This is a commit snapshot of the specification

This document contains the contents of the specification as of commit 58c55e54658a6c2929a0f04ba550c87779a8a51f, and should only be used as a historical reference. This commit may not have even been merged into the specification.

Do not attempt to implement this version of the specification. Do not reference this version as authoritative in any way. Instead, see https://tc39.es/ecma262 for the living specification.

Introduction
1 Scope
+2 Conformance
1. 2.1 Example Normative Optional Clause Heading
2. 2.2 Example Legacy Clause Heading
3. 2.3 Example Legacy Normative Optional Clause Heading
3 Normative References
+4 Overview
1. 4.1 Web Scripting
2. 4.2 Hosts and Implementations
3. +4.3 ECMAScript Overview
  1. 4.3.1 Objects
  2. 4.3.2 The Strict Variant of ECMAScript
4. +4.4 Terms and Definitions
5. 4.5 Organization of This Specification
+5 Notational Conventions
1. +5.1 Syntactic and Lexical Grammars
2. +5.2 Algorithm Conventions
+6 ECMAScript Data Types and Values
1. +6.1 ECMAScript Language Types
2. +6.2 ECMAScript Specification Types
+7 Abstract Operations
1. +7.1 Type Conversion
2. +7.2 Testing and Comparison Operations
3. +7.3 Operations on Objects
4. +7.4 Operations on Iterator Objects
+8 Syntax-Directed Operations
1. 8.1 RS: Evaluation
2. +8.2 Scope Analysis
3. +8.3 Labels
4. +8.4 Function Name Inference
5. +8.5 Contains
  1. 8.5.1 SS: Contains
  2. 8.5.2 SS: ComputedPropertyContains
6. +8.6 Miscellaneous
+9 Executable Code and Execution Contexts
1. +9.1 Environment Records
  1. +9.1.1 The Environment Record Type Hierarchy
  2. +9.1.2 Environment Record Operations
2. +9.2 PrivateEnvironment Records
  1. +9.2.1 PrivateEnvironment Record Operations
    1. 9.2.1.1 NewPrivateEnvironment ( outerPrivateEnv )
    2. 9.2.1.2 ResolvePrivateIdentifier ( privateEnv, identifier )
3. +9.3 Realms
4. +9.4 Execution Contexts
5. +9.5 Jobs and Host Operations to Enqueue Jobs
6. +9.6 Agents
7. 9.7 Agent Clusters
8. 9.8 Forward Progress
9. +9.9 Processing Model of WeakRef and FinalizationRegistry Targets
10. 9.10 ClearKeptObjects ( )
11. 9.11 AddToKeptObjects ( value )
12. 9.12 CleanupFinalizationRegistry ( finalizationRegistry )
13. 9.13 CanBeHeldWeakly ( v )
+10 Ordinary and Exotic Objects Behaviours
1. +10.1 Ordinary Object Internal Methods and Internal Slots
2. +10.2 ECMAScript Function Objects
3. +10.3 Built-in Function Objects
4. +10.4 Built-in Exotic Object Internal Methods and Slots
5. +10.5 Proxy Object Internal Methods and Internal Slots
+11 ECMAScript Language: Source Text
1. +11.1 Source Text
2. +11.2 Types of Source Code
+12 ECMAScript Language: Lexical Grammar
1. 12.1 Unicode Format-Control Characters
2. 12.2 White Space
3. 12.3 Line Terminators
4. 12.4 Comments
5. 12.5 Hashbang Comments
6. 12.6 Tokens
7. +12.7 Names and Keywords
  1. +12.7.1 Identifier Names
  2. 12.7.2 Keywords and Reserved Words
8. 12.8 Punctuators
9. +12.9 Literals
10. +12.10 Automatic Semicolon Insertion
+13 ECMAScript Language: Expressions
1. +13.1 Identifiers
2. +13.2 Primary Expression
3. +13.3 Left-Hand-Side Expressions
4. +13.4 Update Expressions
5. +13.5 Unary Operators
6. +13.6 Exponentiation Operator
  1. 13.6.1 RS: Evaluation
7. +13.7 Multiplicative Operators
  1. 13.7.1 RS: Evaluation
8. +13.8 Additive Operators
  1. +13.8.1 The Addition Operator ( + )
    1. 13.8.1.1 RS: Evaluation
  2. +13.8.2 The Subtraction Operator ( - )
    1. 13.8.2.1 RS: Evaluation
9. +13.9 Bitwise Shift Operators
10. +13.10 Relational Operators
  1. 13.10.1 RS: Evaluation
  2. 13.10.2 InstanceofOperator ( V, target )
11. +13.11 Equality Operators
  1. 13.11.1 RS: Evaluation
12. +13.12 Binary Bitwise Operators
  1. 13.12.1 RS: Evaluation
13. +13.13 Binary Logical Operators
  1. 13.13.1 RS: Evaluation
14. +13.14 Conditional Operator ( ? : )
  1. 13.14.1 RS: Evaluation
15. +13.15 Assignment Operators
16. +13.16 Comma Operator ( , )
  1. 13.16.1 RS: Evaluation
+14 ECMAScript Language: Statements and Declarations
1. +14.1 Statement Semantics
  1. 14.1.1 RS: Evaluation
2. +14.2 Block
3. +14.3 Declarations and the Variable Statement
4. +14.4 Empty Statement
  1. 14.4.1 RS: Evaluation
5. +14.5 Expression Statement
  1. 14.5.1 RS: Evaluation
6. +14.6 The if Statement
  1. 14.6.1 SS: Early Errors
  2. 14.6.2 RS: Evaluation
7. +14.7 Iteration Statements
8. +14.8 The continue Statement
  1. 14.8.1 SS: Early Errors
  2. 14.8.2 RS: Evaluation
9. +14.9 The break Statement
  1. 14.9.1 SS: Early Errors
  2. 14.9.2 RS: Evaluation
10. +14.10 The return Statement
  1. 14.10.1 RS: Evaluation
11. +14.11 The with Statement
  1. 14.11.1 SS: Early Errors
  2. 14.11.2 RS: Evaluation
12. +14.12 The switch Statement
13. +14.13 Labelled Statements
14. +14.14 The throw Statement
  1. 14.14.1 RS: Evaluation
15. +14.15 The try Statement
16. +14.16 The debugger Statement
  1. 14.16.1 RS: Evaluation
+15 ECMAScript Language: Functions and Classes
1. +15.1 Parameter Lists
2. +15.2 Function Definitions
3. +15.3 Arrow Function Definitions
4. +15.4 Method Definitions
5. +15.5 Generator Function Definitions
6. +15.6 Async Generator Function Definitions
7. +15.7 Class Definitions
8. +15.8 Async Function Definitions
9. +15.9 Async Arrow Function Definitions
10. +15.10 Tail Position Calls
+16 ECMAScript Language: Scripts and Modules
1. +16.1 Scripts
2. +16.2 Modules
+17 Error Handling and Language Extensions
1. 17.1 Forbidden Extensions
18 ECMAScript Standard Built-in Objects
+19 The Global Object
1. +19.1 Value Properties of the Global Object
2. +19.2 Function Properties of the Global Object
3. +19.3 Constructor Properties of the Global Object
4. +19.4 Other Properties of the Global Object
+20 Fundamental Objects
1. +20.1 Object Objects
2. +20.2 Function Objects
3. +20.3 Boolean Objects
4. +20.4 Symbol Objects
5. +20.5 Error Objects
+21 Numbers and Dates
1. +21.1 Number Objects
2. +21.2 BigInt Objects
3. +21.3 The Math Object
  1. +21.3.1 Value Properties of the Math Object
  2. +21.3.2 Function Properties of the Math Object
4. +21.4 Date Objects
+22 Text Processing
1. +22.1 String Objects
2. +22.2 RegExp (Regular Expression) Objects
+23 Indexed Collections
1. +23.1 Array Objects
2. +23.2 TypedArray Objects
3. +23.3 Uint8Array Objects
+24 Keyed Collections
1. +24.1 Map Objects
2. +24.2 Set Objects
3. +24.3 WeakMap Objects
4. +24.4 WeakSet Objects
5. +24.5 Abstract Operations for Keyed Collections
  1. 24.5.1 CanonicalizeKeyedCollectionKey ( key )
+25 Structured Data
1. +25.1 ArrayBuffer Objects
2. +25.2 SharedArrayBuffer Objects
3. +25.3 DataView Objects
4. +25.4 The Atomics Object
5. +25.5 The JSON Object
+26 Managing Memory
1. +26.1 WeakRef Objects
2. +26.2 FinalizationRegistry Objects
+27 Control Abstraction Objects
1. +27.1 Iteration
2. +27.2 Promise Objects
3. +27.3 GeneratorFunction Objects
4. +27.4 AsyncGeneratorFunction Objects
5. +27.5 Generator Objects
6. +27.6 AsyncGenerator Objects
7. +27.7 AsyncFunction Objects
+28 Reflection
1. +28.1 The Reflect Object
2. +28.2 Proxy Objects
  1. +28.2.1 The Proxy Constructor
    1. 28.2.1.1 Proxy ( target, handler )
  2. +28.2.2 Properties of the Proxy Constructor
    1. 28.2.2.1 Proxy.revocable ( target, handler )
3. +28.3 Module Namespace Objects
  1. 28.3.1 %Symbol.toStringTag%
+29 Memory Model
1. 29.1 Memory Model Fundamentals
2. 29.2 Agent Events Records
3. 29.3 Chosen Value Records
4. 29.4 Candidate Executions
5. +29.5 Abstract Operations for the Memory Model
6. +29.6 Relations of Candidate Executions
7. +29.7 Properties of Valid Executions
8. 29.8 Races
9. 29.9 Data Races
10. 29.10 Data Race Freedom
11. 29.11 Shared Memory Guidelines
+Annex A (informative) Grammar Summary
1. A.1 Lexical Grammar
2. A.2 Expressions
3. A.3 Statements
4. A.4 Functions and Classes
5. A.5 Scripts and Modules
6. A.6 Number Conversions
7. A.7 Time Zone Offset String Format
8. A.8 Regular Expressions
+Annex B (normative) Additional ECMAScript Features for Web Browsers
1. +B.1 Additional Syntax
  1. B.1.1 HTML-like Comments
  2. +B.1.2 Regular Expressions Patterns
2. +B.2 Additional Built-in Properties
3. +B.3 Other Additional Features
Annex C (informative) The Strict Mode of ECMAScript
+Annex D (informative) Host Layering Points
1. D.1 Host Hooks
2. D.2 Host-defined Fields
3. D.3 Host-defined Objects
4. D.4 Running Jobs
5. D.5 Internal Methods of Exotic Objects
6. D.6 Built-in Objects and Methods
Annex E (informative) Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact
Annex F (informative) Additions and Changes That Introduce Incompatibilities with Prior Editions
Bibliography
Colophon
Copyright & Software License

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Annex C (informative) The Strict Mode of ECMAScript

The strict mode restriction and exceptions

implements, interface, let, package, private, protected, public, static, and yield are reserved words within strict mode code. (12.7.2).
A conforming implementation, when processing strict mode code, must disallow instances of the productions NumericLiteral :: LegacyOctalIntegerLiteral and DecimalIntegerLiteral :: NonOctalDecimalIntegerLiteral .
A conforming implementation, when processing strict mode code, must disallow instances of the productions EscapeSequence :: LegacyOctalEscapeSequence and EscapeSequence :: NonOctalDecimalEscapeSequence .
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the global object. When a simple assignment occurs within strict mode code, its LeftHandSideExpression must not evaluate to an unresolvable Reference. If it does a ReferenceError exception is thrown (6.2.5.6). The LeftHandSideExpression also may not be a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor property with the attribute value { [[Set]]: undefined }, nor to a non-existent property of an object whose [[Extensible]] internal slot is false. In these cases a TypeError exception is thrown (13.15).
An IdentifierReference with the StringValue "eval" or "arguments" may not appear as the LeftHandSideExpression of an Assignment operator (13.15) or of an UpdateExpression (13.4) or as the UnaryExpression operated upon by a Prefix Increment (13.4.4) or a Prefix Decrement (13.4.5) operator.
Arguments objects for strict functions define a non-configurable accessor property "callee" which throws a TypeError exception on access (10.4.4.6).
Arguments objects for strict functions do not dynamically share their array-indexed property values with the corresponding formal parameter bindings of their functions. (10.4.4).
For strict functions, if an arguments object is created the binding of the local identifier arguments to the arguments object is immutable and hence may not be the target of an assignment expression. (10.2.11).
It is a SyntaxError if the StringValue of a BindingIdentifier is either "eval" or "arguments" within strict mode code (13.1.1).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to eval. Instead, a new variable environment is created and that environment is used for declaration binding instantiation for the eval code (19.2.1).
If this is evaluated within strict mode code, then the this value is not coerced to an object. A this value of either undefined or null is not converted to the global object and primitive values are not converted to wrapper objects. The this value passed via a function call (including calls made using Function.prototype.apply and Function.prototype.call) do not coerce the passed this value to an object (10.2.1.2, 20.2.3.1, 20.2.3.3).
When a delete operator occurs within strict mode code, a SyntaxError is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name (13.5.1.1).
When a delete operator occurs within strict mode code, a TypeError is thrown if the property to be deleted has the attribute { [[Configurable]]: false } or otherwise cannot be deleted (13.5.1.2).
Strict mode code may not include a WithStatement. The occurrence of a WithStatement in such a context is a SyntaxError (14.11.1).
It is a SyntaxError if a CatchParameter occurs within strict mode code and the BoundNames of CatchParameter contains either eval or arguments (14.15.1).
It is a SyntaxError if the same BindingIdentifier appears more than once in the FormalParameters of a strict function. An attempt to create such a function using a Function, Generator, or AsyncFunction constructor is a SyntaxError (15.2.1, 20.2.1.1.1).
An implementation may not extend, beyond that defined in this specification, the meanings within strict functions of properties named "caller" or "arguments" of function instances.