterable is either
undefined ornull , return set. - Let adder be ?
Get (set,"add" ). - If
IsCallable (adder) isfalse , throw aTypeError exception. - Let iteratorRecord be ?
GetIterator (iterable). - Repeat,
- Let next be ?
IteratorStep (iteratorRecord). - If next is
false , return set. - Let nextValue be ?
IteratorValue (next). - Let status be
Call (adder, set, « nextValue »). - If status is an
abrupt completion , return ?IteratorClose (iteratorRecord, status).
- Let next be ?
24.4.2 Properties of the WeakSet Constructor
The WeakSet
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
24.4.2.1 WeakSet.prototype
The initial value of WeakSet.prototype is the
This property has the attributes { [[Writable]]:
24.4.3 Properties of the WeakSet Prototype Object
The WeakSet prototype object:
- is %WeakSet.prototype%.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have a [[WeakSetData]] internal slot.
24.4.3.1 WeakSet.prototype.add ( value )
The following steps are taken:
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[WeakSetData]]). - If
Type (value) is not Object, throw aTypeError exception. - Let entries be the
List that is S.[[WeakSetData]]. - For each element e of entries, do
- If e is not
empty andSameValue (e, value) istrue , then- Return S.
- If e is not
- Append value as the last element of entries.
- Return S.
24.4.3.2 WeakSet.prototype.constructor
The initial value of WeakSet.prototype.constructor is the
24.4.3.3 WeakSet.prototype.delete ( value )
The following steps are taken:
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[WeakSetData]]). - If
Type (value) is not Object, returnfalse . - Let entries be the
List that is S.[[WeakSetData]]. - For each element e of entries, do
- If e is not
empty andSameValue (e, value) istrue , then- Replace the element of entries whose value is e with an element whose value is
empty . - Return
true .
- Replace the element of entries whose value is e with an element whose value is
- If e is not
- Return
false .
The value
24.4.3.4 WeakSet.prototype.has ( value )
The following steps are taken:
- Let S be the
this value. - Perform ?
RequireInternalSlot (S, [[WeakSetData]]). - Let entries be the
List that is S.[[WeakSetData]]. - If
Type (value) is not Object, returnfalse . - For each element e of entries, do
- If e is not
empty andSameValue (e, value) istrue , returntrue .
- If e is not
- Return
false .
24.4.3.5 WeakSet.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
24.4.4 Properties of WeakSet Instances
WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.
25 Structured Data
25.1 ArrayBuffer Objects
25.1.1 Notation
The descriptions below in this section,
A read-modify-write modification function is a mathematical function that is notationally represented as an abstract closure that takes two Lists of byte values as arguments and returns a
- They perform all their algorithm steps atomically.
- Their individual algorithm steps are not observable.
To aid verifying that a read-modify-write modification function's algorithm steps constitute a pure, mathematical function, the following editorial conventions are recommended:
- They do not access, directly or transitively via invoked
abstract operations and abstract closures, any language or specification values except their parameters and captured values. - They do not return completion values.
25.1.2 Abstract Operations For ArrayBuffer Objects
25.1.2.1 AllocateArrayBuffer ( constructor, byteLength )
The abstract operation AllocateArrayBuffer takes arguments constructor and byteLength (a non-negative
- Let obj be ?
OrdinaryCreateFromConstructor (constructor,"%ArrayBuffer.prototype%" , « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] »). - Let block be ?
CreateByteDataBlock (byteLength). - Set obj.[[ArrayBufferData]] to block.
- Set obj.[[ArrayBufferByteLength]] to byteLength.
- Return obj.
25.1.2.2 IsDetachedBuffer ( arrayBuffer )
The abstract operation IsDetachedBuffer takes argument arrayBuffer. It performs the following steps when called:
25.1.2.3 DetachArrayBuffer ( arrayBuffer [ , key ] )
The abstract operation DetachArrayBuffer takes argument arrayBuffer and optional argument key. It performs the following steps when called:
Assert :Type (arrayBuffer) is Object and it has [[ArrayBufferData]], [[ArrayBufferByteLength]], and [[ArrayBufferDetachKey]] internal slots.Assert :IsSharedArrayBuffer (arrayBuffer) isfalse .- If key is not present, set key to
undefined . - If
SameValue (arrayBuffer.[[ArrayBufferDetachKey]], key) isfalse , throw aTypeError exception. - Set arrayBuffer.[[ArrayBufferData]] to
null . - Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
- Return
NormalCompletion (null ).
Detaching an ArrayBuffer instance disassociates the
25.1.2.4 CloneArrayBuffer ( srcBuffer, srcByteOffset, srcLength, cloneConstructor )
The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer object), srcByteOffset (a non-negative
Assert :Type (srcBuffer) is Object and it has an [[ArrayBufferData]] internal slot.Assert :IsConstructor (cloneConstructor) istrue .- Let targetBuffer be ?
AllocateArrayBuffer (cloneConstructor, srcLength). - If
IsDetachedBuffer (srcBuffer) istrue , throw aTypeError exception. - Let srcBlock be srcBuffer.[[ArrayBufferData]].
- Let targetBlock be targetBuffer.[[ArrayBufferData]].
- Perform
CopyDataBlockBytes (targetBlock, 0, srcBlock, srcByteOffset, srcLength). - Return targetBuffer.
25.1.2.5 IsUnsignedElementType ( type )
The abstract operation IsUnsignedElementType takes argument type. It verifies if the argument type is an unsigned
- If type is
Uint8 ,Uint8C ,Uint16 ,Uint32 , orBigUint64 , returntrue . - Return
false .
25.1.2.6 IsUnclampedIntegerElementType ( type )
The abstract operation IsUnclampedIntegerElementType takes argument type. It verifies if the argument type is an
- If type is
Int8 ,Uint8 ,Int16 ,Uint16 ,Int32 , orUint32 , returntrue . - Return
false .
25.1.2.7 IsBigIntElementType ( type )
The abstract operation IsBigIntElementType takes argument type. It verifies if the argument type is a BigInt
- If type is
BigUint64 orBigInt64 , returntrue . - Return
false .
25.1.2.8 IsNoTearConfiguration ( type, order )
The abstract operation IsNoTearConfiguration takes arguments type and order. It performs the following steps when called:
- If !
IsUnclampedIntegerElementType (type) istrue , returntrue . - If !
IsBigIntElementType (type) istrue and order is notInit orUnordered , returntrue . - Return
false .
25.1.2.9 RawBytesToNumeric ( type, rawBytes, isLittleEndian )
The abstract operation RawBytesToNumeric takes arguments type (a
- Let elementSize be the Element Size value specified in
Table 60 for Element Type type. - If isLittleEndian is
false , reverse the order of the elements of rawBytes. - If type is
Float32 , then- Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an
IEEE 754-2019 binary32 value. - If value is an
IEEE 754-2019 binary32 NaN value, return theNaN Number value . - Return the
Number value that corresponds to value.
- Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an
- If type is
Float64 , then- Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an
IEEE 754-2019 binary64 value. - If value is an
IEEE 754-2019 binary64 NaN value, return theNaN Number value . - Return the
Number value that corresponds to value.
- Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an
- If !
IsUnsignedElementType (type) istrue , then- Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
- Else,
- Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of a binary little-endian two's complement number of bit length elementSize × 8.
- If !
IsBigIntElementType (type) istrue , return the BigInt value that corresponds to intValue. - Otherwise, return the
Number value that corresponds to intValue.
25.1.2.10 GetValueFromBuffer ( arrayBuffer, byteIndex, type, isTypedArray, order [ , isLittleEndian ] )
The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative
Assert :IsDetachedBuffer (arrayBuffer) isfalse .Assert : There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.- Let block be arrayBuffer.[[ArrayBufferData]].
- Let elementSize be the Element Size value specified in
Table 60 for Element Type type. - If
IsSharedArrayBuffer (arrayBuffer) istrue , then- Let execution be the [[CandidateExecution]] field of the
surrounding agent 'sAgent Record . - Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier (). - If isTypedArray is
true andIsNoTearConfiguration (type, order) istrue , let noTear betrue ; otherwise let noTear befalse . - Let rawValue be a
List of length elementSize whose elements are nondeterministically chosen byte values. - NOTE: In implementations, rawValue is the result of a non-atomic or atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the
memory model to describe observable behaviour of hardware with weak consistency. - Let readEvent be
ReadSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize }. - Append readEvent to eventList.
- Append
Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: rawValue } to execution.[[ChosenValues]].
- Let execution be the [[CandidateExecution]] field of the
- Else, let rawValue be a
List whose elements are bytes from block at indices byteIndex (inclusive) through byteIndex + elementSize (exclusive). Assert : The number of elements in rawValue is elementSize.- If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the
surrounding agent 'sAgent Record . - Return
RawBytesToNumeric (type, rawValue, isLittleEndian).
25.1.2.11 NumericToRawBytes ( type, value, isLittleEndian )
The abstract operation NumericToRawBytes takes arguments type (a
- If type is
Float32 , then- Let rawBytes be a
List whose elements are the 4 bytes that are the result of converting value toIEEE 754-2019 binary32 format using roundTiesToEven mode. If isLittleEndian isfalse , the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value isNaN , rawBytes may be set to any implementation chosenIEEE 754-2019 binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishableNaN value.
- Let rawBytes be a
- Else if type is
Float64 , then- Let rawBytes be a
List whose elements are the 8 bytes that are theIEEE 754-2019 binary64 format encoding of value. If isLittleEndian isfalse , the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value isNaN , rawBytes may be set to any implementation chosenIEEE 754-2019 binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishableNaN value.
- Let rawBytes be a
- Else,
- Let n be the Element Size value specified in
Table 60 for Element Type type. - Let convOp be the abstract operation named in the Conversion Operation column in
Table 60 for Element Type type. - Let intValue be
ℝ (convOp(value)). - If intValue ≥ 0, then
- Let rawBytes be a
List whose elements are the n-byte binary encoding of intValue. If isLittleEndian isfalse , the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
- Let rawBytes be a
- Else,
- Let rawBytes be a
List whose elements are the n-byte binary two's complement encoding of intValue. If isLittleEndian isfalse , the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
- Let rawBytes be a
- Let n be the Element Size value specified in
- Return rawBytes.
25.1.2.12 SetValueInBuffer ( arrayBuffer, byteIndex, type, value, isTypedArray, order [ , isLittleEndian ] )
The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative
Assert :IsDetachedBuffer (arrayBuffer) isfalse .Assert : There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.Assert :Type (value) is BigInt if !IsBigIntElementType (type) istrue ; otherwise,Type (value) is Number.- Let block be arrayBuffer.[[ArrayBufferData]].
- Let elementSize be the Element Size value specified in
Table 60 for Element Type type. - If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the
surrounding agent 'sAgent Record . - Let rawBytes be
NumericToRawBytes (type, value, isLittleEndian). - If
IsSharedArrayBuffer (arrayBuffer) istrue , then- Let execution be the [[CandidateExecution]] field of the
surrounding agent 'sAgent Record . - Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier (). - If isTypedArray is
true andIsNoTearConfiguration (type, order) istrue , let noTear betrue ; otherwise let noTear befalse . - Append
WriteSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes } to eventList.
- Let execution be the [[CandidateExecution]] field of the
- Else, store the individual bytes of rawBytes into block, starting at block[byteIndex].
- Return
NormalCompletion (undefined ).
25.1.2.13 GetModifySetValueInBuffer ( arrayBuffer, byteIndex, type, value, op [ , isLittleEndian ] )
The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer object or a SharedArrayBuffer object), byteIndex (a non-negative
Assert :IsDetachedBuffer (arrayBuffer) isfalse .Assert : There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.Assert :Type (value) is BigInt if !IsBigIntElementType (type) istrue ; otherwise,Type (value) is Number.- Let block be arrayBuffer.[[ArrayBufferData]].
- Let elementSize be the Element Size value specified in
Table 60 for Element Type type. - If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the
surrounding agent 'sAgent Record . - Let rawBytes be
NumericToRawBytes (type, value, isLittleEndian). - If
IsSharedArrayBuffer (arrayBuffer) istrue , then- Let execution be the [[CandidateExecution]] field of the
surrounding agent 'sAgent Record . - Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier (). - Let rawBytesRead be a
List of length elementSize whose elements are nondeterministically chosen byte values. - NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of the
memory model to describe observable behaviour of hardware with weak consistency. - Let rmwEvent be
ReadModifyWriteSharedMemory { [[Order]]:SeqCst , [[NoTear]]:true , [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }. - Append rmwEvent to eventList.
- Append
Chosen Value Record { [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
- Let execution be the [[CandidateExecution]] field of the
- Else,
- Let rawBytesRead be a
List of length elementSize whose elements are the sequence of elementSize bytes starting with block[byteIndex]. - Let rawBytesModified be op(rawBytesRead, rawBytes).
- Store the individual bytes of rawBytesModified into block, starting at block[byteIndex].
- Let rawBytesRead be a
- Return
RawBytesToNumeric (type, rawBytesRead, isLittleEndian).
25.1.3 The ArrayBuffer Constructor
The ArrayBuffer
- is %ArrayBuffer%.
- is the initial value of the
"ArrayBuffer" property of theglobal object . - creates and initializes a new ArrayBuffer object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified ArrayBuffer behaviour must include asupercall to the ArrayBufferconstructor to create and initialize subclass instances with the internal state necessary to support theArrayBuffer.prototypebuilt-in methods.
25.1.3.1 ArrayBuffer ( length )
When the ArrayBuffer function is called with argument length, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - Let byteLength be ?
ToIndex (length). - Return ?
AllocateArrayBuffer (NewTarget, byteLength).
25.1.4 Properties of the ArrayBuffer Constructor
The ArrayBuffer
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
25.1.4.1 ArrayBuffer.isView ( arg )
The isView function takes one argument arg, and performs the following steps:
- If
Type (arg) is not Object, returnfalse . - If arg has a [[ViewedArrayBuffer]] internal slot, return
true . - Return
false .
25.1.4.2 ArrayBuffer.prototype
The initial value of ArrayBuffer.prototype is the
This property has the attributes { [[Writable]]:
25.1.4.3 get ArrayBuffer [ @@species ]
ArrayBuffer[@@species] is an
- Return the
this value.
The value of the
ArrayBuffer prototype methods normally use their
25.1.5 Properties of the ArrayBuffer Prototype Object
The ArrayBuffer prototype object:
- is %ArrayBuffer.prototype%.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.1.5.1 get ArrayBuffer.prototype.byteLength
ArrayBuffer.prototype.byteLength is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[ArrayBufferData]]). - If
IsSharedArrayBuffer (O) istrue , throw aTypeError exception. - If
IsDetachedBuffer (O) istrue , return+0 𝔽. - Let length be O.[[ArrayBufferByteLength]].
- Return
𝔽 (length).
25.1.5.2 ArrayBuffer.prototype.constructor
The initial value of ArrayBuffer.prototype.constructor is
25.1.5.3 ArrayBuffer.prototype.slice ( start, end )
The following steps are taken:
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[ArrayBufferData]]). - If
IsSharedArrayBuffer (O) istrue , throw aTypeError exception. - If
IsDetachedBuffer (O) istrue , throw aTypeError exception. - Let len be O.[[ArrayBufferByteLength]].
- Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let first be 0.
- Else if relativeStart < 0, let first be
max (len + relativeStart, 0). - Else, let first be
min (relativeStart, len). - If end is
undefined , let relativeEnd be len; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let final be 0.
- Else if relativeEnd < 0, let final be
max (len + relativeEnd, 0). - Else, let final be
min (relativeEnd, len). - Let newLen be
max (final - first, 0). - Let ctor be ?
SpeciesConstructor (O,%ArrayBuffer% ). - Let new be ?
Construct (ctor, «𝔽 (newLen) »). - Perform ?
RequireInternalSlot (new, [[ArrayBufferData]]). - If
IsSharedArrayBuffer (new) istrue , throw aTypeError exception. - If
IsDetachedBuffer (new) istrue , throw aTypeError exception. - If
SameValue (new, O) istrue , throw aTypeError exception. - If new.[[ArrayBufferByteLength]] < newLen, throw a
TypeError exception. - NOTE: Side-effects of the above steps may have detached O.
- If
IsDetachedBuffer (O) istrue , throw aTypeError exception. - Let fromBuf be O.[[ArrayBufferData]].
- Let toBuf be new.[[ArrayBufferData]].
- Perform
CopyDataBlockBytes (toBuf, 0, fromBuf, first, newLen). - Return new.
25.1.5.4 ArrayBuffer.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
25.1.6 Properties of ArrayBuffer Instances
ArrayBuffer instances inherit properties from the
ArrayBuffer instances whose [[ArrayBufferData]] is
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other than
25.2 SharedArrayBuffer Objects
25.2.1 Abstract Operations for SharedArrayBuffer Objects
25.2.1.1 AllocateSharedArrayBuffer ( constructor, byteLength )
The abstract operation AllocateSharedArrayBuffer takes arguments constructor and byteLength (a non-negative
- Let obj be ?
OrdinaryCreateFromConstructor (constructor,"%SharedArrayBuffer.prototype%" , « [[ArrayBufferData]], [[ArrayBufferByteLength]] »). - Let block be ?
CreateSharedByteDataBlock (byteLength). - Set obj.[[ArrayBufferData]] to block.
- Set obj.[[ArrayBufferByteLength]] to byteLength.
- Return obj.
25.2.1.2 IsSharedArrayBuffer ( obj )
The abstract operation IsSharedArrayBuffer takes argument obj. It tests whether an object is an ArrayBuffer, a SharedArrayBuffer, or a subtype of either. It performs the following steps when called:
Assert :Type (obj) is Object and it has an [[ArrayBufferData]] internal slot.- Let bufferData be obj.[[ArrayBufferData]].
- If bufferData is
null , returnfalse . - If bufferData is a
Data Block , returnfalse . Assert : bufferData is aShared Data Block .- Return
true .
25.2.2 The SharedArrayBuffer Constructor
The SharedArrayBuffer
- is %SharedArrayBuffer%.
- is the initial value of the
"SharedArrayBuffer" property of theglobal object , if that property is present (see below). - creates and initializes a new SharedArrayBuffer object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified SharedArrayBuffer behaviour must include asupercall to the SharedArrayBufferconstructor to create and initialize subclass instances with the internal state necessary to support theSharedArrayBuffer.prototypebuilt-in methods.
Whenever a
Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its internal [[ArrayBufferData]] slot is never
25.2.2.1 SharedArrayBuffer ( [ length ] )
When the SharedArrayBuffer function is called with optional argument length, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - Let byteLength be ?
ToIndex (length). - Return ?
AllocateSharedArrayBuffer (NewTarget, byteLength).
25.2.3 Properties of the SharedArrayBuffer Constructor
The SharedArrayBuffer
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
25.2.3.1 SharedArrayBuffer.prototype
The initial value of SharedArrayBuffer.prototype is the
This property has the attributes { [[Writable]]:
25.2.3.2 get SharedArrayBuffer [ @@species ]
SharedArrayBuffer[@@species] is an
- Return the
this value.
The value of the
25.2.4 Properties of the SharedArrayBuffer Prototype Object
The SharedArrayBuffer prototype object:
- is %SharedArrayBuffer.prototype%.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.2.4.1 get SharedArrayBuffer.prototype.byteLength
SharedArrayBuffer.prototype.byteLength is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[ArrayBufferData]]). - If
IsSharedArrayBuffer (O) isfalse , throw aTypeError exception. - Let length be O.[[ArrayBufferByteLength]].
- Return
𝔽 (length).
25.2.4.2 SharedArrayBuffer.prototype.constructor
The initial value of SharedArrayBuffer.prototype.constructor is
25.2.4.3 SharedArrayBuffer.prototype.slice ( start, end )
The following steps are taken:
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[ArrayBufferData]]). - If
IsSharedArrayBuffer (O) isfalse , throw aTypeError exception. - Let len be O.[[ArrayBufferByteLength]].
- Let relativeStart be ?
ToIntegerOrInfinity (start). - If relativeStart is -∞, let first be 0.
- Else if relativeStart < 0, let first be
max (len + relativeStart, 0). - Else, let first be
min (relativeStart, len). - If end is
undefined , let relativeEnd be len; else let relativeEnd be ?ToIntegerOrInfinity (end). - If relativeEnd is -∞, let final be 0.
- Else if relativeEnd < 0, let final be
max (len + relativeEnd, 0). - Else, let final be
min (relativeEnd, len). - Let newLen be
max (final - first, 0). - Let ctor be ?
SpeciesConstructor (O,%SharedArrayBuffer% ). - Let new be ?
Construct (ctor, «𝔽 (newLen) »). - Perform ?
RequireInternalSlot (new, [[ArrayBufferData]]). - If
IsSharedArrayBuffer (new) isfalse , throw aTypeError exception. - If new.[[ArrayBufferData]] and O.[[ArrayBufferData]] are the same
Shared Data Block values, throw aTypeError exception. - If new.[[ArrayBufferByteLength]] < newLen, throw a
TypeError exception. - Let fromBuf be O.[[ArrayBufferData]].
- Let toBuf be new.[[ArrayBufferData]].
- Perform
CopyDataBlockBytes (toBuf, 0, fromBuf, first, newLen). - Return new.
25.2.4.4 SharedArrayBuffer.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
25.2.5 Properties of SharedArrayBuffer Instances
SharedArrayBuffer instances inherit properties from the
SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.
25.3 DataView Objects
25.3.1 Abstract Operations For DataView Objects
25.3.1.1 GetViewValue ( view, requestIndex, isLittleEndian, type )
The abstract operation GetViewValue takes arguments view, requestIndex, isLittleEndian, and type. It is used by functions on DataView instances to retrieve values from the view's buffer. It performs the following steps when called:
- Perform ?
RequireInternalSlot (view, [[DataView]]). Assert : view has a [[ViewedArrayBuffer]] internal slot.- Let getIndex be ?
ToIndex (requestIndex). - Set isLittleEndian to !
ToBoolean (isLittleEndian). - Let buffer be view.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Let viewOffset be view.[[ByteOffset]].
- Let viewSize be view.[[ByteLength]].
- Let elementSize be the Element Size value specified in
Table 60 for Element Type type. - If getIndex + elementSize > viewSize, throw a
RangeError exception. - Let bufferIndex be getIndex + viewOffset.
- Return
GetValueFromBuffer (buffer, bufferIndex, type,false ,Unordered , isLittleEndian).
25.3.1.2 SetViewValue ( view, requestIndex, isLittleEndian, type, value )
The abstract operation SetViewValue takes arguments view, requestIndex, isLittleEndian, type, and value. It is used by functions on DataView instances to store values into the view's buffer. It performs the following steps when called:
- Perform ?
RequireInternalSlot (view, [[DataView]]). Assert : view has a [[ViewedArrayBuffer]] internal slot.- Let getIndex be ?
ToIndex (requestIndex). - If !
IsBigIntElementType (type) istrue , let numberValue be ?ToBigInt (value). - Otherwise, let numberValue be ?
ToNumber (value). - Set isLittleEndian to !
ToBoolean (isLittleEndian). - Let buffer be view.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Let viewOffset be view.[[ByteOffset]].
- Let viewSize be view.[[ByteLength]].
- Let elementSize be the Element Size value specified in
Table 60 for Element Type type. - If getIndex + elementSize > viewSize, throw a
RangeError exception. - Let bufferIndex be getIndex + viewOffset.
- Return
SetValueInBuffer (buffer, bufferIndex, type, numberValue,false ,Unordered , isLittleEndian).
25.3.2 The DataView Constructor
The DataView
- is %DataView%.
- is the initial value of the
"DataView" property of theglobal object . - creates and initializes a new DataView object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified DataView behaviour must include asupercall to the DataViewconstructor to create and initialize subclass instances with the internal state necessary to support theDataView.prototypebuilt-in methods.
25.3.2.1 DataView ( buffer [ , byteOffset [ , byteLength ] ] )
When the DataView function is called with at least one argument buffer, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - Perform ?
RequireInternalSlot (buffer, [[ArrayBufferData]]). - Let offset be ?
ToIndex (byteOffset). - If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
- If offset > bufferByteLength, throw a
RangeError exception. - If byteLength is
undefined , then- Let viewByteLength be bufferByteLength - offset.
- Else,
- Let viewByteLength be ?
ToIndex (byteLength). - If offset + viewByteLength > bufferByteLength, throw a
RangeError exception.
- Let viewByteLength be ?
- Let O be ?
OrdinaryCreateFromConstructor (NewTarget,"%DataView.prototype%" , « [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]] »). - If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Set O.[[ViewedArrayBuffer]] to buffer.
- Set O.[[ByteLength]] to viewByteLength.
- Set O.[[ByteOffset]] to offset.
- Return O.
25.3.3 Properties of the DataView Constructor
The DataView
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
25.3.3.1 DataView.prototype
The initial value of DataView.prototype is the
This property has the attributes { [[Writable]]:
25.3.4 Properties of the DataView Prototype Object
The DataView prototype object:
- is %DataView.prototype%.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or [[ByteOffset]] internal slot.
25.3.4.1 get DataView.prototype.buffer
DataView.prototype.buffer is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[DataView]]). Assert : O has a [[ViewedArrayBuffer]] internal slot.- Let buffer be O.[[ViewedArrayBuffer]].
- Return buffer.
25.3.4.2 get DataView.prototype.byteLength
DataView.prototype.byteLength is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[DataView]]). Assert : O has a [[ViewedArrayBuffer]] internal slot.- Let buffer be O.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Let size be O.[[ByteLength]].
- Return
𝔽 (size).
25.3.4.3 get DataView.prototype.byteOffset
DataView.prototype.byteOffset is an
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[DataView]]). Assert : O has a [[ViewedArrayBuffer]] internal slot.- Let buffer be O.[[ViewedArrayBuffer]].
- If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - Let offset be O.[[ByteOffset]].
- Return
𝔽 (offset).
25.3.4.4 DataView.prototype.constructor
The initial value of DataView.prototype.constructor is
25.3.4.5 DataView.prototype.getBigInt64 ( byteOffset [ , littleEndian ] )
When the getBigInt64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - Return ?
GetViewValue (v, byteOffset, littleEndian,BigInt64 ).
25.3.4.6 DataView.prototype.getBigUint64 ( byteOffset [ , littleEndian ] )
When the getBigUint64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - Return ?
GetViewValue (v, byteOffset, littleEndian,BigUint64 ).
25.3.4.7 DataView.prototype.getFloat32 ( byteOffset [ , littleEndian ] )
When the getFloat32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
GetViewValue (v, byteOffset, littleEndian,Float32 ).
25.3.4.8 DataView.prototype.getFloat64 ( byteOffset [ , littleEndian ] )
When the getFloat64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
GetViewValue (v, byteOffset, littleEndian,Float64 ).
25.3.4.9 DataView.prototype.getInt8 ( byteOffset )
When the getInt8 method is called with argument byteOffset, the following steps are taken:
- Let v be the
this value. - Return ?
GetViewValue (v, byteOffset,true ,Int8 ).
25.3.4.10 DataView.prototype.getInt16 ( byteOffset [ , littleEndian ] )
When the getInt16 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
GetViewValue (v, byteOffset, littleEndian,Int16 ).
25.3.4.11 DataView.prototype.getInt32 ( byteOffset [ , littleEndian ] )
When the getInt32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
GetViewValue (v, byteOffset, littleEndian,Int32 ).
25.3.4.12 DataView.prototype.getUint8 ( byteOffset )
When the getUint8 method is called with argument byteOffset, the following steps are taken:
- Let v be the
this value. - Return ?
GetViewValue (v, byteOffset,true ,Uint8 ).
25.3.4.13 DataView.prototype.getUint16 ( byteOffset [ , littleEndian ] )
When the getUint16 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
GetViewValue (v, byteOffset, littleEndian,Uint16 ).
25.3.4.14 DataView.prototype.getUint32 ( byteOffset [ , littleEndian ] )
When the getUint32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
GetViewValue (v, byteOffset, littleEndian,Uint32 ).
25.3.4.15 DataView.prototype.setBigInt64 ( byteOffset, value [ , littleEndian ] )
When the setBigInt64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - Return ?
SetViewValue (v, byteOffset, littleEndian,BigInt64 , value).
25.3.4.16 DataView.prototype.setBigUint64 ( byteOffset, value [ , littleEndian ] )
When the setBigUint64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - Return ?
SetViewValue (v, byteOffset, littleEndian,BigUint64 , value).
25.3.4.17 DataView.prototype.setFloat32 ( byteOffset, value [ , littleEndian ] )
When the setFloat32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
SetViewValue (v, byteOffset, littleEndian,Float32 , value).
25.3.4.18 DataView.prototype.setFloat64 ( byteOffset, value [ , littleEndian ] )
When the setFloat64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
SetViewValue (v, byteOffset, littleEndian,Float64 , value).
25.3.4.19 DataView.prototype.setInt8 ( byteOffset, value )
When the setInt8 method is called with arguments byteOffset and value, the following steps are taken:
- Let v be the
this value. - Return ?
SetViewValue (v, byteOffset,true ,Int8 , value).
25.3.4.20 DataView.prototype.setInt16 ( byteOffset, value [ , littleEndian ] )
When the setInt16 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
SetViewValue (v, byteOffset, littleEndian,Int16 , value).
25.3.4.21 DataView.prototype.setInt32 ( byteOffset, value [ , littleEndian ] )
When the setInt32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
SetViewValue (v, byteOffset, littleEndian,Int32 , value).
25.3.4.22 DataView.prototype.setUint8 ( byteOffset, value )
When the setUint8 method is called with arguments byteOffset and value, the following steps are taken:
- Let v be the
this value. - Return ?
SetViewValue (v, byteOffset,true ,Uint8 , value).
25.3.4.23 DataView.prototype.setUint16 ( byteOffset, value [ , littleEndian ] )
When the setUint16 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
SetViewValue (v, byteOffset, littleEndian,Uint16 , value).
25.3.4.24 DataView.prototype.setUint32 ( byteOffset, value [ , littleEndian ] )
When the setUint32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:
- Let v be the
this value. - If littleEndian is not present, set littleEndian to
false . - Return ?
SetViewValue (v, byteOffset, littleEndian,Uint32 , value).
25.3.4.25 DataView.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
25.3.5 Properties of DataView Instances
DataView instances are ordinary objects that inherit properties from the
The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal slot is used within the specification to identify objects created using the DataView
25.4 The Atomics Object
The Atomics object:
- is %Atomics%.
- is the initial value of the
"Atomics" property of theglobal object . - is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%Object.prototype% . - does not have a [[Construct]] internal method; it cannot be used as a
constructor with thenewoperator. - does not have a [[Call]] internal method; it cannot be invoked as a function.
The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as well as functions that let agents wait for and dispatch primitive events. When used with discipline, the Atomics functions allow multi-
For informative guidelines for programming and implementing shared memory in ECMAScript, please see the notes at the end of the
25.4.1 Abstract Operations for Atomics
25.4.1.1 ValidateIntegerTypedArray ( typedArray [ , waitable ] )
The abstract operation ValidateIntegerTypedArray takes argument typedArray and optional argument waitable (a Boolean). It performs the following steps when called:
- If waitable is not present, set waitable to
false . - Let buffer be ?
ValidateTypedArray (typedArray). - Let typeName be typedArray.[[TypedArrayName]].
- Let type be the Element Type value in
Table 60 for typeName. - If waitable is
true , then- If typeName is not
"Int32Array" or"BigInt64Array" , throw aTypeError exception.
- If typeName is not
- Else,
- If !
IsUnclampedIntegerElementType (type) isfalse and !IsBigIntElementType (type) isfalse , throw aTypeError exception.
- If !
- Return buffer.
25.4.1.2 ValidateAtomicAccess ( typedArray, requestIndex )
The abstract operation ValidateAtomicAccess takes arguments typedArray and requestIndex. It performs the following steps when called:
Assert : typedArray is an Object that has a [[ViewedArrayBuffer]] internal slot.- Let length be typedArray.[[ArrayLength]].
- Let accessIndex be ?
ToIndex (requestIndex). Assert : accessIndex ≥ 0.- If accessIndex ≥ length, throw a
RangeError exception. - Let arrayTypeName be typedArray.[[TypedArrayName]].
- Let elementSize be the Element Size value specified in
Table 60 for arrayTypeName. - Let offset be typedArray.[[ByteOffset]].
- Return (accessIndex × elementSize) + offset.
25.4.1.3 GetWaiterList ( block, i )
A WaiterList is a semantic object that contains an ordered list of those agents that are waiting on a location (block, i) in shared memory; block is a
Initially a WaiterList object has an empty list and no
The
Each WaiterList has a critical section that controls exclusive access to that WaiterList during evaluation. Only a single
The abstract operation GetWaiterList takes arguments block (a
Assert : block is aShared Data Block .Assert : i and i + 3 are valid byte offsets within the memory of block.Assert : i is divisible by 4.- Return the
WaiterList that is referenced by the pair (block, i).
25.4.1.4 EnterCriticalSection ( WL )
The abstract operation EnterCriticalSection takes argument WL (a
Assert : The callingagent is not in thecritical section for anyWaiterList .- Wait until no
agent is in thecritical section for WL, then enter thecritical section for WL (without allowing any otheragent to enter). - If WL has a
Synchronize event , then- NOTE: A WL whose
critical section has been entered at least once has aSynchronize event set byLeaveCriticalSection . - Let execution be the [[CandidateExecution]] field of the
surrounding agent 'sAgent Record . - Let eventsRecord be the
Agent Events Record in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier (). - Let entererEventList be eventsRecord.[[EventList]].
- Let enterEvent be a new
Synchronize event . - Append enterEvent to entererEventList.
- Let leaveEvent be the
Synchronize event in WL. - Append (leaveEvent, enterEvent) to eventsRecord.[[AgentSynchronizesWith]].
- NOTE: A WL whose
EnterCriticalSection has contention when an
25.4.1.5 LeaveCriticalSection ( WL )
The abstract operation LeaveCriticalSection takes argument WL (a
Assert : The callingagent is in thecritical section for WL.- Let execution be the [[CandidateExecution]] field of the calling surrounding's
Agent Record . - Let eventsRecord be the
Agent Events Record in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier (). - Let leaverEventList be eventsRecord.[[EventList]].
- Let leaveEvent be a new
Synchronize event . - Append leaveEvent to leaverEventList.
- Set the
Synchronize event in WL to leaveEvent. - Leave the
critical section for WL.
25.4.1.6 AddWaiter ( WL, W )
The abstract operation AddWaiter takes arguments WL (a
Assert : The callingagent is in thecritical section for WL.Assert : W is not on the list of waiters in anyWaiterList .- Add W to the end of the list of waiters in WL.
25.4.1.7 RemoveWaiter ( WL, W )
The abstract operation RemoveWaiter takes arguments WL (a
Assert : The callingagent is in thecritical section for WL.Assert : W is on the list of waiters in WL.- Remove W from the list of waiters in WL.
25.4.1.8 RemoveWaiters ( WL, c )
The abstract operation RemoveWaiters takes arguments WL (a
Assert : The callingagent is in thecritical section for WL.- Let L be a new empty
List . - Let S be a reference to the list of waiters in WL.
- Repeat, while c > 0 and S is not an empty
List ,- Let W be the first waiter in S.
- Add W to the end of L.
- Remove W from S.
- If c is finite, set c to c - 1.
- Return L.
25.4.1.9 SuspendAgent ( WL, W, timeout )
The abstract operation SuspendAgent takes arguments WL (a
Assert : The callingagent is in thecritical section for WL.Assert : W is equivalent toAgentSignifier ().Assert : W is on the list of waiters in WL.Assert :AgentCanSuspend () istrue .- Perform
LeaveCriticalSection (WL) and suspend W for up to timeout milliseconds, performing the combined operation in such a way that a notification that arrives after thecritical section is exited but before the suspension takes effect is not lost. W can notify either because the timeout expired or because it was notified explicitly by anotheragent callingNotifyWaiter (WL, W), and not for any other reasons at all. - Perform
EnterCriticalSection (WL). - If W was notified explicitly by another
agent callingNotifyWaiter (WL, W), returntrue . - Return
false .
25.4.1.10 NotifyWaiter ( WL, W )
The abstract operation NotifyWaiter takes arguments WL (a
Assert : The callingagent is in thecritical section for WL.- Notify the
agent W.
The embedding may delay notifying W, e.g. for resource management reasons, but W must eventually be notified in order to guarantee forward progress.
25.4.1.11 AtomicReadModifyWrite ( typedArray, index, value, op )
The abstract operation AtomicReadModifyWrite takes arguments typedArray, index, value, and op (a
- Let buffer be ?
ValidateIntegerTypedArray (typedArray). - Let indexedPosition be ?
ValidateAtomicAccess (typedArray, index). - Let arrayTypeName be typedArray.[[TypedArrayName]].
- If typedArray.[[ContentType]] is
BigInt , let v be ?ToBigInt (value). - Otherwise, let v be
𝔽 (?ToIntegerOrInfinity (value)). - If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - NOTE: The above check is not redundant with the check in
ValidateIntegerTypedArray because the call toToBigInt orToIntegerOrInfinity on the preceding lines can have arbitrary side effects, which could cause the buffer to become detached. - Let elementType be the Element Type value in
Table 60 for arrayTypeName. - Return
GetModifySetValueInBuffer (buffer, indexedPosition, elementType, v, op).
25.4.1.12 ByteListBitwiseOp ( op, xBytes, yBytes )
The abstract operation ByteListBitwiseOp takes arguments op (a sequence of Unicode code points), xBytes (a
Assert : op is&,^, or|.Assert : xBytes and yBytes have the same number of elements.- Let result be a new empty
List . - Let i be 0.
- For each element xByte of xBytes, do
- Let yByte be yBytes[i].
- If op is
&, let resultByte be the result of applying the bitwise AND operation to xByte and yByte. - Else if op is
^, let resultByte be the result of applying the bitwise exclusive OR (XOR) operation to xByte and yByte. - Else, op is
|. Let resultByte be the result of applying the bitwise inclusive OR operation to xByte and yByte. - Set i to i + 1.
- Append resultByte to the end of result.
- Return result.
25.4.1.13 ByteListEqual ( xBytes, yBytes )
The abstract operation ByteListEqual takes arguments xBytes (a
- If xBytes and yBytes do not have the same number of elements, return
false . - Let i be 0.
- For each element xByte of xBytes, do
- Let yByte be yBytes[i].
- If xByte ≠ yByte, return
false . - Set i to i + 1.
- Return
true .
25.4.2 Atomics.add ( typedArray, index, value )
The following steps are taken:
- Let type be the Element Type value in
Table 60 for typedArray.[[TypedArrayName]]. - Let isLittleEndian be the value of the [[LittleEndian]] field of the
surrounding agent 'sAgent Record . - Let add be a new
read-modify-write modification function with parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:- Let x be
RawBytesToNumeric (type, xBytes, isLittleEndian). - Let y be
RawBytesToNumeric (type, yBytes, isLittleEndian). - Let T be
Type (x). - Let sum be T::add(x, y).
- Let sumBytes be
NumericToRawBytes (type, sum, isLittleEndian). Assert : sumBytes, xBytes, and yBytes have the same number of elements.- Return sumBytes.
- Let x be
- Return ?
AtomicReadModifyWrite (typedArray, index, value, add).
25.4.3 Atomics.and ( typedArray, index, value )
The following steps are taken:
- Let and be a new
read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:- Return
ByteListBitwiseOp (&, xBytes, yBytes).
- Return
- Return ?
AtomicReadModifyWrite (typedArray, index, value, and).
25.4.4 Atomics.compareExchange ( typedArray, index, expectedValue, replacementValue )
The following steps are taken:
- Let buffer be ?
ValidateIntegerTypedArray (typedArray). - Let block be buffer.[[ArrayBufferData]].
- Let indexedPosition be ?
ValidateAtomicAccess (typedArray, index). - Let arrayTypeName be typedArray.[[TypedArrayName]].
- If typedArray.[[ContentType]] is
BigInt , then - Else,
- Let expected be
𝔽 (?ToIntegerOrInfinity (expectedValue)). - Let replacement be
𝔽 (?ToIntegerOrInfinity (replacementValue)).
- Let expected be
- If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - NOTE: The above check is not redundant with the check in
ValidateIntegerTypedArray because the call toToBigInt orToIntegerOrInfinity on the preceding lines can have arbitrary side effects, which could cause the buffer to become detached. - Let elementType be the Element Type value in
Table 60 for arrayTypeName. - Let elementSize be the Element Size value specified in
Table 60 for Element Type elementType. - Let isLittleEndian be the value of the [[LittleEndian]] field of the
surrounding agent 'sAgent Record . - Let expectedBytes be
NumericToRawBytes (elementType, expected, isLittleEndian). - Let replacementBytes be
NumericToRawBytes (elementType, replacement, isLittleEndian). - If
IsSharedArrayBuffer (buffer) istrue , then- Let execution be the [[CandidateExecution]] field of the
surrounding agent 'sAgent Record . - Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] is
AgentSignifier (). - Let rawBytesRead be a
List of length elementSize whose elements are nondeterministically chosen byte values. - NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of the
memory model to describe observable behaviour of hardware with weak consistency. - NOTE: The comparison of the expected value and the read value is performed outside of the
read-modify-write modification function to avoid needlessly strong synchronization when the expected value is not equal to the read value. - If
ByteListEqual (rawBytesRead, expectedBytes) istrue , then- Let second be a new
read-modify-write modification function with parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:- Return newBytes.
- Let event be
ReadModifyWriteSharedMemory { [[Order]]:SeqCst , [[NoTear]]:true , [[Block]]: block, [[ByteIndex]]: indexedPosition, [[ElementSize]]: elementSize, [[Payload]]: replacementBytes, [[ModifyOp]]: second }.
- Let second be a new
- Else,
- Let event be
ReadSharedMemory { [[Order]]:SeqCst , [[NoTear]]:true , [[Block]]: block, [[ByteIndex]]: indexedPosition, [[ElementSize]]: elementSize }.
- Let event be
- Append event to eventList.
- Append
Chosen Value Record { [[Event]]: event, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
- Let execution be the [[CandidateExecution]] field of the
- Else,
- Let rawBytesRead be a
List of length elementSize whose elements are the sequence of elementSize bytes starting with block[indexedPosition]. - If
ByteListEqual (rawBytesRead, expectedBytes) istrue , then- Store the individual bytes of replacementBytes into block, starting at block[indexedPosition].
- Let rawBytesRead be a
- Return
RawBytesToNumeric (elementType, rawBytesRead, isLittleEndian).
25.4.5 Atomics.exchange ( typedArray, index, value )
The following steps are taken:
- Let second be a new
read-modify-write modification function with parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:- Return newBytes.
- Return ?
AtomicReadModifyWrite (typedArray, index, value, second).
25.4.6 Atomics.isLockFree ( size )
The following steps are taken:
- Let n be ?
ToIntegerOrInfinity (size). - Let AR be the
Agent Record of thesurrounding agent . - If n = 1, return AR.[[IsLockFree1]].
- If n = 2, return AR.[[IsLockFree2]].
- If n = 4, return
true . - If n = 8, return AR.[[IsLockFree8]].
- Return
false .
Atomics.isLockFree() is an optimization primitive. The intuition is that if the atomic step of an atomic primitive (compareExchange, load, store, add, sub, and, or, xor, or exchange) on a datum of size n bytes will be performed without the calling Atomics.isLockFree(n) will return Atomics.isLockFree to determine whether to use locks or atomic operations in critical sections. If an atomic primitive is not lock-free then it is often more efficient for an algorithm to provide its own locking.
Atomics.isLockFree(4) always returns
Regardless of the value of Atomics.isLockFree, all atomic operations are guaranteed to be atomic. For example, they will never have a visible operation take place in the middle of the operation (e.g., "tearing").
25.4.7 Atomics.load ( typedArray, index )
The following steps are taken:
- Let buffer be ?
ValidateIntegerTypedArray (typedArray). - Let indexedPosition be ?
ValidateAtomicAccess (typedArray, index). - If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - NOTE: The above check is not redundant with the check in
ValidateIntegerTypedArray because the call toValidateAtomicAccess on the preceding line can have arbitrary side effects, which could cause the buffer to become detached. - Let arrayTypeName be typedArray.[[TypedArrayName]].
- Let elementType be the Element Type value in
Table 60 for arrayTypeName. - Return
GetValueFromBuffer (buffer, indexedPosition, elementType,true ,SeqCst ).
25.4.8 Atomics.or ( typedArray, index, value )
The following steps are taken:
- Let or be a new
read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:- Return
ByteListBitwiseOp (|, xBytes, yBytes).
- Return
- Return ?
AtomicReadModifyWrite (typedArray, index, value, or).
25.4.9 Atomics.store ( typedArray, index, value )
The following steps are taken:
- Let buffer be ?
ValidateIntegerTypedArray (typedArray). - Let indexedPosition be ?
ValidateAtomicAccess (typedArray, index). - Let arrayTypeName be typedArray.[[TypedArrayName]].
- If arrayTypeName is
"BigUint64Array" or"BigInt64Array" , let v be ?ToBigInt (value). - Otherwise, let v be
𝔽 (?ToIntegerOrInfinity (value)). - If
IsDetachedBuffer (buffer) istrue , throw aTypeError exception. - NOTE: The above check is not redundant with the check in
ValidateIntegerTypedArray because the call toToBigInt orToIntegerOrInfinity on the preceding lines can have arbitrary side effects, which could cause the buffer to become detached. - Let elementType be the Element Type value in
Table 60 for arrayTypeName. - Perform
SetValueInBuffer (buffer, indexedPosition, elementType, v,true ,SeqCst ). - Return v.
25.4.10 Atomics.sub ( typedArray, index, value )
The following steps are taken:
- Let type be the Element Type value in
Table 60 for typedArray.[[TypedArrayName]]. - Let isLittleEndian be the value of the [[LittleEndian]] field of the
surrounding agent 'sAgent Record . - Let subtract be a new
read-modify-write modification function with parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:- Let x be
RawBytesToNumeric (type, xBytes, isLittleEndian). - Let y be
RawBytesToNumeric (type, yBytes, isLittleEndian). - Let T be
Type (x). - Let difference be T::subtract(x, y).
- Let differenceBytes be
NumericToRawBytes (type, difference, isLittleEndian). Assert : differenceBytes, xBytes, and yBytes have the same number of elements.- Return differenceBytes.
- Let x be
- Return ?
AtomicReadModifyWrite (typedArray, index, value, subtract).
25.4.11 Atomics.wait ( typedArray, index, value, timeout )
Atomics.wait puts the calling
- Let buffer be ?
ValidateIntegerTypedArray (typedArray,true ). - If
IsSharedArrayBuffer (buffer) isfalse , throw aTypeError exception. - Let indexedPosition be ?
ValidateAtomicAccess (typedArray, index). - Let arrayTypeName be typedArray.[[TypedArrayName]].
- If arrayTypeName is
"BigInt64Array" , let v be ?ToBigInt64 (value). - Otherwise, let v be ?
ToInt32 (value). - Let q be ?
ToNumber (timeout). - If q is
NaN or+∞ 𝔽, let t be +∞; else if q is-∞ 𝔽, let t be 0; else let t bemax (ℝ (q), 0). - Let B be
AgentCanSuspend (). - If B is
false , throw aTypeError exception. - Let block be buffer.[[ArrayBufferData]].
- Let WL be
GetWaiterList (block, indexedPosition). - Perform
EnterCriticalSection (WL). - Let elementType be the Element Type value in
Table 60 for arrayTypeName. - Let w be !
GetValueFromBuffer (buffer, indexedPosition, elementType,true ,SeqCst ). - If v ≠ w, then
- Perform
LeaveCriticalSection (WL). - Return the String
"not-equal" .
- Perform
- Let W be
AgentSignifier (). - Perform
AddWaiter (WL, W). - Let notified be
SuspendAgent (WL, W, t). - If notified is
true , thenAssert : W is not on the list of waiters in WL.
- Else,
- Perform
RemoveWaiter (WL, W).
- Perform
- Perform
LeaveCriticalSection (WL). - If notified is
true , return the String"ok" . - Return the String
"timed-out" .
25.4.12 Atomics.notify ( typedArray, index, count )
Atomics.notify notifies some agents that are sleeping in the wait queue. The following steps are taken:
- Let buffer be ?
ValidateIntegerTypedArray (typedArray,true ). - Let indexedPosition be ?
ValidateAtomicAccess (typedArray, index). - If count is
undefined , let c be +∞. - Else,
- Let intCount be ?
ToIntegerOrInfinity (count). - Let c be
max (intCount, 0).
- Let intCount be ?
- Let block be buffer.[[ArrayBufferData]].
- Let arrayTypeName be typedArray.[[TypedArrayName]].
- If
IsSharedArrayBuffer (buffer) isfalse , return+0 𝔽. - Let WL be
GetWaiterList (block, indexedPosition). - Let n be 0.
- Perform
EnterCriticalSection (WL). - Let S be
RemoveWaiters (WL, c). - Repeat, while S is not an empty
List ,- Let W be the first
agent in S. - Remove W from the front of S.
- Perform
NotifyWaiter (WL, W). - Set n to n + 1.
- Let W be the first
- Perform
LeaveCriticalSection (WL). - Return
𝔽 (n).
25.4.13 Atomics.xor ( typedArray, index, value )
The following steps are taken:
- Let xor be a new
read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:- Return
ByteListBitwiseOp (^, xBytes, yBytes).
- Return
- Return ?
AtomicReadModifyWrite (typedArray, index, value, xor).
25.4.14 Atomics [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
25.5 The JSON Object
The JSON object:
- is %JSON%.
- is the initial value of the
"JSON" property of theglobal object . - is an
ordinary object . - contains two functions,
parseandstringify, that are used to parse and construct JSON texts. - has a [[Prototype]] internal slot whose value is
%Object.prototype% . - does not have a [[Construct]] internal method; it cannot be used as a
constructor with thenewoperator. - does not have a [[Call]] internal method; it cannot be invoked as a function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this specification is exactly that described by ECMA-404. Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions or extensions to the format.
25.5.1 JSON.parse ( text [ , reviver ] )
The parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON format represents literals, arrays, and objects with a syntax similar to the syntax for ECMAScript literals, Array Initializers, and Object Initializers. After parsing, JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and
The optional reviver parameter is a function that takes two parameters, key and value. It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returns
- Let jsonString be ?
ToString (text). - Parse !
StringToCodePoints (jsonString) as a JSON text as specified in ECMA-404. Throw aSyntaxError exception if it is not a valid JSON text as defined in that specification. - Let scriptString be the
string-concatenation of"(" , jsonString, and");" . - Let script be
ParseText (!StringToCodePoints (scriptString),Script ). Assert : script is aParse Node .- Let completion be the result of evaluating script. The extended
PropertyDefinitionEvaluation semantics defined inB.3.1 must not be used during the evaluation. - Let unfiltered be completion.[[Value]].
Assert : unfiltered is either a String, Number, Boolean, Null, or an Object that is defined by either anArrayLiteral or anObjectLiteral .- If
IsCallable (reviver) istrue , then- Let root be !
OrdinaryObjectCreate (%Object.prototype% ). - Let rootName be the empty String.
- Perform !
CreateDataPropertyOrThrow (root, rootName, unfiltered). - Return ?
InternalizeJSONProperty (root, rootName, reviver).
- Let root be !
- Else,
- Return unfiltered.
The parse function is
Valid JSON text is a subset of the ECMAScript
However, because JSON.parse, the same source text can produce different results when evaluated as a JSON.parse, means that not all texts accepted by JSON.parse are valid as a
25.5.1.1 InternalizeJSONProperty ( holder, name, reviver )
The abstract operation InternalizeJSONProperty takes arguments holder (an Object), name (a String), and reviver (a
This algorithm intentionally does not throw an exception if either [[Delete]] or
- Let val be ?
Get (holder, name). - If
Type (val) is Object, then- Let isArray be ?
IsArray (val). - If isArray is
true , then- Let I be 0.
- Let len be ?
LengthOfArrayLike (val). - Repeat, while I < len,
- Let prop be !
ToString (𝔽 (I)). - Let newElement be ?
InternalizeJSONProperty (val, prop, reviver). - If newElement is
undefined , then- Perform ? val.[[Delete]](prop).
- Else,
- Perform ?
CreateDataProperty (val, prop, newElement).
- Perform ?
- Set I to I + 1.
- Let prop be !
- Else,
- Let keys be ?
EnumerableOwnPropertyNames (val,key ). - For each String P of keys, do
- Let newElement be ?
InternalizeJSONProperty (val, P, reviver). - If newElement is
undefined , then- Perform ? val.[[Delete]](P).
- Else,
- Perform ?
CreateDataProperty (val, P, newElement).
- Perform ?
- Let newElement be ?
- Let keys be ?
- Let isArray be ?
- Return ?
Call (reviver, holder, « name, val »).
It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.
In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.
25.5.2 JSON.stringify ( value [ , replacer [ , space ] ] )
The stringify function returns a String in UTF-16 encoded JSON format representing an ECMAScript value, or
These are the steps in stringifying an object:
- Let stack be a new empty
List . - Let indent be the empty String.
- Let PropertyList and ReplacerFunction be
undefined . - If
Type (replacer) is Object, then- If
IsCallable (replacer) istrue , then- Set ReplacerFunction to replacer.
- Else,
- Let isArray be ?
IsArray (replacer). - If isArray is
true , then- Set PropertyList to a new empty
List . - Let len be ?
LengthOfArrayLike (replacer). - Let k be 0.
- Repeat, while k < len,
- Let prop be !
ToString (𝔽 (k)). - Let v be ?
Get (replacer, prop). - Let item be
undefined . - If
Type (v) is String, set item to v. - Else if
Type (v) is Number, set item to !ToString (v). - Else if
Type (v) is Object, then- If v has a [[StringData]] or [[NumberData]] internal slot, set item to ?
ToString (v).
- If v has a [[StringData]] or [[NumberData]] internal slot, set item to ?
- If item is not
undefined and item is not currently an element of PropertyList, then- Append item to the end of PropertyList.
- Set k to k + 1.
- Let prop be !
- Set PropertyList to a new empty
- Let isArray be ?
- If
- If
Type (space) is Object, then - If
Type (space) is Number, then- Let spaceMV be !
ToIntegerOrInfinity (space). - Set spaceMV to
min (10, spaceMV). - If spaceMV < 1, let gap be the empty String; otherwise let gap be the String value containing spaceMV occurrences of the code unit 0x0020 (SPACE).
- Let spaceMV be !
- Else if
Type (space) is String, then- If the length of space is 10 or less, let gap be space; otherwise let gap be the
substring of space from 0 to 10.
- If the length of space is 10 or less, let gap be space; otherwise let gap be the
- Else,
- Let gap be the empty String.
- Let wrapper be !
OrdinaryObjectCreate (%Object.prototype% ). - Perform !
CreateDataPropertyOrThrow (wrapper, the empty String, value). - Let state be the
Record { [[ReplacerFunction]]: ReplacerFunction, [[Stack]]: stack, [[Indent]]: indent, [[Gap]]: gap, [[PropertyList]]: PropertyList }. - Return ?
SerializeJSONProperty (state, the empty String, wrapper).
The stringify function is
JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then the stringify function must throw a
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.Symbolic primitive values are rendered as follows:
-
The
null value is rendered in JSON text as the String"null" . -
The
undefined value is not rendered. -
The
true value is rendered in JSON text as the String"true" . -
The
false value is rendered in JSON text as the String"false" .
String values are wrapped in QUOTATION MARK (") code units. The code units " and \ are escaped with \ prefixes. Control characters code units are replaced with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).
Finite numbers are stringified as if by calling
Values that do not have a JSON representation (such as
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String representing the key or
25.5.2.1 SerializeJSONProperty ( state, key, holder )
The abstract operation SerializeJSONProperty takes arguments state, key, and holder. It performs the following steps when called:
- Let value be ?
Get (holder, key). - If
Type (value) is Object or BigInt, then- Let toJSON be ?
GetV (value,"toJSON" ). - If
IsCallable (toJSON) istrue , then- Set value to ?
Call (toJSON, value, « key »).
- Set value to ?
- Let toJSON be ?
- If state.[[ReplacerFunction]] is not
undefined , then- Set value to ?
Call (state.[[ReplacerFunction]], holder, « key, value »).
- Set value to ?
- If
Type (value) is Object, then- If value has a [[NumberData]] internal slot, then
- Set value to ?
ToNumber (value).
- Set value to ?
- Else if value has a [[StringData]] internal slot, then
- Set value to ?
ToString (value).
- Set value to ?
- Else if value has a [[BooleanData]] internal slot, then
- Set value to value.[[BooleanData]].
- Else if value has a [[BigIntData]] internal slot, then
- Set value to value.[[BigIntData]].
- If value has a [[NumberData]] internal slot, then
- If value is
null , return"null" . - If value is
true , return"true" . - If value is
false , return"false" . - If
Type (value) is String, returnQuoteJSONString (value). - If
Type (value) is Number, then- If value is finite, return !
ToString (value). - Return
"null" .
- If value is finite, return !
- If
Type (value) is BigInt, throw aTypeError exception. - If
Type (value) is Object andIsCallable (value) isfalse , then- Let isArray be ?
IsArray (value). - If isArray is
true , return ?SerializeJSONArray (state, value). - Return ?
SerializeJSONObject (state, value).
- Let isArray be ?
- Return
undefined .
25.5.2.2 QuoteJSONString ( value )
The abstract operation QuoteJSONString takes argument value. It wraps value in 0x0022 (QUOTATION MARK) code units and escapes certain other code units within it. This operation interprets value as a sequence of UTF-16 encoded code points, as described in
- Let product be the String value consisting solely of the code unit 0x0022 (QUOTATION MARK).
- For each code point C of !
StringToCodePoints (value), do- If C is listed in the “Code Point” column of
Table 61 , then- Set product to the
string-concatenation of product and the escape sequence for C as specified in the “Escape Sequence” column of the corresponding row.
- Set product to the
- Else if C has a numeric value less than 0x0020 (SPACE), or if C has the same numeric value as a
leading surrogate ortrailing surrogate , then- Let unit be the code unit whose numeric value is that of C.
- Set product to the
string-concatenation of product andUnicodeEscape (unit).
- Else,
- Set product to the
string-concatenation of product and !UTF16EncodeCodePoint (C).
- Set product to the
- If C is listed in the “Code Point” column of
- Set product to the
string-concatenation of product and the code unit 0x0022 (QUOTATION MARK). - Return product.
| Code Point | Unicode Character Name | Escape Sequence |
|---|---|---|
| U+0008 | BACKSPACE |
\b
|
| U+0009 | CHARACTER TABULATION |
\t
|
| U+000A | LINE FEED (LF) |
\n
|
| U+000C | FORM FEED (FF) |
\f
|
| U+000D | CARRIAGE RETURN (CR) |
\r
|
| U+0022 | QUOTATION MARK |
\"
|
| U+005C | REVERSE SOLIDUS |
\\
|
25.5.2.3 UnicodeEscape ( C )
The abstract operation UnicodeEscape takes argument C (a code unit). It represents C as a Unicode escape sequence. It performs the following steps when called:
- Let n be the numeric value of C.
Assert : n ≤ 0xFFFF.- Return the
string-concatenation of:- the code unit 0x005C (REVERSE SOLIDUS)
"u" - the String representation of n, formatted as a four-digit lowercase hexadecimal number, padded to the left with zeroes if necessary
25.5.2.4 SerializeJSONObject ( state, value )
The abstract operation SerializeJSONObject takes arguments state and value. It serializes an object. It performs the following steps when called:
- If state.[[Stack]] contains value, throw a
TypeError exception because the structure is cyclical. - Append value to state.[[Stack]].
- Let stepback be state.[[Indent]].
- Set state.[[Indent]] to the
string-concatenation of state.[[Indent]] and state.[[Gap]]. - If state.[[PropertyList]] is not
undefined , then- Let K be state.[[PropertyList]].
- Else,
- Let K be ?
EnumerableOwnPropertyNames (value,key ).
- Let K be ?
- Let partial be a new empty
List . - For each element P of K, do
- Let strP be ?
SerializeJSONProperty (state, P, value). - If strP is not
undefined , then- Let member be
QuoteJSONString (P). - Set member to the
string-concatenation of member and":" . - If state.[[Gap]] is not the empty String, then
- Set member to the
string-concatenation of member and the code unit 0x0020 (SPACE).
- Set member to the
- Set member to the
string-concatenation of member and strP. - Append member to partial.
- Let member be
- Let strP be ?
- If partial is empty, then
- Let final be
"{}" .
- Let final be
- Else,
- If state.[[Gap]] is the empty String, then
- Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
- Let final be the
string-concatenation of"{" , properties, and"}" .
- Else,
- Let separator be the
string-concatenation of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]]. - Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
- Let final be the
string-concatenation of"{" , the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and"}" .
- Let separator be the
- If state.[[Gap]] is the empty String, then
- Remove the last element of state.[[Stack]].
- Set state.[[Indent]] to stepback.
- Return final.
25.5.2.5 SerializeJSONArray ( state, value )
The abstract operation SerializeJSONArray takes arguments state and value. It serializes an array. It performs the following steps when called:
- If state.[[Stack]] contains value, throw a
TypeError exception because the structure is cyclical. - Append value to state.[[Stack]].
- Let stepback be state.[[Indent]].
- Set state.[[Indent]] to the
string-concatenation of state.[[Indent]] and state.[[Gap]]. - Let partial be a new empty
List . - Let len be ?
LengthOfArrayLike (value). - Let index be 0.
- Repeat, while index < len,
- Let strP be ?
SerializeJSONProperty (state, !ToString (𝔽 (index)), value). - If strP is
undefined , then- Append
"null" to partial.
- Append
- Else,
- Append strP to partial.
- Set index to index + 1.
- Let strP be ?
- If partial is empty, then
- Let final be
"[]" .
- Let final be
- Else,
- If state.[[Gap]] is the empty String, then
- Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
- Let final be the
string-concatenation of"[" , properties, and"]" .
- Else,
- Let separator be the
string-concatenation of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]]. - Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
- Let final be the
string-concatenation of"[" , the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and"]" .
- Let separator be the
- If state.[[Gap]] is the empty String, then
- Remove the last element of state.[[Stack]].
- Set state.[[Indent]] to stepback.
- Return final.
The representation of arrays includes only the elements between zero and array.length - 1
25.5.3 JSON [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
26 Managing Memory
26.1 WeakRef Objects
A
26.1.1 The WeakRef Constructor
The WeakRef
- is %WeakRef%.
-
is the initial value of the
"WeakRef" property of theglobal object . -
creates and initializes a new WeakRef object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
-
is designed to be subclassable. It may be used as the value in an
extendsclause of a class definition. Subclass constructors that intend to inherit the specifiedWeakRefbehaviour must include asupercall to theWeakRefconstructor to create and initialize the subclass instance with the internal state necessary to support theWeakRef.prototypebuilt-in methods.
26.1.1.1 WeakRef ( target )
When the WeakRef function is called with argument target, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - If
Type (target) is not Object, throw aTypeError exception. - Let weakRef be ?
OrdinaryCreateFromConstructor (NewTarget,"%WeakRef.prototype%" , « [[WeakRefTarget]] »). - Perform !
AddToKeptObjects (target). - Set weakRef.[[WeakRefTarget]] to target.
- Return weakRef.
26.1.2 Properties of the WeakRef Constructor
The
-
has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
26.1.2.1 WeakRef.prototype
The initial value of WeakRef.prototype is the
This property has the attributes { [[Writable]]:
26.1.3 Properties of the WeakRef Prototype Object
The WeakRef prototype object:
- is %WeakRef.prototype%.
-
has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have a [[WeakRefTarget]] internal slot.
26.1.3.1 WeakRef.prototype.constructor
The initial value of WeakRef.prototype.constructor is
This property has the attributes { [[Writable]]:
26.1.3.2 WeakRef.prototype.deref ( )
The following steps are taken:
- Let weakRef be the
this value. - Perform ?
RequireInternalSlot (weakRef, [[WeakRefTarget]]). - Return !
WeakRefDeref (weakRef).
If the
target = { foo: function() {} };
let weakRef = new WeakRef(target);
... later ...
if (weakRef.deref()) {
weakRef.deref().foo();
}In the above example, if the first deref does not evaluate to
26.1.3.3 WeakRef.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
26.1.4 WeakRef Abstract Operations
26.1.4.1 WeakRefDeref ( weakRef )
The abstract operation WeakRefDeref takes argument weakRef (a
- Let target be weakRef.[[WeakRefTarget]].
- If target is not
empty , then- Perform !
AddToKeptObjects (target). - Return target.
- Perform !
- Return
undefined .
This abstract operation is defined separately from WeakRef.prototype.deref strictly to make it possible to succinctly define liveness.
26.1.5 Properties of WeakRef Instances
26.2 FinalizationRegistry Objects
A
26.2.1 The FinalizationRegistry Constructor
The FinalizationRegistry
- is %FinalizationRegistry%.
-
is the initial value of the
"FinalizationRegistry" property of theglobal object . -
creates and initializes a new FinalizationRegistry object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
-
is designed to be subclassable. It may be used as the value in an
extendsclause of a class definition. Subclass constructors that intend to inherit the specifiedFinalizationRegistrybehaviour must include asupercall to theFinalizationRegistryconstructor to create and initialize the subclass instance with the internal state necessary to support theFinalizationRegistry.prototypebuilt-in methods.
26.2.1.1 FinalizationRegistry ( cleanupCallback )
When the FinalizationRegistry function is called with argument cleanupCallback, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - If
IsCallable (cleanupCallback) isfalse , throw aTypeError exception. - Let finalizationRegistry be ?
OrdinaryCreateFromConstructor (NewTarget,"%FinalizationRegistry.prototype%" , « [[Realm]], [[CleanupCallback]], [[Cells]] »). - Let fn be the
active function object . - Set finalizationRegistry.[[Realm]] to fn.[[Realm]].
- Set finalizationRegistry.[[CleanupCallback]] to cleanupCallback.
- Set finalizationRegistry.[[Cells]] to a new empty
List . - Return finalizationRegistry.
26.2.2 Properties of the FinalizationRegistry Constructor
The
-
has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
26.2.2.1 FinalizationRegistry.prototype
The initial value of FinalizationRegistry.prototype is the
This property has the attributes { [[Writable]]:
26.2.3 Properties of the FinalizationRegistry Prototype Object
The FinalizationRegistry prototype object:
- is %FinalizationRegistry.prototype%.
-
has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have [[Cells]] and [[CleanupCallback]] internal slots.
26.2.3.1 FinalizationRegistry.prototype.constructor
The initial value of FinalizationRegistry.prototype.constructor is
26.2.3.2 FinalizationRegistry.prototype.register ( target, heldValue [ , unregisterToken ] )
The following steps are taken:
- Let finalizationRegistry be the
this value. - Perform ?
RequireInternalSlot (finalizationRegistry, [[Cells]]). - If
Type (target) is not Object, throw aTypeError exception. - If
SameValue (target, heldValue) istrue , throw aTypeError exception. - If
Type (unregisterToken) is not Object, then- If unregisterToken is not
undefined , throw aTypeError exception. - Set unregisterToken to
empty .
- If unregisterToken is not
- Let cell be the
Record { [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }. - Append cell to finalizationRegistry.[[Cells]].
- Return
undefined .
Based on the algorithms and definitions in this specification, cell.[[HeldValue]] is
26.2.3.3 FinalizationRegistry.prototype.unregister ( unregisterToken )
The following steps are taken:
- Let finalizationRegistry be the
this value. - Perform ?
RequireInternalSlot (finalizationRegistry, [[Cells]]). - If
Type (unregisterToken) is not Object, throw aTypeError exception. - Let removed be
false . - For each
Record { [[WeakRefTarget]], [[HeldValue]], [[UnregisterToken]] } cell of finalizationRegistry.[[Cells]], do- If cell.[[UnregisterToken]] is not
empty andSameValue (cell.[[UnregisterToken]], unregisterToken) istrue , then- Remove cell from finalizationRegistry.[[Cells]].
- Set removed to
true .
- If cell.[[UnregisterToken]] is not
- Return removed.
26.2.3.4 FinalizationRegistry.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
26.2.4 Properties of FinalizationRegistry Instances
27 Control Abstraction Objects
27.1 Iteration
27.1.1 Common Iteration Interfaces
An interface is a set of property keys whose associated values match a specific specification. Any object that provides all the properties as described by an interface's specification conforms to that interface. An interface is not represented by a distinct object. There may be many separately implemented objects that conform to any interface. An individual object may conform to multiple interfaces.
27.1.1.1 The Iterable Interface
The Iterable interface includes the property described in
| Property | Value | Requirements |
|---|---|---|
@@iterator
|
A function that returns an Iterator object. | The returned object must conform to the Iterator interface. |
27.1.1.2 The Iterator Interface
An object that implements the Iterator interface must include the property in
| Property | Value | Requirements |
|---|---|---|
|
|
A function that returns an IteratorResult object. |
The returned object must conform to the IteratorResult interface. If a previous call to the next method of an Iterator has returned an IteratorResult object whose next method of that object should also return an IteratorResult object whose |
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
| Property | Value | Requirements |
|---|---|---|
|
|
A function that returns an IteratorResult object. |
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more next method calls to the Iterator. The returned IteratorResult object will typically have a return method. However, this requirement is not enforced.
|
|
|
A function that returns an IteratorResult object. |
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to throw the value passed as the argument. If the method does not throw, the returned IteratorResult object will typically have a |
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-of, yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library functions that accept Iterable objects as arguments also conditionally call them.
27.1.1.3 The AsyncIterable Interface
The AsyncIterable interface includes the properties described in
| Property | Value | Requirements |
|---|---|---|
@@asyncIterator |
A function that returns an AsyncIterator object. | The returned object must conform to the AsyncIterator interface. |
27.1.1.4 The AsyncIterator Interface
An object that implements the AsyncIterator interface must include the properties in
| Property | Value | Requirements |
|---|---|---|
| A function that returns a promise for an IteratorResult object. |
The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. If a previous call to the Additionally, the IteratorResult object that serves as a fulfillment value should have a |
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target AsyncIterator. The for-await-of statement and other common users of AsyncIterators do not pass any arguments, so AsyncIterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
| Property | Value | Requirements |
|---|---|---|
| A function that returns a promise for an IteratorResult object. |
The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller does not intend to make any more Additionally, the IteratorResult object that serves as a fulfillment value should have a |
|
| A function that returns a promise for an IteratorResult object. |
The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to return a rejected promise which rejects with the value passed as the argument. If the returned promise is fulfilled, the IteratorResult fulfillment value will typically have a |
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-await-of and yield* call these methods after performing an existence check.
27.1.1.5 The IteratorResult Interface
The IteratorResult interface includes the properties listed in
| Property | Value | Requirements |
|---|---|---|
|
|
Either |
This is the result status of an iterator next method call. If the end of the iterator was reached |
|
|
Any |
If done is |
27.1.2 The %IteratorPrototype% Object
The %IteratorPrototype% object:
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object .
All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%. The %IteratorPrototype% object provides a place where additional methods that are applicable to all iterator objects may be added.
The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:
Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()))27.1.2.1 %IteratorPrototype% [ @@iterator ] ( )
The following steps are taken:
- Return the
this value.
The value of the
27.1.3 The %AsyncIteratorPrototype% Object
The %AsyncIteratorPrototype% object:
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object .
All objects defined in this specification that implement the AsyncIterator interface also inherit from %AsyncIteratorPrototype%. ECMAScript code may also define objects that inherit from %AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object provides a place where additional methods that are applicable to all async iterator objects may be added.
27.1.3.1 %AsyncIteratorPrototype% [ @@asyncIterator ] ( )
The following steps are taken:
- Return the
this value.
The value of the
27.1.4 Async-from-Sync Iterator Objects
An Async-from-Sync Iterator object is an async iterator that adapts a specific synchronous iterator. There is not a named
27.1.4.1 CreateAsyncFromSyncIterator ( syncIteratorRecord )
The abstract operation CreateAsyncFromSyncIterator takes argument syncIteratorRecord. It is used to create an async iterator
- Let asyncIterator be !
OrdinaryObjectCreate (%AsyncFromSyncIteratorPrototype% , « [[SyncIteratorRecord]] »). - Set asyncIterator.[[SyncIteratorRecord]] to syncIteratorRecord.
- Let nextMethod be !
Get (asyncIterator,"next" ). - Let iteratorRecord be the
Record { [[Iterator]]: asyncIterator, [[NextMethod]]: nextMethod, [[Done]]:false }. - Return iteratorRecord.
27.1.4.2 The %AsyncFromSyncIteratorPrototype% Object
The %AsyncFromSyncIteratorPrototype% object:
- has properties that are inherited by all Async-from-Sync Iterator Objects.
- is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%AsyncIteratorPrototype% . - has the following properties:
27.1.4.2.1 %AsyncFromSyncIteratorPrototype%.next ( [ value ] )
- Let O be the
this value. Assert :Type (O) is Object and O has a [[SyncIteratorRecord]] internal slot.- Let promiseCapability be !
NewPromiseCapability (%Promise% ). - Let syncIteratorRecord be O.[[SyncIteratorRecord]].
- If value is present, then
- Let result be
IteratorNext (syncIteratorRecord, value).
- Let result be
- Else,
- Let result be
IteratorNext (syncIteratorRecord).
- Let result be
IfAbruptRejectPromise (result, promiseCapability).- Return !
AsyncFromSyncIteratorContinuation (result, promiseCapability).
27.1.4.2.2 %AsyncFromSyncIteratorPrototype%.return ( [ value ] )
- Let O be the
this value. Assert :Type (O) is Object and O has a [[SyncIteratorRecord]] internal slot.- Let promiseCapability be !
NewPromiseCapability (%Promise% ). - Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
- Let return be
GetMethod (syncIterator,"return" ). IfAbruptRejectPromise (return, promiseCapability).- If return is
undefined , then- Let iterResult be !
CreateIterResultObject (value,true ). - Perform !
Call (promiseCapability.[[Resolve]],undefined , « iterResult »). - Return promiseCapability.[[Promise]].
- Let iterResult be !
- If value is present, then
- Let result be
Call (return, syncIterator, « value »).
- Let result be
- Else,
- Let result be
Call (return, syncIterator).
- Let result be
IfAbruptRejectPromise (result, promiseCapability).- If
Type (result) is not Object, then- Perform !
Call (promiseCapability.[[Reject]],undefined , « a newly createdTypeError object »). - Return promiseCapability.[[Promise]].
- Perform !
- Return !
AsyncFromSyncIteratorContinuation (result, promiseCapability).
27.1.4.2.3 %AsyncFromSyncIteratorPrototype%.throw ( [ value ] )
- Let O be the
this value. Assert :Type (O) is Object and O has a [[SyncIteratorRecord]] internal slot.- Let promiseCapability be !
NewPromiseCapability (%Promise% ). - Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
- Let throw be
GetMethod (syncIterator,"throw" ). IfAbruptRejectPromise (throw, promiseCapability).- If throw is
undefined , then- Perform !
Call (promiseCapability.[[Reject]],undefined , « value »). - Return promiseCapability.[[Promise]].
- Perform !
- If value is present, then
- Let result be
Call (throw, syncIterator, « value »).
- Let result be
- Else,
- Let result be
Call (throw, syncIterator).
- Let result be
IfAbruptRejectPromise (result, promiseCapability).- If
Type (result) is not Object, then- Perform !
Call (promiseCapability.[[Reject]],undefined , « a newly createdTypeError object »). - Return promiseCapability.[[Promise]].
- Perform !
- Return !
AsyncFromSyncIteratorContinuation (result, promiseCapability).
27.1.4.2.4 Async-from-Sync Iterator Value Unwrap Functions
An async-from-sync iterator value unwrap function is an anonymous built-in function that is used by
When an async-from-sync iterator value unwrap function is called with argument value, the following steps are taken:
- Let F be the
active function object . - Return !
CreateIterResultObject (value, F.[[Done]]).
27.1.4.3 Properties of Async-from-Sync Iterator Instances
Async-from-Sync Iterator instances are ordinary objects that inherit properties from the
| Internal Slot | Description |
|---|---|
| [[SyncIteratorRecord]] |
A |
27.1.4.4 AsyncFromSyncIteratorContinuation ( result, promiseCapability )
The abstract operation AsyncFromSyncIteratorContinuation takes arguments result and promiseCapability (a
- Let done be
IteratorComplete (result). IfAbruptRejectPromise (done, promiseCapability).- Let value be
IteratorValue (result). IfAbruptRejectPromise (value, promiseCapability).- Let valueWrapper be
PromiseResolve (%Promise% , value). IfAbruptRejectPromise (valueWrapper, promiseCapability).- Let steps be the algorithm steps defined in
Async-from-Sync Iterator Value Unwrap Functions . - Let length be the number of non-optional parameters of the function definition in
Async-from-Sync Iterator Value Unwrap Functions . - Let onFulfilled be !
CreateBuiltinFunction (steps, length,"" , « [[Done]] »). - Set onFulfilled.[[Done]] to done.
- Perform !
PerformPromiseThen (valueWrapper, onFulfilled,undefined , promiseCapability). - Return promiseCapability.[[Promise]].
27.2 Promise Objects
A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.
Any Promise object is in one of three mutually exclusive states: fulfilled, rejected, and pending:
-
A promise
pis fulfilled ifp.then(f, r)will immediately enqueue aJob to call the functionf. -
A promise
pis rejected ifp.then(f, r)will immediately enqueue aJob to call the functionr. - A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.
27.2.1 Promise Abstract Operations
27.2.1.1 PromiseCapability Records
A PromiseCapability Record is a
PromiseCapability Records have the fields listed in
| Field Name | Value | Meaning |
|---|---|---|
| [[Promise]] | An object | An object that is usable as a promise. |
| [[Resolve]] |
A |
The function that is used to resolve the given promise object. |
| [[Reject]] |
A |
The function that is used to reject the given promise object. |
27.2.1.1.1 IfAbruptRejectPromise ( value, capability )
IfAbruptRejectPromise is a shorthand for a sequence of algorithm steps that use a
IfAbruptRejectPromise (value, capability).
means the same thing as:
- If value is an
abrupt completion , then- Perform ?
Call (capability.[[Reject]],undefined , « value.[[Value]] »). - Return capability.[[Promise]].
- Perform ?
- Else if value is a
Completion Record , set value to value.[[Value]].
27.2.1.2 PromiseReaction Records
The PromiseReaction is a
PromiseReaction records have the fields listed in
| Field Name | Value | Meaning |
|---|---|---|
| [[Capability]] |
A |
The capabilities of the promise for which this record provides a reaction handler. |
| [[Type]] |
|
The [[Type]] is used when [[Handler]] is |
| [[Handler]] |
A |
The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] is |
27.2.1.3 CreateResolvingFunctions ( promise )
The abstract operation CreateResolvingFunctions takes argument promise. It performs the following steps when called:
- Let alreadyResolved be the
Record { [[Value]]:false }. - Let stepsResolve be the algorithm steps defined in
Promise Resolve Functions . - Let lengthResolve be the number of non-optional parameters of the function definition in
Promise Resolve Functions . - Let resolve be !
CreateBuiltinFunction (stepsResolve, lengthResolve,"" , « [[Promise]], [[AlreadyResolved]] »). - Set resolve.[[Promise]] to promise.
- Set resolve.[[AlreadyResolved]] to alreadyResolved.
- Let stepsReject be the algorithm steps defined in
Promise Reject Functions . - Let lengthReject be the number of non-optional parameters of the function definition in
Promise Reject Functions . - Let reject be !
CreateBuiltinFunction (stepsReject, lengthReject,"" , « [[Promise]], [[AlreadyResolved]] »). - Set reject.[[Promise]] to promise.
- Set reject.[[AlreadyResolved]] to alreadyResolved.
- Return the
Record { [[Resolve]]: resolve, [[Reject]]: reject }.
27.2.1.3.1 Promise Reject Functions
A promise reject function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.
When a promise reject function is called with argument reason, the following steps are taken:
- Let F be the
active function object . Assert : F has a [[Promise]] internal slot whose value is an Object.- Let promise be F.[[Promise]].
- Let alreadyResolved be F.[[AlreadyResolved]].
- If alreadyResolved.[[Value]] is
true , returnundefined . - Set alreadyResolved.[[Value]] to
true . - Return
RejectPromise (promise, reason).
The
27.2.1.3.2 Promise Resolve Functions
A promise resolve function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.
When a promise resolve function is called with argument resolution, the following steps are taken:
- Let F be the
active function object . Assert : F has a [[Promise]] internal slot whose value is an Object.- Let promise be F.[[Promise]].
- Let alreadyResolved be F.[[AlreadyResolved]].
- If alreadyResolved.[[Value]] is
true , returnundefined . - Set alreadyResolved.[[Value]] to
true . - If
SameValue (resolution, promise) istrue , then- Let selfResolutionError be a newly created
TypeError object. - Return
RejectPromise (promise, selfResolutionError).
- Let selfResolutionError be a newly created
- If
Type (resolution) is not Object, then- Return
FulfillPromise (promise, resolution).
- Return
- Let then be
Get (resolution,"then" ). - If then is an
abrupt completion , then- Return
RejectPromise (promise, then.[[Value]]).
- Return
- Let thenAction be then.[[Value]].
- If
IsCallable (thenAction) isfalse , then- Return
FulfillPromise (promise, resolution).
- Return
- Let thenJobCallback be
HostMakeJobCallback (thenAction). - Let job be
NewPromiseResolveThenableJob (promise, resolution, thenJobCallback). - Perform
HostEnqueuePromiseJob (job.[[Job]], job.[[Realm]]). - Return
undefined .
The
27.2.1.4 FulfillPromise ( promise, value )
The abstract operation FulfillPromise takes arguments promise and value. It performs the following steps when called:
Assert : The value of promise.[[PromiseState]] ispending .- Let reactions be promise.[[PromiseFulfillReactions]].
- Set promise.[[PromiseResult]] to value.
- Set promise.[[PromiseFulfillReactions]] to
undefined . - Set promise.[[PromiseRejectReactions]] to
undefined . - Set promise.[[PromiseState]] to
fulfilled . - Return
TriggerPromiseReactions (reactions, value).
27.2.1.5 NewPromiseCapability ( C )
The abstract operation NewPromiseCapability takes argument C. It attempts to use C as a resolve and reject functions. The Promise object plus the resolve and reject functions are used to initialize a new
- If
IsConstructor (C) isfalse , throw aTypeError exception. - NOTE: C is assumed to be a
constructor function that supports the parameter conventions of the Promiseconstructor (see27.2.3.1 ). - Let promiseCapability be the
PromiseCapability Record { [[Promise]]:undefined , [[Resolve]]:undefined , [[Reject]]:undefined }. - Let steps be the algorithm steps defined in
GetCapabilitiesExecutor Functions . - Let length be the number of non-optional parameters of the function definition in
GetCapabilitiesExecutor Functions . - Let executor be !
CreateBuiltinFunction (steps, length,"" , « [[Capability]] »). - Set executor.[[Capability]] to promiseCapability.
- Let promise be ?
Construct (C, « executor »). - If
IsCallable (promiseCapability.[[Resolve]]) isfalse , throw aTypeError exception. - If
IsCallable (promiseCapability.[[Reject]]) isfalse , throw aTypeError exception. - Set promiseCapability.[[Promise]] to promise.
- Return promiseCapability.
This abstract operation supports Promise subclassing, as it is generic on any
27.2.1.5.1 GetCapabilitiesExecutor Functions
A GetCapabilitiesExecutor function is an anonymous built-in function that has a [[Capability]] internal slot.
When a GetCapabilitiesExecutor function is called with arguments resolve and reject, the following steps are taken:
- Let F be the
active function object . Assert : F has a [[Capability]] internal slot whose value is aPromiseCapability Record .- Let promiseCapability be F.[[Capability]].
- If promiseCapability.[[Resolve]] is not
undefined , throw aTypeError exception. - If promiseCapability.[[Reject]] is not
undefined , throw aTypeError exception. - Set promiseCapability.[[Resolve]] to resolve.
- Set promiseCapability.[[Reject]] to reject.
- Return
undefined .
The
27.2.1.6 IsPromise ( x )
The abstract operation IsPromise takes argument x. It checks for the promise brand on an object. It performs the following steps when called:
- If
Type (x) is not Object, returnfalse . - If x does not have a [[PromiseState]] internal slot, return
false . - Return
true .
27.2.1.7 RejectPromise ( promise, reason )
The abstract operation RejectPromise takes arguments promise and reason. It performs the following steps when called:
Assert : The value of promise.[[PromiseState]] ispending .- Let reactions be promise.[[PromiseRejectReactions]].
- Set promise.[[PromiseResult]] to reason.
- Set promise.[[PromiseFulfillReactions]] to
undefined . - Set promise.[[PromiseRejectReactions]] to
undefined . - Set promise.[[PromiseState]] to
rejected . - If promise.[[PromiseIsHandled]] is
false , performHostPromiseRejectionTracker (promise,"reject" ). - Return
TriggerPromiseReactions (reactions, reason).
27.2.1.8 TriggerPromiseReactions ( reactions, argument )
The abstract operation TriggerPromiseReactions takes arguments reactions (a
- For each element reaction of reactions, do
- Let job be
NewPromiseReactionJob (reaction, argument). - Perform
HostEnqueuePromiseJob (job.[[Job]], job.[[Realm]]).
- Let job be
- Return
undefined .
27.2.1.9 HostPromiseRejectionTracker ( promise, operation )
The
An implementation of HostPromiseRejectionTracker must complete normally in all cases. The default implementation of HostPromiseRejectionTracker is to unconditionally return an empty normal completion.
HostPromiseRejectionTracker is called in two scenarios:
- When a promise is rejected without any handlers, it is called with its operation argument set to
"reject" . - When a handler is added to a rejected promise for the first time, it is called with its operation argument set to
"handle" .
A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.
If operation is
27.2.2 Promise Jobs
27.2.2.1 NewPromiseReactionJob ( reaction, argument )
The abstract operation NewPromiseReactionJob takes arguments reaction and argument. It returns a new
- Let job be a new
Job Abstract Closure with no parameters that captures reaction and argument and performs the following steps when called:Assert : reaction is a PromiseReactionRecord .- Let promiseCapability be reaction.[[Capability]].
- Let type be reaction.[[Type]].
- Let handler be reaction.[[Handler]].
- If handler is
empty , then- If type is
Fulfill , let handlerResult beNormalCompletion (argument). - Else,
Assert : type isReject .- Let handlerResult be
ThrowCompletion (argument).
- If type is
- Else, let handlerResult be
HostCallJobCallback (handler,undefined , « argument »). - If promiseCapability is
undefined , thenAssert : handlerResult is not anabrupt completion .- Return
NormalCompletion (empty ).
Assert : promiseCapability is aPromiseCapability Record .- If handlerResult is an
abrupt completion , then- Let status be
Call (promiseCapability.[[Reject]],undefined , « handlerResult.[[Value]] »).
- Let status be
- Else,
- Let status be
Call (promiseCapability.[[Resolve]],undefined , « handlerResult.[[Value]] »).
- Let status be
- Return
Completion (status).
- Let handlerRealm be
null . - If reaction.[[Handler]] is not
empty , then- Let getHandlerRealmResult be
GetFunctionRealm (reaction.[[Handler]].[[Callback]]). - If getHandlerRealmResult is a normal completion, set handlerRealm to getHandlerRealmResult.[[Value]].
- Else, set handlerRealm to
the current Realm Record . - NOTE: handlerRealm is never
null unless the handler isundefined . When the handler is a revoked Proxy and no ECMAScript code runs, handlerRealm is used to create error objects.
- Let getHandlerRealmResult be
- Return the
Record { [[Job]]: job, [[Realm]]: handlerRealm }.
27.2.2.2 NewPromiseResolveThenableJob ( promiseToResolve, thenable, then )
The abstract operation NewPromiseResolveThenableJob takes arguments promiseToResolve, thenable, and then. It performs the following steps when called:
- Let job be a new
Job Abstract Closure with no parameters that captures promiseToResolve, thenable, and then and performs the following steps when called:- Let resolvingFunctions be
CreateResolvingFunctions (promiseToResolve). - Let thenCallResult be
HostCallJobCallback (then, thenable, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »). - If thenCallResult is an
abrupt completion , then- Let status be
Call (resolvingFunctions.[[Reject]],undefined , « thenCallResult.[[Value]] »). - Return
Completion (status).
- Let status be
- Return
Completion (thenCallResult).
- Let resolvingFunctions be
- Let getThenRealmResult be
GetFunctionRealm (then.[[Callback]]). - If getThenRealmResult is a normal completion, let thenRealm be getThenRealmResult.[[Value]].
- Else, let thenRealm be
the current Realm Record . - NOTE: thenRealm is never
null . When then.[[Callback]] is a revoked Proxy and no code runs, thenRealm is used to create error objects. - Return the
Record { [[Job]]: job, [[Realm]]: thenRealm }.
27.2.3 The Promise Constructor
The Promise
- is %Promise%.
- is the initial value of the
"Promise" property of theglobal object . - creates and initializes a new Promise object when called as a
constructor . - is not intended to be called as a function and will throw an exception when called in that manner.
- is designed to be subclassable. It may be used as the value in an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include asupercall to the Promiseconstructor to create and initialize the subclass instance with the internal state necessary to support thePromiseandPromise.prototypebuilt-in methods.
27.2.3.1 Promise ( executor )
When the Promise function is called with argument executor, the following steps are taken:
- If NewTarget is
undefined , throw aTypeError exception. - If
IsCallable (executor) isfalse , throw aTypeError exception. - Let promise be ?
OrdinaryCreateFromConstructor (NewTarget,"%Promise.prototype%" , « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »). - Set promise.[[PromiseState]] to
pending . - Set promise.[[PromiseFulfillReactions]] to a new empty
List . - Set promise.[[PromiseRejectReactions]] to a new empty
List . - Set promise.[[PromiseIsHandled]] to
false . - Let resolvingFunctions be
CreateResolvingFunctions (promise). - Let completion be
Call (executor,undefined , « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »). - If completion is an
abrupt completion , then- Perform ?
Call (resolvingFunctions.[[Reject]],undefined , « completion.[[Value]] »).
- Perform ?
- Return promise.
The executor argument must be a
The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise object. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another Promise object which will provide the value if it is fulfilled.
The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.
The resolve and reject functions passed to an executor function by the Promise
27.2.4 Properties of the Promise Constructor
The Promise
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - has the following properties:
27.2.4.1 Promise.all ( iterable )
The all function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.
- Let C be the
this value. - Let promiseCapability be ?
NewPromiseCapability (C). - Let promiseResolve be
GetPromiseResolve (C). IfAbruptRejectPromise (promiseResolve, promiseCapability).- Let iteratorRecord be
GetIterator (iterable). IfAbruptRejectPromise (iteratorRecord, promiseCapability).- Let result be
PerformPromiseAll (iteratorRecord, C, promiseCapability, promiseResolve). - If result is an
abrupt completion , then- If iteratorRecord.[[Done]] is
false , set result toIteratorClose (iteratorRecord, result). IfAbruptRejectPromise (result, promiseCapability).
- If iteratorRecord.[[Done]] is
- Return
Completion (result).
The all function requires its
27.2.4.1.1 GetPromiseResolve ( promiseConstructor )
The abstract operation GetPromiseResolve takes argument promiseConstructor. It performs the following steps when called:
Assert :IsConstructor (promiseConstructor) istrue .- Let promiseResolve be ?
Get (promiseConstructor,"resolve" ). - If
IsCallable (promiseResolve) isfalse , throw aTypeError exception. - Return promiseResolve.
27.2.4.1.2 PerformPromiseAll ( iteratorRecord, constructor, resultCapability, promiseResolve )
The abstract operation PerformPromiseAll takes arguments iteratorRecord, constructor, resultCapability (a
Assert :IsConstructor (constructor) istrue .Assert :IsCallable (promiseResolve) istrue .- Let values be a new empty
List . - Let remainingElementsCount be the
Record { [[Value]]: 1 }. - Let index be 0.
- Repeat,
- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , then- Set iteratorRecord.[[Done]] to
true . - Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
- If remainingElementsCount.[[Value]] is 0, then
- Let valuesArray be !
CreateArrayFromList (values). - Perform ?
Call (resultCapability.[[Resolve]],undefined , « valuesArray »).
- Let valuesArray be !
- Return resultCapability.[[Promise]].
- Set iteratorRecord.[[Done]] to
- Let nextValue be
IteratorValue (next). - If nextValue is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (nextValue).- Append
undefined to values. - Let nextPromise be ?
Call (promiseResolve, constructor, « nextValue »). - Let steps be the algorithm steps defined in
Promise.allResolve Element Functions - Let length be the number of non-optional parameters of the function definition in
Promise.allResolve Element Functions - Let onFulfilled be !
CreateBuiltinFunction (steps, length,"" , « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »). - Set onFulfilled.[[AlreadyCalled]] to
false . - Set onFulfilled.[[Index]] to index.
- Set onFulfilled.[[Values]] to values.
- Set onFulfilled.[[Capability]] to resultCapability.
- Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
- Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
- Perform ?
Invoke (nextPromise,"then" , « onFulfilled, resultCapability.[[Reject]] »). - Set index to index + 1.
- Let next be
27.2.4.1.3 Promise.all Resolve Element Functions
A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function is called with argument x, the following steps are taken:
- Let F be the
active function object . - If F.[[AlreadyCalled]] is
true , returnundefined . - Set F.[[AlreadyCalled]] to
true . - Let index be F.[[Index]].
- Let values be F.[[Values]].
- Let promiseCapability be F.[[Capability]].
- Let remainingElementsCount be F.[[RemainingElements]].
- Set values[index] to x.
- Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
- If remainingElementsCount.[[Value]] is 0, then
- Let valuesArray be !
CreateArrayFromList (values). - Return ?
Call (promiseCapability.[[Resolve]],undefined , « valuesArray »).
- Let valuesArray be !
- Return
undefined .
The Promise.all resolve element function is
27.2.4.2 Promise.allSettled ( iterable )
The allSettled function returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.
- Let C be the
this value. - Let promiseCapability be ?
NewPromiseCapability (C). - Let promiseResolve be
GetPromiseResolve (C). IfAbruptRejectPromise (promiseResolve, promiseCapability).- Let iteratorRecord be
GetIterator (iterable). IfAbruptRejectPromise (iteratorRecord, promiseCapability).- Let result be
PerformPromiseAllSettled (iteratorRecord, C, promiseCapability, promiseResolve). - If result is an
abrupt completion , then- If iteratorRecord.[[Done]] is
false , set result toIteratorClose (iteratorRecord, result). IfAbruptRejectPromise (result, promiseCapability).
- If iteratorRecord.[[Done]] is
- Return
Completion (result).
The allSettled function requires its
27.2.4.2.1 PerformPromiseAllSettled ( iteratorRecord, constructor, resultCapability, promiseResolve )
The abstract operation PerformPromiseAllSettled takes arguments iteratorRecord, constructor, resultCapability (a
Assert : !IsConstructor (constructor) istrue .Assert :IsCallable (promiseResolve) istrue .- Let values be a new empty
List . - Let remainingElementsCount be the
Record { [[Value]]: 1 }. - Let index be 0.
- Repeat,
- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , then- Set iteratorRecord.[[Done]] to
true . - Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
- If remainingElementsCount.[[Value]] is 0, then
- Let valuesArray be !
CreateArrayFromList (values). - Perform ?
Call (resultCapability.[[Resolve]],undefined , « valuesArray »).
- Let valuesArray be !
- Return resultCapability.[[Promise]].
- Set iteratorRecord.[[Done]] to
- Let nextValue be
IteratorValue (next). - If nextValue is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (nextValue).- Append
undefined to values. - Let nextPromise be ?
Call (promiseResolve, constructor, « nextValue »). - Let stepsFulfilled be the algorithm steps defined in
Promise.allSettledResolve Element Functions - Let lengthFulfilled be the number of non-optional parameters of the function definition in
Promise.allSettledResolve Element Functions - Let onFulfilled be !
CreateBuiltinFunction (stepsFulfilled, lengthFulfilled,"" , « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »). - Let alreadyCalled be the
Record { [[Value]]:false }. - Set onFulfilled.[[AlreadyCalled]] to alreadyCalled.
- Set onFulfilled.[[Index]] to index.
- Set onFulfilled.[[Values]] to values.
- Set onFulfilled.[[Capability]] to resultCapability.
- Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
- Let stepsRejected be the algorithm steps defined in
Promise.allSettledReject Element Functions - Let lengthRejected be the number of non-optional parameters of the function definition in
Promise.allSettledReject Element Functions - Let onRejected be !
CreateBuiltinFunction (stepsRejected, lengthRejected,"" , « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »). - Set onRejected.[[AlreadyCalled]] to alreadyCalled.
- Set onRejected.[[Index]] to index.
- Set onRejected.[[Values]] to values.
- Set onRejected.[[Capability]] to resultCapability.
- Set onRejected.[[RemainingElements]] to remainingElementsCount.
- Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
- Perform ?
Invoke (nextPromise,"then" , « onFulfilled, onRejected »). - Set index to index + 1.
- Let next be
27.2.4.2.2 Promise.allSettled Resolve Element Functions
A Promise.allSettled resolve element function is an anonymous built-in function that is used to resolve a specific Promise.allSettled element. Each Promise.allSettled resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled resolve element function is called with argument x, the following steps are taken:
- Let F be the
active function object . - Let alreadyCalled be F.[[AlreadyCalled]].
- If alreadyCalled.[[Value]] is
true , returnundefined . - Set alreadyCalled.[[Value]] to
true . - Let index be F.[[Index]].
- Let values be F.[[Values]].
- Let promiseCapability be F.[[Capability]].
- Let remainingElementsCount be F.[[RemainingElements]].
- Let obj be !
OrdinaryObjectCreate (%Object.prototype% ). - Perform !
CreateDataPropertyOrThrow (obj,"status" ,"fulfilled" ). - Perform !
CreateDataPropertyOrThrow (obj,"value" , x). - Set values[index] to obj.
- Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
- If remainingElementsCount.[[Value]] is 0, then
- Let valuesArray be !
CreateArrayFromList (values). - Return ?
Call (promiseCapability.[[Resolve]],undefined , « valuesArray »).
- Let valuesArray be !
- Return
undefined .
The Promise.allSettled resolve element function is
27.2.4.2.3 Promise.allSettled Reject Element Functions
A Promise.allSettled reject element function is an anonymous built-in function that is used to reject a specific Promise.allSettled element. Each Promise.allSettled reject element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled reject element function is called with argument x, the following steps are taken:
- Let F be the
active function object . - Let alreadyCalled be F.[[AlreadyCalled]].
- If alreadyCalled.[[Value]] is
true , returnundefined . - Set alreadyCalled.[[Value]] to
true . - Let index be F.[[Index]].
- Let values be F.[[Values]].
- Let promiseCapability be F.[[Capability]].
- Let remainingElementsCount be F.[[RemainingElements]].
- Let obj be !
OrdinaryObjectCreate (%Object.prototype% ). - Perform !
CreateDataPropertyOrThrow (obj,"status" ,"rejected" ). - Perform !
CreateDataPropertyOrThrow (obj,"reason" , x). - Set values[index] to obj.
- Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
- If remainingElementsCount.[[Value]] is 0, then
- Let valuesArray be !
CreateArrayFromList (values). - Return ?
Call (promiseCapability.[[Resolve]],undefined , « valuesArray »).
- Let valuesArray be !
- Return
undefined .
The Promise.allSettled reject element function is
27.2.4.3 Promise.any ( iterable )
The any function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an AggregateError holding the rejection reasons if all of the given promises are rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.
- Let C be the
this value. - Let promiseCapability be ?
NewPromiseCapability (C). - Let promiseResolve be
GetPromiseResolve (C). IfAbruptRejectPromise (promiseResolve, promiseCapability).- Let iteratorRecord be
GetIterator (iterable). IfAbruptRejectPromise (iteratorRecord, promiseCapability).- Let result be
PerformPromiseAny (iteratorRecord, C, promiseCapability, promiseResolve). - If result is an
abrupt completion , then- If iteratorRecord.[[Done]] is
false , set result toIteratorClose (iteratorRecord, result). IfAbruptRejectPromise (result, promiseCapability).
- If iteratorRecord.[[Done]] is
- Return
Completion (result).
The any function requires its Promise
27.2.4.3.1 PerformPromiseAny ( iteratorRecord, constructor, resultCapability, promiseResolve )
The abstract operation PerformPromiseAny takes arguments iteratorRecord, constructor, resultCapability (a
Assert : !IsConstructor (constructor) istrue .Assert : !IsCallable (promiseResolve) istrue .- Let errors be a new empty
List . - Let remainingElementsCount be the
Record { [[Value]]: 1 }. - Let index be 0.
- Repeat,
- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , then- Set iteratorRecord.[[Done]] to
true . - Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
- If remainingElementsCount.[[Value]] is 0, then
- Let error be a newly created
AggregateErrorobject. - Perform !
DefinePropertyOrThrow (error,"errors" , PropertyDescriptor { [[Configurable]]:true , [[Enumerable]]:false , [[Writable]]:true , [[Value]]: !CreateArrayFromList (errors) }). - Return
ThrowCompletion (error).
- Let error be a newly created
- Return resultCapability.[[Promise]].
- Set iteratorRecord.[[Done]] to
- Let nextValue be
IteratorValue (next). - If nextValue is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (nextValue).- Append
undefined to errors. - Let nextPromise be ?
Call (promiseResolve, constructor, « nextValue »). - Let stepsRejected be the algorithm steps defined in
Promise.anyReject Element Functions - Let lengthRejected be the number of non-optional parameters of the function definition in
Promise.anyReject Element Functions - Let onRejected be !
CreateBuiltinFunction (stepsRejected, lengthRejected,"" , « [[AlreadyCalled]], [[Index]], [[Errors]], [[Capability]], [[RemainingElements]] »). - Set onRejected.[[AlreadyCalled]] to
false . - Set onRejected.[[Index]] to index.
- Set onRejected.[[Errors]] to errors.
- Set onRejected.[[Capability]] to resultCapability.
- Set onRejected.[[RemainingElements]] to remainingElementsCount.
- Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
- Perform ?
Invoke (nextPromise,"then" , « resultCapability.[[Resolve]], onRejected »). - Set index to index + 1.
- Let next be
27.2.4.3.2 Promise.any Reject Element Functions
A Promise.any reject element function is an anonymous built-in function that is used to reject a specific Promise.any element. Each Promise.any reject element function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.any reject element function is called with argument x, the following steps are taken:
- Let F be the
active function object . - If F.[[AlreadyCalled]] is
true , returnundefined . - Set F.[[AlreadyCalled]] to
true . - Let index be F.[[Index]].
- Let errors be F.[[Errors]].
- Let promiseCapability be F.[[Capability]].
- Let remainingElementsCount be F.[[RemainingElements]].
- Set errors[index] to x.
- Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
- If remainingElementsCount.[[Value]] is 0, then
- Let error be a newly created
AggregateErrorobject. - Perform !
DefinePropertyOrThrow (error,"errors" , PropertyDescriptor { [[Configurable]]:true , [[Enumerable]]:false , [[Writable]]:true , [[Value]]: !CreateArrayFromList (errors) }). - Return ?
Call (promiseCapability.[[Reject]],undefined , « error »).
- Let error be a newly created
- Return
undefined .
The Promise.any reject element function is
27.2.4.4 Promise.prototype
The initial value of Promise.prototype is the
This property has the attributes { [[Writable]]:
27.2.4.5 Promise.race ( iterable )
The race function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.
- Let C be the
this value. - Let promiseCapability be ?
NewPromiseCapability (C). - Let promiseResolve be
GetPromiseResolve (C). IfAbruptRejectPromise (promiseResolve, promiseCapability).- Let iteratorRecord be
GetIterator (iterable). IfAbruptRejectPromise (iteratorRecord, promiseCapability).- Let result be
PerformPromiseRace (iteratorRecord, C, promiseCapability, promiseResolve). - If result is an
abrupt completion , then- If iteratorRecord.[[Done]] is
false , set result toIteratorClose (iteratorRecord, result). IfAbruptRejectPromise (result, promiseCapability).
- If iteratorRecord.[[Done]] is
- Return
Completion (result).
If the iterable argument is empty or if none of the promises in iterable ever settle then the pending promise returned by this method will never be settled.
The race function expects its resolve method.
27.2.4.5.1 PerformPromiseRace ( iteratorRecord, constructor, resultCapability, promiseResolve )
The abstract operation PerformPromiseRace takes arguments iteratorRecord, constructor, resultCapability (a
Assert :IsConstructor (constructor) istrue .Assert :IsCallable (promiseResolve) istrue .- Repeat,
- Let next be
IteratorStep (iteratorRecord). - If next is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (next).- If next is
false , then- Set iteratorRecord.[[Done]] to
true . - Return resultCapability.[[Promise]].
- Set iteratorRecord.[[Done]] to
- Let nextValue be
IteratorValue (next). - If nextValue is an
abrupt completion , set iteratorRecord.[[Done]] totrue . ReturnIfAbrupt (nextValue).- Let nextPromise be ?
Call (promiseResolve, constructor, « nextValue »). - Perform ?
Invoke (nextPromise,"then" , « resultCapability.[[Resolve]], resultCapability.[[Reject]] »).
- Let next be
27.2.4.6 Promise.reject ( r )
The reject function returns a new promise rejected with the passed argument.
- Let C be the
this value. - Let promiseCapability be ?
NewPromiseCapability (C). - Perform ?
Call (promiseCapability.[[Reject]],undefined , « r »). - Return promiseCapability.[[Promise]].
The reject function expects its
27.2.4.7 Promise.resolve ( x )
The resolve function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by this
- Let C be the
this value. - If
Type (C) is not Object, throw aTypeError exception. - Return ?
PromiseResolve (C, x).
The resolve function expects its
27.2.4.7.1 PromiseResolve ( C, x )
The abstract operation PromiseResolve takes arguments C (a
27.2.4.8 get Promise [ @@species ]
Promise[@@species] is an
- Return the
this value.
The value of the
Promise prototype methods normally use their
27.2.5 Properties of the Promise Prototype Object
The Promise prototype object:
- is %Promise.prototype%.
- has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is an
ordinary object . - does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.
27.2.5.1 Promise.prototype.catch ( onRejected )
When the catch method is called with argument onRejected, the following steps are taken:
- Let promise be the
this value. - Return ?
Invoke (promise,"then" , «undefined , onRejected »).
27.2.5.2 Promise.prototype.constructor
The initial value of Promise.prototype.constructor is
27.2.5.3 Promise.prototype.finally ( onFinally )
When the finally method is called with argument onFinally, the following steps are taken:
- Let promise be the
this value. - If
Type (promise) is not Object, throw aTypeError exception. - Let C be ?
SpeciesConstructor (promise,%Promise% ). Assert :IsConstructor (C) istrue .- If
IsCallable (onFinally) isfalse , then- Let thenFinally be onFinally.
- Let catchFinally be onFinally.
- Else,
- Let stepsThenFinally be the algorithm steps defined in
Then Finally Functions . - Let lengthThenFinally be the number of non-optional parameters of the function definition in
Then Finally Functions . - Let thenFinally be !
CreateBuiltinFunction (stepsThenFinally, lengthThenFinally,"" , « [[Constructor]], [[OnFinally]] »). - Set thenFinally.[[Constructor]] to C.
- Set thenFinally.[[OnFinally]] to onFinally.
- Let stepsCatchFinally be the algorithm steps defined in
Catch Finally Functions . - Let lengthCatchFinally be the number of non-optional parameters of the function definition in
Catch Finally Functions . - Let catchFinally be !
CreateBuiltinFunction (stepsCatchFinally, lengthCatchFinally,"" , « [[Constructor]], [[OnFinally]] »). - Set catchFinally.[[Constructor]] to C.
- Set catchFinally.[[OnFinally]] to onFinally.
- Let stepsThenFinally be the algorithm steps defined in
- Return ?
Invoke (promise,"then" , « thenFinally, catchFinally »).
27.2.5.3.1 Then Finally Functions
A Then Finally function is an anonymous built-in function that has a [[Constructor]] and an [[OnFinally]] internal slot. The value of the [[Constructor]] internal slot is a Promise-like
When a Then Finally function is called with argument value, the following steps are taken:
- Let F be the
active function object . - Let onFinally be F.[[OnFinally]].
Assert :IsCallable (onFinally) istrue .- Let result be ?
Call (onFinally,undefined ). - Let C be F.[[Constructor]].
Assert :IsConstructor (C) istrue .- Let promise be ?
PromiseResolve (C, result). - Let valueThunk be equivalent to a function that returns value.
- Return ?
Invoke (promise,"then" , « valueThunk »).
The
27.2.5.3.2 Catch Finally Functions
A Catch Finally function is an anonymous built-in function that has a [[Constructor]] and an [[OnFinally]] internal slot. The value of the [[Constructor]] internal slot is a Promise-like
When a Catch Finally function is called with argument reason, the following steps are taken:
- Let F be the
active function object . - Let onFinally be F.[[OnFinally]].
Assert :IsCallable (onFinally) istrue .- Let result be ?
Call (onFinally,undefined ). - Let C be F.[[Constructor]].
Assert :IsConstructor (C) istrue .- Let promise be ?
PromiseResolve (C, result). - Let thrower be equivalent to a function that throws reason.
- Return ?
Invoke (promise,"then" , « thrower »).
The
27.2.5.4 Promise.prototype.then ( onFulfilled, onRejected )
When the then method is called with arguments onFulfilled and onRejected, the following steps are taken:
- Let promise be the
this value. - If
IsPromise (promise) isfalse , throw aTypeError exception. - Let C be ?
SpeciesConstructor (promise,%Promise% ). - Let resultCapability be ?
NewPromiseCapability (C). - Return
PerformPromiseThen (promise, onFulfilled, onRejected, resultCapability).
27.2.5.4.1 PerformPromiseThen ( promise, onFulfilled, onRejected [ , resultCapability ] )
The abstract operation PerformPromiseThen takes arguments promise, onFulfilled, and onRejected and optional argument resultCapability (a
Assert :IsPromise (promise) istrue .- If resultCapability is not present, then
- Set resultCapability to
undefined .
- Set resultCapability to
- If
IsCallable (onFulfilled) isfalse , then- Let onFulfilledJobCallback be
empty .
- Let onFulfilledJobCallback be
- Else,
- Let onFulfilledJobCallback be
HostMakeJobCallback (onFulfilled).
- Let onFulfilledJobCallback be
- If
IsCallable (onRejected) isfalse , then- Let onRejectedJobCallback be
empty .
- Let onRejectedJobCallback be
- Else,
- Let onRejectedJobCallback be
HostMakeJobCallback (onRejected).
- Let onRejectedJobCallback be
- Let fulfillReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]:
Fulfill , [[Handler]]: onFulfilledJobCallback }. - Let rejectReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]:
Reject , [[Handler]]: onRejectedJobCallback }. - If promise.[[PromiseState]] is
pending , then - Else if promise.[[PromiseState]] is
fulfilled , then- Let value be promise.[[PromiseResult]].
- Let fulfillJob be
NewPromiseReactionJob (fulfillReaction, value). - Perform
HostEnqueuePromiseJob (fulfillJob.[[Job]], fulfillJob.[[Realm]]).
- Else,
Assert : The value of promise.[[PromiseState]] isrejected .- Let reason be promise.[[PromiseResult]].
- If promise.[[PromiseIsHandled]] is
false , performHostPromiseRejectionTracker (promise,"handle" ). - Let rejectJob be
NewPromiseReactionJob (rejectReaction, reason). - Perform
HostEnqueuePromiseJob (rejectJob.[[Job]], rejectJob.[[Realm]]).
- Set promise.[[PromiseIsHandled]] to
true . - If resultCapability is
undefined , then- Return
undefined .
- Return
- Else,
- Return resultCapability.[[Promise]].
27.2.5.5 Promise.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
27.2.6 Properties of Promise Instances
Promise instances are ordinary objects that inherit properties from the
| Internal Slot | Description |
|---|---|
| [[PromiseState]] |
One of then method.
|
| [[PromiseResult]] |
The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is not |
| [[PromiseFulfillReactions]] |
A |
| [[PromiseRejectReactions]] |
A |
| [[PromiseIsHandled]] | A boolean indicating whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking. |
27.3 GeneratorFunction Objects
GeneratorFunction objects are functions that are usually created by evaluating
27.3.1 The GeneratorFunction Constructor
The GeneratorFunction
- is %GeneratorFunction%.
- is a subclass of
Function. - creates and initializes a new GeneratorFunction object when called as a function rather than as a
constructor . Thus the function callGeneratorFunction (…)is equivalent to the object creation expressionnew GeneratorFunction (…)with the same arguments. - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include asupercall to the GeneratorFunctionconstructor to create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.
27.3.1.1 GeneratorFunction ( p1, p2, … , pn, body )
The last argument specifies the body (executable code) of a generator function; any preceding arguments specify formal parameters.
When the GeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:
- Let C be the
active function object . - Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
- Return ?
CreateDynamicFunction (C, NewTarget,generator , args).
See NOTE for
27.3.2 Properties of the GeneratorFunction Constructor
The GeneratorFunction
- is a standard built-in
function object that inherits from the Functionconstructor . - has a [[Prototype]] internal slot whose value is
%Function% . - has a
"name" property whose value is"GeneratorFunction" . - has the following properties:
27.3.2.1 GeneratorFunction.length
This is a
27.3.2.2 GeneratorFunction.prototype
The initial value of GeneratorFunction.prototype is the
This property has the attributes { [[Writable]]:
27.3.3 Properties of the GeneratorFunction Prototype Object
The GeneratorFunction prototype object:
- is %GeneratorFunction.prototype% (see
Figure 5 ). - is an
ordinary object . - is not a
function object and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed inTable 29 orTable 73 . - has a [[Prototype]] internal slot whose value is
%Function.prototype% .
27.3.3.1 GeneratorFunction.prototype.constructor
The initial value of GeneratorFunction.prototype.constructor is
This property has the attributes { [[Writable]]:
27.3.3.2 GeneratorFunction.prototype.prototype
The initial value of GeneratorFunction.prototype.prototype is the
This property has the attributes { [[Writable]]:
27.3.3.3 GeneratorFunction.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
27.3.4 GeneratorFunction Instances
Every GeneratorFunction instance is an ECMAScript
Each GeneratorFunction instance has the following own properties:
27.3.4.1 length
The specification for the
27.3.4.2 name
The specification for the
27.3.4.3 prototype
Whenever a GeneratorFunction instance is created another
This property has the attributes { [[Writable]]:
Unlike Function instances, the object that is the value of the a GeneratorFunction's
27.4 AsyncGeneratorFunction Objects
AsyncGeneratorFunction objects are functions that are usually created by evaluating
27.4.1 The AsyncGeneratorFunction Constructor
The AsyncGeneratorFunction
- is %AsyncGeneratorFunction%.
- is a subclass of
Function. - creates and initializes a new AsyncGeneratorFunction object when called as a function rather than as a
constructor . Thus the function callAsyncGeneratorFunction (...)is equivalent to the object creation expressionnew AsyncGeneratorFunction (...)with the same arguments. - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified AsyncGeneratorFunction behaviour must include asupercall to the AsyncGeneratorFunctionconstructor to create and initialize subclass instances with the internal slots necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms for defining async generator function objects create direct instances of AsyncGeneratorFunction. There is no syntactic means to create instances of AsyncGeneratorFunction subclasses.
27.4.1.1 AsyncGeneratorFunction ( p1, p2, … , pn, body )
The last argument specifies the body (executable code) of an async generator function; any preceding arguments specify formal parameters.
When the AsyncGeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no "p" arguments, and where body might also not be provided), the following steps are taken:
- Let C be the
active function object . - Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
- Return ?
CreateDynamicFunction (C, NewTarget,asyncGenerator , args).
See NOTE for
27.4.2 Properties of the AsyncGeneratorFunction Constructor
The AsyncGeneratorFunction
- is a standard built-in
function object that inherits from the Functionconstructor . - has a [[Prototype]] internal slot whose value is
%Function% . - has a
"name" property whose value is"AsyncGeneratorFunction" . - has the following properties:
27.4.2.1 AsyncGeneratorFunction.length
This is a
27.4.2.2 AsyncGeneratorFunction.prototype
The initial value of AsyncGeneratorFunction.prototype is the
This property has the attributes { [[Writable]]:
27.4.3 Properties of the AsyncGeneratorFunction Prototype Object
The AsyncGeneratorFunction prototype object:
- is %AsyncGeneratorFunction.prototype%.
- is an
ordinary object . - is not a
function object and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed inTable 29 orTable 74 . - has a [[Prototype]] internal slot whose value is
%Function.prototype% .
27.4.3.1 AsyncGeneratorFunction.prototype.constructor
The initial value of AsyncGeneratorFunction.prototype.constructor is
This property has the attributes { [[Writable]]:
27.4.3.2 AsyncGeneratorFunction.prototype.prototype
The initial value of AsyncGeneratorFunction.prototype.prototype is the
This property has the attributes { [[Writable]]:
27.4.3.3 AsyncGeneratorFunction.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
27.4.4 AsyncGeneratorFunction Instances
Every AsyncGeneratorFunction instance is an ECMAScript
Each AsyncGeneratorFunction instance has the following own properties:
27.4.4.1 length
The value of the
This property has the attributes { [[Writable]]:
27.4.4.2 name
The specification for the
27.4.4.3 prototype
Whenever an AsyncGeneratorFunction instance is created another
This property has the attributes { [[Writable]]:
Unlike function instances, the object that is the value of the an AsyncGeneratorFunction's
27.5 Generator Objects
A Generator object is an instance of a generator function and conforms to both the Iterator and Iterable interfaces.
Generator instances directly inherit properties from the object that is the initial value of the
27.5.1 Properties of the Generator Prototype Object
The Generator prototype object:
- is %GeneratorFunction.prototype.prototype%.
- is an
ordinary object . - is not a Generator instance and does not have a [[GeneratorState]] internal slot.
- has a [[Prototype]] internal slot whose value is
%IteratorPrototype% . - has properties that are indirectly inherited by all Generator instances.
27.5.1.1 Generator.prototype.constructor
The initial value of Generator.prototype.constructor is
This property has the attributes { [[Writable]]:
27.5.1.2 Generator.prototype.next ( value )
The next method performs the following steps:
- Let g be the
this value. - Return ?
GeneratorResume (g, value,empty ).
27.5.1.3 Generator.prototype.return ( value )
The return method performs the following steps:
- Let g be the
this value. - Let C be
Completion { [[Type]]:return , [[Value]]: value, [[Target]]:empty }. - Return ?
GeneratorResumeAbrupt (g, C,empty ).
27.5.1.4 Generator.prototype.throw ( exception )
The throw method performs the following steps:
- Let g be the
this value. - Let C be
ThrowCompletion (exception). - Return ?
GeneratorResumeAbrupt (g, C,empty ).
27.5.1.5 Generator.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
27.5.2 Properties of Generator Instances
Generator instances are initially created with the internal slots described in
| Internal Slot | Description |
|---|---|
| [[GeneratorState]] |
The current execution state of the generator. The possible values are: |
| [[GeneratorContext]] |
The |
| [[GeneratorBrand]] |
A brand used to distinguish different kinds of generators. The [[GeneratorBrand]] of generators declared by ECMAScript source text is always |
27.5.3 Generator Abstract Operations
27.5.3.1 GeneratorStart ( generator, generatorBody )
The abstract operation GeneratorStart takes arguments generator and generatorBody (a
Assert : The value of generator.[[GeneratorState]] isundefined .- Let genContext be the
running execution context . - Set the Generator component of genContext to generator.
- Set the code evaluation state of genContext such that when evaluation is resumed for that
execution context the following steps will be performed:- If generatorBody is a
Parse Node , then- Let result be the result of evaluating generatorBody.
- Else,
Assert : generatorBody is anAbstract Closure with no parameters.- Let result be generatorBody().
Assert : If we return here, the generator either threw an exception or performed either an implicit or explicit return.- Remove genContext from the
execution context stack and restore theexecution context that is at the top of theexecution context stack as therunning execution context . - Set generator.[[GeneratorState]] to
completed . - Once a generator enters the
completed state it never leaves it and its associatedexecution context is never resumed. Any execution state associated with generator can be discarded at this point. - If result.[[Type]] is
normal , let resultValue beundefined . - Else if result.[[Type]] is
return , let resultValue be result.[[Value]]. - Else,
Assert : result.[[Type]] isthrow .- Return
Completion (result).
- Return
CreateIterResultObject (resultValue,true ).
- If generatorBody is a
- Set generator.[[GeneratorContext]] to genContext.
- Set generator.[[GeneratorState]] to
suspendedStart . - Return
NormalCompletion (undefined ).
27.5.3.2 GeneratorValidate ( generator, generatorBrand )
The abstract operation GeneratorValidate takes arguments generator and generatorBrand. It performs the following steps when called:
- Perform ?
RequireInternalSlot (generator, [[GeneratorState]]). - Perform ?
RequireInternalSlot (generator, [[GeneratorBrand]]). - If generator.[[GeneratorBrand]] is not the same value as generatorBrand, throw a
TypeError exception. Assert : generator also has a [[GeneratorContext]] internal slot.- Let state be generator.[[GeneratorState]].
- If state is
executing , throw aTypeError exception. - Return state.
27.5.3.3 GeneratorResume ( generator, value, generatorBrand )
The abstract operation GeneratorResume takes arguments generator, value, and generatorBrand. It performs the following steps when called:
- Let state be ?
GeneratorValidate (generator, generatorBrand). - If state is
completed , returnCreateIterResultObject (undefined ,true ). Assert : state is eithersuspendedStart orsuspendedYield .- Let genContext be generator.[[GeneratorContext]].
- Let methodContext be the
running execution context . - Suspend methodContext.
- Set generator.[[GeneratorState]] to
executing . - Push genContext onto the
execution context stack ; genContext is now therunning execution context . - Resume the suspended evaluation of genContext using
NormalCompletion (value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation. Assert : When we return here, genContext has already been removed from theexecution context stack and methodContext is the currentlyrunning execution context .- Return
Completion (result).
27.5.3.4 GeneratorResumeAbrupt ( generator, abruptCompletion, generatorBrand )
The abstract operation GeneratorResumeAbrupt takes arguments generator, abruptCompletion (a
- Let state be ?
GeneratorValidate (generator, generatorBrand). - If state is
suspendedStart , then- Set generator.[[GeneratorState]] to
completed . - Once a generator enters the
completed state it never leaves it and its associatedexecution context is never resumed. Any execution state associated with generator can be discarded at this point. - Set state to
completed .
- Set generator.[[GeneratorState]] to
- If state is
completed , then- If abruptCompletion.[[Type]] is
return , then- Return
CreateIterResultObject (abruptCompletion.[[Value]],true ).
- Return
- Return
Completion (abruptCompletion).
- If abruptCompletion.[[Type]] is
Assert : state issuspendedYield .- Let genContext be generator.[[GeneratorContext]].
- Let methodContext be the
running execution context . - Suspend methodContext.
- Set generator.[[GeneratorState]] to
executing . - Push genContext onto the
execution context stack ; genContext is now therunning execution context . - Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
Assert : When we return here, genContext has already been removed from theexecution context stack and methodContext is the currentlyrunning execution context .- Return
Completion (result).
27.5.3.5 GetGeneratorKind ( )
The abstract operation GetGeneratorKind takes no arguments. It performs the following steps when called:
- Let genContext be the
running execution context . - If genContext does not have a Generator component, return
non-generator . - Let generator be the Generator component of genContext.
- If generator has an [[AsyncGeneratorState]] internal slot, return
async . - Else, return
sync .
27.5.3.6 GeneratorYield ( iterNextObj )
The abstract operation GeneratorYield takes argument iterNextObj. It performs the following steps when called:
Assert : iterNextObj is an Object that implements the IteratorResult interface.- Let genContext be the
running execution context . Assert : genContext is theexecution context of a generator.- Let generator be the value of the Generator component of genContext.
Assert :GetGeneratorKind () issync .- Set generator.[[GeneratorState]] to
suspendedYield . - Remove genContext from the
execution context stack and restore theexecution context that is at the top of theexecution context stack as therunning execution context . - Set the code evaluation state of genContext such that when evaluation is resumed with a
Completion resumptionValue the following steps will be performed:- Return resumptionValue.
- NOTE: This returns to the evaluation of the
YieldExpression that originally called this abstract operation.
- Return
NormalCompletion (iterNextObj). - NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of genContext.
27.5.3.7 Yield ( value )
The abstract operation Yield takes argument value (an
- Let generatorKind be !
GetGeneratorKind (). - If generatorKind is
async , return ?AsyncGeneratorYield (value). - Otherwise, return ?
GeneratorYield (!CreateIterResultObject (value,false )).
27.5.3.8 CreateIteratorFromClosure ( closure, generatorBrand, generatorPrototype )
The abstract operation CreateIteratorFromClosure takes arguments closure (an
- NOTE: closure can contain uses of the
Yield shorthand to yield an IteratorResult object. - Let internalSlotsList be « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] ».
- Let generator be !
OrdinaryObjectCreate (generatorPrototype, internalSlotsList). - Set generator.[[GeneratorBrand]] to generatorBrand.
- Set generator.[[GeneratorState]] to
undefined . - Perform !
GeneratorStart (generator, closure). - Return generator.
27.6 AsyncGenerator Objects
An AsyncGenerator object is an instance of an async generator function and conforms to both the AsyncIterator and AsyncIterable interfaces.
AsyncGenerator instances directly inherit properties from the object that is the initial value of the
27.6.1 Properties of the AsyncGenerator Prototype Object
The AsyncGenerator prototype object:
- is %AsyncGeneratorFunction.prototype.prototype%.
- is an
ordinary object . - is not an AsyncGenerator instance and does not have an [[AsyncGeneratorState]] internal slot.
- has a [[Prototype]] internal slot whose value is
%AsyncIteratorPrototype% . - has properties that are indirectly inherited by all AsyncGenerator instances.
27.6.1.1 AsyncGenerator.prototype.constructor
The initial value of AsyncGenerator.prototype.constructor is
This property has the attributes { [[Writable]]:
27.6.1.2 AsyncGenerator.prototype.next ( value )
- Let generator be the
this value. - Let completion be
NormalCompletion (value). - Return !
AsyncGeneratorEnqueue (generator, completion,empty ).
27.6.1.3 AsyncGenerator.prototype.return ( value )
- Let generator be the
this value. - Let completion be
Completion { [[Type]]:return , [[Value]]: value, [[Target]]:empty }. - Return !
AsyncGeneratorEnqueue (generator, completion,empty ).
27.6.1.4 AsyncGenerator.prototype.throw ( exception )
- Let generator be the
this value. - Let completion be
ThrowCompletion (exception). - Return !
AsyncGeneratorEnqueue (generator, completion,empty ).
27.6.1.5 AsyncGenerator.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
27.6.2 Properties of AsyncGenerator Instances
AsyncGenerator instances are initially created with the internal slots described below:
| Internal Slot | Description |
|---|---|
| [[AsyncGeneratorState]] | The current execution state of the async generator. The possible values are: |
| [[AsyncGeneratorContext]] | The |
| [[AsyncGeneratorQueue]] | A |
| [[GeneratorBrand]] | A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by ECMAScript source text is always |
27.6.3 AsyncGenerator Abstract Operations
27.6.3.1 AsyncGeneratorRequest Records
The AsyncGeneratorRequest is a
They have the following fields:
| Field Name | Value | Meaning |
|---|---|---|
| [[Completion]] | A |
The completion which should be used to resume the async generator. |
| [[Capability]] | A |
The promise capabilities associated with this request. |
27.6.3.2 AsyncGeneratorStart ( generator, generatorBody )
The abstract operation AsyncGeneratorStart takes arguments generator and generatorBody (a
Assert : generator is an AsyncGenerator instance.Assert : generator.[[AsyncGeneratorState]] isundefined .- Let genContext be the
running execution context . - Set the Generator component of genContext to generator.
- Set the code evaluation state of genContext such that when evaluation is resumed for that
execution context the following steps will be performed:- If generatorBody is a
Parse Node , then- Let result be the result of evaluating generatorBody.
- Else,
Assert : generatorBody is anAbstract Closure with no parameters.- Let result be generatorBody().
Assert : If we return here, the async generator either threw an exception or performed either an implicit or explicit return.- Remove genContext from the
execution context stack and restore theexecution context that is at the top of theexecution context stack as therunning execution context . - Set generator.[[AsyncGeneratorState]] to
completed . - If result is a normal completion, let resultValue be
undefined . - Else,
- Let resultValue be result.[[Value]].
- If result.[[Type]] is not
return , then- Return !
AsyncGeneratorReject (generator, resultValue).
- Return !
- Return !
AsyncGeneratorResolve (generator, resultValue,true ).
- If generatorBody is a
- Set generator.[[AsyncGeneratorContext]] to genContext.
- Set generator.[[AsyncGeneratorState]] to
suspendedStart . - Set generator.[[AsyncGeneratorQueue]] to a new empty
List . - Return
undefined .
27.6.3.3 AsyncGeneratorValidate ( generator, generatorBrand )
The abstract operation AsyncGeneratorValidate takes arguments generator and generatorBrand. It performs the following steps when called:
- Perform ?
RequireInternalSlot (generator, [[AsyncGeneratorContext]]). - Perform ?
RequireInternalSlot (generator, [[AsyncGeneratorState]]). - Perform ?
RequireInternalSlot (generator, [[AsyncGeneratorQueue]]). - If generator.[[GeneratorBrand]] is not the same value as generatorBrand, throw a
TypeError exception.
27.6.3.4 AsyncGeneratorResolve ( generator, value, done )
The abstract operation AsyncGeneratorResolve takes arguments generator, value, and done (a Boolean). It performs the following steps when called:
Assert : generator is an AsyncGenerator instance.- Let queue be generator.[[AsyncGeneratorQueue]].
Assert : queue is not an emptyList .- Let next be the first element of queue.
- Remove the first element from queue.
- Let promiseCapability be next.[[Capability]].
- Let iteratorResult be !
CreateIterResultObject (value, done). - Perform !
Call (promiseCapability.[[Resolve]],undefined , « iteratorResult »). - Perform !
AsyncGeneratorResumeNext (generator). - Return
undefined .
27.6.3.5 AsyncGeneratorReject ( generator, exception )
The abstract operation AsyncGeneratorReject takes arguments generator and exception. It performs the following steps when called:
Assert : generator is an AsyncGenerator instance.- Let queue be generator.[[AsyncGeneratorQueue]].
Assert : queue is not an emptyList .- Let next be the first element of queue.
- Remove the first element from queue.
- Let promiseCapability be next.[[Capability]].
- Perform !
Call (promiseCapability.[[Reject]],undefined , « exception »). - Perform !
AsyncGeneratorResumeNext (generator). - Return
undefined .
27.6.3.6 AsyncGeneratorResumeNext ( generator )
The abstract operation AsyncGeneratorResumeNext takes argument generator. It performs the following steps when called:
Assert : generator is an AsyncGenerator instance.- Let state be generator.[[AsyncGeneratorState]].
Assert : state is notexecuting .- If state is
awaiting-return , returnundefined . - Let queue be generator.[[AsyncGeneratorQueue]].
- If queue is an empty
List , returnundefined . - Let next be the value of the first element of queue.
Assert : next is an AsyncGeneratorRequest record.- Let completion be next.[[Completion]].
- If completion is an
abrupt completion , then- If state is
suspendedStart , then- Set generator.[[AsyncGeneratorState]] to
completed . - Set state to
completed .
- Set generator.[[AsyncGeneratorState]] to
- If state is
completed , then- If completion.[[Type]] is
return , then- Set generator.[[AsyncGeneratorState]] to
awaiting-return . - Let promise be ?
PromiseResolve (%Promise% , completion.[[Value]]). - Let stepsFulfilled be the algorithm steps defined in
AsyncGeneratorResumeNext Return Processor Fulfilled Functions . - Let lengthFulfilled be the number of non-optional parameters of the function definition in
AsyncGeneratorResumeNext Return Processor Fulfilled Functions . - Let onFulfilled be !
CreateBuiltinFunction (stepsFulfilled, lengthFulfilled,"" , « [[Generator]] »). - Set onFulfilled.[[Generator]] to generator.
- Let stepsRejected be the algorithm steps defined in
AsyncGeneratorResumeNext Return Processor Rejected Functions . - Let lengthRejected be the number of non-optional parameters of the function definition in
AsyncGeneratorResumeNext Return Processor Rejected Functions . - Let onRejected be !
CreateBuiltinFunction (stepsRejected, lengthRejected,"" , « [[Generator]] »). - Set onRejected.[[Generator]] to generator.
- Perform !
PerformPromiseThen (promise, onFulfilled, onRejected). - Return
undefined .
- Set generator.[[AsyncGeneratorState]] to
- Else,
Assert : completion.[[Type]] isthrow .- Perform !
AsyncGeneratorReject (generator, completion.[[Value]]). - Return
undefined .
- If completion.[[Type]] is
- If state is
- Else if state is
completed , return !AsyncGeneratorResolve (generator,undefined ,true ). Assert : state is eithersuspendedStart orsuspendedYield .- Let genContext be generator.[[AsyncGeneratorContext]].
- Let callerContext be the
running execution context . - Suspend callerContext.
- Set generator.[[AsyncGeneratorState]] to
executing . - Push genContext onto the
execution context stack ; genContext is now therunning execution context . - Resume the suspended evaluation of genContext using completion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
Assert : result is never anabrupt completion .Assert : When we return here, genContext has already been removed from theexecution context stack and callerContext is the currentlyrunning execution context .- Return
undefined .
27.6.3.6.1 AsyncGeneratorResumeNext Return Processor Fulfilled Functions
An
When an
- Let F be the
active function object . - Set F.[[Generator]].[[AsyncGeneratorState]] to
completed . - Return !
AsyncGeneratorResolve (F.[[Generator]], value,true ).
The
27.6.3.6.2 AsyncGeneratorResumeNext Return Processor Rejected Functions
An
When an
- Let F be the
active function object . - Set F.[[Generator]].[[AsyncGeneratorState]] to
completed . - Return !
AsyncGeneratorReject (F.[[Generator]], reason).
The
27.6.3.7 AsyncGeneratorEnqueue ( generator, completion, generatorBrand )
The abstract operation AsyncGeneratorEnqueue takes arguments generator, completion (a
- Let promiseCapability be !
NewPromiseCapability (%Promise% ). - Let check be
AsyncGeneratorValidate (generator, generatorBrand). - If check is an
abrupt completion , then- Let badGeneratorError be a newly created
TypeError object. - Perform !
Call (promiseCapability.[[Reject]],undefined , « badGeneratorError »). - Return promiseCapability.[[Promise]].
- Let badGeneratorError be a newly created
- Let queue be generator.[[AsyncGeneratorQueue]].
- Let request be AsyncGeneratorRequest { [[Completion]]: completion, [[Capability]]: promiseCapability }.
- Append request to the end of queue.
- Let state be generator.[[AsyncGeneratorState]].
- If state is not
executing , then- Perform !
AsyncGeneratorResumeNext (generator).
- Perform !
- Return promiseCapability.[[Promise]].
27.6.3.8 AsyncGeneratorYield ( value )
The abstract operation AsyncGeneratorYield takes argument value. It performs the following steps when called:
- Let genContext be the
running execution context . Assert : genContext is theexecution context of a generator.- Let generator be the value of the Generator component of genContext.
Assert :GetGeneratorKind () isasync .- Set value to ?
Await (value). - Set generator.[[AsyncGeneratorState]] to
suspendedYield . - Remove genContext from the
execution context stack and restore theexecution context that is at the top of theexecution context stack as therunning execution context . - Set the code evaluation state of genContext such that when evaluation is resumed with a
Completion resumptionValue the following steps will be performed:- If resumptionValue.[[Type]] is not
return , returnCompletion (resumptionValue). - Let awaited be
Await (resumptionValue.[[Value]]). - If awaited.[[Type]] is
throw , returnCompletion (awaited). Assert : awaited.[[Type]] isnormal .- Return
Completion { [[Type]]:return , [[Value]]: awaited.[[Value]], [[Target]]:empty }. - NOTE: When one of the above steps returns, it returns to the evaluation of the
YieldExpression production that originally called this abstract operation.
- If resumptionValue.[[Type]] is not
- Return !
AsyncGeneratorResolve (generator, value,false ). - NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of genContext.
27.6.3.9 CreateAsyncIteratorFromClosure ( closure, generatorBrand, generatorPrototype )
The abstract operation CreateAsyncIteratorFromClosure takes arguments closure (an
- NOTE: closure can contain uses of the
Await shorthand and uses of theYield shorthand to yield an IteratorResult object. - Let internalSlotsList be « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] ».
- Let generator be !
OrdinaryObjectCreate (generatorPrototype, internalSlotsList). - Set generator.[[GeneratorBrand]] to generatorBrand.
- Set generator.[[AsyncGeneratorState]] to
undefined . - Perform !
AsyncGeneratorStart (generator, closure). - Return generator.
27.7 AsyncFunction Objects
AsyncFunction objects are functions that are usually created by evaluating
27.7.1 The AsyncFunction Constructor
The AsyncFunction
- is %AsyncFunction%.
- is a subclass of
Function. - creates and initializes a new AsyncFunction object when called as a function rather than as a
constructor . Thus the function callAsyncFunction(…)is equivalent to the object creation expressionnew AsyncFunction(…)with the same arguments. - is designed to be subclassable. It may be used as the value of an
extendsclause of a class definition. Subclass constructors that intend to inherit the specified AsyncFunction behaviour must include asupercall to the AsyncFunctionconstructor to create and initialize a subclass instance with the internal slots necessary for built-in async function behaviour. All ECMAScript syntactic forms for defining async function objects create direct instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.
27.7.1.1 AsyncFunction ( p1, p2, … , pn, body )
The last argument specifies the body (executable code) of an async function. Any preceding arguments specify formal parameters.
When the AsyncFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no p arguments, and where body might also not be provided), the following steps are taken:
- Let C be the
active function object . - Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
- Return
CreateDynamicFunction (C, NewTarget,async , args).
27.7.2 Properties of the AsyncFunction Constructor
The AsyncFunction
- is a standard built-in
function object that inherits from the Functionconstructor . - has a [[Prototype]] internal slot whose value is
%Function% . - has a
"name" property whose value is"AsyncFunction" . - has the following properties:
27.7.2.1 AsyncFunction.length
This is a
27.7.2.2 AsyncFunction.prototype
The initial value of AsyncFunction.prototype is the
This property has the attributes { [[Writable]]:
27.7.3 Properties of the AsyncFunction Prototype Object
The AsyncFunction prototype object:
- is %AsyncFunction.prototype%.
- is an
ordinary object . - is not a
function object and does not have an [[ECMAScriptCode]] internal slot or any other of the internal slots listed inTable 29 . - has a [[Prototype]] internal slot whose value is
%Function.prototype% .
27.7.3.1 AsyncFunction.prototype.constructor
The initial value of AsyncFunction.prototype.constructor is
This property has the attributes { [[Writable]]:
27.7.3.2 AsyncFunction.prototype [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
27.7.4 AsyncFunction Instances
Every AsyncFunction instance is an ECMAScript
Each AsyncFunction instance has the following own properties:
27.7.4.1 length
The specification for the
27.7.4.2 name
The specification for the
27.7.5 Async Functions Abstract Operations
27.7.5.1 AsyncFunctionStart ( promiseCapability, asyncFunctionBody )
The abstract operation AsyncFunctionStart takes arguments promiseCapability (a
- Let runningContext be the
running execution context . - Let asyncContext be a copy of runningContext.
- NOTE: Copying the execution state is required for the step below to resume its execution. It is ill-defined to resume a currently executing context.
- Set the code evaluation state of asyncContext such that when evaluation is resumed for that
execution context the following steps will be performed:- Let result be the result of evaluating asyncFunctionBody.
Assert : If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.- Remove asyncContext from the
execution context stack and restore theexecution context that is at the top of theexecution context stack as therunning execution context . - If result.[[Type]] is
normal , then- Perform !
Call (promiseCapability.[[Resolve]],undefined , «undefined »).
- Perform !
- Else if result.[[Type]] is
return , then- Perform !
Call (promiseCapability.[[Resolve]],undefined , « result.[[Value]] »).
- Perform !
- Else,
- Return.
- Push asyncContext onto the
execution context stack ; asyncContext is now therunning execution context . - Resume the suspended evaluation of asyncContext. Let result be the value returned by the resumed computation.
Assert : When we return here, asyncContext has already been removed from theexecution context stack and runningContext is the currentlyrunning execution context .Assert : result is a normal completion with a value ofundefined . The possible sources of completion values areAwait or, if the async function doesn't await anything, step4.g above.- Return.
28 Reflection
28.1 The Reflect Object
The Reflect object:
- is %Reflect%.
- is the initial value of the
"Reflect" property of theglobal object . - is an
ordinary object . - has a [[Prototype]] internal slot whose value is
%Object.prototype% . - is not a
function object . - does not have a [[Construct]] internal method; it cannot be used as a
constructor with thenewoperator. - does not have a [[Call]] internal method; it cannot be invoked as a function.
28.1.1 Reflect.apply ( target, thisArgument, argumentsList )
When the apply function is called with arguments target, thisArgument, and argumentsList, the following steps are taken:
- If
IsCallable (target) isfalse , throw aTypeError exception. - Let args be ?
CreateListFromArrayLike (argumentsList). - Perform
PrepareForTailCall (). - Return ?
Call (target, thisArgument, args).
28.1.2 Reflect.construct ( target, argumentsList [ , newTarget ] )
When the construct function is called with arguments target, argumentsList, and newTarget, the following steps are taken:
- If
IsConstructor (target) isfalse , throw aTypeError exception. - If newTarget is not present, set newTarget to target.
- Else if
IsConstructor (newTarget) isfalse , throw aTypeError exception. - Let args be ?
CreateListFromArrayLike (argumentsList). - Return ?
Construct (target, args, newTarget).
28.1.3 Reflect.defineProperty ( target, propertyKey, attributes )
When the defineProperty function is called with arguments target, propertyKey, and attributes, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Let key be ?
ToPropertyKey (propertyKey). - Let desc be ?
ToPropertyDescriptor (attributes). - Return ? target.[[DefineOwnProperty]](key, desc).
28.1.4 Reflect.deleteProperty ( target, propertyKey )
When the deleteProperty function is called with arguments target and propertyKey, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Let key be ?
ToPropertyKey (propertyKey). - Return ? target.[[Delete]](key).
28.1.5 Reflect.get ( target, propertyKey [ , receiver ] )
When the get function is called with arguments target, propertyKey, and receiver, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Let key be ?
ToPropertyKey (propertyKey). - If receiver is not present, then
- Set receiver to target.
- Return ? target.[[Get]](key, receiver).
28.1.6 Reflect.getOwnPropertyDescriptor ( target, propertyKey )
When the getOwnPropertyDescriptor function is called with arguments target and propertyKey, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Let key be ?
ToPropertyKey (propertyKey). - Let desc be ? target.[[GetOwnProperty]](key).
- Return
FromPropertyDescriptor (desc).
28.1.7 Reflect.getPrototypeOf ( target )
When the getPrototypeOf function is called with argument target, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Return ? target.[[GetPrototypeOf]]().
28.1.8 Reflect.has ( target, propertyKey )
When the has function is called with arguments target and propertyKey, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Let key be ?
ToPropertyKey (propertyKey). - Return ? target.[[HasProperty]](key).
28.1.9 Reflect.isExtensible ( target )
When the isExtensible function is called with argument target, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Return ? target.[[IsExtensible]]().
28.1.10 Reflect.ownKeys ( target )
When the ownKeys function is called with argument target, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Let keys be ? target.[[OwnPropertyKeys]]().
- Return
CreateArrayFromList (keys).
28.1.11 Reflect.preventExtensions ( target )
When the preventExtensions function is called with argument target, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Return ? target.[[PreventExtensions]]().
28.1.12 Reflect.set ( target, propertyKey, V [ , receiver ] )
When the set function is called with arguments target, V, propertyKey, and receiver, the following steps are taken:
- If
Type (target) is not Object, throw aTypeError exception. - Let key be ?
ToPropertyKey (propertyKey). - If receiver is not present, then
- Set receiver to target.
- Return ? target.[[Set]](key, V, receiver).
28.1.13 Reflect.setPrototypeOf ( target, proto )
When the setPrototypeOf function is called with arguments target and proto, the following steps are taken:
28.1.14 Reflect [ @@toStringTag ]
The initial value of the
This property has the attributes { [[Writable]]:
28.2 Proxy Objects
28.2.1 The Proxy Constructor
The Proxy
- is %Proxy%.
- is the initial value of the
"Proxy" property of theglobal object . - creates and initializes a new
Proxy exotic object when called as aconstructor . - is not intended to be called as a function and will throw an exception when called in that manner.
28.2.1.1 Proxy ( target, handler )
When Proxy is called with arguments target and handler, it performs the following steps:
- If NewTarget is
undefined , throw aTypeError exception. - Return ?
ProxyCreate (target, handler).
28.2.2 Properties of the Proxy Constructor
The Proxy
- has a [[Prototype]] internal slot whose value is
%Function.prototype% . - does not have a
"prototype" property because Proxy exotic objects do not have a [[Prototype]] internal slot that requires initialization. - has the following properties:
28.2.2.1 Proxy.revocable ( target, handler )
The Proxy.revocable function is used to create a revocable Proxy object. When Proxy.revocable is called with arguments target and handler, the following steps are taken:
- Let p be ?
ProxyCreate (target, handler). - Let steps be the algorithm steps defined in
Proxy Revocation Functions . - Let length be the number of non-optional parameters of the function definition in
Proxy Revocation Functions . - Let revoker be !
CreateBuiltinFunction (steps, length,"" , « [[RevocableProxy]] »). - Set revoker.[[RevocableProxy]] to p.
- Let result be !
OrdinaryObjectCreate (%Object.prototype% ). - Perform !
CreateDataPropertyOrThrow (result,"proxy" , p). - Perform !
CreateDataPropertyOrThrow (result,"revoke" , revoker). - Return result.
28.2.2.1.1 Proxy Revocation Functions
A Proxy revocation function is an anonymous built-in function that has the ability to invalidate a specific Proxy object.
Each Proxy revocation function has a [[RevocableProxy]] internal slot.
When a Proxy revocation function is called, the following steps are taken:
- Let F be the
active function object . - Let p be F.[[RevocableProxy]].
- If p is
null , returnundefined . - Set F.[[RevocableProxy]] to
null . Assert : p is a Proxy object.- Set p.[[ProxyTarget]] to
null . - Set p.[[ProxyHandler]] to
null . - Return
undefined .
The
28.3 Module Namespace Objects
A Module Namespace Object is a
In addition to the properties specified in
28.3.1 @@toStringTag
The initial value of the
This property has the attributes { [[Writable]]:
29 Memory Model
The memory consistency model, or memory model, specifies the possible orderings of
The memory model is defined as relational constraints on events introduced by
This section provides an axiomatic model on events introduced by the
29.1 Memory Model Fundamentals
Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses, defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events agreed upon by all agents in an
No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire, are supported.
A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory
| Field Name | Value | Meaning |
|---|---|---|
| [[Order]] | The weakest ordering guaranteed by the |
|
| [[NoTear]] | A Boolean | Whether this event is allowed to read from multiple write events on equal range as this event. |
| [[Block]] | A |
The block the event operates on. |
| [[ByteIndex]] | A non-negative |
The byte address of the read in [[Block]]. |
| [[ElementSize]] | A non-negative |
The size of the read. |
| Field Name | Value | Meaning |
|---|---|---|
| [[Order]] | The weakest ordering guaranteed by the |
|
| [[NoTear]] | A Boolean | Whether this event is allowed to be read from multiple read events with equal range as this event. |
| [[Block]] | A |
The block the event operates on. |
| [[ByteIndex]] | A non-negative |
The byte address of the write in [[Block]]. |
| [[ElementSize]] | A non-negative |
The size of the write. |
| [[Payload]] | A |
The |
| Field Name | Value | Meaning |
|---|---|---|
| [[Order]] | Read-modify-write events are always sequentially consistent. | |
| [[NoTear]] | Read-modify-write events cannot tear. | |
| [[Block]] | A |
The block the event operates on. |
| [[ByteIndex]] | A non-negative |
The byte address of the read-modify-write in [[Block]]. |
| [[ElementSize]] | A non-negative |
The size of the read-modify-write. |
| [[Payload]] | A |
The |
| [[ModifyOp]] | A |
An abstract closure that returns a modified |
These events are introduced by
Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to directly constrain the permitted orderings of other events.
In addition to
Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of contiguous integers from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping when the events have the same [[Block]], the ranges are not equal and their intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.
Examples of postMessage in a browser), starting and stopping agents, and communicating within the
Events are ordered within candidate executions by the relations defined below.
29.2 Agent Events Records
An Agent Events Record is a
| Field Name | Value | Meaning |
|---|---|---|
| [[AgentSignifier]] | A value that admits equality testing | The |
| [[EventList]] | A |
Events are appended to the list during evaluation. |
| [[AgentSynchronizesWith]] | A |
29.3 Chosen Value Records
A Chosen Value Record is a
| Field Name | Value | Meaning |
|---|---|---|
| [[Event]] | A |
The |
| [[ChosenValue]] | A |
The bytes that were nondeterministically chosen during evaluation. |
29.4 Candidate Executions
A candidate execution of the evaluation of an
| Field Name | Value | Meaning |
|---|---|---|
| [[EventsRecords]] | A |
Maps an |
| [[ChosenValues]] | A |
Maps |
| [[AgentOrder]] | An |
Defined below. |
| [[ReadsBytesFrom]] | A |
Defined below. |
| [[ReadsFrom]] | A |
Defined below. |
| [[HostSynchronizesWith]] | A |
Defined below. |
| [[SynchronizesWith]] | A |
Defined below. |
| [[HappensBefore]] | A |
Defined below. |
An empty candidate execution is a candidate execution
29.5 Abstract Operations for the Memory Model
29.5.1 EventSet ( execution )
The abstract operation EventSet takes argument execution (a
- Let events be an empty Set.
- For each
Agent Events Record aer of execution.[[EventsRecords]], do- For each event E of aer.[[EventList]], do
- Add E to events.
- For each event E of aer.[[EventList]], do
- Return events.
29.5.2 SharedDataBlockEventSet ( execution )
The abstract operation SharedDataBlockEventSet takes argument execution (a
- Let events be an empty Set.
- For each event E of
EventSet (execution), do- If E is a
ReadSharedMemory ,WriteSharedMemory , orReadModifyWriteSharedMemory event, add E to events.
- If E is a
- Return events.
29.5.3 HostEventSet ( execution )
The abstract operation HostEventSet takes argument execution (a
- Let events be an empty Set.
- For each event E of
EventSet (execution), do- If E is not in
SharedDataBlockEventSet (execution), add E to events.
- If E is not in
- Return events.
29.5.4 ComposeWriteEventBytes ( execution, byteIndex, Ws )
The abstract operation ComposeWriteEventBytes takes arguments execution (a
- Let byteLocation be byteIndex.
- Let bytesRead be a new empty
List . - For each element W of Ws, do
Assert : W has byteLocation in its range.- Let payloadIndex be byteLocation - W.[[ByteIndex]].
- If W is a
WriteSharedMemory event, then- Let byte be W.[[Payload]][payloadIndex].
- Else,
Assert : W is aReadModifyWriteSharedMemory event.- Let bytes be
ValueOfReadEvent (execution, W). - Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
- Let byte be bytesModified[payloadIndex].
- Append byte to bytesRead.
- Set byteLocation to byteLocation + 1.
- Return bytesRead.
The read-modify-write modification [[ModifyOp]] is given by the function properties on the Atomics object that introduce
This abstract operation composes a
29.5.5 ValueOfReadEvent ( execution, R )
The abstract operation ValueOfReadEvent takes arguments execution (a
Assert : R is aReadSharedMemory orReadModifyWriteSharedMemory event.- Let Ws be execution.[[ReadsBytesFrom]](R).
Assert : Ws is aList ofWriteSharedMemory orReadModifyWriteSharedMemory events with length equal to R.[[ElementSize]].- Return
ComposeWriteEventBytes (execution, R.[[ByteIndex]], Ws).
29.6 Relations of Candidate Executions
29.6.1 agent-order
For a
- For each pair (E, D) in
EventSet (execution), (E, D) is in execution.[[AgentOrder]] if there is someAgent Events Record aer in execution.[[EventsRecords]] such that E and D are in aer.[[EventList]] and E is before D inList order of aer.[[EventList]].
Each
29.6.2 reads-bytes-from
For a
-
For each
ReadSharedMemory orReadModifyWriteSharedMemory event R inSharedDataBlockEventSet (execution), execution.[[ReadsBytesFrom]](R) is aList of length R.[[ElementSize]] whose elements areWriteSharedMemory orReadModifyWriteSharedMemory events Ws such that all of the following are true.- Each event W with index i in Ws has R.[[ByteIndex]] + i in its range.
- R is not in Ws.
29.6.3 reads-from
For a
- For each pair (R, W) in
SharedDataBlockEventSet (execution), (R, W) is in execution.[[ReadsFrom]] if W is in execution.[[ReadsBytesFrom]](R).
29.6.4 host-synchronizes-with
For a
- If (E, D) is in execution.[[HostSynchronizesWith]], E and D are in
HostEventSet (execution). - There is no cycle in the union of execution.[[HostSynchronizesWith]] and execution.[[AgentOrder]].
For two
The host-synchronizes-with relation allows the postMessage between HTML workers.
29.6.5 synchronizes-with
For a
-
For each pair (R, W) in execution.[[ReadsFrom]], (W, R) is in execution.[[SynchronizesWith]] if R.[[Order]] is
SeqCst , W.[[Order]] isSeqCst , and R and W have equal ranges. -
For each element eventsRecord of execution.[[EventsRecords]], the following is true.
- For each pair (S, Sw) in eventsRecord.[[AgentSynchronizesWith]], (S, Sw) is in execution.[[SynchronizesWith]].
- For each pair (E, D) in execution.[[HostSynchronizesWith]], (E, D) is in execution.[[SynchronizesWith]].
Owing to convention, write events synchronizes-with read events, instead of read events synchronizes-with write events.
Not all
For
29.6.6 happens-before
For a
- For each pair (E, D) in execution.[[AgentOrder]], (E, D) is in execution.[[HappensBefore]].
- For each pair (E, D) in execution.[[SynchronizesWith]], (E, D) is in execution.[[HappensBefore]].
- For each pair (E, D) in
SharedDataBlockEventSet (execution), (E, D) is in execution.[[HappensBefore]] if E.[[Order]] isInit and E and D have overlapping ranges. - For each pair (E, D) in
EventSet (execution), (E, D) is in execution.[[HappensBefore]] if there is an event F such that the pairs (E, F) and (F, D) are in execution.[[HappensBefore]].
Because happens-before is a superset of
29.7 Properties of Valid Executions
29.7.1 Valid Chosen Reads
A
- For each
ReadSharedMemory orReadModifyWriteSharedMemory event R ofSharedDataBlockEventSet (execution), do- Let chosenValueRecord be the element of execution.[[ChosenValues]] whose [[Event]] field is R.
- Let chosenValue be chosenValueRecord.[[ChosenValue]].
- Let readValue be
ValueOfReadEvent (execution, R). - Let chosenLen be the number of elements of chosenValue.
- Let readLen be the number of elements of readValue.
- If chosenLen ≠ readLen, then
- Return
false .
- Return
- If chosenValue[i] ≠ readValue[i] for any
integer value i in the range 0 through chosenLen, exclusive, then- Return
false .
- Return
- Return
true .
29.7.2 Coherent Reads
A
- For each
ReadSharedMemory orReadModifyWriteSharedMemory event R ofSharedDataBlockEventSet (execution), do- Let Ws be execution.[[ReadsBytesFrom]](R).
- Let byteLocation be R.[[ByteIndex]].
- For each element W of Ws, do
- If (R, W) is in execution.[[HappensBefore]], then
- Return
false .
- Return
- If there is a
WriteSharedMemory orReadModifyWriteSharedMemory event V that has byteLocation in its range such that the pairs (W, V) and (V, R) are in execution.[[HappensBefore]], then- Return
false .
- Return
- Set byteLocation to byteLocation + 1.
- If (R, W) is in execution.[[HappensBefore]], then
- Return
true .
29.7.3 Tear Free Reads
A
- For each
ReadSharedMemory orReadModifyWriteSharedMemory event R ofSharedDataBlockEventSet (execution), do- If R.[[NoTear]] is
true , thenAssert : The remainder of dividing R.[[ByteIndex]] by R.[[ElementSize]] is 0.- For each event W such that (R, W) is in execution.[[ReadsFrom]] and W.[[NoTear]] is
true , do- If R and W have equal ranges, and there is an event V such that V and W have equal ranges, V.[[NoTear]] is
true , W is not V, and (R, V) is in execution.[[ReadsFrom]], then- Return
false .
- Return
- If R and W have equal ranges, and there is an event V such that V and W have equal ranges, V.[[NoTear]] is
- If R.[[NoTear]] is
- Return
true .
An event's [[NoTear]] field is
Intuitively, this requirement says when a memory range is accessed in an aligned fashion via an
29.7.4 Sequentially Consistent Atomics
For a
- For each pair (E, D) in execution.[[HappensBefore]], (E, D) is in memory-order.
-
For each pair (R, W) in execution.[[ReadsFrom]], there is no
WriteSharedMemory orReadModifyWriteSharedMemory event V inSharedDataBlockEventSet (execution) such that V.[[Order]] isSeqCst , the pairs (W, V) and (V, R) are in memory-order, and any of the following conditions are true.- The pair (W, R) is in execution.[[SynchronizesWith]], and V and R have equal ranges.
- The pairs (W, R) and (V, R) are in execution.[[HappensBefore]], W.[[Order]] is
SeqCst , and W and V have equal ranges. - The pairs (W, R) and (W, V) are in execution.[[HappensBefore]], R.[[Order]] is
SeqCst , and V and R have equal ranges.
Note 1 This clause additionally constrains
SeqCst events on equal ranges. -
For each
WriteSharedMemory orReadModifyWriteSharedMemory event W inSharedDataBlockEventSet (execution), if W.[[Order]] isSeqCst , then it is not the case that there is an infinite number ofReadSharedMemory orReadModifyWriteSharedMemory events inSharedDataBlockEventSet (execution) with equal range that is memory-order before W.Note 2 This clause together with the forward progress guarantee on agents ensure the liveness condition that
SeqCst writes become visible toSeqCst reads with equal range in finite time.
A
While memory-order includes all events in
29.7.5 Valid Executions
A
- The
host provides ahost-synchronizes-with Relation for execution.[[HostSynchronizesWith]]. - execution.[[HappensBefore]] is a
strict partial order . - execution has valid chosen reads.
- execution has coherent reads.
- execution has tear free reads.
- execution has sequentially consistent atomics.
All programs have at least one valid execution.
29.8 Races
For an execution execution, two events E and D in
- If E is not D, then
- If the pairs (E, D) and (D, E) are not in execution.[[HappensBefore]], then
- If E and D are both
WriteSharedMemory orReadModifyWriteSharedMemory events and E and D do not have disjoint ranges, then- Return
true .
- Return
- If either (E, D) or (D, E) is in execution.[[ReadsFrom]], then
- Return
true .
- Return
- If E and D are both
- If the pairs (E, D) and (D, E) are not in execution.[[HappensBefore]], then
- Return
false .
29.9 Data Races
For an execution execution, two events E and D in
- If E and D are in a race in execution, then
- If E.[[Order]] is not
SeqCst or D.[[Order]] is notSeqCst , then- Return
true .
- Return
- If E and D have overlapping ranges, then
- Return
true .
- Return
- If E.[[Order]] is not
- Return
false .
29.10 Data Race Freedom
An execution execution is data race free if there are no two events in
A program is data race free if all its executions are data race free.
The
29.11 Shared Memory Guidelines
The following are guidelines for ECMAScript programmers working with shared memory.
We recommend programs be kept data race free, i.e., make it so that it is impossible for there to be concurrent non-atomic operations on the same memory location. Data race free programs have interleaving semantics where each step in the evaluation semantics of each
More generally, even if a program is not data race free it may have predictable behaviour, so long as atomic operations are not involved in any data races and the operations that race all have the same access size. The simplest way to arrange for atomics not to be involved in races is to ensure that different memory cells are used by atomic and non-atomic operations and that atomic accesses of different sizes are not used to access the same cells at the same time. Effectively, the program should treat shared memory as strongly typed as much as possible. One still cannot depend on the ordering and timing of non-atomic accesses that race, but if memory is treated as strongly typed the racing accesses will not "tear" (bits of their values will not be mixed).
The following are guidelines for ECMAScript implementers writing compiler transformations for programs using shared memory.
It is desirable to allow most program transformations that are valid in a single-
Let an agent-order slice be the subset of the
Let possible read values of a read event be the set of all values of
Any transformation of an agent-order slice that is valid in the absence of shared memory is valid in the presence of shared memory, with the following exceptions.
-
Atomics are carved in stone: Program transformations must not cause the
SeqCst events in an agent-order slice to be reordered with itsUnordered operations, nor itsSeqCst operations to be reordered with each other, nor may a program transformation remove aSeqCst operation from theagent-order .(In practice, the prohibition on reorderings forces a compiler to assume that every
SeqCst operation is a synchronization and included in the finalmemory-order , which it would usually have to assume anyway in the absence of inter-agent program analysis. It also forces the compiler to assume that every call where the callee's effects on thememory-order are unknown may containSeqCst operations.) -
Reads must be stable: Any given shared memory read must only observe a single value in an execution.
(For example, if what is semantically a single read in the program is executed multiple times then the program is subsequently allowed to observe only one of the values read. A transformation known as rematerialization can violate this rule.)
-
Writes must be stable: All observable writes to shared memory must follow from program semantics in an execution.
(For example, a transformation may not introduce certain observable writes, such as by using read-modify-write operations on a larger location to write a smaller datum, writing a value to memory that the program could not have written, or writing a just-read value back to the location it was read from, if that location could have been overwritten by another
agent after the read.) -
Possible read values must be nonempty: Program transformations cannot cause the possible read values of a shared memory read to become empty.
(Counterintuitively, this rule in effect restricts transformations on writes, because writes have force in
memory model insofar as to be read by read events. For example, writes may be moved and coalesced and sometimes reordered between twoSeqCst operations, but the transformation may not remove every write that updates a location; some write must be preserved.)
Examples of transformations that remain valid are: merging multiple non-atomic reads from the same location, reordering non-atomic reads, introducing speculative non-atomic reads, merging multiple non-atomic writes to the same location, reordering non-atomic writes to different locations, and hoisting non-atomic reads out of loops even if that affects termination. Note in general that aliased TypedArrays make it hard to prove that locations are different.
The following are guidelines for ECMAScript implementers generating machine code for shared memory accesses.
For architectures with memory models no weaker than those of ARM or Power, non-atomic stores and loads may be compiled to bare stores and loads on the target architecture. Atomic stores and loads may be compiled down to instructions that guarantee sequential consistency. If no such instructions exist, memory barriers are to be employed, such as placing barriers on both sides of a bare store or load. Read-modify-write operations may be compiled to read-modify-write instructions on the target architecture, such as LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions on ARM, and load-link/store-conditional instructions on Power.
Specifically, the
- Every atomic operation in the program is assumed to be necessary.
- Atomic operations are never rearranged with each other or with non-atomic operations.
- Functions are always assumed to perform atomic operations.
- Atomic operations are never implemented as read-modify-write operations on larger data, but as non-lock-free atomics if the platform does not have atomic operations of the appropriate size. (We already assume that every platform has normal memory access operations of every interesting size.)
Naive code generation uses these patterns:
- Regular loads and stores compile to single load and store instructions.
- Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or store, and a full fence.
- Lock-free atomic read-modify-write accesses compile to a full fence, an atomic read-modify-write instruction sequence, and a full fence.
- Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of non-atomic load and store instructions, a full fence, and a spinlock release.
That mapping is correct so long as an atomic operation on an address range does not race with a non-atomic write or with an atomic operation of different size. However, that is all we need: the
A number of local improvements to those basic patterns are also intended to be legal:
- There are obvious platform-dependent improvements that remove redundant fences. For example, on x86 the fences around lock-free atomic loads and stores can always be omitted except for the fence following a store, and no fence is needed for lock-free read-modify-write instructions, as these all use LOCK-prefixed instructions. On many platforms there are fences of several strengths, and weaker fences can be used in certain contexts without destroying sequential consistency.
- Most modern platforms support lock-free atomics for all the data sizes required by ECMAScript atomics. Should non-lock-free atomics be needed, the fences surrounding the body of the atomic operation can usually be folded into the lock and unlock steps. The simplest solution for non-lock-free atomics is to have a single lock word per SharedArrayBuffer.
- There are also more complicated platform-dependent local improvements, requiring some code analysis. For example, two back-to-back fences often have the same effect as a single fence, so if code is generated for two atomic operations in sequence, only a single fence need separate them. On x86, even a single fence separating atomic stores can be omitted, as the fence following a store is only needed to separate the store from a subsequent load.
A Grammar Summary
A.1 Lexical Grammar
A.2 Expressions
When processing an instance of the production
the interpretation of
When processing an instance of the production
the interpretation of
In certain circumstances when processing an instance of the production
the interpretation of
A.3 Statements
A.4 Functions and Classes
When processing an instance of the production
the interpretation of
When processing an instance of the production
the interpretation of
A.5 Scripts and Modules
A.6 Number Conversions
All grammar symbols not explicitly defined by the
A.7 Universal Resource Identifier Character Classes
A.8 Regular Expressions
Each \u u u \u
B Additional ECMAScript Features for Web Browsers
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript
This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.
These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.
B.1 Additional Syntax
B.1.1 Numeric Literals
The syntax and semantics of
Syntax
B.1.1.1 Static Semantics
-
The MV of
LegacyOctalIntegerLiteral :: 0 OctalDigit OctalDigit . -
The MV of
LegacyOctalIntegerLiteral :: LegacyOctalIntegerLiteral OctalDigit LegacyOctalIntegerLiteral times 8) plus the MV ofOctalDigit . -
The MV of
DecimalIntegerLiteral :: NonOctalDecimalIntegerLiteral NonOctalDecimalIntegerLiteral . -
The MV of
NonOctalDecimalIntegerLiteral :: 0 NonOctalDigit NonOctalDigit . -
The MV of
NonOctalDecimalIntegerLiteral :: LegacyOctalLikeDecimalIntegerLiteral NonOctalDigit LegacyOctalLikeDecimalIntegerLiteral times 10) plus the MV ofNonOctalDigit . -
The MV of
NonOctalDecimalIntegerLiteral :: NonOctalDecimalIntegerLiteral DecimalDigit NonOctalDecimalIntegerLiteral times 10) plus the MV ofDecimalDigit . -
The MV of
LegacyOctalLikeDecimalIntegerLiteral :: 0 OctalDigit OctalDigit . -
The MV of
LegacyOctalLikeDecimalIntegerLiteral :: LegacyOctalLikeDecimalIntegerLiteral OctalDigit LegacyOctalLikeDecimalIntegerLiteral times 10) plus the MV ofOctalDigit . -
The MV of
NonOctalDigit :: 8 -
The MV of
NonOctalDigit :: 9
B.1.2 String Literals
The syntax and semantics of
Syntax
This definition of
It is possible for string literals to precede a
function invalid() { "\7"; "use strict"; }B.1.2.1 Static Semantics
-
The
SV ofEscapeSequence :: LegacyOctalEscapeSequence LegacyOctalEscapeSequence . -
The MV of
LegacyOctalEscapeSequence :: ZeroToThree OctalDigit ZeroToThree ) plus the MV ofOctalDigit . -
The MV of
LegacyOctalEscapeSequence :: FourToSeven OctalDigit FourToSeven ) plus the MV ofOctalDigit . -
The MV of
LegacyOctalEscapeSequence :: ZeroToThree OctalDigit OctalDigit ZeroToThree ) plus (8 times the MV of the firstOctalDigit ) plus the MV of the secondOctalDigit . -
The
SV ofNonOctalDecimalEscapeSequence :: 8 -
The
SV ofNonOctalDecimalEscapeSequence :: 9 -
The MV of
ZeroToThree :: 0 -
The MV of
ZeroToThree :: 1 -
The MV of
ZeroToThree :: 2 -
The MV of
ZeroToThree :: 3 -
The MV of
FourToSeven :: 4 -
The MV of
FourToSeven :: 5 -
The MV of
FourToSeven :: 6 -
The MV of
FourToSeven :: 7
B.1.3 HTML-like Comments
The syntax and semantics of
Syntax
Similar to a
B.1.4 Regular Expressions Patterns
The syntax of
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [U] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [U] parameter present on the
Syntax
When the same left hand sides occurs with both [+U] and [~U] guards it is to control the disambiguation priority.
B.1.4.1 Static Semantics: Early Errors
The semantics of
- It is a Syntax Error if any source text matches this rule.
Additionally, the rules for the following productions are modified with the addition of the highlighted text:
-
It is a Syntax Error if
IsCharacterClass of the firstClassAtom istrue orIsCharacterClass of the secondClassAtom istrue and this production has a [U] parameter. -
It is a Syntax Error if
IsCharacterClass of the firstClassAtom isfalse andIsCharacterClass of the secondClassAtom isfalse and theCharacterValue of the firstClassAtom is larger than theCharacterValue of the secondClassAtom .
-
It is a Syntax Error if
IsCharacterClass ofClassAtomNoDash istrue orIsCharacterClass ofClassAtom istrue and this production has a [U] parameter. -
It is a Syntax Error if
IsCharacterClass ofClassAtomNoDash isfalse andIsCharacterClass ofClassAtom isfalse and theCharacterValue ofClassAtomNoDash is larger than theCharacterValue ofClassAtom .
B.1.4.2 Static Semantics: IsCharacterClass
The semantics of
- Return
false .
B.1.4.3 Static Semantics: CharacterValue
The semantics of
- Return the code point value of U+005C (REVERSE SOLIDUS).
- Let ch be the code point matched by
ClassControlLetter . - Let i be ch's code point value.
- Return the remainder of dividing i by 32.
- Return the MV of
LegacyOctalEscapeSequence (seeB.1.2 ).
B.1.4.4 Pattern Semantics
The semantics of
Within
Term (
The production
The production
The production
Assertion (
The production
- Evaluate
QuantifiableAssertion to obtain a Matcher m. - Return m.
Assertion (
Atom (
The production
- Let A be the CharSet containing the single character
\U+005C (REVERSE SOLIDUS). - Return !
CharacterSetMatcher (A,false , direction).
The production
- Let ch be the character represented by
ExtendedPatternCharacter . - Let A be a one-element CharSet containing the character ch.
- Return !
CharacterSetMatcher (A,false , direction).
CharacterEscape (
The production
- Let cv be the
CharacterValue of thisCharacterEscape . - Return the character whose character value is cv.
NonemptyClassRanges (
The production
- Evaluate the first
ClassAtom to obtain a CharSet A. - Evaluate the second
ClassAtom to obtain a CharSet B. - Evaluate
ClassRanges to obtain a CharSet C. - Let D be !
CharacterRangeOrUnion (A, B). - Return the union of D and C.
NonemptyClassRangesNoDash (
The production
- Evaluate
ClassAtomNoDash to obtain a CharSet A. - Evaluate
ClassAtom to obtain a CharSet B. - Evaluate
ClassRanges to obtain a CharSet C. - Let D be !
CharacterRangeOrUnion (A, B). - Return the union of D and C.
ClassEscape (
The production
- Let cv be the
CharacterValue of thisClassEscape . - Let c be the character whose character value is cv.
- Return the CharSet containing the single character c.
ClassAtomNoDash (
The production
- Return the CharSet containing the single character
\U+005C (REVERSE SOLIDUS).
\c within a character class where it is not followed by an acceptable control character.B.1.4.4.1 CharacterRangeOrUnion ( A, B )
The abstract operation CharacterRangeOrUnion takes arguments A (a CharSet) and B (a CharSet). It performs the following steps when called:
- If Unicode is
false , then- If A does not contain exactly one character or B does not contain exactly one character, then
- Let C be the CharSet containing the single character
-U+002D (HYPHEN-MINUS). - Return the union of CharSets A, B and C.
- Let C be the CharSet containing the single character
- If A does not contain exactly one character or B does not contain exactly one character, then
- Return !
CharacterRange (A, B).
B.2 Additional Built-in Properties
When the ECMAScript
B.2.1 Additional Properties of the Global Object
The entries in
| Intrinsic Name | Global Name | ECMAScript Language Association |
|---|---|---|
|
|
escape
|
The escape function ( |
|
|
unescape
|
The unescape function ( |
B.2.1.1 escape ( string )
The escape function is a property of the
For those code units being replaced whose value is 0x00FF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose code unit value is greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.
The escape function is the %escape% intrinsic object. When the escape function is called with one argument string, the following steps are taken:
- Set string to ?
ToString (string). - Let length be the number of code units in string.
- Let R be the empty String.
- Let k be 0.
- Repeat, while k < length,
- Let char be the code unit (represented as a 16-bit unsigned
integer ) at index k within string. - If char is one of the code units in
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./" , then- Let S be the String value containing the single code unit char.
- Else if char ≥ 256, then
- Let n be the numeric value of char.
- Let S be the
string-concatenation of:"%u" - the String representation of n, formatted as a four-digit uppercase hexadecimal number, padded to the left with zeroes if necessary
- Else,
Assert : char < 256.- Let n be the numeric value of char.
- Let S be the
string-concatenation of:"%" - the String representation of n, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
- Set R to the
string-concatenation of R and S. - Set k to k + 1.
- Let char be the code unit (represented as a 16-bit unsigned
- Return R.
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.
B.2.1.2 unescape ( string )
The unescape function is a property of the escape function is replaced with the code unit that it represents.
The unescape function is the %unescape% intrinsic object. When the unescape function is called with one argument string, the following steps are taken:
- Set string to ?
ToString (string). - Let length be the number of code units in string.
- Let R be the empty String.
- Let k be 0.
- Repeat, while k ≠ length,
- Let c be the code unit at index k within string.
- If c is the code unit 0x0025 (PERCENT SIGN), then
- Let hexEscape be the empty String.
- Let skip be 0.
- If k ≤ length - 6 and the code unit at index k + 1 within string is the code unit 0x0075 (LATIN SMALL LETTER U), then
- Set hexEscape to the
substring of string from k + 2 to k + 6. - Set skip to 5.
- Set hexEscape to the
- Else if k ≤ length - 3, then
- Set hexEscape to the
substring of string from k + 1 to k + 3. - Set skip to 2.
- Set hexEscape to the
- If hexEscape can be interpreted as an expansion of
HexDigits , then[~Sep] - Let hexIntegerLiteral be the
string-concatenation of"0x" and hexEscape. - Let n be !
ToNumber (hexIntegerLiteral). - Set c to the code unit whose value is
ℝ (n). - Set k to k + skip.
- Let hexIntegerLiteral be the
- Set R to the
string-concatenation of R and c. - Set k to k + 1.
- Return R.
B.2.2 Additional Properties of the Object.prototype Object
B.2.2.1 Object.prototype.__proto__
Object.prototype.__proto__ is an
B.2.2.1.1 get Object.prototype.__proto__
The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:
- Let O be ?
ToObject (this value). - Return ? O.[[GetPrototypeOf]]().
B.2.2.1.2 set Object.prototype.__proto__
The value of the [[Set]] attribute is a built-in function that takes an argument proto. It performs the following steps when called:
- Let O be ?
RequireObjectCoercible (this value). - If
Type (proto) is neither Object nor Null, returnundefined . - If
Type (O) is not Object, returnundefined . - Let status be ? O.[[SetPrototypeOf]](proto).
- If status is
false , throw aTypeError exception. - Return
undefined .
B.2.2.2 Object.prototype.__defineGetter__ ( P, getter )
When the __defineGetter__ method is called with arguments P and getter, the following steps are taken:
- Let O be ?
ToObject (this value). - If
IsCallable (getter) isfalse , throw aTypeError exception. - Let desc be PropertyDescriptor { [[Get]]: getter, [[Enumerable]]:
true , [[Configurable]]:true }. - Let key be ?
ToPropertyKey (P). - Perform ?
DefinePropertyOrThrow (O, key, desc). - Return
undefined .
B.2.2.3 Object.prototype.__defineSetter__ ( P, setter )
When the __defineSetter__ method is called with arguments P and setter, the following steps are taken:
- Let O be ?
ToObject (this value). - If
IsCallable (setter) isfalse , throw aTypeError exception. - Let desc be PropertyDescriptor { [[Set]]: setter, [[Enumerable]]:
true , [[Configurable]]:true }. - Let key be ?
ToPropertyKey (P). - Perform ?
DefinePropertyOrThrow (O, key, desc). - Return
undefined .
B.2.2.4 Object.prototype.__lookupGetter__ ( P )
When the __lookupGetter__ method is called with argument P, the following steps are taken:
- Let O be ?
ToObject (this value). - Let key be ?
ToPropertyKey (P). - Repeat,
- Let desc be ? O.[[GetOwnProperty]](key).
- If desc is not
undefined , then- If
IsAccessorDescriptor (desc) istrue , return desc.[[Get]]. - Return
undefined .
- If
- Set O to ? O.[[GetPrototypeOf]]().
- If O is
null , returnundefined .
B.2.2.5 Object.prototype.__lookupSetter__ ( P )
When the __lookupSetter__ method is called with argument P, the following steps are taken:
- Let O be ?
ToObject (this value). - Let key be ?
ToPropertyKey (P). - Repeat,
- Let desc be ? O.[[GetOwnProperty]](key).
- If desc is not
undefined , then- If
IsAccessorDescriptor (desc) istrue , return desc.[[Set]]. - Return
undefined .
- If
- Set O to ? O.[[GetPrototypeOf]]().
- If O is
null , returnundefined .
B.2.3 Additional Properties of the String.prototype Object
B.2.3.1 String.prototype.substr ( start, length )
The substr method takes two arguments, start and length, and returns a
- Let O be ?
RequireObjectCoercible (this value). - Let S be ?
ToString (O). - Let size be the length of S.
- Let intStart be ?
ToIntegerOrInfinity (start). - If intStart is -∞, set intStart to 0.
- Else if intStart < 0, set intStart to
max (size + intStart, 0). - If length is
undefined , let intLength be size; otherwise let intLength be ?ToIntegerOrInfinity (length). - If intStart is +∞, intLength ≤ 0, or intLength is +∞, return the empty String.
- Let intEnd be
min (intStart + intLength, size). - If intStart ≥ intEnd, return the empty String.
- Return the
substring of S from intStart to intEnd.
The substr function is intentionally generic; it does not require that its
B.2.3.2 String.prototype.anchor ( name )
When the anchor method is called with argument name, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"a" ,"name" , name).
B.2.3.2.1 CreateHTML ( string, tag, attribute, value )
The abstract operation CreateHTML takes arguments string, tag (a String), attribute (a String), and value. It performs the following steps when called:
- Let str be ?
RequireObjectCoercible (string). - Let S be ?
ToString (str). - Let p1 be the
string-concatenation of"<" and tag. - If attribute is not the empty String, then
- Let V be ?
ToString (value). - Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence
""" . - Set p1 to the
string-concatenation of:- p1
- the code unit 0x0020 (SPACE)
- attribute
- the code unit 0x003D (EQUALS SIGN)
- the code unit 0x0022 (QUOTATION MARK)
- escapedV
- the code unit 0x0022 (QUOTATION MARK)
- Let V be ?
- Let p2 be the
string-concatenation of p1 and">" . - Let p3 be the
string-concatenation of p2 and S. - Let p4 be the
string-concatenation of p3,"</" , tag, and">" . - Return p4.
B.2.3.3 String.prototype.big ( )
When the big method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"big" ,"" ,"" ).
B.2.3.4 String.prototype.blink ( )
When the blink method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"blink" ,"" ,"" ).
B.2.3.5 String.prototype.bold ( )
When the bold method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"b" ,"" ,"" ).
B.2.3.6 String.prototype.fixed ( )
When the fixed method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"tt" ,"" ,"" ).
B.2.3.7 String.prototype.fontcolor ( color )
When the fontcolor method is called with argument color, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"font" ,"color" , color).
B.2.3.8 String.prototype.fontsize ( size )
When the fontsize method is called with argument size, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"font" ,"size" , size).
B.2.3.9 String.prototype.italics ( )
When the italics method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"i" ,"" ,"" ).
B.2.3.10 String.prototype.link ( url )
When the link method is called with argument url, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"a" ,"href" , url).
B.2.3.11 String.prototype.small ( )
When the small method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"small" ,"" ,"" ).
B.2.3.12 String.prototype.strike ( )
When the strike method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"strike" ,"" ,"" ).
B.2.3.13 String.prototype.sub ( )
When the sub method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"sub" ,"" ,"" ).
B.2.3.14 String.prototype.sup ( )
When the sup method is called with no arguments, the following steps are taken:
- Let S be the
this value. - Return ?
CreateHTML (S,"sup" ,"" ,"" ).
B.2.3.15 String.prototype.trimLeft ( )
The property
The initial value of the String.prototype.trimStart property.
B.2.3.16 String.prototype.trimRight ( )
The property
The initial value of the String.prototype.trimEnd property.
B.2.4 Additional Properties of the Date.prototype Object
B.2.4.1 Date.prototype.getYear ( )
The getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”
When the getYear method is called with no arguments, the following steps are taken:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , returnNaN . - Return
YearFromTime (LocalTime (t)) -1900 𝔽.
B.2.4.2 Date.prototype.setYear ( year )
The setFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”
When the setYear method is called with one argument year, the following steps are taken:
- Let t be ?
thisTimeValue (this value). - If t is
NaN , set t to+0 𝔽; otherwise, set t toLocalTime (t). - Let y be ?
ToNumber (year). - If y is
NaN , then- Set the [[DateValue]] internal slot of
this Date object toNaN . - Return
NaN .
- Set the [[DateValue]] internal slot of
- Let yi be !
ToIntegerOrInfinity (y). - If 0 ≤ yi ≤ 99, let yyyy be
1900 𝔽 +𝔽 (yi). - Else, let yyyy be y.
- Let d be
MakeDay (yyyy,MonthFromTime (t),DateFromTime (t)). - Let date be
UTC (MakeDate (d,TimeWithinDay (t))). - Set the [[DateValue]] internal slot of
this Date object toTimeClip (date). - Return the value of the [[DateValue]] internal slot of
this Date object .
B.2.4.3 Date.prototype.toGMTString ( )
The toUTCString method is preferred. The toGMTString method is provided principally for compatibility with old code.
The Date.prototype.toGMTString is the same Date.prototype.toUTCString.
B.2.5 Additional Properties of the RegExp.prototype Object
B.2.5.1 RegExp.prototype.compile ( pattern, flags )
When the compile method is called with arguments pattern and flags, the following steps are taken:
- Let O be the
this value. - Perform ?
RequireInternalSlot (O, [[RegExpMatcher]]). - If
Type (pattern) is Object and pattern has a [[RegExpMatcher]] internal slot, then- If flags is not
undefined , throw aTypeError exception. - Let P be pattern.[[OriginalSource]].
- Let F be pattern.[[OriginalFlags]].
- If flags is not
- Else,
- Let P be pattern.
- Let F be flags.
- Return ?
RegExpInitialize (O, P, F).
The compile method completely reinitializes the
B.3 Other Additional Features
B.3.1 __proto__ Property Names in Object Initializers
The following Early Error rule is added to those in
- when
ObjectLiteral appears in a context whereObjectAssignmentPattern is required, - when initially parsing a
CoverParenthesizedExpressionAndArrowParameterList or aCoverCallExpressionAndAsyncArrowHead , or - when parsing text for
JSON.parse .
-
It is a Syntax Error if
PropertyNameList ofPropertyDefinitionList contains any duplicate entries for"__proto__" and at least two of those entries were obtained from productions of the formPropertyDefinition : PropertyName : AssignmentExpression
The
In
is replaced with the following definition:
- Let propKey be the result of evaluating
PropertyName . ReturnIfAbrupt (propKey).- If propKey is the String value
"__proto__" and ifIsComputedPropertyKey (PropertyName ) isfalse , then- Let isProtoSetter be
true .
- Let isProtoSetter be
- Else,
- Let isProtoSetter be
false .
- Let isProtoSetter be
- If
IsAnonymousFunctionDefinition (AssignmentExpression ) istrue and isProtoSetter isfalse , then- Let propValue be ?
NamedEvaluation ofAssignmentExpression with argument propKey.
- Let propValue be ?
- Else,
- Let exprValueRef be the result of evaluating
AssignmentExpression . - Let propValue be ?
GetValue (exprValueRef).
- Let exprValueRef be the result of evaluating
- If isProtoSetter is
true , then- If
Type (propValue) is either Object or Null, then- Return object.[[SetPrototypeOf]](propValue).
- Return
NormalCompletion (empty ).
- If
Assert : enumerable istrue .Assert : object is an ordinary, extensible object with no non-configurable properties.- Return !
CreateDataPropertyOrThrow (object, propKey, propValue).
B.3.2 Labelled Function Declarations
Prior to ECMAScript 2015, the specification of
- It is a Syntax Error if any strict mode source code matches this rule.
The
B.3.3 Block-Level Function Declarations Web Legacy Compatibility Semantics
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a
-
A function is declared and only referenced within a single block
-
One or more
FunctionDeclaration s whoseBindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within aBlock . -
No other declaration of f that is not a
vardeclaration occurs within the function code of g -
All occurrences of f as an
IdentifierReference are within theStatementList of theBlock containing the declaration of f.
-
One or more
-
A function is declared and possibly used within a single
Block but also referenced by an inner function definition that is not contained within that sameBlock .-
One or more
FunctionDeclaration s whoseBindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within aBlock . -
No other declaration of f that is not a
vardeclaration occurs within the function code of g -
There may be occurrences of f as an
IdentifierReference within theStatementList of theBlock containing the declaration of f. -
There is at least one occurrence of f as an
IdentifierReference within another function h that is nested within g and no other declaration of f shadows the references to f from within h. - All invocations of h occur after the declaration of f has been evaluated.
-
One or more
-
A function is declared and possibly used within a single block but also referenced within subsequent blocks.
-
One or more
FunctionDeclaration whoseBindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within aBlock . -
No other declaration of f that is not a
vardeclaration occurs within the function code of g -
There may be occurrences of f as an
IdentifierReference within theStatementList of theBlock containing the declaration of f. -
There is at least one occurrence of f as an
IdentifierReference within the function code of g that lexically follows theBlock containing the declaration of f.
-
One or more
The first use case is interoperable with the semantics of
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains a
B.3.3.1 Changes to FunctionDeclarationInstantiation
During
- If strict is
false , then- For each
FunctionDeclaration f that is directly contained in theStatementList of aBlock ,CaseClause , orDefaultClause , do- Let F be
StringValue of theBindingIdentifier of f. - If replacing the
FunctionDeclaration f with aVariableStatement that has F as aBindingIdentifier would not produce any Early Errors for func and F is not an element of parameterNames, then- NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or another
FunctionDeclaration . - If initializedBindings does not contain F and F is not
"arguments" , then- Perform ! varEnv.CreateMutableBinding(F,
false ). - Perform varEnv.InitializeBinding(F,
undefined ). - Append F to instantiatedVarNames.
- Perform ! varEnv.CreateMutableBinding(F,
- When the
FunctionDeclaration f is evaluated, perform the following steps in place of theFunctionDeclaration Evaluation algorithm provided in15.2.6 :- Let fenv be the
running execution context 's VariableEnvironment. - Let benv be the
running execution context 's LexicalEnvironment. - Let fobj be ! benv.GetBindingValue(F,
false ). - Perform ! fenv.SetMutableBinding(F, fobj,
false ). - Return
NormalCompletion (empty ).
- Let fenv be the
- NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or another
- Let F be
- For each
B.3.3.2 Changes to GlobalDeclarationInstantiation
During
- Let strict be
IsStrict of script. - If strict is
false , then- Let declaredFunctionOrVarNames be a new empty
List . - Append to declaredFunctionOrVarNames the elements of declaredFunctionNames.
- Append to declaredFunctionOrVarNames the elements of declaredVarNames.
- For each
FunctionDeclaration f that is directly contained in theStatementList of aBlock ,CaseClause , orDefaultClause Contained within script, do- Let F be
StringValue of theBindingIdentifier of f. - If replacing the
FunctionDeclaration f with aVariableStatement that has F as aBindingIdentifier would not produce any Early Errors for script, then- If env.HasLexicalDeclaration(F) is
false , then- Let fnDefinable be ? env.CanDeclareGlobalVar(F).
- If fnDefinable is
true , then- NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName nor the name of another
FunctionDeclaration . - If declaredFunctionOrVarNames does not contain F, then
- Perform ? env.CreateGlobalVarBinding(F,
false ). - Append F to declaredFunctionOrVarNames.
- Perform ? env.CreateGlobalVarBinding(F,
- When the
FunctionDeclaration f is evaluated, perform the following steps in place of theFunctionDeclaration Evaluation algorithm provided in15.2.6 :- Let genv be the
running execution context 's VariableEnvironment. - Let benv be the
running execution context 's LexicalEnvironment. - Let fobj be ! benv.GetBindingValue(F,
false ). - Perform ? genv.SetMutableBinding(F, fobj,
false ). - Return
NormalCompletion (empty ).
- Let genv be the
- NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName nor the name of another
- If env.HasLexicalDeclaration(F) is
- Let F be
- Let declaredFunctionOrVarNames be a new empty
B.3.3.3 Changes to EvalDeclarationInstantiation
During
- If strict is
false , then- Let declaredFunctionOrVarNames be a new empty
List . - Append to declaredFunctionOrVarNames the elements of declaredFunctionNames.
- Append to declaredFunctionOrVarNames the elements of declaredVarNames.
- For each
FunctionDeclaration f that is directly contained in theStatementList of aBlock ,CaseClause , orDefaultClause Contained within body, do- Let F be
StringValue of theBindingIdentifier of f. - If replacing the
FunctionDeclaration f with aVariableStatement that has F as aBindingIdentifier would not produce any Early Errors for body, then- Let bindingExists be
false . - Let thisEnv be lexEnv.
Assert : The following loop will terminate.- Repeat, while thisEnv is not the same as varEnv,
- If thisEnv is not an
object Environment Record , then- If thisEnv.HasBinding(F) is
true , then- Let bindingExists be
true .
- Let bindingExists be
- If thisEnv.HasBinding(F) is
- Set thisEnv to thisEnv.[[OuterEnv]].
- If thisEnv is not an
- If bindingExists is
false and varEnv is aglobal Environment Record , then- If varEnv.HasLexicalDeclaration(F) is
false , then- Let fnDefinable be ? varEnv.CanDeclareGlobalVar(F).
- Else,
- Let fnDefinable be
false .
- Let fnDefinable be
- If varEnv.HasLexicalDeclaration(F) is
- Else,
- Let fnDefinable be
true .
- Let fnDefinable be
- If bindingExists is
false and fnDefinable istrue , then- If declaredFunctionOrVarNames does not contain F, then
- If varEnv is a
global Environment Record , then- Perform ? varEnv.CreateGlobalVarBinding(F,
true ).
- Perform ? varEnv.CreateGlobalVarBinding(F,
- Else,
- Let bindingExists be varEnv.HasBinding(F).
- If bindingExists is
false , then- Perform ! varEnv.CreateMutableBinding(F,
true ). - Perform ! varEnv.InitializeBinding(F,
undefined ).
- Perform ! varEnv.CreateMutableBinding(F,
- Append F to declaredFunctionOrVarNames.
- If varEnv is a
- When the
FunctionDeclaration f is evaluated, perform the following steps in place of theFunctionDeclaration Evaluation algorithm provided in15.2.6 :- Let genv be the
running execution context 's VariableEnvironment. - Let benv be the
running execution context 's LexicalEnvironment. - Let fobj be ! benv.GetBindingValue(F,
false ). - Perform ? genv.SetMutableBinding(F, fobj,
false ). - Return
NormalCompletion (empty ).
- Let genv be the
- If declaredFunctionOrVarNames does not contain F, then
- Let bindingExists be
- Let F be
- Let declaredFunctionOrVarNames be a new empty
B.3.3.4 Changes to Block Static Semantics: Early Errors
The rules for the following production in
-
It is a Syntax Error if the
LexicallyDeclaredNames ofStatementList contains any duplicate entries, unless the source code matching this production is notstrict mode code and the duplicate entries are only bound by FunctionDeclarations. -
It is a Syntax Error if any element of the
LexicallyDeclaredNames ofStatementList also occurs in theVarDeclaredNames ofStatementList .
B.3.3.5 Changes to switch Statement Static Semantics: Early Errors
The rules for the following production in
-
It is a Syntax Error if the
LexicallyDeclaredNames ofCaseBlock contains any duplicate entries, unless the source code matching this production is notstrict mode code and the duplicate entries are only bound by FunctionDeclarations. -
It is a Syntax Error if any element of the
LexicallyDeclaredNames ofCaseBlock also occurs in theVarDeclaredNames ofCaseBlock .
B.3.3.6 Changes to BlockDeclarationInstantiation
During
- If env.HasBinding(dn) is
false , then- Perform ! env.CreateMutableBinding(dn,
false ).
- Perform ! env.CreateMutableBinding(dn,
During
- If the binding for fn in env is an uninitialized binding, then
- Perform env.InitializeBinding(fn, fo).
- Else,
Assert : d is aFunctionDeclaration .- Perform env.SetMutableBinding(fn, fo,
false ).
B.3.4 FunctionDeclarations in IfStatement Statement Clauses
The following augments the
This production only applies when parsing
B.3.5 VariableStatements in Catch Blocks
The content of subclause
-
It is a Syntax Error if
BoundNames ofCatchParameter contains any duplicate elements. -
It is a Syntax Error if any element of the
BoundNames ofCatchParameter also occurs in theLexicallyDeclaredNames ofBlock . -
It is a Syntax Error if any element of the
BoundNames ofCatchParameter also occurs in theVarDeclaredNames ofBlock unlessCatchParameter isCatchParameter : BindingIdentifier
The var declarations that bind a name that is also bound by the var declarations will assign to the corresponding catch parameter rather than the var binding.
This modified behaviour also applies to var and function declarations introduced by
Step
- If thisEnv is not the
Environment Record for aCatch clause, throw aSyntaxError exception.
Step
- If thisEnv is not the
Environment Record for aCatch clause, let bindingExists betrue .
B.3.6 Initializers in ForIn Statement Heads
The following augments the
This production only applies when parsing
The
- Return
ContainsDuplicateLabels ofStatement with argument labelSet.
The
- Return
ContainsUndefinedBreakTarget ofStatement with argument labelSet.
The
- Return
ContainsUndefinedContinueTarget ofStatement with arguments iterationSet and « ».
The
- Return
false .
The
- Let names be the
BoundNames ofBindingIdentifier . - Append to names the elements of the
VarDeclaredNames ofStatement . - Return names.
The
- Let declarations be a
List whose sole element isBindingIdentifier . - Append to declarations the elements of the
VarScopedDeclarations ofStatement . - Return declarations.
The
- Let bindingId be
StringValue ofBindingIdentifier . - Let lhs be ?
ResolveBinding (bindingId). - If
IsAnonymousFunctionDefinition (Initializer ) istrue , then- Let value be
NamedEvaluation ofInitializer with argument bindingId.
- Let value be
- Else,
- Let rhs be the result of evaluating
Initializer . - Let value be ?
GetValue (rhs).
- Let rhs be the result of evaluating
- Perform ?
PutValue (lhs, value). - Let keyResult be ?
ForIn/OfHeadEvaluation (« »,Expression ,enumerate ). - Return ?
ForIn/OfBodyEvaluation (BindingIdentifier ,Statement , keyResult,enumerate ,varBinding , labelSet).
B.3.7 The [[IsHTMLDDA]] Internal Slot
An [[IsHTMLDDA]] internal slot may exist on typeof operator
Objects with an [[IsHTMLDDA]] internal slot are never created by this specification. However, the document.all object in web browsers is a document.all.
B.3.7.1 Changes to ToBoolean
The result column in
- If argument has an
[[IsHTMLDDA]] internal slot , returnfalse . - Return
true .
B.3.7.2 Changes to Abstract Equality Comparison
The following steps replace step
- If
Type (x) is Object and x has an[[IsHTMLDDA]] internal slot and y is eithernull orundefined , returntrue . - If x is either
null orundefined andType (y) is Object and y has an[[IsHTMLDDA]] internal slot , returntrue .
B.3.7.3 Changes to the typeof Operator
The following table entry is inserted into
typeof| Type of val | Result |
|---|---|
|
Object (has an |
|
C The Strict Mode of ECMAScript
The strict mode restriction and exceptions
-
implements,interface,let,package,private,protected,public,static, andyieldare reserved words withinstrict mode code . (12.6.2 ). -
A conforming implementation, when processing
strict mode code , must not extend, as described inB.1.1 , the syntax ofNumericLiteral to includeLegacyOctalIntegerLiteral DecimalIntegerLiteral to includeNonOctalDecimalIntegerLiteral -
A conforming implementation, when processing
strict mode code , may not extend the syntax ofEscapeSequence to includeLegacyOctalEscapeSequence NonOctalDecimalEscapeSequence B.1.2 . -
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the
global object . When a simple assignment occurs withinstrict mode code , itsLeftHandSideExpression must not evaluate to an unresolvable Reference. If it does aReferenceError exception is thrown (6.2.4.5 ). TheLeftHandSideExpression also may not be a reference to adata property with the attribute value { [[Writable]]:false }, to anaccessor property with the attribute value { [[Set]]:undefined }, nor to a non-existent property of an object whose [[Extensible]] internal slot has the valuefalse . In these cases aTypeErrorexception is thrown (13.15 ). -
An
IdentifierReference with theStringValue "eval" or"arguments" may not appear as theLeftHandSideExpression of an Assignment operator (13.15 ) or of anUpdateExpression (13.4 ) or as theUnaryExpression operated upon by a Prefix Increment (13.4.4 ) or a Prefix Decrement (13.4.5 ) operator. -
Arguments objects for strict functions define a non-configurable
accessor property "callee" which throws aTypeError exception on access (10.4.4.6 ). -
Arguments objects for strict functions do not dynamically share their
array-indexed property values with the corresponding formal parameter bindings of their functions. (10.4.4 ). -
For strict functions, if an arguments object is created the binding of the local identifier
argumentsto the arguments object is immutable and hence may not be the target of an assignment expression. (10.2.10 ). -
It is a
SyntaxError if theStringValue of aBindingIdentifier is"eval" or"arguments" withinstrict mode code (13.1.1 ). -
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to eval. Instead, a new variable environment is created and that environment is used for declaration binding instantiation for the eval code (
19.2.1 ). -
If
this is evaluated withinstrict mode code , then thethis value is not coerced to an object. Athis value ofundefined ornull is not converted to theglobal object and primitive values are not converted to wrapper objects. Thethis value passed via a function call (including calls made usingFunction.prototype.applyandFunction.prototype.call) do not coerce the passedthis value to an object (10.2.1.2 ,20.2.3.1 ,20.2.3.3 ). -
When a
deleteoperator occurs withinstrict mode code , aSyntaxError is thrown if itsUnaryExpression is a direct reference to a variable, function argument, or function name (13.5.1.1 ). -
When a
deleteoperator occurs withinstrict mode code , aTypeError is thrown if the property to be deleted has the attribute { [[Configurable]]:false } or otherwise cannot be deleted (13.5.1.2 ). -
Strict mode code may not include aWithStatement . The occurrence of aWithStatement in such a context is aSyntaxError (14.11.1 ). -
It is a
SyntaxError if aCatchParameter occurs withinstrict mode code andBoundNames ofCatchParameter contains eitherevalorarguments(14.15.1 ). -
It is a
SyntaxError if the sameBindingIdentifier appears more than once in theFormalParameters of astrict function . An attempt to create such a function using a Function, Generator, or AsyncFunctionconstructor is aSyntaxError (15.2.1 ,20.2.1.1.1 ). -
An implementation may not extend, beyond that defined in this specification, the meanings within strict functions of properties named
"caller" or"arguments" of function instances.
D Host Layering Points
See
D.1 Host Hooks
D.2 Host-defined Fields
[[HostDefined]] on
[[HostDefined]] on Script Records: See
[[HostDefined]] on Module Records: See
[[HostDefined]] on JobCallback Records: See
[[HostSynchronizesWith]] on Candidate Executions: See
[[IsHTMLDDA]]: See
D.3 Host-defined Objects
The
D.4 Running Jobs
Preparation steps before, and cleanup steps after, invocation of
D.5 Internal Methods of Exotic Objects
Any of the essential internal methods in
D.6 Built-in Objects and Methods
Any built-in objects and methods not defined within this specification, except as restricted in
E Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact
Date.prototype.toString when
/.
String.prototype.match and String.prototype.replace was incorrect for cases where the pattern argument was a RegExp value whose global flag is set. The previous specifications stated that for each attempt to match the pattern, if lastIndex did not change it should be incremented by 1. The correct behaviour is that lastIndex should be incremented by one only if the pattern matched the empty String.
Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if
F Additions and Changes That Introduce Incompatibilities with Prior Editions
Space_Separator (Zs) category and thus treated as whitespace in ECMAScript 2015, to be moved to the Format (Cf) category (as of Unicode 6.3.0). This causes whitespace-sensitive methods to behave differently. For example, "\u180E".trim().length was 0 in previous editions, but 1 in ECMAScript 2016 and later. Additionally, ECMAScript 2017 mandated always using the latest version of the Unicode standard.
let followed by the token [ is the start of a
( token of a for statement is immediately followed by the token sequence let [ then the let is treated as the start of a
let [ then the let is treated as the start of a
in
var declaration for the same
eval whose eval code includes a var or FunctionDeclaration declaration that binds the same
Object.freeze is not an object it is treated as if it was a non-extensible
Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument using
Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument using
Object.getPrototypeOf is not an object an attempt is made to coerce the argument using
Object.isExtensible is not an object it is treated as if it was a non-extensible
Object.isFrozen is not an object it is treated as if it was a non-extensible
Object.isSealed is not an object it is treated as if it was a non-extensible
Object.keys is not an object an attempt is made to coerce the argument using
Object.preventExtensions is not an object it is treated as if it was a non-extensible
Object.seal is not an object it is treated as if it was a non-extensible
String.prototype.localeCompare function must treat Strings that are canonically equivalent according to the Unicode standard as being identical. In previous editions implementations were permitted to ignore canonical equivalence and could instead use a bit-wise comparison.
String.prototype.trim method is defined to recognize white space code points that may exists outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code points would not have been recognized as white space.
Atomics.wake has been renamed to Atomics.notify to prevent confusion with Atomics.wait.
await was reduced, which could create an observable difference in resolution order between a then() call and an await expression.
G Colophon
This specification is authored on GitHub in a plaintext source format called Ecmarkup. Ecmarkup is an HTML and Markdown dialect that provides a framework and toolset for authoring ECMAScript specifications in plaintext and processing the specification into a full-featured HTML rendering that follows the editorial conventions for this document. Ecmarkup builds on and integrates a number of other formats and technologies including Grammarkdown for defining syntax and Ecmarkdown for authoring algorithm steps. PDF renderings of this specification are produced by printing the HTML rendering to a PDF.
Prior editions of this specification were authored using Word—the Ecmarkup source text that formed the basis of this edition was produced by converting the ECMAScript 2015 Word document to Ecmarkup using an automated conversion tool.
H Bibliography
-
IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and Electronic Engineers, New York (2019)
Note There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that affect the ECMA-262 specification.
- The Unicode Standard, available at <https://unicode.org/versions/latest>
- Unicode Technical Note #5: Canonical Equivalence in Applications, available at <https://unicode.org/notes/tn5/>
- Unicode Technical Standard #10: Unicode Collation Algorithm, available at <https://unicode.org/reports/tr10/>
- Unicode Standard Annex #15, Unicode Normalization Forms, available at <https://unicode.org/reports/tr15/>
- Unicode Standard Annex #18: Unicode Regular Expressions, available at <https://unicode.org/reports/tr18/>
-
Unicode Standard Annex #24: Unicode
ScriptProperty, available at <https://unicode.org/reports/tr24/> - Unicode Standard Annex #31, Unicode Identifiers and Pattern Syntax, available at <https://unicode.org/reports/tr31/>
- Unicode Standard Annex #44: Unicode Character Database, available at <https://unicode.org/reports/tr44/>
- Unicode Technical Standard #51: Unicode Emoji, available at <https://unicode.org/reports/tr51/>
- IANA Time Zone Database, available at <https://www.iana.org/time-zones>
- ISO 8601:2004(E) Data elements and interchange formats — Information interchange — Representation of dates and times
- RFC 1738 “Uniform Resource Locators (URL)”, available at <https://tools.ietf.org/html/rfc1738>
- RFC 2396 “Uniform Resource Identifiers (URI): Generic Syntax”, available at <https://tools.ietf.org/html/rfc2396>
- RFC 3629 “UTF-8, a transformation format of ISO 10646”, available at <https://tools.ietf.org/html/rfc3629>
- RFC 7231 “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, available at <https://tools.ietf.org/html/rfc7231>
I Copyright & Software License
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