Draft ECMA-262 / October 4, 2024

)

states that the nonterminal WhileStatement represents the token while, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:

states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.

5.1.5.3 Optional Symbols

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:

opt

is a convenient abbreviation for:

and that:

for

(

opt

;

opt

)

is a convenient abbreviation for:

ForStatement

for

(

;

opt

)

for

(

;

opt

)

which in turn is an abbreviation for:

ForStatement

for

(

;

)

for

(

;

)

for

(

;

)

for

(

;

)

so, in this example, the nonterminal ForStatement actually has four alternative right-hand sides.

5.1.5.4 Grammatical Parameters

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:

[Return]

is a convenient abbreviation for:

and that:

[Return, In]

is an abbreviation for:

StatementList_Return_In

Multiple parameters produce a combinatoric 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:

[+In]

is equivalent to saying:

ExpressionStatement_In

and:

[~In]

is equivalent to:

A nonterminal reference may have both a parameter list and an “_opt” suffix. For example:

[+In]

opt

is an abbreviation for:

Initializer_In

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:

[In]

[?In]

is an abbreviation for:

VariableDeclaration_In

Initializer_In

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:

[Return]

[+Return]

is an abbreviation for:

and that:

[Return]

[~Return]

is an abbreviation for:

5.1.5.5 one of

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:

NonZeroDigit

one of

which is merely a convenient abbreviation for:

NonZeroDigit

5.1.5.6 [empty]

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.

5.1.5.7 Lookahead Restrictions

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 non-empty 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:

one of

the definition:

[lookahead ∉ { 1, 3, 5, 7, 9 }]

DecimalDigit

[lookahead ∉ DecimalDigit]

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 goal symbol to use at later positions. As such, when these are used in the syntactic grammar, it is considered an editorial error for a token sequence seq to appear in a lookahead restriction (including as part of a set of sequences) if the choices of lexical goal symbols to use could change whether or not seq would be a prefix of the resulting token sequence.

5.1.5.8 [no LineTerminator here]

If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated position. For example, the production:

ThrowStatement

throw

[no LineTerminator here]

;

indicates that the production may not be used if a LineTerminator occurs in the script between the throw token and the Expression.

Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.

5.1.5.9 but not

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:

Identifier

IdentifierName

but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of code points that could replace IdentifierName provided that the same sequence of code points could not replace ReservedWord.

5.1.5.10 Descriptive Phrases

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:

SourceCharacter

any Unicode code point

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 List. If there are no such additional arguments, that List is empty.

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 lowercase alphabetic characters and the second level of substeps labelled with lowercase roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

Top-level step
1. Substep.
2. Substep.
  1. Subsubstep.
    1. Subsubsubstep
      1. Subsubsubsubstep
        Subsubsubsubsubstep

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 or Parse Node derived from it 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 1, 3, and 4 in the following algorithm:

Let status be SyntaxDirectedOperation of SomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of someParseNode.
Perform SyntaxDirectedOperation of someParseNode with argument "value".

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 chain production's sole right-hand side nonterminal and then returns the result. For example, assume that some algorithm has a step of the form: “Return Evaluation of Block” and that there is a production:

Block

{

}

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

Block

{

}

Return Evaluation of 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 abstract operations or syntax-directed operations.

5.2.3.1 Completion ( `completionRecord` )

The abstract operation Completion takes argument completionRecord (a Completion Record) and returns a Completion Record. It is used to emphasize that a Completion Record is being returned. It performs the following steps when called:

Assert: completionRecord is a Completion Record.
Return completionRecord.

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 created TypeError object).

5.2.3.3 ReturnIfAbrupt

Algorithms steps that say or are otherwise equivalent to:

ReturnIfAbrupt(argument).

mean the same thing as:

Assert: argument is a Completion Record.
If argument is an abrupt completion, return Completion(argument).
Else, 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().
Assert: hygienicTemp is a Completion Record.
If hygienicTemp is an abrupt completion, return Completion(hygienicTemp).
Else, 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:

Assert: argument is a Completion Record.
If argument is an abrupt completion, return Completion(argument).
Else, set argument to argument.[[Value]].
Let result be AbstractOperation(argument).

5.2.3.4 ReturnIfAbrupt Shorthands

Invocations of abstract operations and syntax-directed operations that are prefixed by ? indicate that ReturnIfAbrupt should be applied to the resulting Completion Record. For example, the step:

? 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 abrupt completion and that the resulting Completion Record's [[Value]] field should be used in place of the return value of the operation. For example, the step:

Let val be ! OperationName().

is equivalent to the following steps:

Let val be OperationName().
Assert: val is a normal completion.
Set val to val.[[Value]].

Syntax-directed operations for runtime semantics make use of this shorthand by placing ! or ? before the invocation of the operation:

Perform ! SyntaxDirectedOperation of NonTerminal.

5.2.3.5 Implicit Normal Completion

In algorithms within abstract operations which are declared to return a Completion Record, and within all built-in functions, the returned value is first passed to NormalCompletion, and the result is used instead. This rule does not apply within the Completion algorithm or when the value being returned is clearly marked as a Completion Record in that step; these cases are:

when the result of applying Completion, NormalCompletion, ThrowCompletion, or ReturnCompletion is directly returned
when the result of constructing a Completion Record is directly returned

It is an editorial error if a Completion Record is returned from such an abstract operation through any other means. For example, within these abstract operations,

Return true.

means the same things as any of

Return NormalCompletion(true).

Let completion be NormalCompletion(true).
Return Completion(completion).

Return Completion Record { [[Type]]: normal, [[Value]]: true, [[Target]]: empty }.

Note that, through the ReturnIfAbrupt expansion, the following example is allowed, as within the expanded steps, the result of applying Completion is returned directly in the abrupt case and the implicit NormalCompletion application occurs after unwrapping in the normal case.

Return ? completion.

The following example would be an editorial error because a Completion Record is being returned without being annotated in that step.

Let completion be NormalCompletion(true).
Return completion.

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 Script or Module that may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.

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. Early error rules define early error conditions (see clause 17) that are associated with specific grammar productions. Evaluation of most early error rules are not explicitly invoked within the algorithms of this specification. A conforming implementation must, prior to the first evaluation of a Script or Module, validate all of the early error rules of the productions used to parse that Script or Module. If any of the early error rules are violated the Script or Module is invalid and cannot be evaluated.

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 binary64 (double-precision floating point) values.
BigInts: ECMAScript language 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. 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.

In general, when this specification refers to a numerical value, such as in the phrase, "the length of y" or "the integer represented by the four hexadecimal digits ...", without explicitly specifying a numeric kind, the phrase refers to a mathematical value. Phrases which refer to a Number or a BigInt value are explicitly annotated as such; for example, "the Number value for the number of code points in …" or "the BigInt value for …".

When the term integer is used in this specification, it refers to a mathematical value which is in the set of integers, unless otherwise stated. When the term integral Number is used in this specification, it refers to a finite Number value whose mathematical value is in the set of integers.

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 extended mathematical values, the operators refer to the usual mathematical operations over the extended real numbers; indeterminate forms are not defined and their use in this specification should be considered an editorial error. When applied to Numbers, the operators refer to the relevant operations within IEEE 754-2019. When applied to BigInts, the operators refer to the usual mathematical operations applied to the mathematical value of the BigInt. Numeric operators applied to mixed-type operands (such as a Number and a mathematical value) are not defined and should be considered an editorial error in this specification.

Conversions between mathematical values and Numbers or BigInts are always explicit in this document. A conversion from a mathematical value or extended mathematical value x to a Number is denoted as "the Number value for x" or 𝔽(x), and is defined in 6.1.6.1. A conversion from an integer x to a BigInt is denoted as "the BigInt value for x" or ℤ(x). A conversion from a Number or BigInt x to a mathematical value is denoted as "the mathematical value of x", or ℝ(x). The mathematical value of +0_𝔽 and -0_𝔽 is the mathematical value 0. The mathematical value of non-finite values is not defined. The extended mathematical value of x is the mathematical value of x for finite values, and is +∞ and -∞ for +∞_𝔽 and -∞_𝔽 respectively; it is not defined for NaN.

The mathematical function abs(x) produces the absolute value of x, which is -x if x < 0 and otherwise is x itself.

The mathematical function min(x1, x2, … , xN) produces the mathematically smallest of x1 through xN. The mathematical function max(x1, x2, ..., xN) produces the mathematically largest of x1 through xN. The domain and range of these mathematical functions are the extended mathematical values.

The notation “x modulo y” (y must be finite and non-zero) computes a value k of the same sign as y (or zero) such that abs(k) < abs(y) and x - k = q × y for some integer q.

The phrase "the result of clamping x between lower and upper" (where x is an extended mathematical value and lower and upper are mathematical values such that lower ≤ upper) produces lower if x < lower, produces upper if x > upper, and otherwise produces x.

The mathematical function floor(x) produces the largest integer (closest to +∞) that is not larger than x.

Note

floor(x) = x - (x modulo 1).

The mathematical function truncate(x) removes the fractional part of x by rounding towards zero, producing -floor(-x) if x < 0 and otherwise producing floor(x).

Mathematical functions min, max, abs, floor, and truncate are not defined for Numbers and BigInts, and any usage of those methods that have non-mathematical value arguments would be an editorial error in this specification.

An interval from lower bound a to upper bound b is a possibly-infinite, possibly-empty set of numeric values of the same numeric type. Each bound will be described as either inclusive or exclusive, but not both. There are four kinds of intervals, as follows:

An interval from a (inclusive) to b (inclusive), also called an inclusive interval from a to b, includes all values x of the same numeric type such that a ≤ x ≤ b, and no others.
An interval from a (inclusive) to b (exclusive) includes all values x of the same numeric type such that a ≤ x < b, and no others.
An interval from a (exclusive) to b (inclusive) includes all values x of the same numeric type such that a < x ≤ b, and no others.
An interval from a (exclusive) to b (exclusive) includes all values x of the same numeric type such that a < x < b, and no others.

For example, the interval from 1 (inclusive) to 2 (exclusive) consists of all mathematical values between 1 and 2, including 1 and not including 2. For the purpose of defining intervals, -0_𝔽 < +0_𝔽, so, for example, an inclusive interval with a lower bound of +0_𝔽 includes +0_𝔽 but not -0_𝔽. NaN is never included in an interval.

5.2.6 Value Notation

In this specification, ECMAScript language values are displayed in bold. Examples include null, true, or "hello". These are distinguished from ECMAScript source text such as Function.prototype.apply or let n = 42;.

5.2.7 Identity

In this specification, both specification values and ECMAScript language values are compared for equality. When comparing for equality, values fall into one of two categories. Values without identity are equal to other values without identity if all of their innate characteristics are the same — characteristics such as the magnitude of an integer or the length of a sequence. Values without identity may be manifest without prior reference by fully describing their characteristics. In contrast, each value with identity is unique and therefore only equal to itself. Values with identity are like values without identity but with an additional unguessable, unchangeable, universally-unique characteristic called identity. References to existing values with identity cannot be manifest simply by describing them, as the identity itself is indescribable; instead, references to these values must be explicitly passed from one place to another. Some values with identity are mutable and therefore can have their characteristics (except their identity) changed in-place, causing all holders of the value to observe the new characteristics. A value without identity is never equal to a value with identity.

From the perspective of this specification, the word “is” is used to compare two values for equality, as in “If bool is true, then ...”, and the word “contains” is used to search for a value inside lists using equality comparisons, as in "If list contains a Record r such that r.[[Foo]] is true, then ...". The specification identity of values determines the result of these comparisons and is axiomatic in this specification.

From the perspective of the ECMAScript language, language values are compared for equality using the SameValue abstract operation and the abstract operations it transitively calls. The algorithms of these comparison abstract operations determine language identity of ECMAScript language values.

For specification values, examples of values without specification identity include, but are not limited to: mathematical values and extended mathematical values; ECMAScript source text, surrogate pairs, Directive Prologues, etc; UTF-16 code units; Unicode code points; enums; abstract operations, including syntax-directed operations, host hooks, etc; and ordered pairs. Examples of specification values with specification identity include, but are not limited to: any kind of Records, including Property Descriptors, PrivateElements, etc; Parse Nodes; Lists; Sets and Relations; Abstract Closures; Data Blocks; Private Names; execution contexts and execution context stacks; agent signifiers; and WaiterList Records.

Specification identity agrees with language identity for all ECMAScript language values except Symbol values produced by Symbol.for. The ECMAScript language values without specification identity and without language identity are undefined, null, Booleans, Strings, Numbers, and BigInts. The ECMAScript language values with specification identity and language identity are Symbols not produced by Symbol.for and Objects. Symbol values produced by Symbol.for have specification identity, but not language identity.

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 classified into ECMAScript language types and specification types.

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 undefined. Any variable that has not been assigned a value has the value undefined.

6.1.2 The Null Type

The Null type has exactly one value, called null.

6.1.3 The Boolean Type

The Boolean type represents a logical entity having two values, called true and false.

6.1.4 The String Type

The String type is the set of all ordered sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum length of 2⁵³ - 1 elements. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with non-negative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

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 interval from 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 interval from 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 a trailing 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 a trailing surrogate, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See 11.1.3)
A code unit that is a leading surrogate or trailing surrogate, but is not part of a surrogate pair, is interpreted as a code point with the same value.

The function String.prototype.normalize (see 22.1.3.15) can be used to explicitly normalize a String value. String.prototype.localeCompare (see 22.1.3.12) internally normalizes String values, but no other operations implicitly normalize the strings upon which they operate. Operation results are not language- and/or locale-sensitive unless stated otherwise.

Note

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.

The phrase "the ASCII word characters" denotes the following String value, which consists solely of every letter and number in the Unicode Basic Latin block along with U+005F (LOW LINE):
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_".
For historical reasons, it has significance to various algorithms.

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 integer) and returns a non-negative integer or not-found. It performs the following steps when called:

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 such that fromIndex ≤ i ≤ len - searchLen, in ascending order, do
1. Let candidate be the substring of string from i to i + searchLen.
2. If candidate is searchValue, return i.
Return not-found.

Note 1

If searchValue is the empty String and fromIndex ≤ 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.

Note 2

This algorithm always returns not-found if fromIndex + the length of searchValue > the length of string.

6.1.4.2 StringLastIndexOf ( `string`, `searchValue`, `fromIndex` )

The abstract operation StringLastIndexOf takes arguments string (a String), searchValue (a String), and fromIndex (a non-negative integer) and returns a non-negative integer or not-found. It performs the following steps when called:

Let len be the length of string.
Let searchLen be the length of searchValue.
Assert: fromIndex + searchLen ≤ len.
For each integer i such that 0 ≤ i ≤ fromIndex, in descending order, do
1. Let candidate be the substring of string from i to i + searchLen.
2. If candidate is searchValue, return i.
Return not-found.

Note

If searchValue is the empty String, this algorithm returns fromIndex. The empty String is effectively found at every position within a string, including after the last code unit.

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 (6.1.7).

Each possible Symbol value is unique and immutable.

Each Symbol value immutably holds an associated value called [[Description]] that is either undefined or a String value.

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 (9.3).

Within this specification a well-known symbol is referred to using the standard intrinsic notation where the intrinsic is one of the values listed in Table 1.

Note

Previous editions of this specification used a notation of the form @@name, where the current edition would use %Symbol.name%. In particular, the following names were used: @@asyncIterator, @@hasInstance, @@isConcatSpreadable, @@iterator, @@match, @@matchAll, @@replace, @@search, @@species, @@split, @@toPrimitive, @@toStringTag, and @@unscopables.

Table 1: Well-known Symbols

Specification Name	`[[Description]]`	Value and Purpose
%Symbol.asyncIterator%	"Symbol.asyncIterator"	A method that returns the default AsyncIterator for an object. Called by the semantics of the `for`-`await`-`of` statement.
%Symbol.hasInstance%	"Symbol.hasInstance"	A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the `instanceof` operator.
%Symbol.isConcatSpreadable%	"Symbol.isConcatSpreadable"	A Boolean valued property that if true indicates that an object should be flattened to its array elements by `Array.prototype.concat`.
%Symbol.iterator%	"Symbol.iterator"	A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
%Symbol.match%	"Symbol.match"	A regular expression method that matches the regular expression against a string. Called by the `String.prototype.match` method.
%Symbol.matchAll%	"Symbol.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` method.
%Symbol.replace%	"Symbol.replace"	A regular expression method that replaces matched substrings of a string. Called by the `String.prototype.replace` method.
%Symbol.search%	"Symbol.search"	A regular expression method that returns the index within a string that matches the regular expression. Called by the `String.prototype.search` method.
%Symbol.species%	"Symbol.species"	A function valued property that is the constructor function that is used to create derived objects.
%Symbol.split%	"Symbol.split"	A regular expression method that splits a string at the indices that match the regular expression. Called by the `String.prototype.split` method.
%Symbol.toPrimitive%	"Symbol.toPrimitive"	A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
%Symbol.toStringTag%	"Symbol.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`.
%Symbol.unscopables%	"Symbol.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. The following abstract operations are defined over these numeric types. The "Result" column shows the return type, along with an indication if it is possible for some invocations of the operation to return an abrupt completion.

Table 2: Numeric Type Operations

Operation	Example source	Invoked by the Evaluation semantics of ...	Result
Number::unaryMinus	`-x`	Unary `-` Operator	Number
BigInt::unaryMinus	`-x`	Unary `-` Operator	BigInt
Number::bitwiseNOT	`~x`	Bitwise NOT Operator ( `~` )	Number
BigInt::bitwiseNOT	`~x`	Bitwise NOT Operator ( `~` )	BigInt
Number::exponentiate	`x ** y`	Exponentiation Operator and Math.pow ( `base`, `exponent` )	Number
BigInt::exponentiate	`x ** y`	Exponentiation Operator and Math.pow ( `base`, `exponent` )	either a normal completion containing a BigInt or a throw completion
Number::multiply	`x * y`	Multiplicative Operators	Number
BigInt::multiply	`x * y`	Multiplicative Operators	BigInt
Number::divide	`x / y`	Multiplicative Operators	Number
BigInt::divide	`x / y`	Multiplicative Operators	either a normal completion containing a BigInt or a throw completion
Number::remainder	`x % y`	Multiplicative Operators	Number
BigInt::remainder	`x % y`	Multiplicative Operators	either a normal completion containing a BigInt or a throw completion
Number::add	`x ++` `++ x` `x + y`	Postfix Increment Operator, Prefix Increment Operator, and The Addition Operator ( `+` )	Number
BigInt::add	`x ++` `++ x` `x + y`		BigInt
Number::subtract	`x --` `-- x` `x - y`	Postfix Decrement Operator, Prefix Decrement Operator, and The Subtraction Operator ( `-` )	Number
BigInt::subtract	`x --` `-- x` `x - y`		BigInt
Number::leftShift	`x << y`	The Left Shift Operator ( `<<` )	Number
BigInt::leftShift	`x << y`	The Left Shift Operator ( `<<` )	BigInt
Number::signedRightShift	`x >> y`	The Signed Right Shift Operator ( `>>` )	Number
BigInt::signedRightShift	`x >> y`	The Signed Right Shift Operator ( `>>` )	BigInt
Number::unsignedRightShift	`x >>> y`	The Unsigned Right Shift Operator ( `>>>` )	Number
BigInt::unsignedRightShift	`x >>> y`	The Unsigned Right Shift Operator ( `>>>` )	a throw completion
Number::lessThan	`x < y` `x > y` `x <= y` `x >= y`	Relational Operators, via IsLessThan ( `x`, `y`, `LeftFirst` )	Boolean or undefined (for unordered inputs)
BigInt::lessThan	`x < y` `x > y` `x <= y` `x >= y`		Boolean
Number::equal	`x == y` `x != y` `x === y` `x !== y`	Equality Operators, via IsStrictlyEqual ( `x`, `y` )	Boolean
BigInt::equal	`x == y` `x != y` `x === y` `x !== y`	Equality Operators, via IsStrictlyEqual ( `x`, `y` )	Boolean
Number::sameValue	`Object.is(x, y)`	Object internal methods, via SameValue ( `x`, `y` ), to test exact value equality	Boolean
Number::sameValueZero	`[x].includes(y)`	Array, Map, and Set methods, via SameValueZero ( `x`, `y` ), to test value equality, ignoring the difference between +0_𝔽 and -0_𝔽	Boolean
Number::bitwiseAND	`x & y`	Binary Bitwise Operators	Number
BigInt::bitwiseAND	`x & y`		BigInt
Number::bitwiseXOR	`x ^ y`		Number
BigInt::bitwiseXOR	`x ^ y`		BigInt
Number::bitwiseOR	`x \| y`		Number
BigInt::bitwiseOR	`x \| y`		BigInt
Number::toString	`String(x)`	Many expressions and built-in functions, via ToString ( `argument` )	String
BigInt::toString	`String(x)`		String

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.

Note

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, 2⁶⁴ - 2⁵³ + 3) values, representing the double-precision floating point IEEE 754-2019 binary64 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9,007,199,254,740,990 (that is, 2⁵³ - 2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-defined; to ECMAScript code, all NaN values are indistinguishable from each other.

Note

The bit pattern that might be observed in an ArrayBuffer (see 25.1) or a SharedArrayBuffer (see 25.2) after a Number value has been stored into it is not necessarily the same as the internal representation of that Number value used by the ECMAScript implementation.

There are two other special values, called positive Infinity and negative Infinity. For brevity, these values are also referred to for expository purposes by the symbols +∞_𝔽 and -∞_𝔽, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)

The other 18,437,736,874,454,810,624 (that is, 2⁶⁴ - 2⁵³) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.

Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to for expository purposes by the symbols +0_𝔽 and -0_𝔽, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)

The 18,437,736,874,454,810,622 (that is, 2⁶⁴ - 2⁵³ - 2) finite non-zero values are of two kinds:

18,428,729,675,200,069,632 (that is, 2⁶⁴ - 2⁵⁴) of them are normalized, having the form

s \times m \times 2 e

where s is 1 or -1, m is an integer in the interval from 2⁵² (inclusive) to 2⁵³ (exclusive), and e is an integer in the inclusive interval from -1074 to 971.

The remaining 9,007,199,254,740,990 (that is, 2⁵³ - 2) values are denormalized, having the form

s \times m \times 2 e

where s is 1 or -1, m is an integer in the interval from 0 (exclusive) to 2⁵² (exclusive), and e is -1074.

Note that all the positive and negative integers whose magnitude is no greater than 2⁵³ are representable in the Number type. The integer 0 has two representations in the Number type: +0_𝔽 and -0_𝔽.

A finite number has an odd significand if it is non-zero and the integer m used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.

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 Number value chosen in the following manner. Consider the set of all finite values of the Number type, with -0_𝔽 removed and with two additional values added to it that are not representable in the Number type, namely 2¹⁰²⁴ (which is +1 × 2⁵³ × 2⁹⁷¹) and -2¹⁰²⁴ (which is -1 × 2⁵³ × 2⁹⁷¹). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 2¹⁰²⁴ and -2¹⁰²⁴ are considered to have even significands. Finally, if 2¹⁰²⁴ was chosen, replace it with +∞_𝔽; if -2¹⁰²⁴ was chosen, replace it with -∞_𝔽; if +0_𝔽 was chosen, replace it with -0_𝔽 if and only if x < 0; any other chosen value is used unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754-2019 roundTiesToEven mode.)

The Number value for +∞ is +∞_𝔽, and the Number value for -∞ is -∞_𝔽.

Some ECMAScript operators deal only with integers in specific ranges such as the inclusive interval from -2³¹ to 2³¹ - 1 or the inclusive interval from 0 to 2¹⁶ - 1. These operators accept any value of the Number type but first convert each such value to an integer value in the expected range. See the descriptions of the numeric conversion operations in 7.1.

6.1.6.1.1 Number::unaryMinus ( `x` )

The abstract operation Number::unaryMinus takes argument x (a Number) and returns a Number. It performs the following steps when called:

If x is NaN, return NaN.
Return the negation of 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) and returns an integral Number. It performs the following steps when called:

Let oldValue be ! ToInt32(x).
Return the bitwise complement of 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) and returns a Number. It returns an implementation-approximated value representing the result of raising base to the exponent power. It performs the following steps when called:

If exponent is NaN, return NaN.
If exponent is either +0_𝔽 or -0_𝔽, return 1_𝔽.
If base is NaN, return NaN.
If base is +∞_𝔽, then
1. If exponent > +0_𝔽, return +∞_𝔽. Otherwise, return +0_𝔽.
If base is -∞_𝔽, then
1. If exponent > +0_𝔽, then
  1. If exponent is an odd integral Number, return -∞_𝔽. Otherwise, return +∞_𝔽.
2. Else,
  1. If exponent is an odd integral Number, return -0_𝔽. Otherwise, return +0_𝔽.
If base is +0_𝔽, then
1. If exponent > +0_𝔽, return +0_𝔽. Otherwise, return +∞_𝔽.
If base is -0_𝔽, then
1. If exponent > +0_𝔽, then
  1. If exponent is an odd integral Number, return -0_𝔽. Otherwise, return +0_𝔽.
2. Else,
  1. If exponent is an odd integral Number, return -∞_𝔽. Otherwise, return +∞_𝔽.
Assert: base is finite and is neither +0_𝔽 nor -0_𝔽.
If exponent is +∞_𝔽, then
1. If abs(ℝ(base)) > 1, return +∞_𝔽.
2. If abs(ℝ(base)) = 1, return NaN.
3. If abs(ℝ(base)) < 1, return +0_𝔽.
If exponent is -∞_𝔽, then
1. If abs(ℝ(base)) > 1, return +0_𝔽.
2. If abs(ℝ(base)) = 1, return NaN.
3. If abs(ℝ(base)) < 1, return +∞_𝔽.
Assert: exponent is finite and is neither +0_𝔽 nor -0_𝔽.
If base < -0_𝔽 and exponent is not an integral Number, return NaN.
Return an implementation-approximated Number value representing the result of raising ℝ(base) to the ℝ(exponent) power.

Note

The result of base ** exponent when base is 1_𝔽 or -1_𝔽 and exponent is +∞_𝔽 or -∞_𝔽, or when base is 1_𝔽 and exponent is NaN, differs from IEEE 754-2019. The first edition of ECMAScript specified a result of NaN for this operation, whereas later revisions of IEEE 754 specified 1_𝔽. The historical ECMAScript behaviour is preserved for compatibility reasons.

6.1.6.1.4 Number::multiply ( `x`, `y` )

The abstract operation Number::multiply takes arguments x (a Number) and y (a Number) and returns a Number. It performs multiplication according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the product of x and y. It performs the following steps when called:

If x is NaN or y is NaN, return NaN.
If x is either +∞_𝔽 or -∞_𝔽, then
1. If y is either +0_𝔽 or -0_𝔽, return NaN.
2. If y > +0_𝔽, return x.
3. Return -x.
If y is either +∞_𝔽 or -∞_𝔽, then
1. If x is either +0_𝔽 or -0_𝔽, return NaN.
2. If x > +0_𝔽, return y.
3. Return -y.
If x is -0_𝔽, then
1. If y is -0_𝔽 or y < -0_𝔽, return +0_𝔽.
2. Else, return -0_𝔽.
If y is -0_𝔽, then
1. If x < -0_𝔽, return +0_𝔽.
2. Else, return -0_𝔽.
Return 𝔽(ℝ(x) × ℝ(y)).

Note

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) and returns a Number. It performs division according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the quotient of x and y where x is the dividend and y is the divisor. It performs the following steps when called:

If x is NaN or y is NaN, return NaN.
If x is either +∞_𝔽 or -∞_𝔽, then
1. If y is either +∞_𝔽 or -∞_𝔽, return NaN.
2. If y is +0_𝔽 or y > +0_𝔽, return x.
3. Return -x.
If y is +∞_𝔽, then
1. If x is +0_𝔽 or x > +0_𝔽, return +0_𝔽. Otherwise, return -0_𝔽.
If y is -∞_𝔽, then
1. If x is +0_𝔽 or x > +0_𝔽, return -0_𝔽. Otherwise, return +0_𝔽.
If x is either +0_𝔽 or -0_𝔽, then
1. If y is either +0_𝔽 or -0_𝔽, return NaN.
2. If y > +0_𝔽, return x.
3. Return -x.
If y is +0_𝔽, then
1. If x > +0_𝔽, return +∞_𝔽. Otherwise, return -∞_𝔽.
If y is -0_𝔽, then
1. If x > +0_𝔽, return -∞_𝔽. Otherwise, return +∞_𝔽.
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) and returns 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 is NaN, return NaN.
If n is either +∞_𝔽 or -∞_𝔽, return NaN.
If d is either +∞_𝔽 or -∞_𝔽, return n.
If d is either +0_𝔽 or -0_𝔽, return NaN.
If n is either +0_𝔽 or -0_𝔽, return n.
Assert: n and d are finite and non-zero.
Let quotient be ℝ(n) / ℝ(d).
Let q be truncate(quotient).
Let r be ℝ(n) - (ℝ(d) × q).
If r = 0 and n < -0_𝔽, return -0_𝔽.
Return 𝔽(r).

Note 1

In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

Note 2

The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined by IEEE 754-2019. The IEEE 754-2019 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.

6.1.6.1.7 Number::add ( `x`, `y` )

The abstract operation Number::add takes arguments x (a Number) and y (a Number) and returns a Number. It performs addition according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the sum of its arguments. It performs the following steps when called:

If x is NaN or y is NaN, return NaN.
If x is +∞_𝔽 and y is -∞_𝔽, return NaN.
If x is -∞_𝔽 and y is +∞_𝔽, return NaN.
If x is either +∞_𝔽 or -∞_𝔽, return x.
If y is either +∞_𝔽 or -∞_𝔽, return y.
Assert: x and y are both finite.
If x is -0_𝔽 and y is -0_𝔽, return -0_𝔽.
Return 𝔽(ℝ(x) + ℝ(y)).

Note

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) and returns 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)).

Note

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) and returns an integral 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) and returns an integral 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) and returns an integral 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) and returns a Boolean or undefined. It performs the following steps when called:

If x is NaN, return undefined.
If y is NaN, return undefined.
If x is y, return false.
If x is +0_𝔽 and y is -0_𝔽, return false.
If x is -0_𝔽 and y is +0_𝔽, return false.
If x is +∞_𝔽, return false.
If y is +∞_𝔽, return true.
If y is -∞_𝔽, return false.
If x is -∞_𝔽, return true.
Assert: x and y are finite.
If ℝ(x) < ℝ(y), return true. Otherwise, return false.

6.1.6.1.13 Number::equal ( `x`, `y` )

The abstract operation Number::equal takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:

If x is NaN, return false.
If y is NaN, return false.
If x is y, return true.
If x is +0_𝔽 and y is -0_𝔽, return true.
If x is -0_𝔽 and y is +0_𝔽, return true.
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) and returns a Boolean. It performs the following steps when called:

If x is NaN and y is NaN, return true.
If x is +0_𝔽 and y is -0_𝔽, return false.
If x is -0_𝔽 and y is +0_𝔽, return false.
If x is y, return true.
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) and returns a Boolean. It performs the following steps when called:

If x is NaN and y is NaN, return true.
If x is +0_𝔽 and y is -0_𝔽, return true.
If x is -0_𝔽 and y is +0_𝔽, return true.
If x is y, return true.
Return false.

6.1.6.1.16 NumberBitwiseOp ( `op`, `x`, `y` )

The abstract operation NumberBitwiseOp takes arguments op (&, ^, or |), x (a Number), and y (a Number) and returns an integral Number. It performs the following steps when called:

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 &, then
1. Let result be the result of applying the bitwise AND operation to lBits and rBits.
Else if op is ^, then
1. Let result be the result of applying the bitwise exclusive OR (XOR) operation to lBits and rBits.
Else,
1. Assert: op is |.
2. Let result be the result of applying the bitwise inclusive OR operation to lBits and rBits.
Return the Number value for the integer 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) and returns an integral 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) and returns an integral 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) and returns an integral Number. It performs the following steps when called:

Return NumberBitwiseOp(|, x, y).

6.1.6.1.20 Number::toString ( `x`, `radix` )

The abstract operation Number::toString takes arguments x (a Number) and radix (an integer in the inclusive interval from 2 to 36) and returns a String. It represents x as a String using a positional numeral system with radix radix. The digits used in the representation of a number using radix r are taken from the first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of numbers with magnitude greater than or equal to 1_𝔽 never includes leading zeroes. It performs the following steps when called:

If x is NaN, return "NaN".
If x is either +0_𝔽 or -0_𝔽, return "0".
If x < -0_𝔽, return the string-concatenation of "-" and Number::toString(-x, radix).
If x is +∞_𝔽, return "Infinity".
Let n, k, and s be integers such that k ≥ 1, radix^{k - 1} ≤ s < radix^k, 𝔽(s × radix^{n - k}) is x, and k is as small as possible. Note that k is the number of digits in the representation of s using radix radix, that s is not divisible by radix, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
If radix ≠ 10 or n is in the inclusive interval from -5 to 21, then
1. If n ≥ k, then
  1. Return the string-concatenation of:
    - the code units of the k digits of the representation of s using radix radix
    - n - k occurrences of the code unit 0x0030 (DIGIT ZERO)
2. Else if n > 0, then
  1. Return the string-concatenation of:
    - the code units of the most significant n digits of the representation of s using radix radix
    - the code unit 0x002E (FULL STOP)
    - the code units of the remaining k - n digits of the representation of s using radix radix
3. Else,
  1. Assert: n ≤ 0.
  2. 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 representation of s using radix radix
NOTE: In this case, the input will be represented using scientific E notation, such as 1.2e+3.
Assert: radix is 10.
If n < 0, then
1. Let exponentSign be the code unit 0x002D (HYPHEN-MINUS).
Else,
1. Let exponentSign be the code unit 0x002B (PLUS SIGN).
If k = 1, then
1. Return the string-concatenation of:
  - the code unit of the single digit of s
  - the code unit 0x0065 (LATIN SMALL LETTER E)
  - exponentSign
  - the code units of the decimal representation of abs(n - 1)
Return the string-concatenation of:
- the code unit 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)
- exponentSign
- the code units of the decimal representation of abs(n - 1)

Note 1

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, then ToNumber(ToString(x)) is x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

Note 2

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:

Let n, k, and s be integers such that k ≥ 1, radix^{k - 1} ≤ s < radix^k, 𝔽(s × radix^{n - k}) is x, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × radix^{n - k} is closest in value to ℝ(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 representation of s using radix radix and that s is not divisible by radix.

Note 3

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
https://ampl.com/_archive/first-website/REFS/rounding.pdf. 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 integer value. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits set infinitely to the left.

6.1.6.2.1 BigInt::unaryMinus ( `x` )

The abstract operation BigInt::unaryMinus takes argument x (a BigInt) and returns a BigInt. It performs the following steps when called:

If x = 0_ℤ, return 0_ℤ.
Return -x.

6.1.6.2.2 BigInt::bitwiseNOT ( `x` )

The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt) and returns a BigInt. It returns the one's complement of x. It performs the following steps when called:

Return -x - 1_ℤ.

6.1.6.2.3 BigInt::exponentiate ( `base`, `exponent` )

The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and exponent (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

If exponent < 0_ℤ, throw a RangeError exception.
If base = 0_ℤ and exponent = 0_ℤ, return 1_ℤ.
Return base raised to the power exponent.

6.1.6.2.4 BigInt::multiply ( `x`, `y` )

The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return x × y.

Note

Even if the result has a much larger bit width than the input, the exact mathematical answer is given.

6.1.6.2.5 BigInt::divide ( `x`, `y` )

The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

If y = 0_ℤ, throw a RangeError exception.
Let quotient be ℝ(x) / ℝ(y).
Return ℤ(truncate(quotient)).

6.1.6.2.6 BigInt::remainder ( `n`, `d` )

The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:

If d = 0_ℤ, throw a RangeError exception.
If n = 0_ℤ, return 0_ℤ.
Let quotient be ℝ(n) / ℝ(d).
Let q be ℤ(truncate(quotient)).
Return n - (d × q).

Note

The sign of the result is the sign of the dividend.

6.1.6.2.7 BigInt::add ( `x`, `y` )

The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return x + y.

6.1.6.2.8 BigInt::subtract ( `x`, `y` )

The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return x - y.

6.1.6.2.9 BigInt::leftShift ( `x`, `y` )

The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

If y < 0_ℤ, then
1. Return ℤ(floor(ℝ(x) / 2^-ℝ(y))).
Return x × 2_ℤ^y.

Note

Semantics here should be equivalent to a bitwise shift, treating the BigInt as an infinite length string of binary two's complement digits.

6.1.6.2.10 BigInt::signedRightShift ( `x`, `y` )

The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and y (a BigInt) and returns 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) and returns a throw completion. 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) and returns a Boolean. It performs the following steps when called:

If ℝ(x) < ℝ(y), return true; otherwise return false.

6.1.6.2.13 BigInt::equal ( `x`, `y` )

The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a BigInt) and returns a Boolean. It performs the following steps when called:

If ℝ(x) = ℝ(y), return true; otherwise return false.

6.1.6.2.14 BinaryAnd ( `x`, `y` )

The abstract operation BinaryAnd takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

If x = 1 and y = 1, return 1.
Else, return 0.

6.1.6.2.15 BinaryOr ( `x`, `y` )

The abstract operation BinaryOr takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

If x = 1 or y = 1, return 1.
Else, return 0.

6.1.6.2.16 BinaryXor ( `x`, `y` )

The abstract operation BinaryXor takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:

If x = 1 and y = 0, return 1.
Else if x = 0 and y = 1, return 1.
Else, return 0.

6.1.6.2.17 BigIntBitwiseOp ( `op`, `x`, `y` )

The abstract operation BigIntBitwiseOp takes arguments op (&, ^, or |), x (a BigInt), and y (a BigInt) and returns a BigInt. It performs the following steps when called:

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),
1. Let xDigit be x modulo 2.
2. Let yDigit be y modulo 2.
3. If op is &, then
  1. Set result to result + 2^shift × BinaryAnd(xDigit, yDigit).
4. Else if op is |, then
  1. Set result to result + 2^shift × BinaryOr(xDigit, yDigit).
5. Else,
  1. Assert: op is ^.
  2. Set result to result + 2^shift × BinaryXor(xDigit, yDigit).
6. Set shift to shift + 1.
7. Set x to (x - xDigit) / 2.
8. Set y to (y - yDigit) / 2.
If op is &, then
1. Let tmp be BinaryAnd(x modulo 2, y modulo 2).
Else if op is |, then
1. Let tmp be BinaryOr(x modulo 2, y modulo 2).
Else,
1. Assert: op is ^.
2. Let tmp be BinaryXor(x modulo 2, y modulo 2).
If tmp ≠ 0, then
1. Set result to result - 2^shift.
2. NOTE: This extends the sign.
Return the BigInt value for result.

6.1.6.2.18 BigInt::bitwiseAND ( `x`, `y` )

The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return BigIntBitwiseOp(&, x, y).

6.1.6.2.19 BigInt::bitwiseXOR ( `x`, `y` )

The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return BigIntBitwiseOp(^, x, y).

6.1.6.2.20 BigInt::bitwiseOR ( `x`, `y` )

The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:

Return BigIntBitwiseOp(|, x, y).

6.1.6.2.21 BigInt::toString ( `x`, `radix` )

The abstract operation BigInt::toString takes arguments x (a BigInt) and radix (an integer in the inclusive interval from 2 to 36) and returns a String. It represents x as a String using a positional numeral system with radix radix. The digits used in the representation of a BigInt using radix r are taken from the first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of BigInts other than 0_ℤ never includes leading zeroes. It performs the following steps when called:

If x < 0_ℤ, return the string-concatenation of "-" and BigInt::toString(-x, radix).
Return the String value consisting of the representation of x using radix radix.

6.1.7 The Object Type

Each instance of the Object type, also referred to simply as “an Object”, represents 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.

The properties of an object are uniquely identified using property keys. A property key is either a String or a Symbol. All Strings and Symbols, including the empty String, are valid as property keys. A property name is a property key that is a String.

An integer index is a property name n such that CanonicalNumericIndexString(n) returns an integral Number in the inclusive interval from +0_𝔽 to 𝔽(2⁵³ - 1). An array index is an integer index n such that CanonicalNumericIndexString(n) returns an integral Number in the inclusive interval from +0_𝔽 to 𝔽(2³² - 2).

Note

Every non-negative safe integer has a corresponding integer index. Every 32-bit unsigned integer except 2³² - 1 has a corresponding array index. "-0" is neither an integer index nor an array index.

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 6.1.7.2 for definitions of the multiple forms of objects.

In addition, some objects are callable; these are referred to as functions or function objects and are described further below. All functions in ECMAScript are members of the Object type.

6.1.7.1 Property Attributes

Attributes are used in this specification to define and explain the state of Object properties as described in Table 3. Unless specified explicitly, the initial value of each attribute is its Default Value.

Table 3: Attributes of an Object property

Attribute Name	Types of property for which it is present	Value Domain	Default Value	Description
`[[Value]]`	data property	an ECMAScript language value	undefined	The value retrieved by a get access of the property.
`[[Writable]]`	data property	a Boolean	false	If false, attempts by ECMAScript code to change the property's `[[Value]]` attribute using `[[Set]]` will not succeed.
`[[Get]]`	accessor property	an Object or undefined	undefined	If the value is an Object it must be a function object. The function's `[[Call]]` internal method (Table 5) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
`[[Set]]`	accessor property	an Object or undefined	undefined	If the value is an Object it must be a function object. The function's `[[Call]]` internal method (Table 5) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's `[[Set]]` internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's `[[Get]]` internal method.
`[[Enumerable]]`	data property or accessor property	a Boolean	false	If true, the property will be enumerated by a for-in enumeration (see 14.7.5). Otherwise, the property is said to be non-enumerable.
`[[Configurable]]`	data property or accessor property	a Boolean	false	If false, attempts to delete the property, change it from a data property to an accessor property or from an accessor property to a data property, or make any changes to its attributes (other than replacing an existing `[[Value]]` or setting `[[Writable]]` to false) will fail.

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 TypeError exception is thrown.

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 ECMAScript language type or of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the value undefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.

All objects have an internal slot named [[PrivateElements]], which is a List of PrivateElements. This List represents the values of the private fields, methods, and accessors for the object. Initially, it is an empty List.

Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].

Table 4 summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.

An ordinary object is an object that satisfies all of the following criteria:

For the internal methods listed in Table 4, the object uses those defined in 10.1.
If the object has a [[Call]] internal method, it uses either the one defined in 10.2.1 or the one defined in 10.3.1.
If the object has a [[Construct]] internal method, it uses either the one defined in 10.2.2 or the one defined in 10.3.2.

An exotic object is an object that is not an ordinary object.

This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind of exotic object (such as an Array exotic object or a bound function exotic object), but does not have the same collection of internal methods specified for that kind, is not recognized as that kind of exotic object.

The “Signature” column of Table 4 and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause 6 augmented by the following additional names. “any” means the value may be any ECMAScript language type.

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 Completion Record, either a normal completion that wraps a value of the return type shown in its invocation pattern, or a throw completion.

Table 4: Essential Internal Methods

Internal Method	Signature	Description
`[[GetPrototypeOf]]`	( ) → Object \| Null	Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties.
`[[SetPrototypeOf]]`	(Object \| Null) → Boolean	Associate this object with another object that provides inherited properties. Passing null indicates that there are no inherited properties. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
`[[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 true if the operation was successful or false if the operation was unsuccessful.
`[[GetOwnProperty]]`	(`propertyKey`) → Undefined \| Property Descriptor	Return a Property Descriptor for the own property of this object whose key is `propertyKey`, or undefined if no such property exists.
`[[DefineOwnProperty]]`	(`propertyKey`, `PropertyDescriptor`) → Boolean	Create or alter the own property, whose key is `propertyKey`, to have the state described by `PropertyDescriptor`. Return true if that property was successfully created/updated or false if the property could not be created or updated.
`[[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 this value when evaluating the code.
`[[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 this value when evaluating the code. Returns true if the property value was set or false if it could not be set.
`[[Delete]]`	(`propertyKey`) → Boolean	Remove the own property whose key is `propertyKey` from this object. Return false if the property was not deleted and is still present. Return true if the property was deleted or is not present.
`[[OwnPropertyKeys]]`	( ) → List of property keys	Return a List whose elements are all of the own property keys for the object.

Table 5 summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every constructor must be a function object. Therefore, a constructor may also be referred to as a constructor function or constructor function object.

Table 5: Additional Essential Internal Methods of Function Objects

Internal Method	Signature	Description
`[[Call]]`	(any, a List of any) → any	Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a List whose elements are the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
`[[Construct]]`	(a List of any, Object) → Object	Creates an object. Invoked via the `new` operator or a `super` call. The first argument to the internal method is a List whose elements are the arguments of the constructor invocation or the `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 function object is not necessarily a constructor and such non-constructor function objects do not have a `[[Construct]]` internal method.

The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause 10. If any specified use of an internal method of an exotic object is not supported by an implementation, that usage must throw a TypeError exception when attempted.

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, or true 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 the SameValue algorithm.

Return value:

The value returned by any internal method must be a Completion Record with either:

[[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]] = any ECMAScript language value.

Note 1

An internal method must not return a continue completion, a break completion, or a return completion.

`[[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.

Note 2

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 null). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use the ordinary object definition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.

`[[SetPrototypeOf]]` ( `V` )

The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue 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 return false.

`[[PreventExtensions]]` ( )

The normal return type is Boolean.
If [[PreventExtensions]] returns true, all future calls to [[IsExtensible]] on the target must return false and the target is now considered non-extensible.

`[[GetOwnProperty]]` ( `P` )

The normal return type is either Property Descriptor or Undefined.
If the return value is a Property Descriptor, it must be a fully populated Property Descriptor.
If P is described as a non-configurable, non-writable own data property, all future calls to [[GetOwnProperty]] ( P ) must return Property Descriptor whose [[Value]] is SameValue as P's [[Value]] attribute.
If P's attributes other than [[Writable]] and [[Value]] 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 to true, then the [[Configurable]] attribute must be true.
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).

Note 3

As a consequence of the third invariant, if a property is described as a data property and it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must be true even if no mechanism to change the value is exposed via the other essential internal methods.

`[[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:
1. P is a writable data property. A non-configurable writable data property can be changed into a non-configurable non-writable data property.
2. All attributes of Desc are the SameValue as P's attributes.
[[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 return true.

`[[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 the SameValue as V.
If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.

`[[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 return false unless V is the SameValue as P's [[Value]] attribute.
If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.

`[[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 return false.

`[[OwnPropertyKeys]]` ( )

The normal return type is List.
The returned List must not contain any duplicate entries.
Each element of the returned List must be a property key.
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 realm-specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one per realm.

Within this specification a reference such as %name% means the intrinsic object, associated with the current realm, corresponding to the name. A reference such as %name.a.b% means, as if the "b" property of the value of the "a" property of the intrinsic object %name% was accessed prior to any ECMAScript code being evaluated. Determination of the current realm and its intrinsics is described in 9.4. The well-known intrinsics are listed in Table 6.

Table 6: Well-Known Intrinsic Objects

Intrinsic Name	Global Name	ECMAScript Language Association
%AggregateError%	`AggregateError`	The `AggregateError` constructor (20.5.7.1)
%Array%	`Array`	The Array constructor (23.1.1)
%ArrayBuffer%	`ArrayBuffer`	The ArrayBuffer constructor (25.1.4)
%ArrayIteratorPrototype%		The prototype of Array iterator objects (23.1.5)
%AsyncFromSyncIteratorPrototype%		The prototype of async-from-sync iterator objects (27.1.4)
%AsyncFunction%		The constructor of async function objects (27.7.1)
%AsyncGeneratorFunction%		The constructor of async generator function objects (27.4.1)
%AsyncGeneratorPrototype%		The prototype of async generator objects (27.6)
%AsyncIteratorPrototype%		An object that all standard built-in async iterator objects indirectly inherit from
%Atomics%	`Atomics`	The `Atomics` object (25.4)
%BigInt%	`BigInt`	The BigInt constructor (21.2.1)
%BigInt64Array%	`BigInt64Array`	The BigInt64Array constructor (23.2)
%BigUint64Array%	`BigUint64Array`	The BigUint64Array constructor (23.2)
%Boolean%	`Boolean`	The Boolean constructor (20.3.1)
%DataView%	`DataView`	The DataView constructor (25.3.2)
%Date%	`Date`	The Date constructor (21.4.2)
%decodeURI%	`decodeURI`	The `decodeURI` function (19.2.6.1)
%decodeURIComponent%	`decodeURIComponent`	The `decodeURIComponent` function (19.2.6.2)
%encodeURI%	`encodeURI`	The `encodeURI` function (19.2.6.3)
%encodeURIComponent%	`encodeURIComponent`	The `encodeURIComponent` function (19.2.6.4)
%Error%	`Error`	The Error constructor (20.5.1)
%eval%	`eval`	The `eval` function (19.2.1)
%EvalError%	`EvalError`	The EvalError constructor (20.5.5.1)
%FinalizationRegistry%	`FinalizationRegistry`	The FinalizationRegistry constructor (26.2.1)
%Float32Array%	`Float32Array`	The Float32Array constructor (23.2)
%Float64Array%	`Float64Array`	The Float64Array constructor (23.2)
%ForInIteratorPrototype%		The prototype of For-In iterator objects (14.7.5.10)
%Function%	`Function`	The Function constructor (20.2.1)
%GeneratorFunction%		The constructor of generator function objects (27.3.1)
%GeneratorPrototype%		The prototype of generator objects (27.5)
%Int8Array%	`Int8Array`	The Int8Array constructor (23.2)
%Int16Array%	`Int16Array`	The Int16Array constructor (23.2)
%Int32Array%	`Int32Array`	The Int32Array constructor (23.2)
%isFinite%	`isFinite`	The `isFinite` function (19.2.2)
%isNaN%	`isNaN`	The `isNaN` function (19.2.3)
%IteratorPrototype%		An object that all standard built-in iterator objects indirectly inherit from
%JSON%	`JSON`	The `JSON` object (25.5)
%Map%	`Map`	The Map constructor (24.1.1)
%MapIteratorPrototype%		The prototype of Map iterator objects (24.1.5)
%Math%	`Math`	The `Math` object (21.3)
%Number%	`Number`	The Number constructor (21.1.1)
%Object%	`Object`	The Object constructor (20.1.1)
%parseFloat%	`parseFloat`	The `parseFloat` function (19.2.4)
%parseInt%	`parseInt`	The `parseInt` function (19.2.5)
%Promise%	`Promise`	The Promise constructor (27.2.3)
%Proxy%	`Proxy`	The Proxy constructor (28.2.1)
%RangeError%	`RangeError`	The RangeError constructor (20.5.5.2)
%ReferenceError%	`ReferenceError`	The ReferenceError constructor (20.5.5.3)
%Reflect%	`Reflect`	The `Reflect` object (28.1)
%RegExp%	`RegExp`	The RegExp constructor (22.2.4)
%RegExpStringIteratorPrototype%		The prototype of RegExp String Iterator objects (22.2.9)
%Set%	`Set`	The Set constructor (24.2.2)
%SetIteratorPrototype%		The prototype of Set iterator objects (24.2.6)
%SharedArrayBuffer%	`SharedArrayBuffer`	The SharedArrayBuffer constructor (25.2.3)
%String%	`String`	The String constructor (22.1.1)
%StringIteratorPrototype%		The prototype of String iterator objects (22.1.5)
%Symbol%	`Symbol`	The Symbol constructor (20.4.1)
%SyntaxError%	`SyntaxError`	The SyntaxError constructor (20.5.5.4)
%ThrowTypeError%		A function object that unconditionally throws a new instance of %TypeError%
%TypedArray%		The super class of all typed Array constructors (23.2.1)
%TypeError%	`TypeError`	The TypeError constructor (20.5.5.5)
%Uint8Array%	`Uint8Array`	The Uint8Array constructor (23.2)
%Uint8ClampedArray%	`Uint8ClampedArray`	The Uint8ClampedArray constructor (23.2)
%Uint16Array%	`Uint16Array`	The Uint16Array constructor (23.2)
%Uint32Array%	`Uint32Array`	The Uint32Array constructor (23.2)
%URIError%	`URIError`	The URIError constructor (20.5.5.6)
%WeakMap%	`WeakMap`	The WeakMap constructor (24.3.1)
%WeakRef%	`WeakRef`	The WeakRef constructor (26.1.1)
%WeakSet%	`WeakSet`	The WeakSet constructor (24.4.1)

Note

Additional entries in Table 96.

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 Record, List, Completion Record, Property Descriptor, Environment Record, Abstract Closure, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.

6.2.1 The Enum Specification Type

Enums are values which are internal to the specification and not directly observable from ECMAScript code. Enums are denoted using a sans-serif typeface. For instance, a Completion Record's [[Type]] field takes on values like normal, return, or throw. Enums have no characteristics other than their name. The name of an enum serves no purpose other than to distinguish it from other enums, and implies nothing about its usage or meaning in context.

6.2.2 The List and Record Specification Types

The List type is used to explain the evaluation of argument lists (see 13.3.8) in 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 3^rd 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 « ».

In this specification, the phrase "the list-concatenation of A, B, ..." (where each argument is a possibly empty List) denotes a new List value whose elements are the concatenation of the elements (in order) of each of the arguments (in order).

As applied to a List of Strings, the phrase "sorted according to lexicographic code unit order" means sorting by the numeric value of each code unit up to the length of the shorter string, and sorting the shorter string before the longer string if all are equal, as described in the abstract operation IsLessThan.

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 an ECMAScript language value or specification value. 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]]: false, [[Field3]]: empty } defines a Record value that has three fields, each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.

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]]: false, [[Configurable]]: true }.

6.2.3 The Set and Relation Specification Types

The Set type is used to explain a collection of unordered elements for use in the memory model. It is distinct from the ECMAScript collection type of the same name. To disambiguate, instances of the ECMAScript collection are consistently referred to as "Set objects" within this specification. Values of the Set type are simple collections of elements, where no element appears more than once. Elements may be added to and removed from Sets. Sets may be unioned, intersected, or subtracted from each other.

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 the least Relation 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.

Note 1

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 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.

Note 2

The three properties above are called totality, irreflexivity, and transitivity, respectively.

6.2.4 The Completion Record Specification Type

The Completion Record specification type is used to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.

Completion Records have the fields defined in Table 7.

Table 7: Completion Record Fields

Field Name	Value	Meaning
`[[Type]]`	normal, break, continue, return, or throw	The type of completion that occurred.
`[[Value]]`	any value except a Completion Record	The value that was produced.
`[[Target]]`	a String or empty	The target label for directed control transfers.

The following shorthand terms are sometimes used to refer to Completion Records.

normal completion refers to any Completion Record with a [[Type]] value of normal.
break completion refers to any Completion Record with a [[Type]] value of break.
continue completion refers to any Completion Record with a [[Type]] value of continue.
return completion refers to any Completion Record with a [[Type]] value of return.
throw completion refers to any Completion Record with a [[Type]] value of throw.
abrupt completion refers to any Completion Record with a [[Type]] value other than normal.
a normal completion containing some type of value refers to a normal completion that has a value of that type in its [[Value]] field.

Callable objects that are defined in this specification only return a normal completion or a throw completion. Returning any other kind of Completion Record is considered an editorial error.

Implementation-defined callable objects must return either a normal completion or a throw completion.

6.2.4.1 NormalCompletion ( `value` )

The abstract operation NormalCompletion takes argument value (any value except a Completion Record) and returns a normal completion. It performs the following steps when called:

Return Completion Record { [[Type]]: normal, [[Value]]: value, [[Target]]: empty }.

6.2.4.2 ThrowCompletion ( `value` )

The abstract operation ThrowCompletion takes argument value (an ECMAScript language value) and returns a throw completion. It performs the following steps when called:

Return Completion Record { [[Type]]: throw, [[Value]]: value, [[Target]]: empty }.

6.2.4.3 ReturnCompletion ( `value` )

The abstract operation ReturnCompletion takes argument value (an ECMAScript language value) and returns a return completion. It performs the following steps when called:

Return Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.

6.2.4.4 UpdateEmpty ( `completionRecord`, `value` )

The abstract operation UpdateEmpty takes arguments completionRecord (a Completion Record) and value (any value except a Completion Record) and returns a Completion Record. It performs the following steps when called:

Assert: If completionRecord is either a return completion or a throw completion, then completionRecord.[[Value]] is not empty.
If completionRecord.[[Value]] is not empty, return ? completionRecord.
Return Completion Record { [[Type]]: completionRecord.[[Type]], [[Value]]: value, [[Target]]: completionRecord.[[Target]] }.

6.2.5 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 keyword and other language features. For example, the left-hand operand of an assignment is expected to produce a Reference Record.

A Reference Record is a resolved name or (possibly not-yet-resolved) property binding; its fields are defined by Table 8.

Table 8: Reference Record Fields

Field Name	Value	Meaning
`[[Base]]`	an ECMAScript language value, an Environment Record, or unresolvable	The value or Environment Record which holds the binding. A `[[Base]]` of unresolvable indicates that the binding could not be resolved.
`[[ReferencedName]]`	an ECMAScript language value or a Private Name	The name of the binding. Always a String if `[[Base]]` value is an Environment Record. Otherwise, may be an ECMAScript language value other than a String or a Symbol until ToPropertyKey is performed.
`[[Strict]]`	a Boolean	true if the Reference Record originated in strict mode code, false otherwise.
`[[ThisValue]]`	an ECMAScript language value or empty	If not empty, the Reference Record represents a property binding that was expressed using the `super` keyword; it is called a Super Reference Record and its `[[Base]]` value will never be an Environment Record. In that case, the `[[ThisValue]]` field holds the this value at the time the Reference Record was created.

The following abstract operations are used in this specification to operate upon Reference Records:

6.2.5.1 IsPropertyReference ( `V` )

The abstract operation IsPropertyReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

If V.[[Base]] is unresolvable, return false.
If V.[[Base]] is an Environment Record, return false; otherwise return true.

6.2.5.2 IsUnresolvableReference ( `V` )

The abstract operation IsUnresolvableReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

If V.[[Base]] is unresolvable, return true; otherwise return false.

6.2.5.3 IsSuperReference ( `V` )

The abstract operation IsSuperReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

If V.[[ThisValue]] is not empty, return true; otherwise return false.

6.2.5.4 IsPrivateReference ( `V` )

The abstract operation IsPrivateReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:

If V.[[ReferencedName]] is a Private Name, return true; otherwise return false.

6.2.5.5 GetValue ( `V` )

The abstract operation GetValue takes argument V (a Reference Record or an ECMAScript language value) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:

If V is not a Reference Record, return V.
If IsUnresolvableReference(V) is true, throw a ReferenceError exception.
If IsPropertyReference(V) is true, then
1. Let baseObj be ? ToObject(V.[[Base]]).
2. If IsPrivateReference(V) is true, then
  1. Return ? PrivateGet(baseObj, V.[[ReferencedName]]).
3. If V.[[ReferencedName]] is not a property key, then
  1. Set V.[[ReferencedName]] to ? ToPropertyKey(V.[[ReferencedName]]).
4. Return ? baseObj.[[Get]](V.[[ReferencedName]], GetThisValue(V)).
Else,
1. Let base be V.[[Base]].
2. Assert: base is an Environment Record.
3. Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see 9.1).

Note

The object that may be created in step 3.a is not accessible outside of the above abstract operation and the ordinary object [[Get]] internal method. An implementation might choose to avoid the actual creation of the object.

6.2.5.6 PutValue ( `V`, `W` )

The abstract operation PutValue takes arguments V (a Reference Record or an ECMAScript language value) and W (an ECMAScript language value) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

If V is not a Reference Record, throw a ReferenceError exception.
If IsUnresolvableReference(V) is true, then
1. If V.[[Strict]] is true, throw a ReferenceError exception.
2. Let globalObj be GetGlobalObject().
3. Perform ? Set(globalObj, V.[[ReferencedName]], W, false).
4. Return unused.
If IsPropertyReference(V) is true, then
1. Let baseObj be ? ToObject(V.[[Base]]).
2. If IsPrivateReference(V) is true, then
  1. Return ? PrivateSet(baseObj, V.[[ReferencedName]], W).
3. If V.[[ReferencedName]] is not a property key, then
  1. Set V.[[ReferencedName]] to ? ToPropertyKey(V.[[ReferencedName]]).
4. Let succeeded be ? baseObj.[[Set]](V.[[ReferencedName]], W, GetThisValue(V)).
5. If succeeded is false and V.[[Strict]] is true, throw a TypeError exception.
6. Return unused.
Else,
1. Let base be V.[[Base]].
2. Assert: base is an Environment Record.
3. Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see 9.1).

Note

The object that may be created in step 3.a is not accessible outside of the above abstract operation and the ordinary object [[Set]] internal method. An implementation might choose to avoid the actual creation of that object.

6.2.5.7 GetThisValue ( `V` )

The abstract operation GetThisValue takes argument V (a Reference Record) and returns an ECMAScript language value. It performs the following steps when called:

Assert: IsPropertyReference(V) is true.
If IsSuperReference(V) is true, return V.[[ThisValue]]; otherwise return V.[[Base]].

6.2.5.8 InitializeReferencedBinding ( `V`, `W` )

The abstract operation InitializeReferencedBinding takes arguments V (a Reference Record) and W (an ECMAScript language value) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

Assert: IsUnresolvableReference(V) is false.
Let base be V.[[Base]].
Assert: base is an Environment Record.
Return ? base.InitializeBinding(V.[[ReferencedName]], W).

6.2.5.9 MakePrivateReference ( `baseValue`, `privateIdentifier` )

The abstract operation MakePrivateReference takes arguments baseValue (an ECMAScript language value) and privateIdentifier (a String) and returns a Reference Record. It performs the following steps when called:

Let privateEnv be the running execution context's PrivateEnvironment.
Assert: privateEnv is not null.
Let privateName be ResolvePrivateIdentifier(privateEnv, privateIdentifier).
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: privateName, [[Strict]]: true, [[ThisValue]]: empty }.

6.2.6 The Property Descriptor Specification Type

The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. A Property Descriptor is a Record with zero or more fields, where each field's name is an attribute name and its value is a corresponding attribute value as specified in 6.1.7.1. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.

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 (in which case it is a generic 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 corresponding fields defined in Table 3.

The following abstract operations are used in this specification to operate upon Property Descriptor values:

6.2.6.1 IsAccessorDescriptor ( `Desc` )

The abstract operation IsAccessorDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

If Desc is undefined, return false.
If Desc has a [[Get]] field, return true.
If Desc has a [[Set]] field, return true.
Return false.

6.2.6.2 IsDataDescriptor ( `Desc` )

The abstract operation IsDataDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

If Desc is undefined, return false.
If Desc has a [[Value]] field, return true.
If Desc has a [[Writable]] field, return true.
Return false.

6.2.6.3 IsGenericDescriptor ( `Desc` )

The abstract operation IsGenericDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:

If Desc is undefined, return false.
If IsAccessorDescriptor(Desc) is true, return false.
If IsDataDescriptor(Desc) is true, return false.
Return true.

6.2.6.4 FromPropertyDescriptor ( `Desc` )

The abstract operation FromPropertyDescriptor takes argument Desc (a Property Descriptor or undefined) and returns an Object or undefined. It performs the following steps when called:

If Desc is undefined, return undefined.
Let obj be OrdinaryObjectCreate(%Object.prototype%).
Assert: obj is an extensible ordinary object with no own properties.
If Desc has a [[Value]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "value", Desc.[[Value]]).
If Desc has a [[Writable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "writable", Desc.[[Writable]]).
If Desc has a [[Get]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "get", Desc.[[Get]]).
If Desc has a [[Set]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "set", Desc.[[Set]]).
If Desc has an [[Enumerable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "enumerable", Desc.[[Enumerable]]).
If Desc has a [[Configurable]] field, then
1. Perform ! CreateDataPropertyOrThrow(obj, "configurable", Desc.[[Configurable]]).
Return obj.

6.2.6.5 ToPropertyDescriptor ( `Obj` )

The abstract operation ToPropertyDescriptor takes argument Obj (an ECMAScript language value) and returns either a normal completion containing a Property Descriptor or a throw completion. It performs the following steps when called:

If Obj is not an Object, throw a TypeError exception.
Let desc be a new Property Descriptor that initially has no fields.
Let hasEnumerable be ? HasProperty(Obj, "enumerable").
If hasEnumerable is true, then
1. Let enumerable be ToBoolean(? Get(Obj, "enumerable")).
2. Set desc.[[Enumerable]] to enumerable.
Let hasConfigurable be ? HasProperty(Obj, "configurable").
If hasConfigurable is true, then
1. Let configurable be ToBoolean(? Get(Obj, "configurable")).
2. Set desc.[[Configurable]] to configurable.
Let hasValue be ? HasProperty(Obj, "value").
If hasValue is true, then
1. Let value be ? Get(Obj, "value").
2. Set desc.[[Value]] to value.
Let hasWritable be ? HasProperty(Obj, "writable").
If hasWritable is true, then
1. Let writable be ToBoolean(? Get(Obj, "writable")).
2. Set desc.[[Writable]] to writable.
Let hasGet be ? HasProperty(Obj, "get").
If hasGet is true, then
1. Let getter be ? Get(Obj, "get").
2. If IsCallable(getter) is false and getter is not undefined, throw a TypeError exception.
3. Set desc.[[Get]] to getter.
Let hasSet be ? HasProperty(Obj, "set").
If hasSet is true, then
1. Let setter be ? Get(Obj, "set").
2. If IsCallable(setter) is false and setter is not undefined, throw a TypeError exception.
3. Set desc.[[Set]] to setter.
If desc has a [[Get]] field or desc has a [[Set]] field, then
1. If desc has a [[Value]] field or desc has a [[Writable]] field, throw a TypeError exception.
Return desc.

6.2.6.6 CompletePropertyDescriptor ( `Desc` )

The abstract operation CompletePropertyDescriptor takes argument Desc (a Property Descriptor) and returns unused. It performs the following steps when called:

Let like be the Record { [[Value]]: undefined, [[Writable]]: false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false }.
If IsGenericDescriptor(Desc) is true or IsDataDescriptor(Desc) is true, then
1. If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
2. If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
Else,
1. If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
2. 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 unused.

6.2.7 The Environment Record Specification Type

The Environment Record type is used to explain the behaviour of name resolution in nested functions and blocks. This type and the operations upon it are defined in 9.1.

6.2.8 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 abstract operations, invocations perform the algorithm steps described by the Abstract Closure.

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 Completion Record, that Completion Record must be either a normal completion or a throw completion.

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:
1. Return x + addend.
Let val be closure(1).
Assert: val is 42.

6.2.9 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 integer in the inclusive interval from 0 to 255. A Data Block value is created with a fixed number of bytes that each have the initial value 0.

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-based integer-indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to access its 3^rd byte.

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 memory model. Abstract operations below introduce Shared Data Block events and act as the interface between evaluation semantics and the event semantics of the memory model. The events form a candidate execution, on which the memory model acts as a filter. Please consult the memory model for full semantics.

Shared Data Block events are modelled by Records, defined in the memory model.

The following abstract operations are used in this specification to operate upon Data Block values:

6.2.9.1 CreateByteDataBlock ( `size` )

The abstract operation CreateByteDataBlock takes argument size (a non-negative integer) and returns either a normal completion containing a Data Block or a throw completion. It performs the following steps when called:

If size > 2⁵³ - 1, throw a RangeError exception.
Let db be a new Data Block value consisting of size bytes. If it is impossible to create such a Data Block, throw a RangeError exception.
Set all of the bytes of db to 0.
Return db.

6.2.9.2 CreateSharedByteDataBlock ( `size` )

The abstract operation CreateSharedByteDataBlock takes argument size (a non-negative integer) and returns either a normal completion containing a Shared Data Block or a throw completion. It performs the following steps when called:

Let db be a new Shared Data Block value consisting of size bytes. If it is impossible to create such a Shared Data Block, throw a RangeError exception.
Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
Let zero be « 0 ».
For each index i of db, do
1. Append WriteSharedMemory { [[Order]]: init, [[NoTear]]: true, [[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero } to eventsRecord.[[EventList]].
Return db.

6.2.9.3 CopyDataBlockBytes ( `toBlock`, `toIndex`, `fromBlock`, `fromIndex`, `count` )

The abstract operation CopyDataBlockBytes takes arguments toBlock (a Data Block or a Shared Data Block), toIndex (a non-negative integer), fromBlock (a Data Block or a Shared Data Block), fromIndex (a non-negative integer), and count (a non-negative integer) and returns unused. It performs the following steps when called:

Assert: fromBlock and toBlock are distinct 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,
1. If fromBlock is a Shared Data Block, then
  1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
  2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
  3. Let bytes be a List whose sole element is a nondeterministically chosen byte value.
  4. 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.
  5. Let readEvent be ReadSharedMemory { [[Order]]: unordered, [[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
  6. Append readEvent to eventsRecord.[[EventList]].
  7. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
  8. If toBlock is a Shared Data Block, then
    1. Append WriteSharedMemory { [[Order]]: unordered, [[NoTear]]: true, [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]: 1, [[Payload]]: bytes } to eventsRecord.[[EventList]].
  9. Else,
    1. Set toBlock[toIndex] to bytes[0].
2. Else,
  1. Assert: toBlock is not a Shared Data Block.
  2. Set toBlock[toIndex] to fromBlock[fromIndex].
3. Set toIndex to toIndex + 1.
4. Set fromIndex to fromIndex + 1.
5. Set count to count - 1.
Return unused.

6.2.10 The PrivateElement Specification Type

The PrivateElement type is a Record used in the specification of private class fields, methods, and accessors. Although Property Descriptors are not used for private elements, private fields behave similarly to non-configurable, non-enumerable, writable data properties, private methods behave similarly to non-configurable, non-enumerable, non-writable data properties, and private accessors behave similarly to non-configurable, non-enumerable accessor properties.

Values of the PrivateElement type are Record values whose fields are defined by Table 9. Such values are referred to as PrivateElements.

Table 9: PrivateElement Fields

Field Name	Values of the `[[Kind]]` field for which it is present	Value	Meaning
`[[Key]]`	All	a Private Name	The name of the field, method, or accessor.
`[[Kind]]`	All	field, method, or accessor	The kind of the element.
`[[Value]]`	field and method	an ECMAScript language value	The value of the field.
`[[Get]]`	accessor	a function object or undefined	The getter for a private accessor.
`[[Set]]`	accessor	a function object or undefined	The setter for a private accessor.

6.2.11 The ClassFieldDefinition Record Specification Type

The ClassFieldDefinition type is a Record used in the specification of class fields.

Values of the ClassFieldDefinition type are Record values whose fields are defined by Table 10. Such values are referred to as ClassFieldDefinition Records.

Table 10: ClassFieldDefinition Record Fields

Field Name	Value	Meaning
`[[Name]]`	a Private Name, a String, or a Symbol	The name of the field.
`[[Initializer]]`	an ECMAScript function object or empty	The initializer of the field, if any.

6.2.12 Private Names

The Private Name specification type is used to describe a globally unique value (one which differs from any other Private Name, even if they are otherwise indistinguishable) which represents the key of a private class element (field, method, or accessor). Each Private Name has an associated immutable [[Description]] which is a String value. A Private Name may be installed on any ECMAScript object with PrivateFieldAdd or PrivateMethodOrAccessorAdd, and then read or written using PrivateGet and PrivateSet.

6.2.13 The ClassStaticBlockDefinition Record Specification Type

A ClassStaticBlockDefinition Record is a Record value used to encapsulate the executable code for a class static initialization block.

ClassStaticBlockDefinition Records have the fields listed in Table 11.

Table 11: ClassStaticBlockDefinition Record Fields

Field Name	Value	Meaning
`[[BodyFunction]]`	an ECMAScript function object	The function object to be called during static initialization of a class.

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 abstract operations are defined throughout this specification.

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 abstract operations. The conversion abstract operations are polymorphic; they can accept a value of any ECMAScript language type. But no other specification types are used with these operations.

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 (an ECMAScript language value) and optional argument preferredType (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. 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:

If input is an Object, then
1. Let exoticToPrim be ? GetMethod(input, %Symbol.toPrimitive%).
2. If exoticToPrim is not undefined, then
  1. If preferredType is not present, then
    1. Let hint be "default".
  2. Else if preferredType is string, then
    1. Let hint be "string".
  3. Else,
    1. Assert: preferredType is number.
    2. Let hint be "number".
  4. Let result be ? Call(exoticToPrim, input, « hint »).
  5. If result is not an Object, return result.
  6. Throw a TypeError exception.
3. If preferredType is not present, let preferredType be number.
4. Return ? OrdinaryToPrimitive(input, preferredType).
Return input.

Note

When ToPrimitive is called without a hint, then it generally behaves as if the hint were number. However, objects may over-ride this behaviour by defining a %Symbol.toPrimitive% method. Of the objects defined in this specification only Dates (see 21.4.4.45) and Symbol objects (see 20.4.3.5) over-ride the default ToPrimitive behaviour. Dates treat the absence of a hint as if the hint were string.

7.1.1.1 OrdinaryToPrimitive ( `O`, `hint` )

The abstract operation OrdinaryToPrimitive takes arguments O (an Object) and hint (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. It performs the following steps when called:

If hint is string, then
1. Let methodNames be « "toString", "valueOf" ».
Else,
1. Let methodNames be « "valueOf", "toString" ».
For each element name of methodNames, do
1. Let method be ? Get(O, name).
2. If IsCallable(method) is true, then
  1. Let result be ? Call(method, O).
  2. If result is not an Object, return result.
Throw a TypeError exception.

7.1.2 ToBoolean ( `argument` )

The abstract operation ToBoolean takes argument argument (an ECMAScript language value) and returns a Boolean. It converts argument to a value of type Boolean. It performs the following steps when called:

If argument is a Boolean, return argument.
If argument is one of undefined, null, +0_𝔽, -0_𝔽, NaN, 0_ℤ, or the empty String, return false.
NOTE: This step is replaced in section B.3.6.1.
Return true.

7.1.3 ToNumeric ( `value` )

The abstract operation ToNumeric takes argument value (an ECMAScript language value) and returns either a normal completion containing either a Number or a BigInt, or a throw completion. It returns value converted to a Number or a BigInt. It performs the following steps when called:

Let primValue be ? ToPrimitive(value, number).
If primValue is a BigInt, return primValue.
Return ? ToNumber(primValue).

7.1.4 ToNumber ( `argument` )

The abstract operation ToNumber takes argument argument (an ECMAScript language value) and returns either a normal completion containing a Number or a throw completion. It converts argument to a value of type Number. It performs the following steps when called:

If argument is a Number, return argument.
If argument is either a Symbol or a BigInt, throw a TypeError exception.
If argument is undefined, return NaN.
If argument is either null or false, return +0_𝔽.
If argument is true, return 1_𝔽.
If argument is a String, return StringToNumber(argument).
Assert: argument is an Object.
Let primValue be ? ToPrimitive(argument, number).
Assert: primValue is not an Object.
Return ? ToNumber(primValue).

7.1.4.1 ToNumber Applied to the String Type

The abstract operation StringToNumber specifies how to convert a String value to a Number value, using the following grammar.

Syntax

:::

opt

opt

opt

:::

opt

:::

:::

NonDecimalIntegerLiteral

[~Sep]

StrDecimalLiteral

:::

:::

Infinity

[~Sep]

[~Sep]

opt

[~Sep]

opt

[~Sep]

[~Sep]

opt

[~Sep]

[~Sep]

opt

All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (12.9.3)

Note

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral:

A StringNumericLiteral may include leading and/or trailing white space and/or line terminators.
A StringNumericLiteral that is decimal may have any number of leading 0 digits.
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.
Infinity and -Infinity are recognized as a StringNumericLiteral but not as a NumericLiteral.
A StringNumericLiteral cannot include a BigIntLiteralSuffix.
A StringNumericLiteral cannot include a NumericLiteralSeparator.

7.1.4.1.1 StringToNumber ( `str` )

The abstract operation StringToNumber takes argument str (a String) and returns a Number. It performs the following steps when called:

Let literal be ParseText(str, StringNumericLiteral).
If literal is a List of errors, return NaN.
Return the StringNumericValue of literal.

7.1.4.1.2 Runtime Semantics: StringNumericValue

The syntax-directed operation StringNumericValue takes no arguments and returns a Number.

Note

The conversion of a StringNumericLiteral to a Number value is similar overall to the determination of the NumericValue of a NumericLiteral (see 12.9.3), but some of the details are different.

It is defined piecewise over the following productions:

StringNumericLiteral

:::

StrWhiteSpace

opt

Return +0_𝔽.

:::

opt

opt

Return the StringNumericValue of StrNumericLiteral.

StrNumericLiteral

:::

NonDecimalIntegerLiteral

Return 𝔽(MV of NonDecimalIntegerLiteral).

StrDecimalLiteral

:::

Let a be the StringNumericValue of StrUnsignedDecimalLiteral.
If a is +0_𝔽, return -0_𝔽.
Return -a.

:::

Infinity

Return +∞_𝔽.

:::

opt

opt

Let a be the MV of the first DecimalDigits.
If the second DecimalDigits is present, then
1. Let b be the MV of the second DecimalDigits.
2. Let n be the number of code points in the second DecimalDigits.
Else,
1. Let b be 0.
2. Let n be 0.
If ExponentPart is present, let e be the MV of ExponentPart. Otherwise, let e be 0.
Return RoundMVResult((a + (b × 10^-n)) × 10^e).

:::

opt

Let b be the MV of DecimalDigits.
If ExponentPart is present, let e be the MV of ExponentPart. Otherwise, let e be 0.
Let n be the number of code points in DecimalDigits.
Return RoundMVResult(b × 10^{e - n}).

:::