This is not the official ECMAScript Language Specification.

This is a draft of the next edition of the standard. See also:

For copyright information, see Ecma International’s legal disclaimer in the document itself.

ECMA-262

6th Edition / Draft July 18, 2014

Draft

ECMAScript Language Specification

Draft

Report Errors and Issues at: https://bugs.ecmascript.org

Product: Draft for 6th Edition

Component: choose an appropriate one

Version: Rev 26, July 18, 2014 Draft

Ecma/TC39/2014/0xx

Contents

  1. Introduction
  2. 1 Scope
  3. 2 Conformance
  4. 3 Normative references
  5. 4 Overview
    1. 4.1 Web Scripting
    2. 4.2 ECMAScript Overview
      1. 4.2.1 Objects
      2. 4.2.2 The Strict Variant of ECMAScript
    3. 4.3 Terms and definitions
      1. 4.3.1 type
      2. 4.3.2 primitive value
      3. 4.3.3 object
      4. 4.3.4 constructor
      5. 4.3.5 prototype
      6. 4.3.6 ordinary object
      7. 4.3.7 exotic object
      8. 4.3.8 standard object
      9. 4.3.9 built-in object
      10. 4.3.10 undefined value
      11. 4.3.11 Undefined type
      12. 4.3.12 null value
      13. 4.3.13 Null type
      14. 4.3.14 Boolean value
      15. 4.3.15 Boolean type
      16. 4.3.16 Boolean object
      17. 4.3.17 String value
      18. 4.3.18 String type
      19. 4.3.19 String object
      20. 4.3.20 Number value
      21. 4.3.21 Number type
      22. 4.3.22 Number object
      23. 4.3.23 Infinity
      24. 4.3.24 NaN
      25. 4.3.25 Symbol value
      26. 4.3.26 Symbol type
      27. 4.3.27 Symbol object
      28. 4.3.28 function
      29. 4.3.29 built-in function
      30. 4.3.30 property
      31. 4.3.31 method
      32. 4.3.32 built-in method
      33. 4.3.33 attribute
      34. 4.3.34 own property
      35. 4.3.35 inherited property
    4. 4.4 Organization of This Specification
  6. 5 Notational Conventions
    1. 5.1 Syntactic and Lexical Grammars
      1. 5.1.1 Context-Free Grammars
      2. 5.1.2 The Lexical and RegExp Grammars
      3. 5.1.3 The Numeric String Grammar
      4. 5.1.4 The Syntactic Grammar
      5. 5.1.5 Grammar Notation
    2. 5.2 Algorithm Conventions
    3. 5.3 Static Semantic Rules
  7. 6 ECMAScript Data Types and Values
    1. 6.1 ECMAScript Language Types
      1. 6.1.1 The Undefined Type
      2. 6.1.2 The Null Type
      3. 6.1.3 The Boolean Type
      4. 6.1.4 The String Type
      5. 6.1.5 The Symbol Type
      6. 6.1.6 The Number Type
      7. 6.1.7 The Object Type
    2. 6.2 ECMAScript Specification Types
      1. 6.2.1 The List and Record Specification Type
      2. 6.2.2 The Completion Record Specification Type
      3. 6.2.3 The Reference Specification Type
      4. 6.2.4 The Property Descriptor Specification Type
      5. 6.2.5 The Lexical Environment and Environment Record Specification Types
      6. 6.2.6 Data Blocks
  8. 7 Abstract Operations
    1. 7.1 Type Conversion and Testing
      1. 7.1.1 ToPrimitive ( input [, PreferredType] )
      2. 7.1.2 ToBoolean ( argument )
      3. 7.1.3 ToNumber ( argument )
      4. 7.1.4 ToInteger ( argument )
      5. 7.1.5 ToInt32 ( argument ) — Signed 32 Bit Integer
      6. 7.1.6 ToUint32 ( argument ) — Unsigned 32 Bit Integer
      7. 7.1.7 ToInt16 ( argument ) — Signed 16 Bit Integer
      8. 7.1.8 ToUint16 ( argument ) — Unsigned 16 Bit Integer
      9. 7.1.9 ToInt8 ( argument ) — Signed 8 Bit Integer
      10. 7.1.10 ToUint8 ( argument ) — Unsigned 8 Bit Integer
      11. 7.1.11 ToUint8Clamp ( argument ) — Unsigned 8 Bit Integer, Clamped
      12. 7.1.12 ToString ( argument )
      13. 7.1.13 ToObject ( argument )
      14. 7.1.14 ToPropertyKey ( argument )
      15. 7.1.15 ToLength ( argument )
      16. 7.1.16 CanonicalNumericIndexString ( argument )
    2. 7.2 Testing and Comparison Operations
      1. 7.2.1 CheckObjectCoercible ( argument )
      2. 7.2.2 IsCallable ( argument )
      3. 7.2.3 SameValue(x, y)
      4. 7.2.4 SameValueZero(x, y)
      5. 7.2.5 IsConstructor ( argument )
      6. 7.2.6 IsPropertyKey ( argument )
      7. 7.2.7 IsExtensible (O)
      8. 7.2.8 IsInteger ( argument )
      9. 7.2.9 Abstract Relational Comparison
      10. 7.2.10 Abstract Equality Comparison
      11. 7.2.11 Strict Equality Comparison
    3. 7.3 Operations on Objects
      1. 7.3.1 Get (O, P)
      2. 7.3.2 Put (O, P, V, Throw)
      3. 7.3.3 CreateDataProperty (O, P, V)
      4. 7.3.4 CreateDataPropertyOrThrow (O, P, V)
      5. 7.3.5 DefinePropertyOrThrow (O, P, desc)
      6. 7.3.6 DeletePropertyOrThrow (O, P)
      7. 7.3.7 GetMethod (O, P)
      8. 7.3.8 HasProperty (O, P)
      9. 7.3.9 HasOwnProperty (O, P)
      10. 7.3.10 Invoke(O,P, [args])
      11. 7.3.11 SetIntegrityLevel (O, level)
      12. 7.3.12 TestIntegrityLevel (O, level)
      13. 7.3.13 CreateArrayFromList (elements)
      14. 7.3.14 CreateListFromArrayLike (obj)
      15. 7.3.15 OrdinaryHasInstance (C, O)
      16. 7.3.16 GetPrototypeFromConstructor ( constructor, intrinsicDefaultProto )
      17. 7.3.17 CreateFromConstructor (F)
      18. 7.3.18 Construct (F, argumentsList)
      19. 7.3.19 GetOption (options, P)
      20. 7.3.20 EnumerableOwnNames (O)
      21. 7.3.21 GetFunctionRealm ( obj ) Abstract Operation
    4. 7.4 Operations on Iterator Objects
      1. 7.4.1 CheckIterable ( obj )
      2. 7.4.2 GetIterator ( obj, method )
      3. 7.4.3 IteratorNext ( iterator, value )
      4. 7.4.4 IteratorComplete ( iterResult )
      5. 7.4.5 IteratorValue ( iterResult )
      6. 7.4.6 IteratorStep ( iterator )
      7. 7.4.7 CreateIterResultObject (value, done)
      8. 7.4.8 CreateListIterator (list)
      9. 7.4.9 CreateEmptyIterator ( )
      10. 7.4.10 CreateCompoundIterator ( iterator1, iterator2 )
    5. 7.5 Operations on Promise Objects
      1. 7.5.1 PromiseNew ( executor ) Abstract Operation
      2. 7.5.2 PromiseBuiltinCapability () Abstract Operation
      3. 7.5.3 PromiseOf (value) Abstract Operation
      4. 7.5.4 PromiseAll (promiseList) Abstract Operation
      5. 7.5.5 PromiseCatch (promise, rejectedAction) Abstract Operation
      6. 7.5.6 PromiseThen (promise, resolvedAction, rejectedAction) Abstract Operation
  9. 8 Executable Code and Execution Contexts
    1. 8.1 Lexical Environments
      1. 8.1.1 Environment Records
      2. 8.1.2 Lexical Environment Operations
    2. 8.2 Code Realms
      1. 8.2.1 CreateRealm ( ) Abstract Operation
      2. 8.2.2 CreateIntrinsics ( realmRec ) Abstract Operation
      3. 8.2.3 SetRealmGlobalObj ( realmRec, globalObj ) Abstract Operation
      4. 8.2.4 SetDefaultGlobalBindings ( realmRec ) Abstract Operation
    3. 8.3 Execution Contexts
      1. 8.3.1 ResolveBinding ( name ) Abstract Operation
      2. 8.3.2 GetThisEnvironment ( ) Abstract Operation
      3. 8.3.3 ResolveThisBinding ( ) Abstract Operation
      4. 8.3.4 GetGlobalObject ( ) Abstract Operation
    4. 8.4 Jobs and Job Queues
      1. 8.4.1 EnqueueJob ( queueName, job, arguments) Abstract Operation
      2. 8.4.2 NextJob result
    5. 8.5 Initialization
      1. 8.5.1 InitializeFirstRealm ( realm ) Abstract Operation
  10. 9 Ordinary and Exotic Objects Behaviours
    1. 9.1 Ordinary Object Internal Methods and Internal Slots
      1. 9.1.1 [[GetPrototypeOf]] ( )
      2. 9.1.2 [[SetPrototypeOf]] (V)
      3. 9.1.3 [[IsExtensible]] ( )
      4. 9.1.4 [[PreventExtensions]] ( )
      5. 9.1.5 [[GetOwnProperty]] (P)
      6. 9.1.6 [[DefineOwnProperty]] (P, Desc)
      7. 9.1.7 [[HasProperty]](P)
      8. 9.1.8 [[Get]] (P, Receiver)
      9. 9.1.9 [[Set]] ( P, V, Receiver)
      10. 9.1.10 [[Delete]] (P)
      11. 9.1.11 [[Enumerate]] ()
      12. 9.1.12 [[OwnPropertyKeys]] ( )
      13. 9.1.13 ObjectCreate(proto, internalSlotsList) Abstract Operation
      14. 9.1.14 OrdinaryCreateFromConstructor ( constructor, intrinsicDefaultProto, internalSlotsList )
    2. 9.2 ECMAScript Function Objects
      1. 9.2.1 [[GetOwnProperty]] (P)
      2. 9.2.2 [[Call]] ( thisArgument, argumentsList)
      3. 9.2.3 [[Construct]] ( argumentsList)
      4. 9.2.4 FunctionAllocate (functionPrototype, strict) Abstract Operation
      5. 9.2.5 FunctionInitialize (F, kind, Strict, ParameterList, Body, Scope) Abstract Operation
      6. 9.2.6 FunctionCreate (kind, ParameterList, Body, Scope, Strict) Abstract Operation
      7. 9.2.7 GeneratorFunctionCreate (kind, ParameterList, Body, Scope, Strict) Abstract Operation
      8. 9.2.8 AddRestrictedFunctionProperties ( F, realm ) Abstract Operation
      9. 9.2.9 MakeConstructor (F, writablePrototype, prototype) Abstract Operation
      10. 9.2.10 MakeMethod ( F, methodName, homeObject) Abstract Operation
      11. 9.2.11 SetFunctionName (F, name, prefix) Abstract Operation
      12. 9.2.12 CloneMethod(function, newHome, newName) Abstract Operation
      13. 9.2.13 FunctionDeclarationInstantiation(func, argumentsList, env ) Abstract Operation
    3. 9.3 Built-in Function Objects
      1. 9.3.1 [[Call]] ( thisArgument, argumentsList)
      2. 9.3.2 CreateBuiltinFunction(realm, steps, prototype, internalSlotsList) Abstract Operation
    4. 9.4 Built-in Exotic Object Internal Methods and Data Fields
      1. 9.4.1 Bound Function Exotic Objects
      2. 9.4.2 Array Exotic Objects
      3. 9.4.3 String Exotic Objects
      4. 9.4.4 Arguments Exotic Objects
      5. 9.4.5 Integer Indexed Exotic Objects
      6. 9.4.6 Module Exotic Objects
    5. 9.5 Proxy Object Internal Methods and Internal Slots
      1. 9.5.1 [[GetPrototypeOf]] ( )
      2. 9.5.2 [[SetPrototypeOf]] (V)
      3. 9.5.3 [[IsExtensible]] ( )
      4. 9.5.4 [[PreventExtensions]] ( )
      5. 9.5.5 [[GetOwnProperty]] (P)
      6. 9.5.6 [[DefineOwnProperty]] (P, Desc)
      7. 9.5.7 [[HasProperty]] (P)
      8. 9.5.8 [[Get]] (P, Receiver)
      9. 9.5.9 [[Set]] ( P, V, Receiver)
      10. 9.5.10 [[Delete]] (P)
      11. 9.5.11 [[Enumerate]] ()
      12. 9.5.12 [[OwnPropertyKeys]] ( )
      13. 9.5.13 [[Call]] (thisArgument, argumentsList)
      14. 9.5.14 [[Construct]] Internal Method
      15. 9.5.15 ProxyCreate(target, handler) Abstract Operation
  11. 10 ECMAScript Language: Source Code
    1. 10.1 Source Text
      1. 10.1.1 Static Semantics: UTF-16Encoding
      2. 10.1.2 Static Semantics: UTF16Decode(lead, trail)
    2. 10.2 Types of Source Code
      1. 10.2.1 Strict Mode Code
      2. 10.2.2 Non-ECMAScript Functions
  12. 11 ECMAScript Language: Lexical Grammar
    1. 11.1 Unicode Format-Control Characters
    2. 11.2 White Space
    3. 11.3 Line Terminators
    4. 11.4 Comments
    5. 11.5 Tokens
    6. 11.6 Names and Keywords
      1. 11.6.1 Identifier Names
      2. 11.6.2 Reserved Words
    7. 11.7 Punctuators
    8. 11.8 Literals
      1. 11.8.1 Null Literals
      2. 11.8.2 Boolean Literals
      3. 11.8.3 Numeric Literals
      4. 11.8.4 String Literals
      5. 11.8.5 Regular Expression Literals
      6. 11.8.6 Template Literal Lexical Components
    9. 11.9 Automatic Semicolon Insertion
      1. 11.9.1 Rules of Automatic Semicolon Insertion
      2. 11.9.2 Examples of Automatic Semicolon Insertion
  13. 12 ECMAScript Language: Expressions
    1. 12.1 Identifiers
      1. 12.1.1 Static Semantics: Early Errors
      2. 12.1.2 Static Semantics: BoundNames
      3. 12.1.3 Static Semantics: StringValue
      4. 12.1.4 Runtime Semantics: BindingInitialization
      5. 12.1.5 Runtime Semantics: Evaluation
    2. 12.2 Primary Expression
      1. 12.2.0 Semantics
      2. 12.2.1 The this Keyword
      3. 12.2.2 Identifier Reference
      4. 12.2.3 Literals
      5. 12.2.4 Array Initializer
      6. 12.2.5 Object Initializer
      7. 12.2.6 Function Defining Expressions
      8. 12.2.7 Generator Comprehensions
      9. 12.2.8 Regular Expression Literals
      10. 12.2.9 Template Literals
      11. 12.2.10 The Grouping Operator
    3. 12.3 Left-Hand-Side Expressions
      1. 12.3.1 Static Semantics
      2. 12.3.2 Property Accessors
      3. 12.3.3 The new Operator
      4. 12.3.4 Function Calls
      5. 12.3.5 The super Keyword
      6. 12.3.6 Argument Lists
      7. 12.3.7 Tagged Templates
    4. 12.4 Postfix Expressions
      1. 12.4.1 Static Semantics: Early Errors
      2. 12.4.2 Static Semantics: IsFunctionDefinition
      3. 12.4.3 Static Semantics: IsValidSimpleAssignmentTarget
      4. 12.4.4 Postfix Increment Operator
      5. 12.4.5 Postfix Decrement Operator
    5. 12.5 Unary Operators
      1. 12.5.1 Static Semantics: Early Errors
      2. 12.5.2 Static Semantics: IsFunctionDefinition
      3. 12.5.3 Static Semantics: IsValidSimpleAssignmentTarget
      4. 12.5.4 The delete Operator
      5. 12.5.5 The void Operator
      6. 12.5.6 The typeof Operator
      7. 12.5.7 Prefix Increment Operator
      8. 12.5.8 Prefix Decrement Operator
      9. 12.5.9 Unary + Operator
      10. 12.5.10 Unary - Operator
      11. 12.5.11 Bitwise NOT Operator ( ~ )
      12. 12.5.12 Logical NOT Operator ( ! )
    6. 12.6 Multiplicative Operators
      1. 12.6.1 Static Semantics: IsFunctionDefinition
      2. 12.6.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.6.3 Runtime Semantics: Evaluation
    7. 12.7 Additive Operators
      1. 12.7.1 Static Semantics: IsFunctionDefinition
      2. 12.7.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.7.3 The Addition operator ( + )
      4. 12.7.4 The Subtraction Operator ( - )
      5. 12.7.5 Applying the Additive Operators to Numbers
    8. 12.8 Bitwise Shift Operators
      1. 12.8.1 Static Semantics: IsFunctionDefinition
      2. 12.8.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.8.3 The Left Shift Operator ( << )
      4. 12.8.4 The Signed Right Shift Operator ( >> )
      5. 12.8.5 The Unsigned Right Shift Operator ( >>> )
    9. 12.9 Relational Operators
      1. 12.9.1 Static Semantics: IsFunctionDefinition
      2. 12.9.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.9.3 Runtime Semantics: Evaluation
      4. 12.9.4 Runtime Semantics: InstanceofOperator(O, C)
    10. 12.10 Equality Operators
      1. 12.10.1 Static Semantics: IsFunctionDefinition
      2. 12.10.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.10.3 Runtime Semantics: Evaluation
    11. 12.11 Binary Bitwise Operators
      1. 12.11.1 Static Semantics: IsFunctionDefinition
      2. 12.11.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.11.3 Runtime Semantics: Evaluation
    12. 12.12 Binary Logical Operators
      1. 12.12.1 Static Semantics: IsFunctionDefinition
      2. 12.12.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.12.3 Runtime Semantics: Evaluation
    13. 12.13 Conditional Operator ( ? : )
      1. 12.13.1 Static Semantics: IsFunctionDefinition
      2. 12.13.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.13.3 Runtime Semantics: Evaluation
    14. 12.14 Assignment Operators
      1. 12.14.1 Static Semantics: Early Errors
      2. 12.14.2 Static Semantics: IsFunctionDefinition
      3. 12.14.3 Static Semantics: IsValidSimpleAssignmentTarget
      4. 12.14.4 Runtime Semantics: Evaluation
      5. 12.14.5 Destructuring Assignment
    15. 12.15 Comma Operator ( , )
      1. 12.15.1 Static Semantics: IsFunctionDefinition
      2. 12.15.2 Static Semantics: IsValidSimpleAssignmentTarget
      3. 12.15.3 Runtime Semantics: Evaluation
  14. 13 ECMAScript Language: Statements and Declarations
    1. 13.0 Statement Semantics
      1. 13.0.1 Static Semantics: VarDeclaredNames
      2. 13.0.2 Static Semantics: VarScopedDeclarations
      3. 13.0.3 Runtime Semantics: LabelledEvaluation
      4. 13.0.4 Runtime Semantics: Evaluation
    2. 13.1 Block
      1. 13.1.1 Static Semantics: Early Errors
      2. 13.1.2 Static Semantics: LexicallyDeclaredNames
      3. 13.1.3 Static Semantics: LexicallyScopedDeclarations
      4. 13.1.4 Static Semantics: TopLevelLexicallyDeclaredNames
      5. 13.1.5 Static Semantics: TopLevelLexicallyScopedDeclarations
      6. 13.1.6 Static Semantics: TopLevelVarDeclaredNames
      7. 13.1.7 Static Semantics: TopLevelVarScopedDeclarations
      8. 13.1.8 Static Semantics: VarDeclaredNames
      9. 13.1.9 Static Semantics: VarScopedDeclarations
      10. 13.1.10 Runtime Semantics: Evaluation
      11. 13.1.11 Runtime Semantics: BlockDeclarationInstantiation( code, env )
    3. 13.2 Declarations and the Variable Statement
      1. 13.2.1 Let and Const Declarations
      2. 13.2.2 Variable Statement
      3. 13.2.3 Destructuring Binding Patterns
    4. 13.3 Empty Statement
      1. 13.3.1 Runtime Semantics: Evaluation
    5. 13.4 Expression Statement
      1. 13.4.1 Runtime Semantics: Evaluation
    6. 13.5 The if Statement
      1. 13.5.1 Static Semantics: Early Errors
      2. 13.5.2 Static Semantics: VarDeclaredNames
      3. 13.5.3 Static Semantics: VarScopedDeclarations
      4. 13.5.4 Runtime Semantics: Evaluation
    7. 13.6 Iteration Statements
      1. 13.6.0 Semantics
      2. 13.6.1 The do-while Statement
      3. 13.6.2 The while Statement
      4. 13.6.3 The for Statement
      5. 13.6.4 The for-in and for-of Statements
    8. 13.7 The continue Statement
      1. 13.7.1 Static Semantics: Early Errors
      2. 13.7.2 Runtime Semantics: Evaluation
    9. 13.8 The break Statement
      1. 13.8.1 Static Semantics: Early Errors
      2. 13.8.2 Runtime Semantics: Evaluation
    10. 13.9 The return Statement
      1. 13.9.1 Runtime Semantics: Evaluation
    11. 13.10 The with Statement
      1. 13.10.1 Static Semantics: Early Errors
      2. 13.10.2 Static Semantics: VarDeclaredNames
      3. 13.10.3 Static Semantics: VarScopedDeclarations
      4. 13.10.4 Runtime Semantics: Evaluation
    12. 13.11 The switch Statement
      1. 13.11.1 Static Semantics: Early Errors
      2. 13.11.2 Static Semantics: LexicallyDeclaredNames
      3. 13.11.3 Static Semantics: LexicallyScopedDeclarations
      4. 13.11.4 Static Semantics: VarDeclaredNames
      5. 13.11.5 Static Semantics: VarScopedDeclarations
      6. 13.11.6 Runtime Semantics: CaseBlockEvaluation
      7. 13.11.7 Runtime Semantics: CaseSelectorEvaluation
      8. 13.11.8 Runtime Semantics: Evaluation
    13. 13.12 Labelled Statements
      1. 13.12.1 Static Semantics: Early Errors
      2. 13.12.2 Static Semantics: CurrentLabelSet
      3. 13.12.3 Static Semantics: IsLabelledFunction ( stmt )
      4. 13.12.4 Static Semantics: LexicallyDeclaredNames
      5. 13.12.5 Static Semantics: LexicallyScopedDeclarations
      6. 13.12.6 Static Semantics: TopLevelLexicallyDeclaredNames
      7. 13.12.7 Static Semantics: TopLevelLexicallyScopedDeclarations
      8. 13.12.8 Static Semantics: TopLevelVarDeclaredNames
      9. 13.12.9 Static Semantics: TopLevelVarScopedDeclarations
      10. 13.12.10 Static Semantics: VarDeclaredNames
      11. 13.12.11 Static Semantics: VarScopedDeclarations
      12. 13.12.12 Runtime Semantics: LabelledEvaluation
    14. 13.13 The throw Statement
      1. 13.13.1 Runtime Semantics: Evaluation
    15. 13.14 The try Statement
      1. 13.14.1 Static Semantics: Early Errors
      2. 13.14.2 Static Semantics: VarDeclaredNames
      3. 13.14.3 Static Semantics: VarScopedDeclarations
      4. 13.14.4 Runtime Semantics: BindingInitialization
      5. 13.14.5 Runtime Semantics: CatchClauseEvaluation
      6. 13.14.6 Runtime Semantics: Evaluation
    16. 13.15 The debugger statement
      1. 13.15.1 Runtime Semantics: Evaluation
  15. 14 ECMAScript Language: Functions and Classes
    1. 14.1 Function Definitions
      1. 14.1.1 Directive Prologues and the Use Strict Directive
      2. 14.1.2 Static Semantics: Early Errors
      3. 14.1.3 Static Semantics: BoundNames
      4. 14.1.4 Static Semantics: Contains
      5. 14.1.5 Static Semantics: ContainsExpression
      6. 14.1.6 Static Semantics: ExpectedArgumentCount
      7. 14.1.7 Static Semantics: HasInitializer
      8. 14.1.8 Static Semantics: HasName
      9. 14.1.9 Static Semantics: IsAnonymousFunctionDefinition ( production) Abstract Operation
      10. 14.1.10 Static Semantics: IsConstantDeclaration
      11. 14.1.11 Static Semantics: IsFunctionDefinition
      12. 14.1.12 Static Semantics: IsSimpleParameterList
      13. 14.1.13 Static Semantics: IsStrict
      14. 14.1.14 Static Semantics: LexicallyDeclaredNames
      15. 14.1.15 Static Semantics: LexicallyScopedDeclarations
      16. 14.1.16 Static Semantics: ReferencesSuper
      17. 14.1.17 Static Semantics: VarDeclaredNames
      18. 14.1.18 Static Semantics: VarScopedDeclarations
      19. 14.1.19 Runtime Semantics: EvaluateBody
      20. 14.1.20 Runtime Semantics: IteratorBindingInitialization
      21. 14.1.21 Runtime Semantics: InstantiateFunctionObject
      22. 14.1.22 Runtime Semantics: Evaluation
    2. 14.2 Arrow Function Definitions
      1. 14.2.1 Static Semantics: Early Errors
      2. 14.2.2 Static Semantics: BoundNames
      3. 14.2.3 Static Semantics: Contains
      4. 14.2.4 Static Semantics: ContainsExpression
      5. 14.2.5 Static Semantics: CoveredFormalsList
      6. 14.2.6 Static Semantics: ExpectedArgumentCount
      7. 14.2.7 Static Semantics: HasInitializer
      8. 14.2.8 Static Semantics: HasName
      9. 14.2.9 Static Semantics: IsSimpleParameterList
      10. 14.2.10 Static Semantics: LexicallyDeclaredNames
      11. 14.2.11 Static Semantics: LexicallyScopedDeclarations
      12. 14.2.12 Static Semantics: ReferencesSuper
      13. 14.2.13 Static Semantics: VarDeclaredNames
      14. 14.2.14 Static Semantics: VarScopedDeclarations
      15. 14.2.15 Runtime Semantics: IteratorBindingInitialization
      16. 14.2.16 Runtime Semantics: EvaluateBody
      17. 14.2.17 Runtime Semantics: Evaluation
    3. 14.3 Method Definitions
      1. 14.3.1 Static Semantics: Early Errors
      2. 14.3.2 Static Semantics: ComputedPropertyContains
      3. 14.3.3 Static Semantics: ExpectedArgumentCount
      4. 14.3.4 Static Semantics: HasComputedPropertyKey
      5. 14.3.5 Static Semantics: PropName
      6. 14.3.6 Static Semantics: ReferencesSuper
      7. 14.3.7 Static Semantics: SpecialMethod
      8. 14.3.8 Runtime Semantics: DefineMethod
      9. 14.3.9 Runtime Semantics: PropertyDefinitionEvaluation
    4. 14.4 Generator Function Definitions
      1. 14.4.1 Static Semantics: Early Errors
      2. 14.4.2 Static Semantics: BoundNames
      3. 14.4.3 Static Semantics: ComputedPropertyContains
      4. 14.4.4 Static Semantics: Contains
      5. 14.4.5 Static Semantics: HasComputedPropertyKey
      6. 14.4.6 Static Semantics: HasName
      7. 14.4.7 Static Semantics: IsConstantDeclaration
      8. 14.4.8 Static Semantics: IsFunctionDefinition
      9. 14.4.9 Static Semantics: PropName
      10. 14.4.10 Static Semantics: ReferencesSuper
      11. 14.4.11 Runtime Semantics: EvaluateBody
      12. 14.4.12 Runtime Semantics: InstantiateFunctionObject
      13. 14.4.13 Runtime Semantics: PropertyDefinitionEvaluation
      14. 14.4.14 Runtime Semantics: Evaluation
    5. 14.5 Class Definitions
      1. 14.5.1 Static Semantics: Early Errors
      2. 14.5.2 Static Semantics: BoundNames
      3. 14.5.3 Static Semantics: ConstructorMethod
      4. 14.5.4 Static Semantics: Contains
      5. 14.5.5 Static Semantics: ComputedPropertyContains
      6. 14.5.6 Static Semantics: HasName
      7. 14.5.7 Static Semantics: IsConstantDeclaration
      8. 14.5.8 Static Semantics: IsFunctionDefinition
      9. 14.5.9 Static Semantics: IsStatic
      10. 14.5.10 Static Semantics: NonConstructorMethodDefinitions
      11. 14.5.11 Static Semantics: PrototypePropertyNameList
      12. 14.5.12 Static Semantics: PropName
      13. 14.5.13 Static Semantics: StaticPropertyNameList
      14. 14.5.14 Runtime Semantics: ClassDefinitionEvaluation
      15. 14.5.15 Runtime Semantics: Evaluation
    6. 14.6 Tail Position Calls
      1. 14.6.1 Static Semantics: IsInTailPosition(nonterminal) Abstract Operation
      2. 14.6.2 Static Semantics: HasProductionInTailPosition
      3. 14.6.3 Runtime Semantics: PrepareForTailCall ( )
  16. 15 ECMAScript Language: Scripts and Modules
    1. 15.1 Scripts
      1. 15.1.1 Static Semantics: Early Errors
      2. 15.1.2 Static Semantics: IsStrict
      3. 15.1.3 Static Semantics: LexicallyDeclaredNames
      4. 15.1.4 Static Semantics: LexicallyScopedDeclarations
      5. 15.1.5 Static Semantics: VarDeclaredNames
      6. 15.1.6 Static Semantics: VarScopedDeclarations
      7. 15.1.7 Runtime Semantics: ScriptEvaluation
      8. 15.1.8 Runtime Semantics: GlobalDeclarationInstantiation
      9. 15.1.9 Runtime Semantics: ScriptEvaluationJob ( source )
    2. 15.2 Modules
      1. 15.2.0 Module Static Semantics
      2. 15.2.1 Imports
      3. 15.2.2 Exports
      4. 15.2.3 Runtime Semantics: Loader State
      5. 15.2.4 Runtime Semantics: Module Loading
      6. 15.2.5 Runtime Semantics: Module Linking
      7. 15.2.6 Runtime Semantics: Module Evaluation
  17. 16 Error Handling and Language Extensions
  18. 17 ECMAScript Standard Built-in Objects
  19. 18 The Global Object
    1. 18.1 Value Properties of the Global Object
      1. 18.1.1 Infinity
      2. 18.1.2 NaN
      3. 18.1.3 undefined
    2. 18.2 Function Properties of the Global Object
      1. 18.2.1 eval (x)
      2. 18.2.2 isFinite (number)
      3. 18.2.3 isNaN (number)
      4. 18.2.4 parseFloat (string)
      5. 18.2.5 parseInt (string , radix)
      6. 18.2.6 URI Handling Functions
    3. 18.3 Constructor Properties of the Global Object
      1. 18.3.1 Array ( . . . )
      2. 18.3.2 ArrayBuffer ( . . . )
      3. 18.3.3 Boolean ( . . . )
      4. 18.3.4 DataView ( . . . )
      5. 18.3.5 Date ( . . . )
      6. 18.3.6 Error ( . . . )
      7. 18.3.7 EvalError ( . . . )
      8. 18.3.8 Float32Array ( . . . )
      9. 18.3.9 Float64Array ( . . . )
      10. 18.3.10 Function ( . . . )
      11. 18.3.11 Int8Array ( . . . )
      12. 18.3.12 Int16Array ( . . . )
      13. 18.3.13 Int32Array ( . . . )
      14. 18.3.14 Map ( . . . )
      15. 18.3.15 Number ( . . . )
      16. 18.3.16 Object ( . . . )
      17. 18.3.17 RangeError ( . . . )
      18. 18.3.18 ReferenceError ( . . . )
      19. 18.3.19 RegExp ( . . . )
      20. 18.3.20 Set ( . . . )
      21. 18.3.21 String ( . . . )
      22. 18.3.22 Symbol ( . . . )
      23. 18.3.23 SyntaxError ( . . . )
      24. 18.3.24 TypeError ( . . . )
      25. 18.3.25 Uint8Array ( . . . )
      26. 18.3.26 Uint8ClampedArray ( . . . )
      27. 18.3.27 Uint16Array ( . . . )
      28. 18.3.28 Uint32Array ( . . . )
      29. 18.3.29 URIError ( . . . )
      30. 18.3.30 WeakMap ( . . . )
      31. 18.3.31 WeakSet ( . . . )
    4. 18.4 Other Properties of the Global Object
      1. 18.4.1 JSON
      2. 18.4.2 Math
      3. 18.4.3 Proxy ( . . . )
      4. 18.4.4 Reflect
      5. 18.4.5 System
  20. 19 Fundamental Objects
    1. 19.1 Object Objects
      1. 19.1.1 The Object Constructor
      2. 19.1.2 Properties of the Object Constructor
      3. 19.1.3 Properties of the Object Prototype Object
      4. 19.1.4 Properties of Object Instances
    2. 19.2 Function Objects
      1. 19.2.1 The Function Constructor
      2. 19.2.2 Properties of the Function Constructor
      3. 19.2.3 Properties of the Function Prototype Object
      4. 19.2.4 Function Instances
    3. 19.3 Boolean Objects
      1. 19.3.1 The Boolean Constructor
      2. 19.3.2 Properties of the Boolean Constructor
      3. 19.3.3 Properties of the Boolean Prototype Object
      4. 19.3.4 Properties of Boolean Instances
    4. 19.4 Symbol Objects
      1. 19.4.1 The Symbol Constructor class
      2. 19.4.2 Properties of the Symbol Constructor
      3. 19.4.3 Properties of the Symbol Prototype Object
      4. 19.4.4 Properties of Symbol Instances
    5. 19.5 Error Objects
      1. 19.5.1 The Error Constructor
      2. 19.5.2 Properties of the Error Constructor
      3. 19.5.3 Properties of the Error Prototype Object
      4. 19.5.4 Properties of Error Instances
      5. 19.5.5 Native Error Types Used in This Standard
      6. 19.5.6 NativeError Object Structure
  21. 20 Numbers and Dates
    1. 20.1 Number Objects
      1. 20.1.1 The Number Constructor
      2. 20.1.2 Properties of the Number Constructor
      3. 20.1.3 Properties of the Number Prototype Object
      4. 20.1.4 Properties of Number Instances
    2. 20.2 The Math Object
      1. 20.2.1 Value Properties of the Math Object
      2. 20.2.2 Function Properties of the Math Object
    3. 20.3 Date Objects
      1. 20.3.1 Overview of Date Objects and Definitions of Abstract Operations
      2. 20.3.2 The Date Constructor
      3. 20.3.3 Properties of the Date Constructor
      4. 20.3.4 Properties of the Date Prototype Object
      5. 20.3.5 Properties of Date Instances
  22. 21 Text Processing
    1. 21.1 String Objects
      1. 21.1.1 The String Constructor
      2. 21.1.2 Properties of the String Constructor
      3. 21.1.3 Properties of the String Prototype Object
      4. 21.1.4 Properties of String Instances
      5. 21.1.5 String Iterator Objects
    2. 21.2 RegExp (Regular Expression) Objects
      1. 21.2.1 Patterns
      2. 21.2.2 Pattern Semantics
      3. 21.2.3 The RegExp Constructor
      4. 21.2.4 Properties of the RegExp Constructor
      5. 21.2.5 Properties of the RegExp Prototype Object
      6. 21.2.6 Properties of RegExp Instances
  23. 22 Indexed Collections
    1. 22.1 Array Objects
      1. 22.1.1 The Array Constructor
      2. 22.1.2 Properties of the Array Constructor
      3. 22.1.3 Properties of the Array Prototype Object
      4. 22.1.4 Properties of Array Instances
      5. 22.1.5 Array Iterator Objects
    2. 22.2 TypedArray Objects
      1. 22.2.1 The %TypedArray% Intrinsic Object
      2. 22.2.2 Properties of the %TypedArray% Intrinsic Object
      3. 22.2.3 Properties of the %TypedArrayPrototype% Object
      4. 22.2.4 The TypedArray Constructors
      5. 22.2.5 Properties of the TypedArray Constructors
      6. 22.2.6 Properties of TypedArray Prototype Objects
      7. 22.2.7 Properties of TypedArray Instances
  24. 23 Keyed Collection
    1. 23.1 Map Objects
      1. 23.1.1 The Map Constructor
      2. 23.1.2 Properties of the Map Constructor
      3. 23.1.3 Properties of the Map Prototype Object
      4. 23.1.4 Properties of Map Instances
      5. 23.1.5 Map Iterator Objects
    2. 23.2 Set Objects
      1. 23.2.1 The Set Constructor
      2. 23.2.2 Properties of the Set Constructor
      3. 23.2.3 Properties of the Set Prototype Object
      4. 23.2.4 Properties of Set Instances
      5. 23.2.5 Set Iterator Objects
    3. 23.3 WeakMap Objects
      1. 23.3.1 The WeakMap Constructor
      2. 23.3.2 Properties of the WeakMap Constructor
      3. 23.3.3 Properties of the WeakMap Prototype Object
      4. 23.3.4 Properties of WeakMap Instances
    4. 23.4 WeakSet Objects
      1. 23.4.1 The WeakSet Constructor
      2. 23.4.2 Properties of the WeakSet Constructor
      3. 23.4.3 Properties of the WeakSet Prototype Object
      4. 23.4.4 Properties of WeakSet Instances
  25. 24 Structured Data
    1. 24.1 ArrayBuffer Objects
      1. 24.1.1 Abstract Operations For ArrayBuffer Objects
      2. 24.1.2 The ArrayBuffer Constructor
      3. 24.1.3 Properties of the ArrayBuffer Constructor
      4. 24.1.4 Properties of the ArrayBuffer Prototype Object
      5. 24.1.5 Properties of the ArrayBuffer Instances
    2. 24.2 DataView Objects
      1. 24.2.1 Abstract Operations For DataView Objects
      2. 24.2.2 The DataView Constructor
      3. 24.2.3 Properties of the DataView Constructor
      4. 24.2.4 Properties of the DataView Prototype Object
      5. 24.2.5 Properties of DataView Instances
    3. 24.3 The JSON Object
      1. 24.3.1 JSON.parse ( text [ , reviver ] )
      2. 24.3.2 JSON.stringify ( value [ , replacer [ , space ] ] )
      3. 24.3.3 JSON [ @@toStringTag ]
  26. 25 Control Abstraction Objects
    1. 25.1 Common Iteration Interfaces
      1. 25.1.1 The Iterable Interface
      2. 25.1.2 The Iterator Interface
      3. 25.1.3 The IteratorResult Interface
    2. 25.2 GeneratorFunction Objects
      1. 25.2.1 The GeneratorFunction Constructor
      2. 25.2.2 Properties of the GeneratorFunction Constructor
      3. 25.2.3 Properties of the GeneratorFunction Prototype Object
      4. 25.2.4 GeneratorFunction Instances
    3. 25.3 Generator Objects
      1. 25.3.1 Properties of Generator Prototype
      2. 25.3.2 Properties of Generator Instances
      3. 25.3.3 Generator Abstract Operations
    4. 25.4 Promise Objects
      1. 25.4.1 Promise Abstract Operations
      2. 25.4.2 Promise Jobs
      3. 25.4.3 The Promise Constructor
      4. 25.4.4 Properties of the Promise Constructor
      5. 25.4.5 Properties of the Promise Prototype Object
      6. 25.4.6 Properties of Promise Instances
  27. 26 Reflection
    1. 26.1 The Reflect Object
      1. 26.1.1 Reflect.apply ( target, thisArgument, argumentsList )
      2. 26.1.2 Reflect.construct ( target, argumentsList )
      3. 26.1.3 Reflect.defineProperty ( target, propertyKey, attributes )
      4. 26.1.4 Reflect.deleteProperty ( target, propertyKey )
      5. 26.1.5 Reflect.enumerate ( target )
      6. 26.1.6 Reflect.get ( target, propertyKey [ , receiver ])
      7. 26.1.7 Reflect.getOwnPropertyDescriptor ( target, propertyKey )
      8. 26.1.8 Reflect.getPrototypeOf ( target )
      9. 26.1.9 Reflect.has ( target, propertyKey )
      10. 26.1.10 Reflect.isExtensible (target)
      11. 26.1.11 Reflect.ownKeys ( target )
      12. 26.1.12 Reflect.preventExtensions ( target )
      13. 26.1.13 Reflect.set ( target, propertyKey, V [ , receiver ] )
      14. 26.1.14 Reflect.setPrototypeOf ( target, proto )
    2. 26.2 Realm Objects
      1. 26.2.1 The Reflect.Realm Constructor
      2. 26.2.2 Properties of the Reflect.Realm Constructor
      3. 26.2.3 Properties of the Reflect.Realm Prototype Object
      4. 26.2.4 Properties of Reflect.Realm Instances
    3. 26.3 Loader Objects
      1. 26.3.1 The Reflect.Loader Constructor
      2. 26.3.2 Properties of the Loader Constructor
      3. 26.3.3 Properties of the Reflect.Loader Prototype Object
      4. 26.3.4 Properties of Reflect.Loader Instances
      5. 26.3.5 Loader Iterator Objects
    4. 26.4 The System Object
    5. 26.5 Proxy Objects
      1. 26.5.1 The Proxy Constructor Function
      2. 26.5.2 Properties of the Proxy Constructor Function
  28. Annex A (informative) Grammar Summary
    1. A.1 Lexical Grammar
    2. A.2 Expressions
    3. A.3 Statements
    4. A.4 Functions and Scripts
    5. A.5 Number Conversions
    6. A.6 Universal Resource Identifier Character Classes
    7. A.7 Regular Expressions
  29. Annex B (normative) Additional ECMAScript Features for Web Browsers
    1. B.1 Additional Syntax
      1. B.1.1 Numeric Literals
      2. B.1.2 String Literals
      3. B.1.3 HTML-like Comments
      4. B.1.4 Regular Expressions Patterns
    2. B.2 Additional Built-in Properties
      1. B.2.1 Additional Properties of the Global Object
      2. B.2.2 Additional Properties of the Object.prototype Object
      3. B.2.3 Additional Properties of the String.prototype Object
      4. B.2.4 Additional Properties of the Date.prototype Object
      5. B.2.5 Additional Properties of the RegExp.prototype Object
    3. B.3 Other Additional Features
      1. B.3.1 __proto__ Property Names in Object Initializers
      2. B.3.2 Labelled Function Declarations
      3. B.3.3 Block-Level Function Declarations Web Legacy Compatibility Semantics
      4. B.3.4 FunctionDeclarations in IfStatement Statement Clauses.
      5. B.3.5 VariableStatements in Catch blocks
  30. Annex C (informative) The Strict Mode of ECMAScript
  31. Annex D (informative) Corrections and Clarifications with Possible Compatibility Impact
    1. D.1 In Edition 6
    2. D.2 In Edition 5.1
    3. D.3 In Edition 5
  32. Annex E (informative) Additions and Changes That Introduce Incompatibilities with Prior Editions
    1. E.1 In the 6th Edition
    2. E.2 In the 5th Edition
  33. Bibliography

Introduction

This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.

The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.

That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.

The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of forthcoming internationalization facilities and future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.

After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. Although that work was not completed and not published as the fourth edition of ECMAScript, it informs continuing evolution of the language. The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security.

The edition 5.1 of the ECMAScript Standard is fully aligned with the third edition of the international standard ISO/IEC 16262:2011.

This present sixth edition of the Standard………

ECMAScript is a vibrant language and the evolution of the language is not complete. Significant technical enhancement will continue with future editions of this specification.

This Ecma Standard has been adopted by the General Assembly of <month> <year>.

"DISCLAIMER

This draft document may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published, and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this section are included on all such copies and derivative works. However, this document itself may not be modified in any way, including by removing the copyright notice or references to Ecma International, except as needed for the purpose of developing any document or deliverable produced by Ecma International.

This disclaimer is valid only prior to final version of this document. After approval all rights on the standard are reserved by Ecma International.

The limited permissions are granted through the standardization phase and will not be revoked by Ecma International or its successors or assigns during this time.

This document and the information contained herein is provided on an "AS IS" basis and ECMA INTERNATIONAL DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY OWNERSHIP RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."

ECMAScript Language Specification

1 Scope

This Standard defines the ECMAScript scripting language.

2 Conformance

A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.

A conforming implementation of ECMAScript must interpret source code input in conformance with the Unicode Standard, Version 5.1.0 or later and ISO/IEC 10646. If the adopted ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the Unicode set, collection 10646.

A conforming implementation of ECMAScript that provides an application programming interface that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.

A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may provide properties not described in this specification, and values for those properties, for objects that are described in this specification.

A conforming implementation of ECMAScript may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of the “future reserved words” listed in subclause 11.6.2.2 of this specification.

3 Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

IEEE Std 754-2008: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and Electronic Engineers, New York (2008)

ISO/IEC 10646:2003: Information Technology – Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor

The Unicode Standard, Version 5.0, as amended by Unicode 5.1.0, or successor

Unicode Standard Annex #15, Unicode Normalization Forms, version Unicode 5.1.0, or successor

Unicode Standard Annex #31, Unicode Identifiers and Pattern Syntax, version Unicode 5.1.0, or successor.

ECMA-402, ECMAScript Internationalization API Specification.
http://www.ecma-international.org/publications/standards/Ecma-402.htm

ECMA-404, The JSON Data Interchange Format.
http://www.ecma-international.org/publications/standards/Ecma-404.htm

4 Overview

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.

A scripting language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and non-professional programmers. ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general purpose programming language.

ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript is now used both as a general propose programming language and to provide core scripting capabilities for a variety of host environments. Therefore the core language is specified in this document apart from any particular host environment.

Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular C, Java™, Self, and Scheme as described in:

ISO/IEC 9899:1996, Programming Languages – C.

Gosling, James, Bill Joy and Guy Steele. The Java™ Language Specification. Addison Wesley Publishing Co., 1996.

Ungar, David, and Smith, Randall B. Self: The Power of Simplicity. OOPSLA '87 Conference Proceedings, pp. 227–241, Orlando, FL, October 1987.

IEEE Standard for the Scheme Programming Language. IEEE Std 1178-1990.

4.1 Web Scripting

A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction and there is no need for a main program.

A web server provides a different host environment for server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customized user interface for a Web-based application.

Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution environment.

4.2 ECMAScript Overview

The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.

ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. In ECMAScript, an object is a collection of properties each with zero or more attributes that determine how each property can be used—for example, when the Writable attribute for a property is set to false, any attempt by executed ECMAScript code to change the value of the property fails. Properties are containers that hold other objects, primitive values, or functions. A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, Symbol and String; an object is a member of the remaining built-in type Object; and a function is a callable object. A function that is associated with an object via a property is a method.

ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include the global object, the Object object, the Function object, the Array object, the String object, the Boolean object, the Number object, the Math object, the Date object, the RegExp object, the JSON object, and the Error objects Error, EvalError, RangeError, ReferenceError, SyntaxError, TypeError and URIError.

ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.

4.2.1 Objects

ECMAScript objects are not fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial values to their properties. Each constructor is a function that has a property named “prototype” that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new Date(2009,11) creates a new Date object. Invoking a constructor without using new has consequences that depend on the constructor. For example, Date() produces a string representation of the current date and time rather than an object.

Every object created by a constructor has an implicit reference (called the object’s prototype) to the value of its constructor’s “prototype” property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.

An image of lots of boxes and arrows.
Figure 1 — Object/Prototype Relationships

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, while structure, behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:

CF is a constructor (and also an object). Five objects have been created by using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named q1 and q2. The dashed lines represent the implicit prototype relationship; so, for example, cf3’s prototype is CFp. The constructor, CF, has two properties itself, named P1 and P2, which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named CFP1 in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp’s implicit prototype chain that are not named q1, q2, or CFP1. Notice that there is no implicit prototype link between CF and CFp.

Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object’s properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.

Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern of constructor functions, prototype objects, and methods. The ECMAScript built-in object themselves follow such a class-like pattern. The ECMAScript language includes syntatic class definitions that permit programmers to concisely define objects that conform to the same class-like abstraction pattern used by the built-in objects.

4.2.2 The Strict Variant of ECMAScript

The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.

The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript code units. Because strict mode is selected at the level of a syntactic code unit, strict mode only imposes restrictions that have local effect within such a code unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple code units. A complete ECMAScript program may be composed for both strict mode and non-strict mode ECMAScript code units. In this case, strict mode only applies when actually executing code that is defined within a strict mode code unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict mode variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode code units into a single composite program.

4.3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

4.3.1 type

set of data values as defined in clause 6 of this specification

4.3.2 primitive value

member of one of the types Undefined, Null, Boolean, Number, Symbol, or String as defined in clause 6

NOTE A primitive value is a datum that is represented directly at the lowest level of the language implementation.

4.3.3 object

member of the type Object

NOTE An object is a collection of properties and has a single prototype object. The prototype may be the null value.

4.3.4 constructor

function object that creates and initializes objects

NOTE The value of a constructor’s “prototype” property is a prototype object that is used to implement inheritance and shared properties.

4.3.5 prototype

object that provides shared properties for other objects

NOTE When a constructor creates an object, that object implicitly references the constructor’s “prototype” property for the purpose of resolving property references. The constructor’s “prototype” property can be referenced by the program expression constructor.prototype, and properties added to an object’s prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the Object.create built-in function.

4.3.6 ordinary object

object that has the default behaviour for the essential internal methods that must be supported by all objects.

4.3.7 exotic object

object that has does not have the default behaviour for one or more of the essential internal methods that must be supported by all objects.

NOTE Any object that is not an ordinary object is an exotic object.

4.3.8 standard object

object whose semantics are defined by this specification.

4.3.9 built-in object

object supplied by an ECMAScript implementation, independent of the host environment, that is present at the start of the execution of an ECMAScript program

NOTE Standard built-in objects are defined in this specification, and an ECMAScript implementation may specify and define others. A built-in constructor is a built-in object that is also a constructor.

4.3.10 undefined value

primitive value used when a variable has not been assigned a value

4.3.11 Undefined type

type whose sole value is the undefined value

4.3.12 null value

primitive value that represents the intentional absence of any object value

4.3.13 Null type

type whose sole value is the null value

4.3.14 Boolean value

member of the Boolean type

NOTE There are only two Boolean values, true and false.

4.3.15 Boolean type

type consisting of the primitive values true and false

4.3.16 Boolean object

member of the Object type that is an instance of the standard built-in Boolean constructor

NOTE A Boolean object is created by using the Boolean constructor in a new expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.

4.3.17 String value

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer

NOTE A String value is a member of the String type. Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.

4.3.18 String type

set of all possible String values

4.3.19 String object

member of the Object type that is an instance of the standard built-in String constructor

NOTE A String object is created by using the String constructor in a new expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the String constructor as a function (21.1.1.1).

4.3.20 Number value

primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value

NOTE A Number value is a member of the Number type and is a direct representation of a number.

4.3.21 Number type

set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and negative infinity

4.3.22 Number object

member of the Object type that is an instance of the standard built-in Number constructor

NOTE A Number object is created by using the Number constructor in a new expression, supplying a Number value as an argument. The resulting object has an internal slot whose value is the Number value. A Number object can be coerced to a Number value by calling the Number constructor as a function (20.1.1.1).

4.3.23 Infinity

number value that is the positive infinite Number value

4.3.24 NaN

number value that is an IEEE 754 “Not-a-Number” value

4.3.25 Symbol value

primitive value that represents a unique, non-String Object property key.

4.3.26 Symbol type

set of all possible Symbol values

4.3.27 Symbol object

member of the Object type that is an instance of the standard built-in Symbol constructor

4.3.28 function

member of the Object type that may be invoked as a subroutine

NOTE In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function’s code may or may not be written in ECMAScript.

4.3.29 built-in function

built-in object that is a function

NOTE Examples of built-in functions include parseInt and Math.exp. An implementation may provide implementation-dependent built-in functions that are not described in this specification.

4.3.30 property

association between a key and a value that is a part of an object. The key be either a String value or a Symbol value.

NOTE Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.

4.3.31 method

function that is the value of a property

NOTE When a function is called as a method of an object, the object is passed to the function as its this value.

4.3.32 built-in method

method that is a built-in function

NOTE Standard built-in methods are defined in this specification, and an ECMAScript implementation may specify and provide other additional built-in methods.

4.3.33 attribute

internal value that defines some characteristic of a property

4.3.34 own property

property that is directly contained by its object

4.3.35 inherited property

property of an object that is not an own property but is a property (either own or inherited) of the object’s prototype

4.4 Organization of This Specification

The remainder of this specification is organized as follows:

Clause 5 defines the notational conventions used throughout the specification.

Clauses 6−8.5.1 define the execution environment within which ECMAScript programs operate.

Clauses 10−16 define the actual ECMAScript programming language includings its syntactic encoding and the execution semantics of all language features.

Clauses 17−26 define the ECMAScript standard library. It includes the definitions of all of the standard objects that are available for use by ECMAScript programs as they execute.

5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

5.1.2 The Lexical and RegExp Grammars

A lexical grammar for ECMAScript is given in clause 11. This grammar has as its terminal symbols Unicode code points that conform to the rules for SourceCharacter defined in 10.1. It defines a set of productions, starting from the goal symbol InputElementDiv or InputElementRegExp, that describe how sequences of such code points are translated into a sequence of input elements.

Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (11.9). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form “/**/” regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.

A RegExp grammar for ECMAScript is given in 21.2.1. This grammar also has as its terminal symbols the code points as defined by SourceCharacter. It defines a set of productions, starting from the goal symbol Pattern, that describe how sequences of code points are translated into regular expression patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.

5.1.3 The Numeric String Grammar

Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical grammar having to do with numeric literals and has as its terminal symbols SourceCharacter. This grammar appears in 7.1.3.1.

Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.

5.1.4 The Syntactic Grammar

The syntactic grammar for ECMAScript is given in clauses 11, 12, 13, 14, and 15. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from the goal symbol Script, that describe how sequences of tokens can form syntactically correct independent components of an ECMAScript programs.

When a stream of code points is to be parsed as an ECMAScript script, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The script is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal Script, with no tokens left over.

Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.

The syntactic grammar as presented in clauses 12, 13, 14 and 15 is actually not a complete account of which token sequences are accepted as correct ECMAScript scripts. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a line terminator character appears in certain “awkward” places.

In certain cases in order to avoid ambiguities the syntactic grammar uses generalized productions that permit token sequences that are not valid ECMAScript scripts. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive supplemental grammar is provided that further restricts the acceptable token sequences. In certain contexts, when explicitly specific, the input elements corresponding to such a production are parsed again using a goal symbol of a supplemental grammar. The script is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the supplemental goal symbol, with no tokens left over.

5.1.5 Grammar Notation

Terminal symbols of the lexical, RegExp, and numeric string grammars, and some of the terminal symbols of the other grammars, are shown in fixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a script exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin range, as opposed to any similar-looking code points from other Unicode ranges.

Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

WhileStatement :
while ( Expression ) Statement

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:

ArgumentList :
AssignmentExpression
ArgumentList , AssignmentExpression

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.

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:

VariableDeclaration :
BindingIdentifier Initializeropt

is a convenient abbreviation for:

VariableDeclaration :
BindingIdentifier
BindingIdentifier Initializer

and that:

IterationStatement :
for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement

is a convenient abbreviation for:

IterationStatement :
for ( LexicalDeclaration ; Expressionopt ) Statement
for ( LexicalDeclaration Expression ; Expressionopt ) Statement

which in turn is an abbreviation for:

IterationStatement :
for ( LexicalDeclaration ; ) Statement
for ( LexicalDeclaration ; Expression ) Statement
for ( LexicalDeclaration Expression ; ) Statement
for ( LexicalDeclaration Expression ; Expression ) Statement

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

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, preceeded by an underscore, appended to the parameterized nonterminal symbol. This means that:

StatementList[Return] :
ReturnStatement
ExpressionStatement

is a convenient abbreviation for:

StatementList :
ReturnStatement
ExpressionStatement
StatementList_Return :
ReturnStatement
ExpressionStatement

and that:

StatementList[Return, In] :
ReturnStatement
ExpressionStatement

is an abbreviation for:

StatementList :
ReturnStatement
ExpressionStatement
StatementList_Return :
ReturnStatement
ExpressionStatement
StatementList_In :
ReturnStatement
ExpressionStatement
StatementList_Return_In :
ReturnStatement
ExpressionStatement

Multiple parameters produce a combinatory number of productions, not all of which are necessarily referenced in a complete grammar.

References to nonterminals on the right hand side of a production can also be parameterized. For example:

StatementList :
ReturnStatement
ExpressionStatement[In]

is equivalent to saying:

StatementList :
ReturnStatement
ExpressionStatement_In

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

VariableDeclaration :
BindingIdentifier Initializer[In]opt

is an abbreviation for:

VariableDeclaration :
BindingIdentifier
BindingIdentifier 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:

VariableDeclaration[In] :
BindingIdentifier Initializer[?In]

is an abbreviation for:

VariableDeclaration :
BindingIdentifier Initializer
VariableDeclaration_In :
BindingIdentifier 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:

StatementList[Return] :
[+Return] ReturnStatement
ExpressionStatement

is an abbreviation for:

StatementList :
ExpressionStatement
StatementList_Return :
ReturnStatement
ExpressionStatement

and that

StatementList[Return] :
[~Return] ReturnStatement
ExpressionStatement

is an abbreviation for:

StatementList :
ReturnStatement
ExpressionStatement
StatementList_Return :
ExpressionStatement

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
1 2 3 4 5 6 7 8 9

which is merely a convenient abbreviation for:

NonZeroDigit ::
1
2
3
4
5
6
7
8
9

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.

If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it indicates that the production may not be used if the immediately following input token is a member of the given set. The set can be written as a list of terminals enclosed in curly braces. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand. If the set consists of a single terminal the phrase “[lookahead ≠ terminal]” may be used.

For example, given the definitions

DecimalDigit :: one of
0 1 2 3 4 5 6 7 8 9
DecimalDigits ::
DecimalDigit
DecimalDigits DecimalDigit

the definition

LookaheadExample ::
n [lookahead ∉ {1, 3, 5, 7, 9}] DecimalDigits
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.

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] Expression ;

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.

The lexical grammar has multiple goal symbols and the appropriate goal symbol to use depends upon the syntactic grammar context. If a phrase of the form “[Lexical goal LexicalGoalSymbol]” appears on the right-hand-side of a syntactic production then the next token must be lexically recognized using the indicated goal symbol. In the absence of such a phrase the default lexical goal symbol is used.

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-code point token, it represents the sequence of code points that would make up such a token.

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:

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.

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, in which case the names and usage of the parameters must be provided as part of the algorithm’s definition. 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.

Algorithms may be associated with productions of 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 script souce code.

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.

Unless explicitly specified otherwise, all chain productions have an implicit associated definition for every algorithm that might be applied to that production’s left-hand side nonterminal. The implicit definition simply reapplies the same algorithm name with the same parameters, if any, to the chain production’s sole right-hand side nonterminal and then result. For example, assume there is a production:

Block :
{ StatementList }

but there is no corresponding Evaluation algorithm that is explicitly specified for that production. If in some algorithm there is a statement of the form: “Return the result of evaluating Block” it is implicit that an Evaluation algorithm exists of the form:

Runtime Semantics: Evaluation

Block : { StatementList }
  1. Return the result of evaluating StatementList.

For clarity of expression, algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

  1. Top-level step
    1. Substep.
    2. Substep.
      1. Subsubstep.
        1. Subsubsubstep
          1. Subsubsubsubstep
            1. 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 may assert 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.

Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers, which do not include infinities and do not include a negative zero that is distinguished from positive zero. Algorithms in this standard that model floating-point arithmetic include explicit steps, where necessary, to handle infinities and signed zero and to perform rounding. If a mathematical operation or function is applied to a floating-point number, it should be understood as being applied to the exact mathematical value represented by that floating-point number; such a floating-point number must be finite, and if it is +0 or −0 then the corresponding mathematical value is simply 0.

The mathematical function abs(x) produces the absolute value of x, which is x if x is negative (less than zero) and otherwise is x itself.

The mathematical function sign(x) produces 1 if x is positive and −1 if x is negative. The sign function is not used in this standard for cases when x is zero.

The mathematical function min(x1, x2, ..., xn) produces the mathematically smallest of x1 through xn.

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

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

NOTE floor(x) = x−(x modulo 1).

5.3 Static Semantic Rules

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 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.

Unless otherwise specified every grammar production alternative in this specification implicitly has a definition for a static semantic rule named Contains which takes an argument named symbol whose value is a terminal or nonterminal of the grammar that includes the associated production. The default definition of Contains is:

  1. For each terminal and nonterminal grammar symbol, sym, in the definition of this production do
    1. If sym is the same grammar symbol as symbol, return true.
    2. If sym is a nonterminal, then
      1. Let contained be the result of sym Contains symbol.
      2. If contained is true, return true.
  2. Return false.

The above definition is explicitly over-ridden for specific productions.

A special kind of static semantic rule is an Early Error Rule. Early error rules define early error conditions (see clause 16) 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, validate all of the early error rules of the productions used to parse that Script. If any of the early error rules are violated the Script is invalid and cannot be evaluated.

6 ECMAScript Data Types and Values

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.

Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause.

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, 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 finite ordered sequences of zero or more 16-bit unsigned integer values (“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 nonnegative 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.

Where ECMAScript operations interpret String values, each element is interpreted as a single UTF-16 code unit. However, ECMAScript does not place any restrictions or requirements on the sequence of code units in a String value, so they may be ill-formed when interpreted as UTF-16 code unit sequences. Operations that do not interpret String contents treat them as sequences of undifferentiated 16-bit unsigned integers. No operations ensure that Strings are in a normalized form. Only operations that are explicitly specified to be language or locale sensitive produce language-sensitive results

NOTE The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source code is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.

Some operations interpret String contents as UTF-16 encoded Unicode code points. In that case the interpretation is:

  • A code unit in the range 0 to 0xD7FF or in the range 0xE000 to 0xFFFF is interpreted as a code point with the same value.

  • A sequence of two code units, where the first code unit c1 is in the range 0xD800 to 0xDBFF and the second code unit c2 is in the range 0xDC00 to 0xDFFF, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c20xDC00) + 0x10000.

  • A code unit that is in the range 0xD800 to 0xDFFF, but is not part of a surrogate pair, is interpreted as a code point with the same value.

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 values 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 Code Realms (8.1.2.5).

Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed in Table 1.

Table 1— Well-known Symbols
Specification Name [[Description]] Value and Purpose
@@create "Symbol.create" A method used to allocate an object. Called from the [[Construct]] internal method.
@@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.
@@isConcatSpreadable "Symbol.isConcatSpreadable" A Boolean value that if true indicates that an object should be flatten to its array elements by Array.prototype.concat.
@@isRegExp "Symbol.isRegExp" A Boolean value that if true indicates that an object may be used as a regular expression.
@@iterator "Symbol.iterator" A method that returns the default iterator for an object. Called by the semantics of the for-of statement.
@@toPrimitive "Symbol.toPrimitive" A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
@@toStringTag "Symbol.toStringTag" A string value that is used in the creation of the default string description of an object. Called by the built-in method Object.prototype.toString.
@@unscopables "Symbol.unscopables" An Array of string values that are property names that are excluded from the with environment bindings of the associated objects.

6.1.6 The Number Type

The Number type has exactly 18437736874454810627 (that is, 264−253+3) values, representing the double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990 (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-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.

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 +∞ and −∞, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)

The other 18437736874454810624 (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 and −0, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)

The 18437736874454810622 (that is, 264−253−2) finite nonzero values are of two kinds:

18428729675200069632 (that is, 264−254) of them are normalized, having the form

s × m × 2e

where s is +1 or −1, m is a positive integer less than 253 but not less than 252, and e is an integer ranging from −1074 to 971, inclusive.

The remaining 9007199254740990 (that is, 253−2) values are denormalized, having the form

s × m × 2e

where s is +1 or −1, m is a positive integer less than 252, 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 (indeed, the integer 0 has two representations, +0 and -0).

A finite number has an odd significand if it is nonzero 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 nonzero 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 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 +∞; if −21024 was chosen, replace it with −∞; if +0 was chosen, replace it with −0 if and only if x is less than zero; 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 “round to nearest” mode.)

Some ECMAScript operators deal only with integers in the range −231 through 231−1, inclusive, or in the range 0 through 232−1, inclusive. These operators accept any value of the Number type but first convert each such value to one of 232 integer values. See the descriptions of the ToInt32 and ToUint32 operators in 7.1.5 and 7.1.6, respectively.

6.1.7 The Object Type

An Object is logically a collection of properties. Each property is either a data property, or an accessor property:

  • A data property associates a key value with an ECMAScript language value and a set of Boolean attributes.

  • An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve an ECMAScript language value that is associated with the property.

Properties are identified using key values. A key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty string, are valid as property keys.

An integer index is a String-valued property key that is a canonical numeric String (see 7.1.16) and whose numeric value is either +0 or a positive integer ≤ 253−1. An array index is an integer index whose numeric value i is in the range +0 ≤ i < 232−1.

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 properties 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. Ordinary objects are the most common form of objects and have the default object semantics. An exotic object is any form of object whose property semantics differ in any way from the default semantics.

6.1.7.1 Property Attributes

Attributes are used in this specification to define and explain the state of Object properties. A data property associates a key value with the attributes listed in Table 2.

Table 2 — Attributes of a Data Property
Attribute Name Value Domain Description
[[Value]] Any ECMAScript language type The value retrieved by a get access of the property.
[[Writable]] Boolean If false, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]] Boolean If true, the property will be enumerated by a for-in enumeration (see 13.6.3.6). Otherwise, the property is said to be non-enumerable.
[[Configurable]] Boolean If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]], or changing [[Writable]] to false) will fail.

An accessor property associates a key value with the attributes listed in Table 3.

Table 3 — Attributes of an Accessor Property
Attribute Name Value Domain Description
[[Get]] Object or Undefined If the value is an Object it must be a function Object. The function’s [[Call]] internal method (Table 6) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]] Object or Undefined If the value is an Object it must be a function Object. The function’s [[Call]] internal method (Table 6) 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]] Boolean If true, the property is to be enumerated by a for-in enumeration (see 13.6.3.6). Otherwise, the property is said to be non-enumerable.
[[Configurable]] Boolean If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the initial values of a property’s attributes are not explicitly specified by this specification, the default value defined in Table 4 is used.

Table 4 — Default Attribute Values
Attribute Name Default Value
[[Value]] undefined
[[Get]] undefined
[[Set]] undefined
[[Writable]] false
[[Enumerable]] false
[[Configurable]] false

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. 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 ECMA 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.

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

Table 5 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.

The “Signature” column of Table 5 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. An internal method implicitly returns a Completion Record as described in 6.2.2. In addition to its parameters, an internal method always has access to the object upon which it is invoked as a method.

Table 5 — Essential Internal Methods
Internal Method Signature Description
[[GetPrototypeOf]] ()Object or Null Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties.
[[SetPrototypeOf]] (Object or Null)Boolean Associate with an object 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 an object.
[[PreventExtensions]] ( )Boolean Control whether new properties may be added to an object. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
[[GetOwnProperty]]

(propertyKey) →

Undefined or Property Descriptor

Returns a Property Descriptor for the own property of this object whose key is propertyKey, or undefined if no such property exists.
[[HasProperty]] (propertyKey) Boolean Returns a Boolean value indicating whether the object already has either an own or inherited property whose key is propertyKey.
[[Get]] (propertyKey, Receiver) any Retrieve the value of an object’s property using the propertyKey parameter. 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 Try to set the value of an object’s property indentified by 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 indicating that the property value was set or false indicating that it could not be set.
[[Delete]] (propertyKey) Boolean Removes the own property indentified by the propertyKey parameter from the object. Return false if the property was not deleted and is still present. Return true if the property was deleted or was not present.
[[DefineOwnProperty]] (propertyKey, PropertyDescriptor) Boolean Creates or alters the named own property to have the state described by a Property Descriptor. Returns true indicating that the property was successfully created/updated or false indicating that the property could not be created or updated.
[[Enumerate]] ()Object Returns an iterator object over the keys of the string-keyed enumerable properties of the object.
[[OwnPropertyKeys]] ()Array of propertyKey Returns an Array object whose elements are all of the own property keys for the object.

Table 6 summarizes additional essential internal methods that are supported by objects that may be called as functions.

Table 6 — Additional Essential Internal Methods of Function Objects
Internal Method Signature Description
[[Call]] (any, a List of any) any Executes code associated with the object. Invoked via a function call expression. The arguments to the internal method are a this value and a list containing the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
[[Construct]] (a List of any) Object Creates an object. Invoked via the new operator. The arguments to the internal method are the arguments passed to the new operator. 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 method for ordinary objects and standard exotic objects are specified in clause 8.5.1. If any specified use of an exotic object's internal methods 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.

Definitions:

● The target of an internal method is the object the internal method is called upon.

● 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 SameValue algorithm specified in 7.2.3.

[[GetPrototypeOf]] ( )

● The Type of the return value must be either Object or Null.

● If target is non-extensible, and [[GetPrototypeOf]] returns a value v, then any future calls to [[GetPrototypeOf]] should return the SameValue as v.

● An object’s prototype chain must have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result must eventually lead to the value null.

[[SetPrototypeOf]] (V)

● The Type of the return value must be Boolean.

● If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue as the target’s observed [[GetPrototypeOf]] value.

[[PreventExtensions]] ( )

● The Type of the return value must be 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 Type of the return value must be either Object or Undefined.

● If the Type of the return value is Object, that object must be a complete property descriptor (see 6.2.4.6).

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

● If a property P is described as a data property with Desc.[[Value]] equal to v and Desc.[[Writable]] and Desc.[[Configurable]] are both false, then the SameValue must be returned for the Desc.[[Value]] attribute of the property on all future calls to [[GetOwnProperty]] ( P ).

● If P’s attributes other than [[Writable]] may change over time or if the property might disappear, 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)

[[DefineOwnProperty]] (P, Desc)

● The Type of the return value must be 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 non-configurable writable own data property. A non-configurable writable data property can be changed into a non-configurable non-writable data property.

2. All attributes in 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 Type of the return value must be Boolean.

● If P was previously observed as a non-configurable data or accessor own property of the target, [[HasProperty]] must return true.

[[Get]] (P, Receiver)

● 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.

● 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 Type of the return value must be 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 Type of the return value must be Boolean.

● If P was previously observed to be a non-configurable own data or accessor property of the target, [[Delete]] must return false.

[[Enumerate]] ( )

● The Type of the return value must be Object.

[[OwnPropertyKeys]] ( )

● The Type of the return value must be Object.

● The return value must be an exotic Array object.

● The returned array must contain at least the keys of all non-configurable own properties that have previously been observed.

● If the object is non-extensible, the returned array must contain only the keys of all own properties of the object that are observable using [[GetOwnProperty]].

[[Construct]] ( )

● The Type of the return value must be Object.

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. Determination of the current Realm and its intrinsics is described in 8.1.2.5. The well-known intrinsics are listed in Table 7.

Table 7 — Well-known Intrinsic Objects
Intrinsic Name Global Name ECMAScript Language Association
%ObjectPrototype% The initial value of the "prototype" data property of the intrinsic %Object%. (19.1.3)
%ThrowTypeError% A function that, when called, throws a TypeError exception.
%FunctionPrototype% The initial value of the "prototype" data property of the intrinsic %Function%.
%Object% "Object" The Object constructor (19.1.1)
%ObjProto_toString% The initial value of the "toString" data property of the intrinsic %ObjectPrototype%. (19.1.3.6)
%Function% "Function" The Function constructor (19.2.1)
%Array% "Array" The Array constructor (22.1.1)
%ArrayPrototype% The initial value of the "prototype" data property of the intrinsic %Array%.
%ArrayProto_values% The initial value of the "values" data property of the intrinsic %ArrayPrototype%. (22.1.3.29)
%ArrayIteratorPrototype% The prototype object used for
Iterator objects created by the CreateArrayIterator abstract operation.
%String% "String" The String constructor (21.1.1)
%StringPrototype% The initial value of the "prototype" data property of the intrinsic %String%.
%StringIteratorPrototype% The prototype object used for
Iterator objects created by the CreateStringIterator abstract operation
%Boolean% "Boolean" The initial value of the global object property named "Boolean".
%BooleanPrototype% The initial value of the "prototype" data property of the intrinsic %Boolean%.
%Number% "Number" The initial value of the global object property named "Number".
%NumberPrototype% The initial value of the "prototype" data property of the intrinsic %Number%.
%Date% "Date" The initial value of the global object property named "Date".
%DatePrototype% The initial value of the "prototype" data property of the intrinsic %Date%.
%RegExp% "RegExp" The initial value of the global object property named "RegExp".
%RegExpPrototype% The initial value of the "prototype" data property of the intrinsic %RegExp%.
%Map% "Map" The initial value of the global object property named "Map".
%MapPrototype% The initial value of the "prototype" data property of the intrinsic %Map%.
%MapIteratorPrototype% The prototype object used for
Iterator objects created by the CreateMapIterator abstract operation
%WeakMap% "WeakMap" The initial value of the global object property named "WeakMap".
%WeakMapPrototype% The initial value of the "prototype" data property of the intrinsic %WeakMap%.
%Set% "Set" The initial value of the global object property named "Set".
%SetPrototype% The initial value of the "prototype" data property of the intrinsic %Set%.
%WeakSet% "WeakSet" The initial value of the global object property named "WeakSet".
%WeakSetPrototype% The initial value of the "prototype" data property of the intrinsic %WeakSet%.
%SetIteratorPrototype% The prototype object used for
Iterator objects created by the CreateSetIterator abstract operation
%GeneratorFunction% The constructor of generator functions.
%Generator% The initial value of the prototype property of the %GeneratorFunction intrinsic
%GeneratorPrototype% The initial value of the prototype property of the %Generator% intrinsic
%Error%
%EvalError%
%RangeError%
%ReferenceError%
%SyntaxError%
%TypeError%
%URIError%
%ErrorPrototype%
%EvalErrorPrototype%
%RangeErrorPrototype%
%ReferenceErrorPrototype%
%SyntaxErrorPrototype%
%TypeErrorPrototype%
%URIErrorPrototype%
%ArrayBuffer%
%ArrayBufferPrototype% The initial value of the "prototype" data property of the intrinsic %ArrayBuffer%.
%TypedArray%
%TypedArrayPrototype% The initial value of the "prototype" data property of the intrinsic %TypedArray%.
%Int8Array%
%Int8ArrayPrototype%
%DataView%
%DataViewPrototype%
%ThrowTypeError% A function object that unconditionally throws a new instance of %TypeError%.
%Realm%
%RealmPrototype%
%Promise%
%PromisePrototype%
%Loader%
%LoaderPrototype%
%LoaderIteratorPrototype%
%ReturnUndefined%
%Symbol%

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 are Reference, List, Completion, Property Descriptor, Lexical Environment, Environment Record, 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 List and Record Specification Type

The List type is used to explain the evaluation of argument lists (see 12.3.6) 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 convience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3rd element of the List arguments.

The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is either an ECMAScript value or an abstract value represented by a name associated with the Record type. Field names are always enclosed in double brackets, for example [[value]]

For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, {[[field1]]: 42, [[field2]]: 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}.

6.2.2 The Completion Record Specification Type

The Completion type is a Record 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.

Values of the Completion type are Record values whole fields are defined as by Table 8.

Table 8 — Completion Record Fields
Field Name Value Meaning
[[type]] One of normal, break, continue, return, or throw The type of completion that occurred.
[[value]] any ECMAScript language value or empty The value that was produced.
[[target]] any ECMAScript string or empty The target label for directed control transfers.

The term “abrupt completion” refers to any completion with a [[type]] value other than normal.

6.2.2.1 NormalCompletion

The abstract operation NormalCompletion with a single argument, such as:

  1. Return NormalCompletion(argument).

Is a shorthand that is defined as follows:

  1. Return Completion{[[type]]: normal, [[value]]: argument, [[target]]:empty}.

6.2.2.2 Implicit Completion Values

The algorithms of this specification often implicitly return Completion Records whose [[type]] is normal. Unless it is otherwise obvious from the context, an algorithm statement that returns a value that is not a Completion Record, such as:

  1. Return "Infinity".

Generally means the same thing as:

  1. Return NormalCompletion("Infinity").

A “return” statement without a value in an algorithm step means the same thing as:

  1. Return NormalCompletion(undefined).

Similarly, any reference to a Completion Record value that is in a context that does not explicitly require a complete Completion Record value is equivalent to an explicit reference to the [[value]] field of the Completion Record value unless the Completion Record is an abrupt completion.

6.2.2.3 Throw an Exception

Algorithms steps that say to throw an exception, such as

  1. Throw a TypeError exception.

mean the same things as:

  1. Return Completion{[[type]]: throw, [[value]]: a newly created TypeError object, [[target]]:empty}.

6.2.2.4 ReturnIfAbrupt

Algorithms steps that say

  1. ReturnIfAbrupt(argument).

mean the same thing as:

  1. If argument is an abrupt completion, then return argument.
  2. Else if argument is a Completion Record, then let argument be argument.[[value]].

6.2.3 The Reference Specification Type

NOTE The Reference 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.

A Reference is a resolved name or property binding. A Reference consists of three components, the base value, the referenced name and the Boolean valued strict reference flag. The base value is either undefined, an Object, a Boolean, a String, a Symbol, a Number, or an environment record (8.1.1). A base value of undefined indicates that the Reference could not be resolved to a binding. The referenced name is a String or Symbol value.

A Super Reference is a Reference that is used to represents a name binding that was expressed using the super keyword. A Super Reference has an additional thisValue component and its base value will never be an environment record.

The following abstract operations are used in this specification to access the components of references:

  • GetBase(V). Returns the base value component of the reference V.

  • GetReferencedName(V). Returns the referenced name component of the reference V.

  • IsStrictReference(V). Returns the strict reference flag component of the reference V.

  • HasPrimitiveBase(V). Returns true if Type(base) is a Boolean, String, Symbol, or Number.

  • IsPropertyReference(V). Returns true if either the base value is an object or HasPrimitiveBase(V) is true; otherwise returns false.

  • IsUnresolvableReference(V). Returns true if the base value is undefined and false otherwise.

  • IsSuperReference(V). Returns true if this reference has a thisValue component.

The following abstract operations are used in this specification to operate on references:

6.2.3.1 GetValue (V)

  1. ReturnIfAbrupt(V).
  2. If Type(V) is not Reference, return V.
  3. Let base be GetBase(V).
  4. If IsUnresolvableReference(V), throw a ReferenceError exception.
  5. If IsPropertyReference(V), then
    1. If HasPrimitiveBase(V) is true, then
      1. Assert: In this case, base will never be null or undefined.
      2. Let base be ToObject(base).
    2. Return the result of calling the [[Get]] internal method of base passing GetReferencedName(V) and GetThisValue(V) as the arguments.
  6. Else base must be an environment record,
    1. Return the result of calling the GetBindingValue (see 8.1.1) concrete method of base passing GetReferencedName(V) and IsStrictReference(V) as arguments.

NOTE The object that may be created in step 5.a.ii 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.

6.2.3.2 PutValue (V, W)

  1. ReturnIfAbrupt(V).
  2. ReturnIfAbrupt(W).
  3. If Type(V) is not Reference, throw a ReferenceError exception.
  4. Let base be GetBase(V).
  5. If IsUnresolvableReference(V), then
    1. If IsStrictReference(V) is true, then
      1. Throw ReferenceError exception.
    2. Let globalObj be the result of the abstract operation GetGlobalObject.
    3. Return Put(globalObj,GetReferencedName(V), W, false).
  6. Else if IsPropertyReference(V), then
    1. If HasPrimitiveBase(V) is true, then
      1. Assert: In this case, base will never be null or undefined.
      2. Set base to ToObject(base).
    2. Let succeeded be the result of calling the [[Set]] internal method of base passing GetReferencedName(V), W, and GetThisValue(V) as arguments.
    3. ReturnIfAbrupt(succeeded).
    4. If succeeded is false and IsStrictReference(V) is true, then throw a TypeError exception.
    5. Return.
  7. Else base must be a Reference whose base is an environment record.
    1. Return the result of calling the SetMutableBinding (8.1.1) concrete method of base, passing GetReferencedName(V), W, and IsStrictReference(V) as arguments.

NOTE The object that may be created in step 6.a.ii is not accessible outside of the above algorithm and the ordinary object [[Set]] internal method. An implementation might choose to avoid the actual creation of that object.

6.2.3.3 GetThisValue (V)

  1. Assert: IsPropertyReference(V) is true.
  2. If IsSuperReference(V), then
    1. Return the value of the thisValue component of the reference V.
  3. Return GetBase(V).

6.2.4 The Property Descriptor Specification Type

The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. Values of the Property Descriptor type are Records. Each field’s name is an attribute name and its value is a corresponding attribute value as specified in 6.1.7.1. In addition, any field may be present or absent. 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. A generic Property Descriptor is a Property Descriptor value that is neither a data Property Descriptor nor an accessor Property Descriptor. A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the fields that correspond to the property attributes defined in either Table 2 or Table 3.

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

6.2.4.1 IsAccessorDescriptor ( Desc )

When the abstract operation IsAccessorDescriptor is called with Property Descriptor Desc, the following steps are taken:

  1. If Desc is undefined, then return false.
  2. If both Desc.[[Get]] and Desc.[[Set]] are absent, then return false.
  3. Return true.

6.2.4.2 IsDataDescriptor ( Desc )

When the abstract operation IsDataDescriptor is called with Property Descriptor Desc, the following steps are taken:

  1. If Desc is undefined, then return false.
  2. If both Desc.[[Value]] and Desc.[[Writable]] are absent, then return false.
  3. Return true.

6.2.4.3 IsGenericDescriptor ( Desc )

When the abstract operation IsGenericDescriptor is called with Property Descriptor Desc, the following steps are taken:

  1. If Desc is undefined, then return false.
  2. If IsAccessorDescriptor(Desc) and IsDataDescriptor(Desc) are both false, then return true.
  3. Return false.

6.2.4.4 FromPropertyDescriptor ( Desc )

When the abstract operation FromPropertyDescriptor is called with Property Descriptor Desc, the following steps are taken:

  1. If Desc is undefined, then return undefined.
  2. Let obj be ObjectCreate(%ObjectPrototype%).
  3. Assert: obj is an extensible ordinary object with no own properties.
  4. If Desc has a [[Value]] field, then
    1. Call OrdinaryDefineOwnProperty with arguments obj, "value", and PropertyDescriptor{[[Value]]: Desc.[[Value]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}
  5. If Desc has a [[Writable]] field, then
    1. Call OrdinaryDefineOwnProperty with arguments obj, "writable", and PropertyDescriptor{[[Value]]: Desc.[[Writable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
  6. If Desc has a [[Get]] field, then
    1. Call OrdinaryDefineOwnProperty with arguments obj, "get", and PropertyDescriptor{[[Value]]: Desc.[[Get]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
  7. If Desc has a [[Set]] field, then
    1. Call OrdinaryDefineOwnProperty with arguments obj, "set", and PropertyDescriptor{[[Value]]: Desc.[[Set]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
  8. If Desc has an [[Enumerable]] field, then
    1. Call OrdinaryDefineOwnProperty with arguments obj, "enumerable", and PropertyDescriptor{[[Value]]: Desc.[[Enumerable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
  9. If Desc has a [[Configurable]] field, then
    1. Call OrdinaryDefineOwnProperty with arguments obj , "configurable", and PropertyDescriptor{[[Value]]: Desc.[[Configurable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
  10. Return obj.

6.2.4.5 ToPropertyDescriptor ( Obj )

When the abstract operation ToPropertyDescriptor is called with object Obj, the following steps are taken:

  1. ReturnIfAbrupt(Obj).
  2. If Type(Obj) is not Object throw a TypeError exception.
  3. Let desc be a new Property Descriptor that initially has no fields.
  4. If HasProperty(Obj, "enumerable") is true, then
    1. Let enum be Get(Obj, "enumerable").
    2. ReturnIfAbrupt(enum).
    3. Set the [[Enumerable]] field of desc to ToBoolean(enum).
  5. If HasProperty(Obj, "configurable") is true, then
    1. Let conf be Get(Obj, "configurable").
    2. ReturnIfAbrupt(conf).
    3. Set the [[Configurable]] field of desc to ToBoolean(conf).
  6. If HasProperty(Obj, "value") is true, then
    1. Let value be Get(Obj, "value").
    2. ReturnIfAbrupt(value).
    3. Set the [[Value]] field of desc to value.
  7. If HasProperty(Obj, "writable") is true, then
    1. Let writable be Get(Obj, "writable").
    2. ReturnIfAbrupt(writable).
    3. Set the [[Writable]] field of desc to ToBoolean(writable).
  8. If HasProperty(Obj, "get") is true, then
    1. Let getter be Get(Obj, "get").
    2. ReturnIfAbrupt(getter).
    3. If IsCallable(getter) is false and getter is not undefined, then throw a TypeError exception.
    4. Set the [[Get]] field of desc to getter.
  9. If HasProperty(Obj, "set") is true, then
    1. Let setter be Get(Obj, "set").
    2. ReturnIfAbrupt(setter).
    3. If IsCallable(setter) is false and setter is not undefined, then throw a TypeError exception.
    4. Set the [[Set]] field of desc to setter.
  10. If either desc.[[Get]] or desc.[[Set]] are present, then
    1. If either desc.[[Value]] or desc.[[Writable]] are present, then throw a TypeError exception.
  11. Return desc.

6.2.4.6 CompletePropertyDescriptor ( Desc )

When the abstract operation CompletePropertyDescriptor is called with Property Descriptor Desc the following steps are taken:

  1. ReturnIfAbrupt(Desc).
  2. Assert: Desc is a Property Descriptor
  3. Let like be Record{[[Value]]: undefined, [[Writable]]: false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false}.
  4. If either IsGenericDescriptor(Desc) or IsDataDescriptor(Desc) is true, then
    1. If Desc does not have a [[Value]] field, then set Desc.[[Value]] to like.[[Value]].
    2. If Desc does not have a [[Writable]] field, then set Desc.[[Writable]] to like.[[Writable]].
  5. Else,
    1. If Desc does not have a [[Get]] field, then set Desc.[[Get]] to like.[[Get]].
    2. If Desc does not have a [[Set]] field, then set Desc.[[Set]] to like.[[Set]].
  6. If Desc does not have an [[Enumerable]] field, then set Desc.[[Enumerable]] to like.[[Enumerable]].
  7. If Desc does not have a [[Configurable]] field, then set Desc.[[Configurable]] to like.[[Configurable]].
  8. Return Desc.

6.2.5 The Lexical Environment and Environment Record Specification Types

The Lexical Environment and Environment Record types are used to explain the behaviour of name resolution in nested functions and blocks. These types and the operations upon them are defined in 8.1.

6.2.6 Data Blocks

The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. 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 express to the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-origined integer indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to express access to its 3rd byte.

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

6.2.6.1 CreateByteDataBlock(size)

When the abstract operation CreateByteDataBlock is called with integer argument size, the following steps are taken:

  1. Assert: size≥0.
  2. Let db be a new Data Block value consisting of size bytes. If it is impossible to create such a Data Block, then throw a RangeError exception.
  3. Set all of the bytes of db to 0.
  4. Return db.

6.2.6.2 CopyDataBlockBytes(toBlock, toIndex, fromBlock, fromIndex, count)

When the abstract operation CopyDataBlockBytes is called the following steps are taken:

  1. Assert: fromBlock and toBlock are distinct Data Block values.
  2. Assert: fromIndex, toIndex, and count are positive integer values.
  3. Let fromSize be the number of bytes in fromBlock.
  4. Assert: fromIndex+countfromSize.
  5. Let toSize be the number of bytes in toBlock.
  6. Assert: toIndex+counttoSize.
  7. Repeat, while count>0
    1. Set toBlock[toIndex] to the value of fromBlock[fromIndex].
    2. Increment toIndex and fromIndex each by 1.
    3. Decrement count by 1.
  8. Return NormalCompletion(empty)

7 Abstract Operations

These operations are not a part of the ECMAScript language; they are defined here to 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 and Testing

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 or of a Completion Record value. But no other specification types are used with these operations.

7.1.1 ToPrimitive ( input [, PreferredType] )

The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType. The abstract operation ToPrimitive 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. Conversion occurs according to Table 9:

Table 9 — ToPrimitive Conversions
Input Type Result
Completion Record If input is an abrupt completion, return input. Otherwise return ToPrimitive(input.[[value]]) also passing the optional hint PreferredType.
Undefined Return input (no conversion).
Null Return input (no conversion).
Boolean Return input (no conversion).
Number Return input (no conversion).
String Return input (no conversion).
Symbol Return input (no conversion).
Object Perform the steps following this table.

When Type(input) is Object, the following steps are taken:

  1. If PreferredType was not passed, let hint be "default".
  2. Else if PreferredType is hint String, let hint be "string".
  3. Else PreferredType is hint Number, let hint be "number".
  4. Let exoticToPrim be GetMethod(input, @@toPrimitive).
  5. ReturnIfAbrupt(exoticToPrim).
  6. If exoticToPrim is not undefined, then
    1. Let result be the result of calling the [[Call]] internal method of exoticToPrim, with input as thisArgument and (hint) as argumentsList.
    2. ReturnIfAbrupt(result).
    3. If Type(result) is not Object, then return result.
    4. Throw a TypeError exception.
  7. If hint is "default" then, let hint be "number".
  8. Return OrdinaryToPrimitive(input,hint).

When the abstract operation OrdinaryToPrimitive is called with arguments O and hint, the following steps are taken:

  1. Assert: Type(O) is Object
  2. Assert: Type(hint) is String and its value is either "string" or "number".
  3. If hint is "string", then
    1. Let methodNames be the List ( "toString", "valueOf").
  4. Else,
    1. Let methodNames be the List ( "valueOf", "toString").
  5. For each name in methodNames in List order, do
    1. Let method be Get(O, name).
    2. ReturnIfAbrupt(method).
    3. If IsCallable(method) is true then,
      1. Let result be the result of calling the [[Call]] internal method of method, with O as thisArgument and an empty List as argumentsList.
      2. ReturnIfAbrupt(result).
      3. If Type(result) is not Object, then return result.
  6. Throw a TypeError exception.

NOTE When ToPrimitive is called with no hint, then it generally behaves as if the hint were Number. However, objects may over-ride this behaviour by defining a @@toPrimitive method. Of the objects defined in this specification only Date objects (see 20.3) and Symbol objects (see 19.4.3.4) over-ride the default ToPrimitive behaviour. Date objects treat no hint as if the hint were String.

7.1.2 ToBoolean ( argument )

The abstract operation ToBoolean converts argument to a value of type Boolean according to Table 10:

Table 10 — ToBoolean Conversions
Argument Type Result
Completion Record If argument is an abrupt completion, return argument. Otherwise return ToBoolean(argument.[[value]])
Undefined Return false
Null Return false
Boolean Return argument (no conversion).
Number Return false if argument is +0, −0, or NaN; otherwise return true.
String Return false if argument is the empty String (its length is zero); otherwise return true.
Symbol Return true
Object Return true

7.1.3 ToNumber ( argument )

The abstract operation ToNumber converts argument to a value of type Number according to Table 11:

Table 11 — ToNumber Conversions
Argument Type Result
Completion Record If argument is an abrupt completion, return argument. Otherwise return ToNumber(argument.[[value]])
Undefined Return NaN
Null Return +0
Boolean Return 1 if argument is true. Return +0 if argument is false.
Number Return argument (no conversion).
String See grammar and conversion algorithm below.
Symbol Throw a TypeError exception.
Object

Apply the following steps:

  1. Let primValue be ToPrimitive(argument, hint Number).
  2. Return ToNumber(primValue).

7.1.3.1 ToNumber Applied to the String Type

ToNumber applied to Strings applies the following grammar to the input String interpreted as a sequence of UTF-16 encoded code points. If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.

NOTE The terminal symbols of this grammar are all composed of Unicode BMP code points so the result will be NaN if the string contains the UTF-16 encoding of any supplementary code points or any unpaired surrogate code points

Syntax

StringNumericLiteral :::
StrWhiteSpaceopt
StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
StrWhiteSpace :::
StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar :::
WhiteSpace
LineTerminator
StrNumericLiteral :::
StrDecimalLiteral
HexIntegerLiteral
StrDecimalLiteral :::
StrUnsignedDecimalLiteral
+ StrUnsignedDecimalLiteral
- StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral :::
Infinity
DecimalDigits . DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
DecimalDigits ExponentPartopt
DecimalDigits :::
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit ::: one of
0 1 2 3 4 5 6 7 8 9
ExponentPart :::
ExponentIndicator SignedInteger
ExponentIndicator ::: one of
e E
SignedInteger :::
DecimalDigits
+ DecimalDigits
- DecimalDigits
HexIntegerLiteral :::
0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit
HexDigit ::: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

NOTE Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (see 11.8.3):

  • 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.

  • Infinity and –Infinity are recognized as a StringNumericLiteral but not as a NumericLiteral.

7.1.3.1.1 Runtime Semantics: MV’s

The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric literal (see 11.8.3), but some of the details are different, so the process for converting a String numeric literal to a value of Number type is given here in full. This value is determined in two steps: first, a mathematical value (MV) is derived from the String numeric literal; second, this mathematical value is rounded as described below.

  • The MV of StringNumericLiteral ::: [empty] is 0.

  • The MV of StringNumericLiteral ::: StrWhiteSpace is 0.

  • The MV of StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt is the MV of StrNumericLiteral, no matter whether white space is present or not.

  • The MV of StrNumericLiteral ::: StrDecimalLiteral is the MV of StrDecimalLiteral.

  • The MV of StrNumericLiteral ::: HexIntegerLiteral is the MV of HexIntegerLiteral.

  • The MV of StrDecimalLiteral ::: StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.

  • The MV of StrDecimalLiteral ::: + StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.

  • The MV of StrDecimalLiteral ::: - StrUnsignedDecimalLiteral is the negative of the MV of StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral is 0, the negative of this MV is also 0. The rounding rule described below handles the conversion of this signless mathematical zero to a floating-point +0 or −0 as appropriate.)

  • The MV of StrUnsignedDecimalLiteral ::: Infinity is 1010000 (a value so large that it will round to +∞).

  • The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . is the MV of DecimalDigits.

  • The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits is the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10n), where n is the number of code points in the second DecimalDigits.

  • The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . ExponentPart is the MV of DecimalDigits times 10e, where e is the MV of ExponentPart.

  • The MV of StrUnsignedDecimalLiteral ::: DecimalDigits . DecimalDigits ExponentPart is (the MV of the first DecimalDigits plus (the MV of the second DecimalDigits times 10n)) times 10e, where n is the number of code points in the second DecimalDigits and e is the MV of ExponentPart.

  • The MV of StrUnsignedDecimalLiteral ::: . DecimalDigits is the MV of DecimalDigits times 10n, where n is the number of code points in DecimalDigits.

  • The MV of StrUnsignedDecimalLiteral ::: . DecimalDigits ExponentPart is the MV of DecimalDigits times 10en, where n is the number of code points in DecimalDigits and e is the MV of ExponentPart.

  • The MV of StrUnsignedDecimalLiteral ::: DecimalDigits is the MV of DecimalDigits.

  • The MV of StrUnsignedDecimalLiteral ::: DecimalDigits ExponentPart is the MV of DecimalDigits times 10e, where e is the MV of ExponentPart.

  • The MV of DecimalDigits ::: DecimalDigit is the MV of DecimalDigit.

  • The MV of DecimalDigits ::: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.

  • The MV of ExponentPart ::: ExponentIndicator SignedInteger is the MV of SignedInteger.

  • The MV of SignedInteger ::: DecimalDigits is the MV of DecimalDigits.

  • The MV of SignedInteger ::: + DecimalDigits is the MV of DecimalDigits.

  • The MV of SignedInteger ::: - DecimalDigits is the negative of the MV of DecimalDigits.

  • The MV of DecimalDigit ::: 0 or of HexDigit ::: 0 is 0.

  • The MV of DecimalDigit ::: 1 or of HexDigit ::: 1 is 1.

  • The MV of DecimalDigit ::: 2 or of HexDigit ::: 2 is 2.

  • The MV of DecimalDigit ::: 3 or of HexDigit ::: 3 is 3.

  • The MV of DecimalDigit ::: 4 or of HexDigit ::: 4 is 4.

  • The MV of DecimalDigit ::: 5 or of HexDigit ::: 5 is 5.

  • The MV of DecimalDigit ::: 6 or of HexDigit ::: 6 is 6.

  • The MV of DecimalDigit ::: 7 or of HexDigit ::: 7 is 7.

  • The MV of DecimalDigit ::: 8 or of HexDigit ::: 8 is 8.

  • The MV of DecimalDigit ::: 9 or of HexDigit ::: 9 is 9.

  • The MV of HexDigit ::: a or of HexDigit ::: A is 10.

  • The MV of HexDigit ::: b or of HexDigit ::: B is 11.

  • The MV of HexDigit ::: c or of HexDigit ::: C is 12.

  • The MV of HexDigit ::: d or of HexDigit ::: D is 13.

  • The MV of HexDigit ::: e or of HexDigit ::: E is 14.

  • The MV of HexDigit ::: f or of HexDigit ::: F is 15.

  • The MV of HexIntegerLiteral ::: 0x HexDigit is the MV of HexDigit.

  • The MV of HexIntegerLiteral ::: 0X HexDigit is the MV of HexDigit.

  • The MV of HexIntegerLiteral ::: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the MV of HexDigit.

Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0 unless the first non white space code point in the String numeric literal is ‘-’, in which case the rounded value is −0. Otherwise, the rounded value must be the Number value for the MV (in the sense defined in 6.1.6), unless the literal includes a StrUnsignedDecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th digit position. A digit is significant if it is not part of an ExponentPart and

  • it is not 0; or
  • there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

7.1.4 ToInteger ( argument )

The abstract operation ToInteger converts argument to an integral numeric value. This abstract operation functions as follows:

  1. Let number be ToNumber(argument).
  2. ReturnIfAbrupt(number).
  3. If number is NaN, return +0.
  4. If number is +0, −0, +∞, or −∞, return number.
  5. Return the result of computing sign(number) × floor(abs(number)).

7.1.5 ToInt32 ( argument ) — Signed 32 Bit Integer

The abstract operation ToInt32 converts argument to one of 232 integer values in the range −231 through 231−1, inclusive. This abstract operation functions as follows:

  1. Let number be ToNumber(argument).
  2. ReturnIfAbrupt(number).
  3. If number is NaN, +0, −0, +∞, or −∞, return +0.
  4. Let int be sign(number) × floor(abs(number)).
  5. Let int32bit be int modulo 232.
  6. If int32bit ≥ 231, return int32bit − 232, otherwise return int32bit.

NOTE Given the above definition of ToInt32:

  • The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.

  • ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that + and − are mapped to +0.)

  • ToInt32 maps −0 to +0.

7.1.6 ToUint32 ( argument ) — Unsigned 32 Bit Integer

The abstract operation ToUint32 converts argument to one of 232 integer values in the range 0 through 232−1, inclusive. This abstract operation functions as follows:

  1. Let number be ToNumber(argument).
  2. ReturnIfAbrupt(number).
  3. If number is NaN, +0, −0, +, or −, return +0.
  4. Let int be sign(number) × floor(abs(number)).
  5. Let int32bit be int modulo 232.
  6. Return int32bit.

NOTE Given the above definition of ToUint32:

  • Step 6 is the only difference between ToUint32 and ToInt32.

  • The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.

  • ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.)

  • ToUint32 maps −0 to +0.

7.1.7 ToInt16 ( argument ) — Signed 16 Bit Integer

The abstract operation ToInt16 converts argument to one of 216 integer values in the range −32768 through 32767, inclusive. This abstract operation functions as follows:

  1. Let number be ToNumber(argument).
  2. ReturnIfAbrupt(number).
  3. If number is NaN, +0, −0, +∞, or −∞, return +0.
  4. Let int be sign(number) × floor(abs(number)).
  5. Let int16bit be int modulo 216.
  6. If int16bit ≥ 215, return int16bit − 216, otherwise return int16bit.

7.1.8 ToUint16 ( argument ) — Unsigned 16 Bit Integer

The abstract operation ToUint16 converts argument to one of 216 integer values in the range 0 through 216−1, inclusive. This abstract operation functions as follows:

  1. Let number be ToNumber(argument).
  2. ReturnIfAbrupt(number).
  3. If number is NaN, +0, −0, +, or −, return +0.
  4. Let int be sign(number) × floor(abs(number)).
  5. Let int16bit be int modulo 216.
  6. Return int16bit.

NOTE Given the above definition of ToUint16:

  • The substitution of 216 for 232 in step 5 is the only difference between ToUint32 and ToUint16.
  • ToUint16 maps −0 to +0.

7.1.9 ToInt8 ( argument ) — Signed 8 Bit Integer

The abstract operation ToInt8 converts argument to one of 28 integer values in the range −128 through 127, inclusive. This abstract operation functions as follows:

  1. Let number be ToNumber(argument).
  2. ReturnIfAbrupt(number).
  3. If number is NaN, +0, −0, +∞, or −∞, return +0.
  4. Let int be sign(number) × floor(abs(number)).
  5. Let int8bit be int modulo 28.
  6. If int8bit ≥ 27, return int8bit − 28, otherwise return int8bit.

7.1.10 ToUint8 ( argument ) — Unsigned 8 Bit Integer

The abstract operation ToUint8 converts argument to one of 28 integer values in the range 0 through 255, inclusive. This abstract operation functions as follows:

  1. Let number be ToNumber(argument).
  2. ReturnIfAbrupt(number).
  3. If number is NaN, +0, −0, +, or −, return +0.
  4. Let int be sign(number) × floor(abs(number)).
  5. Let int8bit be int modulo 28.
  6. Return int8bit.

7.1.11 ToUint8Clamp ( argument ) — Unsigned 8 Bit Integer, Clamped

The abstract operation ToUint8Clamp converts argument to one of 28 integer values in the range 0 through 255, inclusive. This abstract operation functions as follows:

  1. Let number be ToNumber(argument).
  2. ReturnIfAbrupt(number).
  3. If number is NaN, return +0.
  4. If number ≤ 0, return +0.
  5. If number ≥ 255, return 255.
  6. Let f be floor(number).
  7. If f + 0.5 < number, then return f + 1.
  8. If number < f + 0.5, then return f.
  9. If f is odd, then return f + 1.
  10. Return f.

NOTE Note that unlike the other ECMAScript integer conversion abstract operation, ToUint8Clamp rounds rather than truncates non-integer values and does not convert + to 0. ToUint8Clamp does “round half to even” tie-breaking. This differs from Math.round which does “round half up” tie-breaking.

7.1.12 ToString ( argument )

The abstract operation ToString converts argument to a value of type String according to Table 12:

Table 12 — ToString Conversions
Argument Type Result
Completion Record If argument is an abrupt completion, return argument. Otherwise return ToString(argument.[[value]])
Undefined "undefined"
Null "null"
Boolean

If argument is true, then return "true".

If argument is false, then return "false".

Number See 7.1.12.1.
String Return argument (no conversion)
Symbol Throw a TypeError exception.
Object

Apply the following steps:

1. Let primValue be ToPrimitive(argument, hint String).

2. Return ToString(primValue).

7.1.12.1 ToString Applied to the Number Type

The abstract operation ToString converts a Number m to String format as follows:

  1. If m is NaN, return the String "NaN".
  2. If m is +0 or −0, return the String "0".
  3. If m is less than zero, return the String concatenation of the String "-" and ToString(−m).
  4. If m is +∞, return the String "Infinity".
  5. Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1s < 10k, the Number value for s × 10n−k is m, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
  6. If kn ≤ 21, return the String consisting of the code points of the k digits of the decimal representation of s (in order, with no leading zeroes), followed by n−k occurrences of the code point U+0030 (DIGIT ZERO).
  7. If 0 < n ≤ 21, return the String consisting of code points of the the most significant n digits of the decimal representation of s, followed by the code point U+002E (FULL STOP), followed by the code points of the remaining k−n digits of the decimal representation of s.
  8. If −6 < n ≤ 0, return the String consisting of the code point U+0030 (DIGIT ZERO), followed by a the code point U+002E (FULL STOP), followed by −n occurrences of the code point U+0030 (DIGIT ZERO), followed by the code points of the k digits of the decimal representation of s.
  9. Otherwise, if k = 1, return the String consisting of the code point of the single digit of s, followed by code point U+0065 (LATIN SMALL LETTER E), followed by the code point U+002B (PLUS SIGN) or the code point U+002D (HYPHEN-MINUS) according to whether n−1 is positive or negative, followed by the code points of the decimal representation of the integer abs(n−1) (with no leading zeroes).
  10. Return the String consisting of the code point of the most significant digit of the decimal representation of s, followed by code point U+002E (FULL STOP), followed by the code points of the remaining k−1 digits of the decimal representation of s, followed by code point U+0065 (LATIN SMALL LETTER E), followed by code point U+002B (PLUS SIGN) or the code point U+002D (HYPHEN-MINUS) according to whether n−1 is positive or negative, followed by the code points of the decimal representation of the integer abs(n−1) (with no leading zeroes).

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, then ToNumber(ToString(x)) is exactly the same Number value as 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:

Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1s < 10k, the Number value for s × 10nk is m, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10nk is closest in value to m. If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.

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). November 30, 1990. Available as
http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz. 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.

7.1.13 ToObject ( argument )

The abstract operation ToObject converts argument to a value of type Object according to Table 13:

Table 13 — ToObject Conversions
Argument Type Result
Completion Record If argument is an abrupt completion, return argument. Otherwise return ToObject(argument.[[value]])
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return a new Boolean object whose [[BooleanData]] internal slot is set to the value of argument. See 19.3 for a description of Boolean objects.
Number Return a new Number object whose [[NumberData]] internal slot is set to the value of argument. See 20.1 for a description of Number objects.
String Return a new String object whose [[StringData]] internal slot is set to the value of argument. See 21.1 for a description of String objects.
Symbol Return a new Symbol object whose [[SymbolData]] internal slot is set to the value of argument. See 19.4 for a description of Symbol objects.
Object Return argument (no conversion).

7.1.14 ToPropertyKey ( argument )

The abstract operation ToPropertyKey converts argument to a value that can be used as a property key by performing the following steps:

  1. ReturnIfAbrupt(argument).
  2. If Type(argument) is Symbol, then
    1. Return argument.
  3. Return ToString(argument).

7.1.15 ToLength ( argument )

The abstract operation ToLength converts argument to an integer suitable for use as the length of an array-like object. It performs the following steps:

  1. ReturnIfAbrupt(argument).
  2. Let len be ToInteger(argument).
  3. ReturnIfAbrupt(len).
  4. If len ≤ +0, then return +0.
  5. Return min(len, 253-1).

7.1.16 CanonicalNumericIndexString ( argument )

The abstract operation CanonicalNumericIndexString returns argument converted to a numeric value if it is a String representation of a Number that would be produced by ToString, or the string "-0". Otherwise, it returns undefined. This abstract operation functions as follows:

  1. Assert: Type(argument) is String.
  2. If argument is "-0", then return −0.
  3. Let n be ToNumber(argument).
  4. If SameValue(ToString(n), argument) is false, then return undefined.
  5. Return n.

A canonical numeric string is any String value for which the CanonicalNumericIndexString abstraction operation does not return undefined.

7.2 Testing and Comparison Operations

7.2.1 CheckObjectCoercible ( argument )

The abstract operation CheckObjectCoercible throws an error if argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 14:

Table 14 — CheckObjectCoercible Results
Argument Type Result
Completion Record If argument is an abrupt completion, return argument. Otherwise return CheckObjectCoercible(argument.[[value]])
Undefined Throw a TypeError exception.
Null Throw a TypeError exception.
Boolean Return argument
Number Return argument
String Return argument
Symbol Return argument
Object Return argument

7.2.2 IsCallable ( argument )

The abstract operation IsCallable determines if argument, which must be an ECMAScript language value or a Completion Record, is a callable function Object according to Table 15:

Table 15 — IsCallable Results
Argument Type Result
Completion Record If argument is an abrupt completion, return argument. Otherwise return IsCallable(argument.[[value]])
Undefined Return false.
Null Return false.
Boolean Return false.
Number Return false.
String Return false.
Symbol Return false.
Object If argument has a [[Call]] internal method, then return true, otherwise return false.

7.2.3 SameValue(x, y)

The internal comparison abstract operation SameValue(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:

  1. ReturnIfAbrupt(x).
  2. ReturnIfAbrupt(y).
  3. If Type(x) is different from Type(y), return false.
  4. If Type(x) is Undefined, return true.
  5. If Type(x) is Null, return true.
  6. If Type(x) is Number, then
    1. If x is NaN and y is NaN, return true.
    2. If x is +0 and y is -0, return false.
    3. If x is -0 and y is +0, return false.
    4. If x is the same Number value as y, return true.
    5. Return false.
  7. If Type(x) is String, then
    1. If x and y are exactly the same sequence of code units (same length and same code units in corresponding positions) return true; otherwise, return false.
  8. If Type(x) is Boolean, then
    1. If x and y are both true or both false, then return true; otherwise, return false.
  9. If Type(x) is Symbol, then
    1. If x and y are both the same Symbol value, then return true; otherwise, return false.
  10. Return true if x and y are the same Object value. Otherwise, return false.

7.2.4 SameValueZero(x, y)

The internal comparison abstract operation SameValueZero(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:

  1. ReturnIfAbrupt(x).
  2. ReturnIfAbrupt(y).
  3. If Type(x) is different from Type(y), return false.
  4. If Type(x) is Undefined, return true.
  5. If Type(x) is Null, return true.
  6. If Type(x) is Number, then
    1. If x is NaN and y is NaN, return true.
    2. If x is +0 and y is -0, return true.
    3. If x is -0 and y is +0, return true.
    4. If x is the same Number value as y, return true.
    5. Return false.
  7. If Type(x) is String, then
    1. If x and y are exactly the same sequence of code units (same length and same code units in corresponding positions) return true; otherwise, return false.
  8. If Type(x) is Boolean, then
    1. If x and y are both true or both false, then return true; otherwise, return false.
  9. If Type(x) is Symbol, then
    1. If x and y are both the same Symbol value, then return true; otherwise, return false.
  10. Return true if x and y are the same Object value. Otherwise, return false.

NOTE SameValueZero differs from SameValue only in its treatment of +0 and -0.

7.2.5 IsConstructor ( argument )

The abstract operation IsConstructor determines if argument, which must be an ECMAScript language value or a Completion Record, is a function object with a [[Construct]] internal method.

  1. ReturnIfAbrupt(argument).
  2. If Type(argument) is not Object, return false.
  3. If argument has a [[Construct]] internal method, return true.
  4. Return false.

7.2.6 IsPropertyKey ( argument )

The abstract operation IsPropertyKey determines if argument, which must be an ECMAScript language value or a Completion Record, is a value that may be used as a property key.

  1. ReturnIfAbrupt(argument).
  2. If Type(argument) is String, return true.
  3. If Type(argument) is Symbol, return true.
  4. Return false.

7.2.7 IsExtensible (O)

The abstract operation IsExtensible is used to determine whether additional properties can be added to the object that is O. A Boolean value is returned. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Return the result of calling the [[IsExtensible]] internal method of O.

7.2.8 IsInteger ( argument )

The abstract operation IsInteger determines if argument is a finite integer numeric value.

  1. ReturnIfAbrupt(argument).
  2. If Type(argument) is not Number, return false.
  3. If argument is NaN, +∞, or −∞, return false.
  4. If floor(abs(argument)) ≠ abs(argument), then return false.
  5. Return true.

7.2.9 Abstract Relational Comparison

The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named LeftFirst as a parameter. The flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of LeftFirst is true and indicates that the x parameter corresponds to an expression that occurs to the left of the y parameter’s corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. Such a comparison is performed as follows:

  1. ReturnIfAbrupt(x).
  2. ReturnIfAbrupt(y).
  3. If the LeftFirst flag is true, then
    1. Let px be ToPrimitive(x, hint Number).
    2. ReturnIfAbrupt(px).
    3. Let py be ToPrimitive(y, hint Number).
    4. ReturnIfAbrupt(py).
  4. Else the order of evaluation needs to be reversed to preserve left to right evaluation
    1. Let py be ToPrimitive(y, hint Number).
    2. ReturnIfAbrupt(py).
    3. Let px be ToPrimitive(x, hint Number).
    4. ReturnIfAbrupt(px).
  5. If both px and py are Strings, then
    1. If py is a prefix of px, return false. (A String value p is a prefix of String value q if q can be the result of concatenating p and some other String r. Note that any String is a prefix of itself, because r may be the empty String.)
    2. If px is a prefix of py, return true.
    3. Let k be the smallest nonnegative integer such that the code unit at position k within px is different from the code unit at position k within py. (There must be such a k, for neither String is a prefix of the other.)
    4. Let m be the integer that is the code unit value at position k within px.
    5. Let n be the integer that is the code unit value at position k within py.
    6. If m < n, return true. Otherwise, return false.
  6. Else,
    1. Let nx be ToNumber(px). Because px and py are primitive values evaluation order is not important.
    2. ReturnIfAbrupt(nx).
    3. Let ny be ToNumber(py).
    4. ReturnIfAbrupt(ny).
    5. If nx is NaN, return undefined.
    6. If ny is NaN, return undefined.
    7. If nx and ny are the same Number value, return false.
    8. If nx is +0 and ny is −0, return false.
    9. If nx is −0 and ny is +0, return false.
    10. If nx is +∞, return false.
    11. If ny is +∞, return true.
    12. If ny is −∞, return false.
    13. If nx is −∞, return true.
    14. If the mathematical value of nx is less than the mathematical value of ny —note that these mathematical values are both finite and not both zero—return true. Otherwise, return false.

NOTE 1 Step 5 differs from step 11 in the algorithm for the addition operator + (12.7.3) in using “and” instead of “or”.

NOTE 2 The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.

7.2.10 Abstract Equality Comparison

The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:

  1. ReturnIfAbrupt(x).
  2. ReturnIfAbrupt(y).
  3. If Type(x) is the same as Type(y), then
    1. Return the result of performing Strict Equality Comparison x === y.
  4. If x is null and y is undefined, return true.
  5. If x is undefined and y is null, return true.
  6. If Type(x) is Number and Type(y) is String,
    return the result of the comparison x == ToNumber(y).
  7. If Type(x) is String and Type(y) is Number,
    return the result of the comparison ToNumber(x) == y.
  8. If Type(x) is Symbol or Type(y) is Symbol, return false.
  9. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
  10. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
  11. If Type(x) is either String or Number and Type(y) is Object,
    return the result of the comparison x == ToPrimitive(y).
  12. If Type(x) is Object and Type(y) is either String or Number,
    return the result of the comparison ToPrimitive(x) == y.
  13. Return false.

7.2.11 Strict Equality Comparison

The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:

  1. If Type(x) is different from Type(y), return false.
  2. If Type(x) is Undefined, return true.
  3. If Type(x) is Null, return true.
  4. If Type(x) is Number, then
    1. If x is NaN, return false.
    2. If y is NaN, return false.
    3. If x is the same Number value as y, return true.
    4. If x is +0 and y is −0, return true.
    5. If x is −0 and y is +0, return true.
    6. Return false.
  5. If Type(x) is String, then
    1. If x and y are exactly the same sequence of code units (same length and same code units in corresponding positions), return true.
    2. Else, return false.
  6. If Type(x) is Boolean, then
    1. If x and y are both true or both false, return true.
    2. Else, return false.
  7. If x and y are the same Symbol value, return true.
  8. If x and y are the same Object value, return true.
  9. Return false.

NOTE This algorithm differs from the SameValue Algorithm (7.2.3) in its treatment of signed zeroes and NaNs.

7.3 Operations on Objects

7.3.1 Get (O, P)

The abstract operation Get is used to retrieve the value of a specific property of an object. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Return the result of calling the [[Get]] internal method of O passing P and O as the arguments.

7.3.2 Put (O, P, V, Throw)

The abstract operation Put is used to set the value of a specific property of an object. The operation is called with arguments O, P, V, and Throw where O is the object, P is the property key, V is the new value for the property and Throw is a Boolean flag. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Assert: Type(Throw) is Boolean.
  4. Let success be the result of calling the [[Set]] internal method of O passing P, V, and O as the arguments.
  5. ReturnIfAbrupt(success).
  6. If success is false and Throw is true, then throw a TypeError exception.
  7. Return success.

7.3.3 CreateDataProperty (O, P, V)

The abstract operation CreateDataProperty is used to create a new own property of an object. The operation is called with arguments O, P, and V where O is the object, P is the property key, and V is the value for the property. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Let newDesc be the PropertyDescriptor{[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
  4. Return the result of calling the [[DefineOwnProperty]] internal method of O passing P and newDesc as arguments.

NOTE This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or O is not extensible [[DefineOwnProperty]] will return false.

7.3.4 CreateDataPropertyOrThrow (O, P, V)

The abstract operation CreateDataPropertyOrThrow is used to create a new own property of an object. It throws a TypeError exception if the requested property update cannot be performed. The operation is called with arguments O, P, and V where O is the object, P is the property key, and V is the value for the property. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Let success be CreateDataProperty(O, P, V).
  4. ReturnIfAbrupt(success).
  5. If success is false, then throw a TypeError exception.
  6. Return success.

NOTE This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or O is not extensible [[DefineOwnProperty]] will return false causing this operation to throw a TypeError exception.

7.3.5 DefinePropertyOrThrow (O, P, desc)

The abstract operation DefinePropertyOrThrow is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw a TypeError exception if the requested property update cannot be performed. The operation is called with arguments O, P, and desc where O is the object, P is the property key, and desc is the Property Descriptor for the property. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Let success be the result of calling the [[DefineOwnProperty]] internal method of O passing P and desc as arguments.
  4. ReturnIfAbrupt(success).
  5. If success is false, then throw a TypeError exception.
  6. Return success.

7.3.6 DeletePropertyOrThrow (O, P)

The abstract operation DeletePropertyOrThrow is used to remove a specific own property of an object. It throws an exception if the property is not configurable. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Let success be the result of calling the [[Delete]] internal method of O passing P as the argument.
  4. ReturnIfAbrupt(success).
  5. If success is false, then throw a TypeError exception.
  6. Return success.

7.3.7 GetMethod (O, P)

The abstract operation GetMethod is used to get the value of a specific property of an object when the value of the property is expected to be a function. The operation is called with arguments O and P where O is the object, P is the property key. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Let func be the result of calling the [[Get]] internal method of O passing P and O as the arguments.
  4. ReturnIfAbrupt(func).
  5. If func is either undefined or null, then return undefined.
  6. If IsCallable(func) is false, then throw a TypeError exception.
  7. Return func.

7.3.8 HasProperty (O, P)

The abstract operation HasProperty is used to determine whether an object has a property with the specified property key. The property may be either an own or inherited. A Boolean value is returned. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Return the result of calling the [[HasProperty]] internal method of O with argument P.

7.3.9 HasOwnProperty (O, P)

The abstract operation HasOwnProperty is used to determine whether an object has an own property with the specified property key. A Boolean value is returned. The operation is called with arguments O and P where O is the object and P is the property key. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: IsPropertyKey(P) is true.
  3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the argument.
  4. ReturnIfAbrupt(desc).
  5. If desc is undefined, return false.
  6. Return true.

7.3.10 Invoke(O,P, [args])

The abstract operation Invoke is used to call a method property of an object. The operation is called with arguments O, P , and optionally args where O serves as both the lookup point for the property and the this value of the call, P is the property key, and args is the list of arguments values passed to the method. If args is not present, an empty List is used as its value. This abstract operation performs the following steps:

  1. Assert: P is a valid property key.
  2. If args was not passed, then let args be a new empty List.
  3. Let obj be ToObject(O).
  4. ReturnIfAbrupt(obj).
  5. Let func be the result of calling the [[Get]] internal method of obj passing P and O as the arguments.
  6. ReturnIfAbrupt(func).
  7. If IsCallable(func) is false, then throw a TypeError exception.
  8. Return the result of calling the [[Call]] internal method of func passing O as thisArgument and args as argumentsList.

7.3.11 SetIntegrityLevel (O, level)

The abstract operation SetIntegrityLevel is used to fix the set of own properties of an object. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: level is either "sealed" or "frozen".
  3. Let keysArray be the result of calling the [[OwnPropertyKeys]] internal method of O.
  4. Let keys be CreateListFromArrayLike(keysArray).
  5. ReturnIfAbrupt(keys).
  6. Let pendingException be undefined.
  7. If level is "sealed", then
    1. Repeat for each element k of keys,
      1. Let status be DefinePropertyOrThrow(O, k, PropertyDescriptor{ [[Configurable]]: false}).
      2. If status is an abrupt completion, then
        1. If pendingException is undefined, then set pendingException to status.
  8. Else level is "frozen",
    1. Repeat for each element k of keys,
      1. Let status be the result of calling the [[GetOwnProperty]] internal method of O with k.
      2. If status is an abrupt completion, then
        1. If pendingException is undefined, then set pendingException to status.
      3. Else,
        1. Let currentDesc be status.[[value]].
        2. If currentDesc is not undefined, then
          1. If IsAccessorDescriptor(currentDesc) is true, then
            1. Let desc be the PropertyDescriptor{[[Configurable]]: false}.
          2. Else,
            1. Let desc be the PropertyDescriptor { [[Configurable]]: false, [[Writable]]: false }.
          3. Let status be DefinePropertyOrThrow(O, k, desc).
          4. If status is an abrupt completion, then
            1. If pendingException is undefined, then set pendingException to status.
  9. If pendingException is not undefined, then return pendingException.
  10. Return the result of calling the [[PreventExtensions]] internal method of O.

7.3.12 TestIntegrityLevel (O, level)

The abstract operation TestIntegrityLevel is used to determine if the set of own properties of an object are fixed. This abstract operation performs the following steps:

  1. Assert: Type(O) is Object.
  2. Assert: level is either "sealed" or "frozen".
  3. Let status be IsExtensible(O).
  4. ReturnIfAbrupt(status).
  5. If status is true, then return false
  6. NOTE If the object is extensible, none of its properties are examined.
  7. Let keysArray be the result of calling the [[OwnPropertyKeys]] internal method of O.
  8. Let keys be CreateListFromArrayLike(keysArray).
  9. ReturnIfAbrupt(keys).
  10. Let pendingException be undefined.
  11. Let configurable be false.
  12. Let writable be false.
  13. Repeat for each element k of keys,
    1. Let status be the result of calling the [[GetOwnProperty]] internal method of O with k.
    2. If status is an abrupt completion, then
      1. If pendingException is undefined, then set pendingException to status.
    3. Else,
      1. Let currentDesc be status.[[value]].
      2. If currentDesc is not undefined, then
        1. Set configurable to configurable logically ored with currentDesc.[[Configurable]].
        2. If IsDataDescriptor(currentDesc) is true, then
          1. Set writable to writable logically ored with currentDesc.[[Writable]].
  14. If pendingException is not undefined, then return pendingException.
  15. If level is "frozen" and writable is true, then return false.
  16. If configurable is true, then return false.
  17. Return true.

7.3.13 CreateArrayFromList (elements)

The abstract operation CreateArrayFromList is used to create an Array object whose elements are provided by a List. This abstract operation performs the following steps:

  1. Assert: elements is a List whose elements are all ECMAScript language values.
  2. Let array be ArrayCreate(0) (see 9.4.2.2).
  3. Let n be 0.
  4. For each element e of elements
    1. Let status be the result of CreateDataProperty(array, ToString(n), e).
    2. Assert: status is true.
    3. Increment n by 1.
  5. Return array.

7.3.14 CreateListFromArrayLike (obj)

The abstract operation CreateListFromArrayLike is used to create a List value whose elements are provided by the indexed properties of an array-like object. This abstract operation performs the following steps:

  1. ReturnIfAbrupt(obj).
  2. If Type(obj) is not Object, then throw a TypeError exception.
  3. Let len be Get(obj, "length").
  4. Let n be ToLength(len).
  5. ReturnIfAbrupt(n).
  6. Let list be an empty List.
  7. Let index be 0.
  8. Repeat while index < n
    1. Let indexName be ToString(index).
    2. Let next be Get(obj, indexName).
    3. ReturnIfAbrupt(next).
    4. Append next as the last element of list.
    5. Set index to index + 1.
  9. Return list.

7.3.15 OrdinaryHasInstance (C, O)

The abstract operation OrdinaryHasInstance implements the default algorithm for determining if an object O inherits from the instance object inheritance path provided by constructor C. This abstract operation performs the following steps:

  1. If IsCallable(C) is false, return false.
  2. If C has a [[BoundTargetFunction]] internal slot, then
    1. Let BC be the value of C’s [[BoundTargetFunction]] internal slot.
    2. Return InstanceofOperator(O,BC) (see 12.9.4).
  3. If Type(O) is not Object, return false.
  4. Let P be Get(C, "prototype").
  5. ReturnIfAbrupt(P).
  6. If Type(P) is not Object, throw a TypeError exception.
  7. Repeat
    1. Set O to the result of calling the [[GetPrototypeOf]] internal method of O with no arguments.
    2. ReturnIfAbrupt(O).
    3. If O is null, return false.
    4. If SameValue(P, O) is true, return true.

7.3.16 GetPrototypeFromConstructor ( constructor, intrinsicDefaultProto )

The abstract operation GetPrototypeFromConstructor determines the [[Prototype]] value that should be used to create an object corresponding to a specific constructor. The value is retrieved from the constructor’s prototype property, if it exists. Otherwise the supplied default is used for [[Prototype]]. This abstract operation performs the following steps:

  1. Assert: intrinsicDefaultProto is a string value that is this specification’s name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
  2. If IsConstructor (constructor) is false, then throw a TypeError exception.
  3. Let proto be Get(constructor, "prototype").
  4. ReturnIfAbrupt(proto).
  5. If Type(proto) is not Object, then
    1. Let realm be GetFunctionRealm(constructor).
    2. Let proto be realm’s intrinsic object named intrinsicDefaultProto.
  6. Return proto.

NOTE If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the Code Realm of the constructor function rather than from the running execution context. This accounts for the possibility that a built-in @@create method from a different Code Realm might be installed on constructor.

7.3.17 CreateFromConstructor (F)

When the abstract operation CreateFromConstructor is called with Object F the following steps are taken:

  1. Let creator be GetMethod (F, @@create).
  2. ReturnIfAbrupt(creator).
  3. If creator is undefined, then return undefined.
  4. Let obj be the result of calling the [[Call]] internal method of creator with arguments F and an empty List.
  5. ReturnIfAbrupt(obj).
  6. If Type(obj) is not Object, then throw a TypeError exception.
  7. Return obj.

NOTE This operation is equivalent to: F[Symbol.create]()followed by an error check.

7.3.18 Construct (F, argumentsList)

When the abstract operation Construct is called with Object F and List argumentsList the following steps are taken:

  1. Assert: Type(F) is Object.
  2. Let obj be CreateFromConstructor(F).
  3. ReturnIfAbrupt(obj).
  4. If obj is undefined, then
    1. Let obj be OrdinaryCreateFromConstructor(F, "%ObjectPrototype%").
    2. ReturnIfAbrupt(obj).
    3. Assert: Type(obj) is Object.
  5. Let result be the result of calling the [[Call]] internal method of F, providing obj and argumentsList as the arguments.
  6. ReturnIfAbrupt(result).
  7. If Type(result) is Object then return result.
  8. Return obj.

NOTE This operation is equivalent to: new F(...argumentsList)

7.3.19 GetOption (options, P)

The abstract operation GetOption is used to retrieve the value of a specific property of an object in situation where the object may not be present. The operation is called with arguments options and P where options is the object and P is the property key. This abstract operation performs the following steps:

  1. Assert: IsPropertyKey(P) is true.
  2. If options is undefined, then return undefined.
  3. If Type(options) is not Object, then throw a TypeError exception.
  4. Return the result of calling the [[Get]] internal method of options passing P and options as the arguments.

7.3.20 EnumerableOwnNames (O)

When the abstract operation EnumerableOwnNames is called with Object O the following steps are taken:

  1. Assert: Type(O) is Object.
  2. Let ownKey be the result of calling the [[OwnPropertyKeys]] internal method of O with no arguments.
  3. ReturnIfAbrupt(ownKeys).
  4. Let length be ToLength(Get(ownKeys, "length")).
  5. ReturnIfAbrupt(length).
  6. Let names be a new empty List.
  7. Let k be 0.
  8. Repeat, while k < length
    1. Let key be Get(ownKeys, ToString(k)).
    2. ReturnIfAbrupt(key).
    3. If Type(key) is String, then
      1. Let desc be the resulting of calling the [[GetOwnProperty]] internal method of O with argument key.
      2. ReturnIfAbrupt(desc).
      3. If desc is not undefined, then
        1. If desc.[[Enumerable]] is true, then append key to names.
    4. Let k be k + 1.
  9. Return names.

7.3.21 GetFunctionRealm ( obj ) Abstract Operation

The abstract operation GetFunctionRealm with argument obj performs the following steps:

  1. Assert: obj is a callable object.
  2. If obj has a [[Realm]] internal slot, then
    1. Return obj’s [[Realm]] internal slot.
  3. If obj is a Bound Function exotic object, then
    1. Let target be objs [[BoundTargetFunction]] internal slot.
    2. Return GetFunctionRealm(target).
  4. If obj is a Proxy exotic object, then
    1. Let proxyTarget be the value of obj’s [[ProxyTarget]] internal slot.
    2. Return GetFunctionRealm(proxyTarget).
  5. Return the running execution context’s Realm.

NOTE Step 5 will only be reached if target is a non-standard exotic function object that does not have a [[Realm]] internal slot.

7.4 Operations on Iterator Objects

See Commmon Iteration Interfaces (25.1).

7.4.1 CheckIterable ( obj )

The abstract operation CheckIterable with argument obj performs the following steps:

  1. If Type(obj) is not Object, then return undefined.
  2. Return Get(obj, @@iterator).

7.4.2 GetIterator ( obj, method )

The abstract operation GetIterator with argument obj and optional argument method performs the following steps:

  1. ReturnIfAbrupt(obj).
  2. If method was not passed, then
    1. Let method be CheckIterable(obj).
    2. ReturnIfAbrupt(method).
  3. If IsCallable(method) is false, then throw a TypeError exception.
  4. Let iterator be the result of calling the [[Call]] internal method of method with obj as thisArgument and an empty List as argumentsList.
  5. ReturnIfAbrupt(iterator).
  6. If Type(iterator) is not Object, then throw a TypeError exception.
  7. Return iterator.

7.4.3 IteratorNext ( iterator, value )

The abstract operation IteratorNext with argument iterator and optional argument value performs the following steps:

  1. If value was not passed,
    1. Let result be Invoke(iterator, "next", ( )).
  2. Else,
    1. Let result be Invoke(iterator, "next", (value)).
  3. ReturnIfAbrupt(result).
  4. If Type(result) is not Object, then throw a TypeError exception.
  5. Return result.

7.4.4 IteratorComplete ( iterResult )

The abstract operation IteratorComplete with argument iterResult performs the following steps:

  1. Assert: Type(iterResult) is Object.
  2. Let done be Get(iterResult, "done").
  3. Return ToBoolean(done).

7.4.5 IteratorValue ( iterResult )

The abstract operation IteratorValue with argument iterResult performs the following steps:

  1. Assert: Type(iterResult) is Object.
  2. Return Get(iterResult, "value").

7.4.6 IteratorStep ( iterator )

The abstract operation IteratorStep with argument iterator requests the next value from iterator and returns either false indicating that the iterator has reached its end or the IteratorResult object if a next value is available. IteratorStep performs the following steps:

  1. Let result be IteratorNext(iterator).
  2. ReturnIfAbrupt(result).
  3. Let done be IteratorComplete(result).
  4. ReturnIfAbrupt(done).
  5. If done is true, then return false.
  6. Return result.

7.4.7 CreateIterResultObject (value, done)

The abstract operation CreateIterResultObject with arguments value and done creates an object that supports the IteratorResult interface by performing the following steps:

  1. Assert: Type(done) is Boolean.
  2. Let obj be ObjectCreate(%ObjectPrototype%).
  3. Perform CreateDataProperty(obj, "value", value).
  4. Perform CreateDataProperty(obj, "done", done).
  5. Return obj.

7.4.8 CreateListIterator (list)

The abstract operation CreateListIterator with argument list creates an Iterator (25.1.2) object whose next method returns the successive elements of list. It performs the following steps:

  1. Let iterator be ObjectCreate(%ObjectPrototype%, ([[IteratorNext]], [[IteratedList]], [[ListIteratorNextIndex]])).
  2. Set iterator’s [[IteratedList]] internal slot to list.
  3. Set iterator’s [[ListIteratorNextIndex]] internal slot to 0.
  4. Let next be a new built-in function object as defined in ListIterator next (7.4.8.1).
  5. Set iterator’s [[IteratedNext]] internal slot to next.
  6. Let status be the result of CreateDataProperty(iterator, "next", next).
  7. Return iterator.

7.4.8.1 ListIterator next( )

The ListIterator next method is a standard built-in function object (clause 17) that performs the following steps:

  1. Let O be the this value.
  2. Let f be the active function object.
  3. If O does not have a [[IteratedNext]] internal slot, then throw a TypeError exception.
  4. Let next be the value of the [[IteratedNext]] internal slot of O.
  5. If SameValue(f, next) is false, then throw a TypeError exception.
  6. If O does not have a [[IteratedList]] internal slot, then throw a TypeError exception.
  7. Let list be the value of the [[IteratedList]] internal slot of O.
  8. Let index be the value of the [[ListIteratorNextIndex]] internal slot of O.
  9. Let len be the number of elements of list.
  10. If indexlen, then
    1. Return CreateIterResultObject(undefined, true).
  11. Set the value of the [[ListIteratorNextIndex]] internal slot of O to index+1.
  12. Return CreateIterResultObject(list[index], false).

NOTE A ListIterator next method will throw an exception if applied to any object other than the one with which it was originally associated.

7.4.9 CreateEmptyIterator ( )

The abstract operation CreateEmptyIterator with no arguments creates an Iterator object whose next method always reports that the iterator is done. It performs the following steps:

  1. Let empty be a List with no elements.
  2. Return CreateListIterator(empty).

7.4.10 CreateCompoundIterator ( iterator1, iterator2 )

The abstract operation CreateCompoundIterator with arguments iterator1 and iterator2 creates an Iterator (25.1.2) object whose next method returns the successive elements of iterator1 followed by the successive elements of iterator2. It performs the following steps:

  1. Let iterator be ObjectCreate(%ObjectPrototype%, ([[Iterator1]], [[Iterator2]], [[State]], [[IteratorNext]])).
  2. Set iterator’s [[Iterator1]] internal slot to iterator1.
  3. Set iterator’s [[Iterator2]] internal slot to iterator2.
  4. Set iterator’s [[State]] internal slot to 1.
  5. Let next be a new built-in function object as defined in CompundIterator next (7.4.10.1).
  6. Set iterator’s [[IteratedNext]] internal slot to next.
  7. Let status be the result of CreateDataProperty(iterator, "next", next).
  8. Return iterator.

7.4.10.1 CompoundIterator next( )

The CompoundIterator next method is a standard built-in function object that performs the following steps:

  1. Let O be the this value.
  2. Let f be the active function object.
  3. If O does not have a [[IteratedNext]] internal slot, then throw a TypeError exception.
  4. Let next be the value of the [[IteratedNext]] internal slot of O.
  5. If SameValue(f, next) is false, then throw a TypeError exception.
  6. If O does not have a [[Iterator1]] internal slot, then throw a TypeError exception.
  7. Assert: O is an object created and initialized by CreateCompoundIterator.
  8. Let state be the value of O’s [[State] internal slot.
  9. If state = 1, then
    1. Let iterator1 be the value of O’s [[Iterator1] internal slot.
    2. Let result1 be IteratorStep(iterator1).
    3. If result1 is not false, then,
      1. Return result1.
    4. Set O’s [[State] internal slot to 2.
  10. Let iterator2 be the value of O’s [[Iterator2] internal slot.
  11. Return IteratorNext(iterator2).

NOTE A CompoundIterator next method will throw an exception if applied to any object other than the one with which it was originally associated.

7.5 Operations on Promise Objects

Promise Objects (25.4) serve as a placeholder for the eventual result of a deferred (and possibly asynchronous) computation.

Within this specification the adjective “eventual” mean a value or a Promise object that will ultimately resolves to the value. For example, “Returns an eventual String” is equivalent to “Returns either a String or a Promise object that will eventually resolves to a String”. A “resolved value” is the final value of an “eventual value”.

NOTE The Promise related abstract operations defined in this subclause are used by specification algorithms when they perform or respond to asynchronous operations. They ensure that the actual built-in Promise operations are used by the algorithms, even if ECMAScript code has modified the properties of %Promise% or %PromisePrototype%.

7.5.1 PromiseNew ( executor ) Abstract Operation

The abstract operation PromiseNew allocates and initializes a new promise object for use by specification algorithm. The executor argument initiates the deferred computation.

  1. Let promise be AllocatePromise(%Promise%).
  2. Return InitializePromise(promise, executor).

7.5.2 PromiseBuiltinCapability () Abstract Operation

The abstract operation PromiseBuiltinCapability allocates a PromiseCapability record (25.4.1.1) for a builtin promise object for use by specification algorithm.

  1. Let promise be AllocatePromise(%Promise%).
  2. Return CreatePromiseCapabilityRecord(promise, %Promise%).

NOTE This abstract operation is the same as the default built-in behaviour of NewPromiseCapability abstract operation (25.4.1.4).

7.5.3 PromiseOf (value) Abstract Operation

The abstract operation PromiseOf returns a new Promise that resolves to the argument value.

  1. Assert: IsPromise(value) is false.
  2. Let capability be PromiseBuiltinCapability( ).
  3. ReturnIfAbrupt(capability).
  4. Let resolveResult be the result of calling the [[Call]] internal method of capability.[[Resolve]] with undefined as thisArgument and (value) as argumentsList.
  5. ReturnIfAbrupt(resolveResult).
  6. Return capability.[[Promise]].

NOTE This abstract operation is the same as the default built-in behaviour of the Promise.resolve method (25.4.4.5). However, PromiseOf does not accept a Promise is its argument.

7.5.4 PromiseAll (promiseList) Abstract Operation

7.5.5 PromiseCatch (promise, rejectedAction) Abstract Operation

7.5.6 PromiseThen (promise, resolvedAction, rejectedAction) Abstract Operation

8 Executable Code and Execution Contexts

8.1 Lexical Environments

A Lexical Environment is a specification type used to define the association of Identifiers to specific variables and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment consists of an Environment Record and a possibly null reference to an outer Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement and a new Lexical Environment is created each time such code is evaluated.

An Environment Record records the identifier bindings that are created within the scope of its associated Lexical Environment.

The outer environment reference is used to model the logical nesting of Lexical Environment values. The outer reference of a (inner) Lexical Environment is a reference to the Lexical Environment that logically surrounds the inner Lexical Environment. An outer Lexical Environment may, of course, have its own outer Lexical Environment. A Lexical Environment may serve as the outer environment for multiple inner Lexical Environments. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Lexical Environments of each of the nested functions will have as their outer Lexical Environment the Lexical Environment of the current evaluation of the surrounding function.

A global environment is a Lexical Environment which does not have an outer environment. The global environment’s outer environment reference is null. A global environment’s environment record may be prepopulated with identifier bindings and includes an associated global object whose properties provide some of the global environment’s identifier bindings. This global object is the value of a global environment’s this binding. As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.

A method environment is a Lexical Environment that corresponds to the invocation of an ECMAScript function object that establishes a new this binding. A method environment also captures the state necessary to support super method invocations.

Lexical Environments and Environment Record values are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.

8.1.1 Environment Records

There are two primary kinds of Environment Record values used in this specification: declarative environment records and object environment records. Declarative environment records are used to define the effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly associate identifier bindings with ECMAScript language values. Object environment records are used to define the effect of ECMAScript elements such as WithStatement that associate identifier bindings with the properties of some object. Global Environment Records and Function Environment Records are specializations that are used for specifically for Script global declarations and for top-level declarations within functions.

For specification purposes Environment Record values can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with three concrete subclasses, declarative environment record, object environment record, and global environment record. Function environment records are a subclass of declarative environment record. The abstract class includes the abstract specification methods defined in Table 16. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 16 — Abstract Methods of Environment Records
Method Purpose
HasBinding(N) Determine if an environment record has a binding for an identifier. Return true if it does and false if it does not. The String value N is the text of the identifier.
CreateMutableBinding(N, D) Create a new but uninitialized mutable binding in an environment record. The String value N is the text of the bound name. If the optional Boolean argument D is true the binding is may be subsequently deleted.
CreateImmutableBinding(N) Create a new but uninitialized immutable binding in an environment record. The String value N is the text of the bound name.
InitializeBinding(N,V) Set the value of an already existing but uninitialized binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N,V, S) Set the value of an already existing mutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception. S is used to identify strict mode references.
GetBindingValue(N,S) Returns the value of an already existing binding from an environment record. The String value N is the text of the bound name. S is used to identify strict mode references. If S is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.
DeleteBinding(N) Delete a binding from an environment record. The String value N is the text of the bound name If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
HasThisBinding() Determine if an environment record establishes a this binding. Return true if it does and false if it does not.
HasSuperBinding() Determine if an environment record establishes a super method binding. Return true if it does and false if it does not.
WithBaseObject () If this environment record is associated with a with statement, return the with object. Otherwise, return undefined.

8.1.1.1 Declarative Environment Records

Each declarative environment record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A declarative environment record binds the set of identifiers defined by the declarations contained within its scope.

The behaviour of the concrete specification methods for Declarative Environment Records is defined by the following algorithms.

8.1.1.1.1 HasBinding(N)

The concrete environment record method HasBinding for declarative environment records simply determines if the argument identifier is one of the identifiers bound by the record:

  1. Let envRec be the declarative environment record for which the method was invoked.
  2. If envRec has a binding for the name that is the value of N, return true.
  3. Return false.

8.1.1.1.2 CreateMutableBinding (N, D)

The concrete Environment Record method CreateMutableBinding for declarative environment records creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If Boolean argument D is provided and has the value true the new binding is marked as being subject to deletion.

  1. Let envRec be the declarative environment record for which the method was invoked.
  2. Assert: envRec does not already have a binding for N.
  3. Create a mutable binding in envRec for N and record that it is uninitialized. If D is true record that the newly created binding may be deleted by a subsequent DeleteBinding call.
  4. Return NormalCompletion(empty).

8.1.1.1.3 CreateImmutableBinding (N)

The concrete Environment Record method CreateImmutableBinding for declarative environment records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this environment record for N.

  1. Let envRec be the declarative environment record for which the method was invoked.
  2. Assert: envRec does not already have a binding for N.
  3. Create an immutable binding in envRec for N and record that it is uninitialized.

8.1.1.1.4 InitializeBinding (N,V)

The concrete Environment Record method InitializeBinding for declarative environment records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

  1. Let envRec be the declarative environment record for which the method was invoked.
  2. Assert: envRec must have an uninitialized binding for N.
  3. Set the bound value for N in envRec to V.
  4. Record that the binding for N in envRec has been initialized.

8.1.1.1.5 SetMutableBinding (N,V,S)

The concrete Environment Record method SetMutableBinding for declarative environment records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N must already exist. If the binding is an immutable binding, a TypeError is thrown if S is true.

  1. Let envRec be the declarative environment record for which the method was invoked.
  2. Assert: envRec must have a binding for N.
  3. If the binding for N in envRec has not yet been initialized throw a ReferenceError exception.
  4. Else if the binding for N in envRec is a mutable binding, change its bound value to V.
  5. Else this must be an attempt to change the value of an immutable binding so if S is true throw a TypeError exception.
  6. Return NormalCompletion(empty).

8.1.1.1.6 GetBindingValue(N,S)

The concrete Environment Record method GetBindingValue for declarative environment records simply returns the value of its bound identifier whose name is the value of the argument N. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.

  1. Let envRec be the declarative environment record for which the method was invoked.
  2. Assert: envRec has a binding for N.
  3. If the binding for N in envRec is an uninitialized binding, then throw a ReferenceError exception.
  4. Return the value currently bound to N in envRec.

8.1.1.1.7 DeleteBinding (N)

The concrete Environment Record method DeleteBinding for declarative environment records can only delete bindings that have been explicitly designated as being subject to deletion.

  1. Let envRec be the declarative environment record for which the method was invoked.
  2. If envRec does not have a binding for the name that is the value of N, return true.
  3. If the binding for N in envRec cannot be deleted, return false.
  4. Remove the binding for N from envRec.
  5. Return true.

8.1.1.1.8 HasThisBinding ()

Regular Declarative Environment Records do not provide a this binding.

  1. Return false.

8.1.1.1.9 HasSuperBinding ()

Regular Declarative Environment Records do not provide a super binding.

  1. Return false.

8.1.1.1.10 WithBaseObject()

Declarative Environment Records always return undefined as their WithBaseObject.

  1. Return undefined.

8.1.1.2 Object Environment Records

Each object environment record is associated with an object called its binding object. An object environment record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an object environment record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the value false. Immutable bindings do not exist for object environment records.

Object environment records also have a possibly empty List of strings called unscopables. The strings in this List are excluded from the environment records set of bound names, regardless of whether or not they exist as property keys of its binding object.

Object environment records created for with statements (13.10) can provide their binding object as an implicit this value for use in function calls. The capability is controlled by a withEnvironment Boolean value that is associated with each object environment record. By default, the value of withEnvironment is false for any object environment record.

The behaviour of the concrete specification methods for Object Environment Records is defined by the following algorithms.

8.1.1.2.1 HasBinding(N)

The concrete Environment Record method HasBinding for object environment records determines if its associated binding object has a property whose name is the value of the argument N:

  1. Let envRec be the object environment record for which the method was invoked.
  2. Let bindings be the binding object for envRec.
  3. Repeat, while bindings is not null,
    1. Let hasOwn be HasOwnProperty(bindings, N).
    2. ReturnIfAbrupt(hasOwn).
    3. If hasOwn is true, then
      1. Let hasUnscopables be HasOwnProperty(bindings, @@unscopables).
      2. ReturnIfAbrupt(hasUnscopables).
      3. If hasUnscopables is false, then return true.
      4. Let unscopables be Get(bindings, @@unscopables).
      5. ReturnIfAbrupt(unscopables).
      6. If Type(unscopables) is not Object, then return true.
      7. Let isBlocked be HasOwnProperty(unscopables, N).
      8. ReturnIfAbrupt(isBlocked).
      9. If isBlocked is false, then return true.
    4. Let bindings be the result of calling the [[GetPrototypeOf]] internal method of bindings with no arguments.
    5. ReturnIfAbrupt(bindings).
  4. Return false.

8.1.1.2.2 CreateMutableBinding (N, D)

The concrete Environment Record method CreateMutableBinding for object environment records creates in an environment record’s associated binding object a property whose name is the String value and initializes it to the value undefined. If Boolean argument D is provided and has the value true the new property’s [[Configurable]] attribute is set to true, otherwise it is set to false.

  1. Let envRec be the object environment record for which the method was invoked.
  2. Let bindings be the binding object for envRec.
  3. If D is true then let configValue be true otherwise let configValue be false.
  4. Return DefinePropertyOrThrow(bindings, N, PropertyDescriptor{[[Value]]:undefined, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: configValue}).

NOTE Normally envRec will not have a binding for N but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.

8.1.1.2.3 CreateImmutableBinding (N)

The concrete Environment Record method CreateImmutableBinding is never used within this specification in association with Object environment records.

8.1.1.2.4 InitializeBinding (N,V)

The concrete Environment Record method InitializeBinding for object environment records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

  1. Let envRec be the object environment record for which the method was invoked.
  2. Assert: envRec must have an uninitialized binding for N.
  3. Record that the binding for N in envRec has been initialized.
  4. Return the result of calling the SetMutableBinding concrete method of envRec with N, V, and false as arguments.

8.1.1.2.5 SetMutableBinding (N,V,S)

The concrete Environment Record method SetMutableBinding for object environment records attempts to set the value of the environment record’s associated binding object’s property whose name is the value of the argument N to the value of argument V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

  1. Let envRec be the object environment record for which the method was invoked.
  2. Let bindings be the binding object for envRec.
  3. Return Put(bindings, N, V, and S).

8.1.1.2.6 GetBindingValue(N,S)

The con

crete Environment Record method GetBindingValue for object environment records returns the value of its associated binding object’s property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon the value of the S argument:

  1. Let envRec be the object environment record for which the method was invoked.
  2. Let bindingThis be the binding object for envRec.
  3. Let bindings be bindingThis.
  4. Repeat, while bindings is not null,
    1. Let hasOwn be HasOwnProperty(bindings, N).
    2. ReturnIfAbrupt(hasOwn).
    3. If hasOwn is true, then
      1. Let hasUnscopables be HasOwnProperty(bindings, @@unscopables).
      2. ReturnIfAbrupt(hasUnscopables).
      3. Let isBlocked be false.
      4. If hasUnscopables is true, then
        1. Let unscopables be Get(bindings, @@unscopables).
        2. ReturnIfAbrupt(unscopables).
        3. If Type(unscopables) is Object, then
          1. Let isBlocked be HasOwnProperty(unscopables, N).
          2. ReturnIfAbrupt(isBlocked).
      5. If isBlocked is false, then
        1. Return the result of calling the [[Get]] internal method of bindings with arguments N and bindingThis.
    4. Let bindings be the result of calling the [[GetPrototypeOf]] internal method of bindings with no arguments.
    5. ReturnIfAbrupt(bindings).
  5. If S is false, return the value undefined, otherwise throw a ReferenceError exception.

8.1.1.2.7 DeleteBinding (N)

The concrete Environment Record method DeleteBinding for object environment records can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true.

  1. Let envRec be the object environment record for which the method was invoked.
  2. Let bindings be the binding object for envRec.
  3. Return the result of calling the [[Delete]] internal method of bindings passing N as the argument.

8.1.1.2.8 HasThisBinding ()

Regular Object Environment Records do not provide a this binding.

  1. Return false.

8.1.1.2.9 HasSuperBinding ()

Regular Object Environment Records do not provide a super binding.

  1. Return false.

8.1.1.2.10 WithBaseObject()

Object Environment Records return undefined as their WithBaseObject unless their withEnvironment flag is true.

  1. Let envRec be the object environment record for which the method was invoked.
  2. If the withEnvironment flag of envRec is true, return the binding object for envRec.
  3. Otherwise, return undefined.

8.1.1.3 Function Environment Records

A function environment record is a declarative environment record that is used to represent the outer most scope of a function that provides a this binding. In addition to its identifier bindings, a function environment record contains the this value used within its scope. If such a function references super, its function environment record also contains the state that is used to perform super method invocations from within the function.

Function environment records store their this binding as the value of their thisValue. If the associated function references super, the environment record stores in HomeObject the object that the function is bound to as a method and in MethodName the property key used for unqualified super invocations from within the function. The default value for HomeObject and MethodName is undefined.

Methods environment records support all of Declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, declarative environment records support the methods listed in Table 17:

Table 17 — Additional Methods of Function Environment Records
Method Purpose
GetThisBinding() Return the value of this environment record’s this binding.
GetSuperBase() Return the object that is the base for super property accesses bound in this environment record. The object is derived from this environment record’s HomeObject binding. If the value is Empty, return undefined.
GetMethodName() Return the value of this environment record’s MethodName binding.

The behaviour of the additional concrete specification methods for Function Environment Records is defined by the following algorithms:

8.1.1.3.1 HasThisBinding ()

Function Environment Records always provide a this binding.

  1. Return true.

8.1.1.3.2 HasSuperBinding ()

  1. If this environment record’s HomeObject has the value Empty, then return false. Otherwise, return true.

8.1.1.3.3 GetThisBinding ()

  1. Return the value of this environment record’s thisValue.

8.1.1.3.4 GetSuperBase ()

  1. Let home be the value of this environment record’s HomeObject.
  2. If home has the value Empty, then return undefined.
  3. Assert: Type(home) is Object.
  4. Return the result of calling home’s [[GetPrototypeOf]] internal method.

8.1.1.3.5 GetMethodName ()

  1. Return the value of this environment record’s MethodName.

8.1.1.4 Global Environment Records

A global environment record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common Realm (8.1.2.5). A global environment record provides the bindings for built-in globals (clause 18), properties of the global object, and for all declarations that are not function code and that occur within Script productions.

A global environment record is logically a single record but it is specified as a composite encapsulating an object environment record and a declarative environment record. The object environment record has as its base object the global object of the associated Realm. This global object is also the value of the global environment record’s thisValue. The object environment record component of a global environment record contains the bindings for all built-in globals (clause 18) and all bindings introduced by a FunctionDeclaration, GeneratorDeclaration, or VariableStatement contained in global code. The bindings for all other ECMAScript declarations in global code are contained in the declarative environment record component of the global environment record.

Properties may be created directly on a global object. Hence, the object environment record component of a global environment record may contain both bindings created explicitly by FunctionDeclaration, GeneratorDeclaration, or VariableDeclaration declarations and binding created implicitly as properties of the global object. In order to identify which bindings were explicitly created using declarations, a global environment record maintains a list of the names bound using its CreateGlobalVarBindings and CreateGlobalFunctionBindings concrete methods.

Global environment records have the additional state components listed in Table 18 and the additional methods listed in Table 19.

Table 18 — Components of Global Environment Records
Component Purpose
ObjectRecord An Object Environment Record whose base object is the global object. It contains global built-in bindings as well as FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration bindings in global code for the associated Realm.
DeclarativeRecord A Declarative Environment Record that contains bindings for all declarations in global code for the associated Realm code except for FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration bindings.
VarNames A List containing the string names bound by FunctionDeclaration, GeneratorDeclaration, and VariableDeclaration declarations in global code for the associated Realm.
Table 19 — Additional Methods of Global Environment Records
Method Purpose
GetThisBinding() Return the value of this environment record’s this binding.
HasVarDeclaration (N) Determines if the argument identifier has a binding in this environment record that was created using a VariableDeclaration, FunctionDeclaration, or GeneratorDeclaration.
HasLexicalDeclaration (N) Determines if the argument identifier has a binding in this environment record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration.
CanDeclareGlobalVar (N) Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N.
CanDeclareGlobalFunction (N) Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.
CreateGlobalVarBinding(N, D) Used to create global var bindings in the ObjectRecord component of a global environment record. The binding will be a mutable binding. The corresponding global object property will have attribute values approate for a var. The String value N is the text of the bound name. D is true the binding is may be subsequently deleted. This is logically equivalent to CreateMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D) Used to create and initialize global function bindings in the ObjectRecord component of a global environment record. The binding will be a mutable binding. The corresponding global object property will have attribute values approate for a function.The String value N is the text of the bound name. V is the initial value of the binding. If the optional Boolean argument D is true the binding is may be subsequently deleted. This is logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.

The behaviour of the concrete specification methods for Global Environment Records is defined by the following algorithms.

8.1.1.4.1 HasBinding(N)

The concrete environment record method HasBinding for global environment records simply determines if the argument identifier is one of the identifiers bound by the record:

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let DclRec be envRec’s DeclarativeRecord.
  3. If the result of calling DclRec’s HasBinding concrete method with argument N is true, return true.
  4. Let ObjRec be envRec’s ObjectRecord.
  5. Return the result of calling ObjRec’s HasBinding concrete method with argument N.

8.1.1.4.2 CreateMutableBinding (N, D)

The concrete environment record method CreateMutableBinding for global environment records creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If Boolean argument D is provided and has the value true the new binding is marked as being subject to deletion.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let DclRec be envRec’s DeclarativeRecord.
  3. Let alreadyThere be the result of calling the HasBinding concrete method of of DclRec with argument N.
  4. ReturnIfAbrupt(alreadyThere).
  5. Return the result of calling the CreateMutableBinding concrete method of DclRec with arguments N and D.

8.1.1.4.3 CreateImmutableBinding (N)

The concrete Environment Record method CreateImmutableBinding for global environment records creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this environment record for N.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let DclRec be envRec’s DeclarativeRecord.
  3. Let alreadyThere be the result of calling the HasBinding concrete method of of DclRec with argument N.
  4. ReturnIfAbrupt(alreadyThere).
  5. Return the result of calling the CreateImmutableBinding concrete method of DclRec with argument N.

8.1.1.4.4 InitializeBinding (N,V)

The concrete Environment Record method InitializeBinding for global environment records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let DclRec be envRec’s DeclarativeRecord.
  3. If the result of calling DclRec’s HasBinding concrete method with argument N is true, then
    1. Return the result of calling DclRec’s InitializeBinding concrete method with arguments N and V.
  4. Assert: If the binding exists it must be in the object environment record.
  5. Let ObjRec be envRec’s ObjectRecord.
  6. Return the result of calling ObjRec’s InitializeBinding concrete method with arguments N and V.

8.1.1.4.5 SetMutableBinding (N,V,S)

The concrete Environment Record method SetMutableBinding for global environment records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. If the binding is an immutable binding, a TypeError is thrown if S is true. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let DclRec be envRec’s DeclarativeRecord.
  3. If the result of calling DclRec’s HasBinding concrete method with argument N is true, then
    1. Return the result of calling the SetMutableBinding concrete method of DclRec with arguments N, V, and S.
  4. Let ObjRec be envRec’s ObjectRecord.
  5. Return the result of calling the SetMutableBinding concrete method of ObjRec with arguments N, V, and S.

8.1.1.4.6 GetBindingValue(N,S)

The concrete Environment Record method GetBindingValue for global environment records simply returns the value of its bound identifier whose name is the value of the argument N. If the binding is an uninitialized binding throw a ReferenceError exception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let DclRec be envRec’s DeclarativeRecord.
  3. If the result of calling DclRec’s HasBinding concrete method with argument N is true, then
    1. Return the result of calling the GetBindingValue concrete method of DclRec with arguments N and S.
  4. Let ObjRec be envRec’s ObjectRecord.
  5. Return the result of calling the GetBindingValue concrete method of ObjRec with arguments N, and S.

8.1.1.4.7 DeleteBinding (N)

The concrete Environment Record method DeleteBinding for global environment records can only delete bindings that have been explicitly designated as being subject to deletion.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let DclRec be envRec’s DeclarativeRecord.
  3. If the result of calling DclRec’s HasBinding concrete method with argument N is true, then
    1. Return the result of calling the DeleteBinding concrete method of DclRec with argument N.
  4. Let ObjRec be envRec’s ObjectRecord.
  5. If the result of calling ObjRec’s HasBinding concrete method with argument N is true, then
    1. Let status be the result of calling the DeleteBinding concrete method of ObjRec with argument N.
    2. ReturnIfAbrupt(status).
    3. If status is true, then
      1. Let varNames be envRec’s VarNames List.
      2. If N is an element of varNames, then remove that element from the varNames.
    4. Return status.
  6. Return true.

8.1.1.4.8 HasThisBinding ()

Global Environment Records always provide a this binding whose value is the associated global object.

  1. Return true.

8.1.1.4.9 HasSuperBinding ()

  1. Return false.

8.1.1.4.10 WithBaseObject()

Global Environment Records always return undefined as their WithBaseObject.

  1. Return undefined.

8.1.1.4.11 GetThisBinding ()

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let ObjRec be envRec’s ObjectRecord.
  3. Let bindings be the binding object for ObjRec.
  4. Return bindings.

8.1.1.4.12 HasVarDeclaration (N)

The concrete environment record method HasVarDeclaration for global environment records determines if the argument identifier has a binding in this record that was created using a VariableStatement or a FunctionDeclaration :

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let varDeclaredNames be envRec’s VarNames List.
  3. If varDeclaredNames contains the value of N, return true.
  4. Return false.

8.1.1.4.13 HasLexicalDeclaration (N)

The concrete environment record method HasLexicalDeclaration for global environment records determines if the argument identifier has a binding in this record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration :

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let DclRec be envRec’s DeclarativeRecord.
  3. Return the result of calling DclRec’s HasBinding concrete method with argument N.

8.1.1.4.14 CanDeclareGlobalVar (N)

The concrete environment record method CanDeclareGlobalVar for global environment records determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundent var declarations and var declarations for pre-existing global object properties are allowed.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let ObjRec be envRec’s ObjectRecord.
  3. If the result of calling ObjRec’s HasBinding concrete method with argument N is true, return true.
  4. Let bindings be the binding object for ObjRec.
  5. Let extensible be IsExtensible(bindings).
  6. Return extensible.

8.1.1.4.15 CanDeclareGlobalFunction (N)

The concrete environment record method CanDeclareGlobalFunction for global environment records determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let ObjRec be envRec’s ObjectRecord.
  3. Let globalObject be the binding object for ObjRec.
  4. Let extensible be IsExtensible(globalObject).
  5. ReturnIfAbrupt(extensible).
  6. If the result of calling ObjRec’s HasBinding concrete method with argument N is false, then return extensible.
  7. Let existingProp be the result of calling the [[GetOwnProperty]] internal method of globalObject with argument N.
  8. ReturnIfAbrupt(existingProp).
  9. If existingProp is undefined, then return extensible.
  10. If existingProp.[[Configurable]] is true, then return true.
  11. If IsDataDescriptor(existingProp) is true and existingProp has attribute values {[[Writable]]: true, [[Enumerable]]: true}, then return true.
  12. Return false.

8.1.1.4.16 CreateGlobalVarBinding (N, D)

The concrete Environment Record method CreateGlobalVarBinding for global environment records creates a mutable binding in the associated object environment record and records the bound name in the associated VarNames List. If a binding already exists, it is reused.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let ObjRec be envRec’s ObjectRecord.
  3. If the result of calling ObjRec’s HasBinding concrete method with argument N is false, then
    1. Let status be the result of calling the CreateMutableBinding concrete method of ObjRec with arguments N and D.
    2. ReturnIfAbrupt(status).
  4. Let varDeclaredNames be envRec’s VarNames List.
  5. If varDeclaredNames does not contain the value of N, then
    1. Append N to varDeclaredNames.
  6. Return NormalCompletion(empty).

8.1.1.4.17 CreateGlobalFunctionBinding (N, V, D)

The concrete Environment Record method CreateGlobalFunctionBinding for global environment records creates a mutable binding in the associated object environment record and records the bound name in the associated VarNames List. If a binding already exists, it is replaced.

  1. Let envRec be the global environment record for which the method was invoked.
  2. Let ObjRec be envRec’s ObjectRecord.
  3. Let globalObject be the binding object for ObjRec.
  4. Let existingProp be the result of calling the [[GetOwnProperty]] internal method of globalObject with argument N.
  5. ReturnIfAbrupt(existingProp).
  6. If existingProp is undefined or existingProp.[[Configurable]] is true, then
    1. Let desc be the PropertyDescriptor{[[Value]]:V, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: D}.
  7. Else,
    1. Let desc be the PropertyDescriptor{[[Value]]:V }.
  8. Let status be DefinePropertyOrThrow(globalObject, N, desc).
  9. ReturnIfAbrupt(status).
  10. Let varDeclaredNames be envRec’s VarNames List.
  11. If varDeclaredNames does not contain the value of N, then
    1. Append N to varDeclaredNames.
  12. Return NormalCompletion(empty).

NOTE Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values.

8.1.2 Lexical Environment Operations

The following abstract operations are used in this specification to operate upon lexical environments:

8.1.2.1 GetIdentifierReference (lex, name, strict) Abstract Operation

The abstract operation GetIdentifierReference is called with a Lexical Environment lex, a String name, and a Boolean flag strict. The value of lex may be null. When called, the following steps are performed:

  1. If lex is the value null, then
    1. Return a value of type Reference whose base value is undefined, whose referenced name is name, and whose strict reference flag is strict.
  2. Let envRec be lex’s environment record.
  3. Let exists be the result of calling the HasBinding concrete method of envRec passing name as the argument.
  4. ReturnIfAbrupt(exists).
  5. If exists is true, then
    1. Return a value of type Reference whose base value is envRec, whose referenced name is name, and whose strict reference flag is strict.
  6. Else
    1. Let outer be the value of lex’s outer environment reference.
    2. Return GetIdentifierReference(outer, name, strict).

8.1.2.2 NewDeclarativeEnvironment (E) Abstract Operation

When the abstract operation NewDeclarativeEnvironment is called with either a Lexical Environment or null as argument E the following steps are performed:

  1. Let env be a new Lexical Environment.
  2. Let envRec be a new declarative environment record containing no bindings.
  3. Set env’s environment record to be envRec.
  4. Set the outer lexical environment reference of env to E.
  5. Return env.

8.1.2.3 NewObjectEnvironment (O, E) Abstract Operation

When the abstract operation NewObjectEnvironment is called with an Object O and a Lexical Environment E (or null) as arguments, the following steps are performed:

  1. Let env be a new Lexical Environment.
  2. Let envRec be a new object environment record containing O as the binding object.
  3. Set env’s environment record to envRec.
  4. Set the outer lexical environment reference of env to E.
  5. Return env.

8.1.2.4 NewFunctionEnvironment (F, T) Abstract Operation

When the abstract operation NewFunctionEnvironment is called with an ECMAScript function Object F and an ECMAScript value T as arguments, the following steps are performed:

  1. Assert: The value of F’s [[ThisMode]] internal slot is not lexical.
  2. Let env be a new Lexical Environment.
  3. Let envRec be a new Function environment record containing containing no bindings.
  4. Set envRec’s thisValue to T.
  5. If F’s [[NeedsSuper]] internal slot is true, then
    1. Let home be the value of F’s [[HomeObject]] internal slot.
    2. If home is undefined, then throw a ReferenceError exception.
    3. Set envRec’s HomeObject to home.
    4. Set envRec’s MethodName to the value of F’s [[MethodName]] internal slot.
  6. Else,
    1. Set envRec’s HomeObject to Empty.
  7. Set env’s environment record to be envRec.
  8. Set the outer lexical environment reference of env to the value of F’s [[Environment]] internal slot.
  9. Return env.

8.1.2.5 NewGlobalEnvironment ( G ) Abstract Operation

When the abstract operation NewGlobalEnvironment is called with an ECMAScript Object G as its argument, the following steps are performed:

  1. Let env be a new Lexical Environment.
  2. Let objRec be a new object environment record containing G as the binding object.
  3. Set objRec’s unscopables to an empty List.
  4. Let dclRec be a new declarative environment record containing no bindings.
  5. Let globalRec be a new global environment record.
  6. Set globalRec’s ObjectRecord to objRec.
  7. Set globalRec’s DeclarativeRecord to dclRec.
  8. Set globalRec’s VarNames to a new empty List.
  9. Set env’s environment record to globalRec.
  10. Set the outer lexical environment reference of env to null
  11. Return env.

8.2 Code Realms

Before it is evaluated, all ECMAScript code must be associated with a Realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, a Loader object that can associate new ECMAScript code with the realm, and other associated state and resources.

A Realm is specified as a Record with the fields specified in Table 20:

Table 20 — Realm Record Fields
Field Name Value Meaning
[[intrinsics]] A record whose field names are intrinsic keys and whose values are objects These are the intrinsic values used by code associated with this Realm
[[globalThis]] An object The global object for this Realm
[[globalEnv]] An ECMAScript environment The global environment for this Realm
[[directEvalTranslate]] undefined or an object that is callable as a function.
[[nonEvalFallback]] undefined or an object that is callable as a function.
[[indirectEval]] undefined or an object that is callable as a function.
[[loader]] any ECMAScript identifier or empty The Loader object that can associate ECMAScript code with this Realm

8.2.1 CreateRealm ( ) Abstract Operation

The abstract operation CreateRealm with no arguments performs the following steps:

  1. Let realmRec be a new Record.
  2. Let intrinsics be CreateIntrinsics(realmRec).
  3. Set realmRec.[[globalThis]] to undefined.
  4. Set realmRec.[[globalEnv]] to undefined.
  5. Set each of realmRec.[[directEvalTranslate]], realmRec.[[nonEvalFallback]], and realmRec.[[indirectEval]] to undefined.
  6. Return realmRec.

8.2.2 CreateIntrinsics ( realmRec ) Abstract Operation

When the abstract operation CreateIntrinsics with argument realmRec performs the following:

  1. Let intrinsics be a new Record.
  2. Set realmRec.[[intrinsics]] to intrinsics.
  3. Let objProto be ObjectCreate(null).
  4. Set intrinsics.[[%ObjectPrototype%]] to objProto.
  5. Let throwerSteps be the algorithm steps of the %ThrowTypeError% function (9.2.8.1).
  6. Let thrower be CreateBuiltinFunction(realmRec, throwerSteps, null).
  7. Set intrinsics.[[%ThrowTypeError%]] to thrower.
  8. Let noSteps be an empty sequence of algorithm steps.
  9. Let funcProto be the CreateBuiltinFunction(realmRec, noSteps, objProto).
  10. Set intrinsics.[[%FunctionPrototype%]] to funcProto.
  11. Call the [[SetPrototypeOf]] internal method of thrower with argument funcProto.
  12. Set fields of intrinsics with the values listed in Table 7 that have not already been handled above. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses 18-26. All object property values are newly created object values. All values that are built-in function objects are created by performing CreateBuiltinFunction(realmRec, <steps>, <prototype>, <slots>) where <steps> is the definition of that function provided by this specification, <prototype> is the specified value of the function’s [[Prototype]] internal slot and <slots> is a list of the names, if any, of the functions specified internal slots. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
  13. Return intrinsics.

8.2.3 SetRealmGlobalObj ( realmRec, globalObj ) Abstract Operation

The abstract operation SetRealmGlobalObj with arguments realmRec and globalObj performs the following steps:

  1. If globalObj is undefined, then
    1. Let globalObj be ObjectCreate(realmRec.[[instrinsics]].[[%ObjectPrototype%]]).
  2. Assert: Type(globalObj) is Object.
  3. Set realmRec.[[globalThis]] to newGlobal.
  4. Let newGlobalEnv be NewGlobalEnvironment(newGlobal).
  5. Set realmRec.[[globalEnv]] to newGlobalEnv.
  6. Return realmRec.

8.2.4 SetDefaultGlobalBindings ( realmRec ) Abstract Operation

The abstract operation SetDefaultGlobalBindings with argument realmRec performs the following steps:

  1. Let global be realmRec.[[globalThis]].
  2. For each property of the Global Object specified in clause 18, do
    1. Let name be the string value of the property name.
    2. Let desc be the fully populated data property descriptor for the property containing the specified attributes for the property. For properties whose values are functions, the value of the [[Value]] attribute is the corresponding intrinsic function object from realmRec.
    3. Let status be DefinePropertyOrThrow(global, name, desc).
    4. ReturnIfAbrupt(status).
  3. Return global.

8.3 Execution Contexts

An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context that is actually executing code. This is known as the running execution context. A stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.

An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 21.

Table 21 —State Components for All Execution Contexts
Component Purpose
code evaluation state Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context.
Function If this execution context is evaluating the code of a function object, then the value of this is the that function. If the context is evaluation the code of a Script the value is null.
Realm The Realm from which associated code accesses ECMAScript resources.

Evaluation of code by the running execution context may be suspended at various points defined within this specification. Once the running execution context has been suspended a different execution context may become the running execution context and commence evaluating its code. At some later time a suspended execution context may again become the running execution context and continue evaluating its code at the point where it had previously been suspended. Transition of the running execution context status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of the running execution context.

The value of the Realm component of the running execution context is also called the current Realm. The value of the Function component of the running execution context is also called the active Function.

Execution contexts for ECMAScript code have the additional state components listed in Table 22.

Table 22 — Additional State Components for ECMAScript Code Execution Contexts
Component Purpose
LexicalEnvironment Identifies the Lexical Environment used to resolve identifier references made by code within this execution context.
VariableEnvironment Identifies the Lexical Environment whose environment record holds bindings created by VariableStatements within this execution context.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value. The value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during execution of code within an execution context.

Execution contexts representing the evaluation of generator objects have the additional state components listed in Table 23.

Table 23 — Additional State Components for Generator Execution Contexts
Component Purpose
Generator The GeneratorObject that this execution context is evaluating.

In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of the running execution context.

An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.

8.3.1 ResolveBinding ( name ) Abstract Operation

The ResolveBinding abstract operation is used to determine the binding of name passed as a string value using the LexicalEnvironment of the running execution context. During execution of ECMAScript code, ResolveBinding is performed using the following algorithm:

  1. Let env be the running execution context’s LexicalEnvironment.
  2. If the syntactic production that is being evaluated is contained in strict mode code, then let strict be true, else let strict be false.
  3. Return GetIdentifierReference(env, name, strict ).

NOTE The result of ResolveBinding is always a Reference value with its referenced name component equal to the name argument.

8.3.2 GetThisEnvironment ( ) Abstract Operation

The abstract operation GetThisEnvironment finds the lexical environment that currently supplies the binding of the keyword this. GetThisEnvironment performs the following steps:

  1. Let lex be the running execution context’s LexicalEnvironment.
  2. Repeat
    1. Let envRec be lex’s environment record.
    2. Let exists be the result of calling the HasThisBinding concrete method of envRec.
    3. If exists is true, then return envRec.
    4. Let outer be the value of lex’s outer environment reference.
    5. Let lex be outer.

NOTE The loop in step 2 will always terminate because the list of environments always ends with the global environment which has a this binding.

8.3.3 ResolveThisBinding ( ) Abstract Operation

The abstract operation ResolveThisBinding determines the binding of the keyword this using the LexicalEnvironment of the running execution context. ResolveThisBinding performs the following steps:

  1. Let env be GetThisEnvironment( ).
  2. Return the result of calling the GetThisBinding concrete method of env.

8.3.4 GetGlobalObject ( ) Abstract Operation

The abstract operation GetGlobalObject returns the global object used by the currently running execution context. GetGlobalObject performs the following steps:

  1. Let ctx be the running execution context.
  2. Let currentRealm be ctx’s Realm.
  3. Return currentRealm.[[globalThis]].

8.4 Jobs and Job Queues

A Job is an abstract operation that initiates an ECAMScript computation when no other ECMAScript computation is currently in progress. A Job abstract operation may be defined to accept an arbitrary set of job parameters.

Execution of a Job can be initiated only when there is no running execution context and the execution context stack is empty. A PendingJob is a request for the future execution of a Job. A PendingJob is an internal Record whose fields are specified in Table 24.

Table 24 — PendingJob Record Fields
Field Name Value Meaning
[[Job]] The name of a Job abstract operation This is the abstract operation that is performed when execution of this PendingJob is initiated. Jobs are abstract operations that use NextJob rather than Return to indicate that they have completed.
[[Arguments]] A List. The List of argument values that are to be passed to [[Job]] when it is activated.
[[Realm]] A Realm Record The Realm for the initial execution context when this Pending Job is initiated.
[[HostDefined]] Any, default value is undefined. Field reserved for use by host environments that need to associate additional information with a pending Job.

A Job Queue is a FIFO queue of PendingJob records. Each Job Queue has a name and the full set of available Job Queues are defined by an ECMAScript implementation. Every ECMAScript implementation has at least the Job Queues defined in Table 25.

Table 25 — Required Job Queues
Name Purpose
ScriptJobs Jobs that validate and evaluate ECMAScript Script and Module code units. See clauses 10 and 15.
PromiseJobs Jobs that are responses to the settlement of a Promise (see 25.4).

A request for the future execution of a Job is made by enqueueing, on a Job Queue, a PendingJob record that includes a Job abstract operation name and any necessary argument values. When there is no running execution context and the execution context stack is empty, the ECMAScript implementation removes the first PendingJob from a Job Queue and uses the information contained in it to create an execution context and starts execution of the associated Job abstract operation.

The PendingJob records from a single Job Queue are always initiated in FIFO order. This specification does not define the order in which multiple Job Queues are serviced. An ECMAScript implementation may interweave the FIFO evaluation of the PendingJob records of a Job Queue with the evaluation of the PendingJob records of one or more other Job Queues. An implementation must define what occurs when there are no running execution context and all Job Queues are empty.

NOTE Typically an ECMAScript implementation will have its Job Queues pre-initialized with at least one PendingJob and one of those Jobs will be the first to be executed. An implementation might choose to free all resources and terminate if the current Job completes and all Job Queues are empty. Alternatively, it might choose to wait for a some implementation specific agent or mechanism to enqueue new PendingJob requests.

The following abstract operations are used to create and manage Jobs and Job Queues:

8.4.1 EnqueueJob ( queueName, job, arguments) Abstract Operation

The abstract operation requires three arguments: queueName, job, and arguments. It performs the following steps:

  1. Assert: Type(queueName) is String and its value is the name of a Job Queue recognized by this implementation.
  2. Assert: job is the name of a Job.
  3. Assert: arguments is a List that has the same number of elements as the number of parameters required by job.
  4. Let callerContext be the running execution context.
  5. Let callerRealm be callerContext’s Realm.
  6. Let pending be PendingJob{ [[Job]]: job, [[Arguments]]: arguments, [[Realm]]: callerRealm, [[HostDefined]]: undefined }.
  7. Perform any implementation or host environment defined processing of pending. This may including modify the [[HostDefined]] field or any other field of pending.
  8. Add pending at the back of the Job Queue named by queueName.
  9. Return NormalCompletion(empty).

8.4.2 NextJob result

An algorithm step such as:

  1. NextJob result.

is used in Job abstract operations in place of:

  1. Return result.

Job abstract operations must not contain a Return step or a ReturnIfAbrupt step. The NextJob result operation is equivalent to the following steps:

  1. If result is an abrupt completion, then perform implementation defined unhandled exception processing.
  2. Suspend the running execution context.
  3. Assert: The execution context stack is now empty.
  4. Let nextQueue be a non-empty Job Queue chosen in an implementation defined manner. If all Job Queues are empty, the result is implementation defined.
  5. Let nextPending be the PendingJob record at the front of nextQueue. Remove that record from nextQueue.
  6. Let newContext be a new execution context.
  7. Set newContext’s Realm to nextPending.[[Realm]].
  8. Push newContext onto the execution context stack; newContext is now the running execution context.
  9. Perform any implementation or host environment defined job initialization using nextPending.
  10. Perform the abstract operation named by nextPending.[[Job]] using the elements of nextPending.[[Arguments]] as its arguments.

8.5 Initialization

An ECMAScript implementation performs the following steps prior to the execution of any Jobs or the evaluation of any ECMAScript code:

  1. Let realm be CreateRealm().
  2. Let newContext be a new execution context.
  3. Set the Function of newContext to null.
  4. Set the Realm of newContext to realm.
  5. Push newContext onto the execution context stack; newContext is now the running execution context.
  6. Let status be InitializeFirstRealm(realm).
  7. If status is an abrupt completion, then
    1. Assert: The first realm could not be created.
    2. Terminate ECMAScript execution.
  8. In an implementation dependent manner, obtain the SourceCharacter sequence (see 10) for zero or more ECMAScript scripts. For each such sequence source do,
    1. EnqueueJob("ScriptJobs", ScriptEvaluationJob, (source)).
  9. NextJob NormalCompletion(undefined).

8.5.1 InitializeFirstRealm ( realm ) Abstract Operation

The abstract operation InitializeFirstRealm with parameter realm performs the following steps:

  1. Let intrinsics be CreateIntrinsics(realm).
  2. If this implementation requires use of an exotic object to serve as realm’s global object, then let global be such an object created in an implementation defined manner. Otherwise, let global be undefined indicating that an ordinary object should be created as the global object.
  3. Perform SetRealmGlobalObject(realm, global).
  4. Let globalObj be SetDefaultGlobalBindings(realm).
  5. ReturnIfAbrupt(globalObj).
  6. Create any implementation defined global object properties on globalObj.
  7. Return NormalCompletion(undefined).

9 Ordinary and Exotic Objects Behaviours

9.1 Ordinary Object Internal Methods and Internal Slots

All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either null or an object and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not for set access. Accessor properties are inherited for both get access and set access.

Every ordinary object has a Boolean-valued [[Extensible]] internal slot that controls whether or not properties may be added to the object. If the value of the [[Extensible]] internal slot is false then additional properties may not be added to the object. In addition, if [[Extensible]] is false the value of the [[Prototype]] internal slot of the object may not be modified. Once the value of an object’s [[Extensible]] internal slot has been set to false it may not be subsequently changed to true.

In the following algorithm descriptions, assume O is an ordinary object, P is a property key value, V is any ECMAScript language value, and Desc is a Property Descriptor record.

9.1.1 [[GetPrototypeOf]] ( )

When the [[GetPrototypeOf]] internal method of O is called the following steps are taken:

  1. Return the value of the [[Prototype]] internal slot of O.

9.1.2 [[SetPrototypeOf]] (V)

When the [[SetPrototypeOf]] internal method of O is called with argument V the following steps are taken:

  1. Assert: Either Type(V) is Object or Type(V) is Null.
  2. Let extensible be the value of the [[Extensible]] internal slot of O.
  3. Let current be the value of the [[Prototype]] internal slot of O.
  4. If SameValue(V, current), then return true.
  5. If extensible is false, then return false.
  6. If V is not null, then
    1. Let p be V.
    2. Repeat, while p is not null
      1. If SameValue(p, O) is true, then return false.
      2. Let nextp be the result of calling the [[GetPrototypeOf]] internal method of p with no arguments.
      3. ReturnIfAbrupt(nextp).
      4. Let p be nextp.
  7. Let extensible be the value of the [[Extensible]] internal slot of O.
  8. If extensible is false, then
    1. Let current2 be the value of the [[Prototype]] internal slot of O.
    2. If SameValue(V, current2) is true, then return true.
    3. Return false.
  9. Set the value of the [[Prototype]] internal slot of O to V.
  10. Return true.

9.1.3 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of O is called the following steps are taken:

  1. Return the value of the [[Extensible]] internal slot of O.

9.1.4 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of O is called the following steps are taken:

  1. Set the value of the [[Extensible]] internal slot of O to false.
  2. Return true.

9.1.5 [[GetOwnProperty]] (P)

When the [[GetOwnProperty]] internal method of O is called with property key P, the following steps are taken:

  1. Return OrdinaryGetOwnProperty(O, P).

9.1.5.1 OrdinaryGetOwnProperty (O, P)

When the abstract operation OrdinaryGetOwnProperty is called with Object O and with property key P, the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. If O does not have an own property with key P, return undefined.
  3. Let D be a newly created Property Descriptor with no fields.
  4. Let X be O’s own property whose key is P.
  5. If X is a data property, then
    1. Set D.[[Value]] to the value of X’s [[Value]] attribute.
    2. Set D.[[Writable]] to the value of X’s [[Writable]] attribute
  6. Else X is an accessor property, so
    1. Set D.[[Get]] to the value of X’s [[Get]] attribute.
    2. Set D.[[Set]] to the value of X’s [[Set]] attribute.
  7. Set D.[[Enumerable]] to the value of X’s [[Enumerable]] attribute.
  8. Set D.[[Configurable]] to the value of X’s [[Configurable]] attribute.
  9. Return D.

9.1.6 [[DefineOwnProperty]] (P, Desc)

When the [[DefineOwnProperty]] internal method of O is called with property key P and Property Descriptor Desc, the following steps are taken:

  1. Return OrdinaryDefineOwnProperty(O, P, Desc).

9.1.6.1 OrdinaryDefineOwnProperty (O, P, Desc)

When the abstract operation OrdinaryDefineOwnProperty is called with Object O, property key P, and Property Descriptor Desc the following steps are taken:

  1. Let current be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
  2. ReturnIfAbrupt(current).
  3. Let extensible be the value of the [[Extensible]] internal slot of O.
  4. Return ValidateAndApplyPropertyDescriptor(O, P, extensible, Desc, current).

9.1.6.2 IsCompatiblePropertyDescriptor (Extensible, Desc, Current)

When the abstract operation IsCompatiblePropertyDescriptor is called with Boolean value Extensible, and Property Descriptors Desc, and Current the following steps are taken:

  1. Return ValidateAndApplyPropertyDescriptor(undefined, undefined, Extensible, Desc, Current).

9.1.6.3 ValidateAndApplyPropertyDescriptor (O, P, extensible, Desc, current)

When the abstract operation ValidateAndApplyPropertyDescriptor is called with Object O, property key P, Boolean value extensible, and Property Descriptors Desc, and current the following steps are taken:

This algorithm contains steps that test various fields of the Property Descriptor Desc for specific values. The fields that are tested in this manner need not actually exist in Desc. If a field is absent then its value is considered to be false.

NOTE If undefined is passed as the O argument only validation is performed and no object updates are performed.

  1. Assert: If O is not undefined then P is a valid property key.
  2. If current is undefined, then
    1. If extensible is false, then return false.
    2. Assert: extensible is true.
    3. If IsGenericDescriptor(Desc) or IsDataDescriptor(Desc) is true, then
      1. If O is not undefined, then create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
    4. Else Desc must be an accessor Property Descriptor,
      1. If O is not undefined, then create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to its default value.
    5. Return true.
  3. Return true, if every field in Desc is absent.
  4. Return true, if every field in Desc also occurs in current and the value of every field in Desc is the same value as the corresponding field in current when compared using the SameValue algorithm.
  5. If the [[Configurable]] field of current is false then
    1. Return false, if the [[Configurable]] field of Desc is true.
    2. Return false, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current and Desc are the Boolean negation of each other.
  6. If IsGenericDescriptor(Desc) is true, then no further validation is required.
  7. Else if IsDataDescriptor(current) and IsDataDescriptor(Desc) have different results, then
    1. Return false, if the [[Configurable]] field of current is false.
    2. If IsDataDescriptor(current) is true, then
      1. If O is not undefined, then convert the property named P of object O from a data property to an accessor property. Preserve the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
    3. Else,
      1. If O is not undefined, then convert the property named P of object O from an accessor property to a data property. Preserve the existing values of the converted property’s [[Configurable]] and [[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
  8. Else if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, then
    1. If the [[Configurable]] field of current is false, then
      1. Return false, if the [[Writable]] field of current is false and the [[Writable]] field of Desc is true.
      2. If the [[Writable]] field of current is false, then
        1. Return false, if the [[Value]] field of Desc is present and SameValue(Desc.[[Value]], current.[[Value]]) is false.
    2. Else the [[Configurable]] field of current is true, so any change is acceptable.
  9. Else IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true,
    1. If the [[Configurable]] field of current is false, then
      1. Return false, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]], current.[[Set]]) is false.
      2. Return false, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]]) is false.
  10. If O is not undefined, then
    1. For each field of Desc that is present, set the corresponding attribute of the property named P of object O to the value of the field.
  11. Return true.

NOTE Step 8.b allows any field of Desc to be different from the corresponding field of current if current’s [[Configurable]] field is true. This even permits changing the [[Value]] of a property whose [[Writable]] attribute is false. This is allowed because a true [[Configurable]] attribute would permit an equivalent sequence of calls where [[Writable]] is first set to true, a new [[Value]] is set, and then [[Writable]] is set to false.

9.1.7 [[HasProperty]](P)

When the [[HasProperty]] internal method of O is called with property key P, the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let hasOwn be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
  3. ReturnIfAbrupt(hasOwn).
  4. If hasOwn is not undefined, then return true.
  5. Let parent be the result of calling the [[GetPrototypeOf]] internal method of O.
  6. ReturnIfAbrupt(parent).
  7. If parent is not null, then
    1. Return the result of calling the [[HasProperty]] internal method of parent with argument P.
  8. Return false.

9.1.8 [[Get]] (P, Receiver)

When the [[Get]] internal method of O is called with property key P and ECMAScript language value Receiver the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
  3. ReturnIfAbrupt(desc).
  4. If desc is undefined, then
    1. Let parent be the result of calling the [[GetPrototypeOf]] internal method of O.
    2. ReturnIfAbrupt(parent).
    3. If parent is null, then return undefined.
    4. Return the result of calling the [[Get]] internal method of parent with arguments P and Receiver.
  5. If IsDataDescriptor(desc) is true, return desc.[[Value]].
  6. Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]].
  7. If getter is undefined, return undefined.
  8. Return the result of calling the [[Call]] internal method of getter with Receiver as the thisArgument and an empty List as argumentsList.

9.1.9 [[Set]] ( P, V, Receiver)

When the [[Set]] internal method of O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
  3. ReturnIfAbrupt(ownDesc).
  4. If ownDesc is undefined, then
    1. Let parent be the result of calling the [[GetPrototypeOf]] internal method of O.
    2. ReturnIfAbrupt(parent).
    3. If parent is not null, then
      1. Return the result of calling the [[Set]] internal method of parent with arguments P, V, and Receiver.
    4. Else,
      1. Let ownDesc be the PropertyDescriptor{[[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
  5. If IsDataDescriptor(ownDesc) is true, then
    1. If ownDesc.[[Writable]] is false, return false.
    2. If Type(Receiver) is not Object, return false.
    3. Let existingDescriptor be the result of calling the [[GetOwnProperty]] internal method of Receiver with argument P.
    4. ReturnIfAbrupt(existingDescriptor).
    5. If existingDescriptor is not undefined, then
      1. Let valueDesc be the PropertyDescriptor{[[Value]]: V}.
      2. Return the result of calling the [[DefineOwnProperty]] internal method of Receiver with arguments P and valueDesc.
    6. Else Receiver does not currently have a property P,
      1. Return CreateDataProperty(Receiver, P, V).
  6. If IsAccessorDescriptor(ownDesc) is true, then
    1. Let setter be ownDesc.[[Set]].
    2. If setter is undefined, return false.
    3. Let setterResult be the result of calling the [[Call]] internal method of setter providing Receiver as thisArgument and a new List containing V as argumentsList.
    4. ReturnIfAbrupt(setterResult).
    5. Return true.

9.1.10 [[Delete]] (P)

When the [[Delete]] internal method of O is called with property key P the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
  3. ReturnIfAbrupt(desc).
  4. If desc is undefined, then return true.
  5. If desc.[[Configurable]] is true, then
    1. Remove the own property with name P from O.
    2. Return true.
  6. Return false.

9.1.11 [[Enumerate]] ()

When the [[Enumerate]] internal method of O is called the following steps are taken:

  1. Return an Iterator object (25.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

The iterator’s next method processes object properties to deterime whether the property key should be returned as an iterator value. Processed properties do not include properties whose property key is a Symbol. Properties of the object being enumerated may be deleted during enumeration. A property that is deleted before it is processed by the iterator’s next method is ignored. If new properties are added to the object being enumerated during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be return by the iterator’s next method at most once in any enumeration.

Enumerating the properties of an object includes processing properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if has the same name as a property that has already been processed by the iterator’s next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed.

The following is an informative definition of an ECMAScript generator function that conforms to these rules:

function* enumerate(obj) {
if (Object(obj)!== obj) return undefined;
let visited = new Set;
while (obj !== null) {
for (name of Object.getOwnPropertyNames(obj)) {
//any new properties added to obj by vistor are ignored.
if (!visited.has(name)) {
let desc = Object.getOwnPropertyDescriptor(obj,name);
if (desc) {
visited.add(name);
if (desc.enumerable) yield name;
}
}
}
obj = Object.getPrototypeOf(obj);
}
}

9.1.12 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of O is called the following steps are taken:

  1. Let keys be a new empty List.
  2. For each own property key P of O that is an integer index, in ascending numeric index order
    1. Add P as the last element of keys.
  3. For each own property key P of O that is a String but is not an integer index, in property creation order
    1. Add P as the last element of keys.
  4. For each own property key P of O that is a Symbol, in property creation order
    1. Add P as the last element of keys.
  5. Return CreateArrayFromList(keys).

9.1.13 ObjectCreate(proto, internalSlotsList) Abstract Operation

The abstract operation ObjectCreate with argument proto (an object or null) is used to specify the runtime creation of new ordinary objects. The optional argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, an empty List is used. This abstract operation performs the following steps:

  1. If internalSlotsList was not provided, let internalSlotsList be an empty List.
  2. Let obj be a newly created object with an internal slot for each name in internalSlotsList.
  3. Set obj’s essential internal methods to the default ordinary object definitions specified in 9.1.
  4. Set the [[Prototype]] internal slot of obj to proto.
  5. Set the [[Extensible]] internal slot of obj to true.
  6. Return obj.

9.1.14 OrdinaryCreateFromConstructor ( constructor, intrinsicDefaultProto, internalSlotsList )

The abstract operation OrdinaryCreateFromConstructor creates an ordinary object whose [[Prototype]] value is retrieved from a constructor’s prototype property, if it exists. Otherwise the supplied default is used for [[Prototype]]. The optional internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. If the list is not provided, an empty List is used. This abstract operation performs the following steps:

  1. Assert: intrinsicDefaultProto is a string value that is this specification’s name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
  2. Let proto be GetPrototypeFromConstructor(constructor, intrinsicDefaultProto).
  3. ReturnIfAbrupt(proto).
  4. Return ObjectCreate(proto, internalSlotsList).

9.2 ECMAScript Function Objects

ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScript function object is an ordinary object and has the same internal slots and (except as noted below) and the same internal methods as other ordinary objects. The code of an ECMAScript function object may be either strict mode code (10.2.1) or non-strict mode code.

ECMAScript function objects have the additional internal slots listed in Table 26.

ECMAScript function objects whose code is not strict mode code (10.2.1) provide an alternative definition for the [[GetOwnProperty]] internal method. This alternative prevents the value of strict mode function from being revealed as the value of a function object property named "caller". The alternative definition exist solely to preclude a non-standard legacy feature of some ECMAScript implementations from revealing information about strict mode callers. If an implementation does not provide such a feature, it need not implement this alternative internal method for ECMAScript function objects. ECMAScript function objects are considered to be ordinary objects even though they may use the alternative definition of [[GetOwnProperty]].

Table 26 — Internal Slots of ECMAScript Function Objects
Internal Slot Type Description
[[Environment]] Lexical Environment The Lexical Environment that the function was closed over. Used as the outer environment when evaluating the code of the function.
[[FormalParameters]] Parse Node The root parse node of the source code that defines the function’s formal parameter list.
[[FunctionKind]] String Either "normal" or "generator".
[[Code]] Parse Node The root parse node of the source code that defines the function’s body.
[[Realm]] Realm Record The Code Realm in which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.
[[ThisMode]] (lexical, strict, global) Defines how this references are interpreted within the formal parameters and code body of the function. lexical means that this refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined is interpreted as a reference to the global object.
[[Strict]] Boolean true if this is a strict mode function, false if this is not a strict mode function.
[[NeedsSuper]] Boolean true if this function uses super.
[[HomeObject]] Object If the function uses super, this is the object whose [[GetPrototypeOf]] provides the object where super property lookups begin.
[[MethodName]] String or Symbol If the function uses super, this is the property key that is used for unqualified references to super.

All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method. ECMAScript function objects whose code is not strict mode code have the [[Get]] and [[GetOwnProperty]] internal methods defined here.

9.2.1 [[GetOwnProperty]] (P)

When the [[GetOwnProperty]] internal method of non-strict ECMAScript function object F is called with property key P, the following steps are taken:

  1. Let v be OrdinaryGetOwnProperty(F, P).
  2. If IsDataDescriptor(v) is true, then
    1. If P is "caller" then,
      1. Let callerValue be v.[[Value]].
      2. If callerValue is an ECMAScript Function object, then
        1. If callerValue’s [[Strict]] internal slot is true, then set v.[[Value]] to null.
  3. Return v.

If an implementation does not provide a built-in caller property for non-strict ECMAScript function objects then it must not use this definition. Instead the ordinary object [[GetOwnProperty]] internal method is used.

9.2.2 [[Call]] ( thisArgument, argumentsList)

The [[Call]] internal method for an ECMAScript function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

  1. If F’s [[Code]] internal slot has the value undefined, then throw a TypeError exception.
  2. Let callerContext be the running execution context.
  3. If callerContext is not already suspended, then Suspend callerContext.
  4. Let calleeContext be a new ECMAScript Code execution context.
  5. Set the Function of calleeContext to F.
  6. Let calleeRealm be the value of F’s [[Realm]] internal slot.
  7. Set the Realm of calleeContext to calleeRealm.
  8. Let thisMode be the value of F’s [[ThisMode]] internal slot.
  9. Let needsThisWrapper be false.
  10. If thisMode is lexical, then
    1. Let localEnv be the result of calling NewDeclarativeEnvironment passing the value of the [[Environment]] internal slot of F as the argument.
  11. Else,
    1. If thisMode is strict, then let thisValue be thisArgument.
    2. Else
      1. if thisArgument is null or undefined, then
        1. Let thisValue be calleeRealm.[[globalThis]].
      2. Else
        1. if Type(thisArgument) is not Object, then let needsThisWrapper be true.
        2. Let thisValue be thisArgument.
    3. Let localEnv be NewFunctionEnvironment(F, thisValue).
    4. ReturnIfAbrupt(localEnv).
    5. NOTE Any exception objects produced by NewFunctionEnvironment are associated with callerReam.
  12. Set the LexicalEnvironment of calleeContext to localEnv.
  13. Set the VariableEnvironment of calleeContext to localEnv.
  14. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
  15. If needsThisWrapper is true then,
    1. Let wrapperedThis be ToObject(thisArgument).
    2. Assert: wrapperedThis is not an abrupt completion.
    3. NOTE Wrappering deferred until calleeContext is running so that ToObject produces objects using calleeRealm.
    4. Let functionEnv be localEnv’s environment record.
    5. Set functionEnv’s thisValue to wrapperedThis.
  16. Let status be the result of performing FunctionDeclarationInstantiation using the function F, argumentsList , and localEnv as described in 9.2.13.
  17. If status is an abrupt completion, then
    1. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
    2. Return status.
  18. Let result be the result of EvaluateBody of the production that is the value of F's [[Code]] internal slot passing F as the argument.
  19. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  20. Return result.

NOTE 1 Most ECMAScript functions use a Function Environment Record as their LexicalEnvironment. ECMAScript functions that are arrow functions use a Declarative Environment Record as their LexicalEnvironment.

NOTE 2 When calleeContext is removed from the execution context stack it must not be destroyed because it may have been suspended and retained by a generator object for later resumption.

9.2.3 [[Construct]] ( argumentsList)

The [[Construct]] internal method for an ECMAScript Function object F is called with a single parameter argumentsList which is a possibly empty List of ECMAScript language values. The following steps are taken:

  1. If F’s [[Code]] internal slot has the value undefined, then throw a TypeError exception.
  2. Return Construct(F, argumentsList).

9.2.4 FunctionAllocate (functionPrototype, strict) Abstract Operation

The abstract operation FunctionAllocate requires the two arguments functionPrototype and strict. It also accepts one optional argument, functionKind. FunctionAllocate performs the following steps:

  1. Assert: Type(functionPrototype) is Object.
  2. Assert: If functionKind is present, its value is either "normal", "non-constructor"or "generator".
  3. If functionKind is not present, then let functionKind be "normal".
  4. If functionKind is "non-constructor", then
    1. Let functionKind be "normal".
    2. Let needsConstruct be false.
  5. Else let needsConstruct be true.
  6. Let F be a newly created ECMAScript function object with the internal slots listed in Table 26. All of those internal slots are initialized to undefined.
  7. Set F’s essential internal methods except for [[GetOwnProperty]] to the default ordinary object definitions specified in 9.1.
  8. If strict is true, set F’s [[GetOwnProperty]] internal method to the default ordinary object definitions specified in 9.1.
  9. Else, set F’s [[GetOwnProperty]] internal method to the definitions specified in 9.2.1.
  10. Set F’s [[Call]] internal method to the definition specified in 9.2.2.
  11. If needsConstruct is true, then
    1. Set F’s [[Construct]] internal method to the definition specified in 9.2.3.
  12. Set the [[Strict]] internal slot of F to strict.
  13. Set the [[FunctionKind]] internal slot of F to functionKind.
  14. Set the [[Prototype]] internal slot of F to functionPrototype.
  15. Set the [[Extensible]] internal slot of F to true.
  16. Set the [[Realm]] internal slot of F to the running execution context’s Realm.
  17. Return F.

9.2.5 FunctionInitialize (F, kind, Strict, ParameterList, Body, Scope) Abstract Operation

The abstract operation FunctionInitialize requires the arguments: a function object F, kind which is one of (Normal, Method, Arrow), a Boolean Strict, a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope. FunctionInitialize performs the following steps:

  1. Let len be the ExpectedArgumentCount of ParameterList.
  2. Let realm be the value of F’s [[Realm]] internal slot.
  3. Let status be DefinePropertyOrThrow(F, "length", PropertyDescriptor{[[Value]]: len, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true}).
  4. ReturnIfAbrupt(status).
  5. If Strict is true, then
    1. Let status be AddRestrictedFunctionProperties(F, realm).
    2. ReturnIfAbrupt(status).
  6. Set the [[Strict]] internal slot of F to Strict.
  7. Set the [[Environment]] internal slot of F to the value of Scope.
  8. Set the [[FormalParameters]] internal slot of F to ParameterList .
  9. Set the [[Code]] internal slot of F to Body.
  10. If kind is Arrow, then set the [[ThisMode]] internal slot of F to lexical.
  11. Else if Strict is true, then set the [[ThisMode]] internal slot of F to strict.
  12. Else set the [[ThisMode]] internal slot of F to global.
  13. Return F.

9.2.6 FunctionCreate (kind, ParameterList, Body, Scope, Strict) Abstract Operation

The abstract operation FunctionCreate requires the arguments: kind which is one of (Normal, Method, Arrow), a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope, a Boolean flag Strict, and optionally, an object functionPrototype. FunctionCreate performs the following steps:

  1. If the functionPrototype argument was not passed, then
    1. Let functionPrototype be the intrinsic object %FunctionPrototype%.
  2. If kind is not Normal, then let allocKind be "non-constructor".
  3. Else let allocKind be "normal".
  4. Let F be FunctionAllocate(functionPrototype, Strict, allocKind).
  5. Return FunctionInitialize(F, kind, Strict, ParameterList, Body, Scope).

9.2.7 GeneratorFunctionCreate (kind, ParameterList, Body, Scope, Strict) Abstract Operation

The abstract operation GeneratorFunctionCreate requires the arguments: kind which is one of (Normal, Method, Arrow), a parameter list production specified by ParameterList, a body production specified by Body, a Lexical Environment specified by Scope, and a Boolean flag Strict. GeneratorFunctionCreate performs the following steps:

  1. Let functionPrototype be the intrinsic object %Generator%.
  2. Let F be FunctionAllocate(functionPrototype, Strict, "generator").
  3. Return FunctionInitialize(F, kind, Strict, ParameterList, Body, Scope).

9.2.8 AddRestrictedFunctionProperties ( F, realm ) Abstract Operation

The abstract operation AddRestrictedFunctionProperties is called with a function object F and Realm Record realm as its argument. It performs the following steps:

  1. If realm.[[intrinsics]].[[%ThrowTypeError%]] exists and has been initialized, then
    1. Let thrower be realm.[[intrinsics]].[[%ThrowTypeError%]].
  2. Else,
    1. NOTE %ThrowTypeError% must be the first intrinsic built-in function created for any Realm. If realm.[[intrinsics]].[[%ThrowTypeError%]] has not been initialized, F must be %ThrowTypeError%.
    2. Let thrower be F.
  3. Let status be DefinePropertyOrThrow(F, "caller", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true}).
  4. Assert: status is not an abrupt completion.
  5. Return DefinePropertyOrThrow(F , "arguments", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: true}).
  6. Assert: The above returned value is not an abrupt completion.

9.2.8.1 %ThrowTypeError% ( )

The %ThrowTypeError% intrinsic is an anonymous built-in function object that is defined once for each Realm. When %ThrowTypeError% is called it performs the following steps:

  1. Throw a TypeError exception.

The value of the [[Extensible]] internal slot of a %ThrowTypeError% function is false.

The length property of a %ThrowTypeError% function has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

9.2.9 MakeConstructor (F, writablePrototype, prototype) Abstract Operation

The abstract operation MakeConstructor requires a Function argument F and optionally, a Boolean writablePrototype and an object prototype. If prototype is provided it is assumed to already contain, if needed, a "constructor" property whose value is F. This operation converts F into a constructor by performing the following steps:

  1. Assert: F is an ECMAScript function object.
  2. Assert: F has a [[Constructor]] internal method.
  3. Let installNeeded be false.
  4. If the prototype argument was not provided, then
    1. Let installNeeded be true.
    2. Let prototype be ObjectCreate(%ObjectPrototype%).
  5. If the writablePrototype argument was not provided, then
    1. Let writablePrototype be true.
  6. If installNeeded, then
    1. Let status be DefinePropertyOrThrow(prototype, "constructor", PropertyDescriptor{[[Value]]: F, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: writablePrototype }).
    2. ReturnIfAbrupt(status).
  7. Let status be DefinePropertyOrThrow(F, "prototype", PropertyDescriptor{[[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]: false, [[Configurable]]: false}).
  8. ReturnIfAbrupt(status).
  9. Return NormalCompletion(undefined).

9.2.10 MakeMethod ( F, methodName, homeObject) Abstract Operation

The abstract operation MakeMethod with arguments F, methodName and homeObject configures F as a method by performing the following steps:

  1. Assert: F is an ECMAScript function object.
  2. Assert: methodName is either undefined or a property key.
  3. Assert: Type(homeObject ) is either Undefined or Object.
  4. Set the [[NeedsSuper]] internal slot of F to true.
  5. Set the [[HomeObject]] internal slot of F to homeObject.
  6. Set the [[MethodName]] internal slot of F to methodName.
  7. Return NormalCompletion(undefined).

9.2.11 SetFunctionName (F, name, prefix) Abstract Operation

The abstract operation SetFunctionName requires a Function argument F, a String or Symbol argument name and optionally a String argument prefix. This operation adds a name property to F by performing the following steps:

  1. Assert: F is an extensible ECMAScript function object that does not have a name own property.
  2. Assert: Type(name) is either Symbol or String.
  3. If Type(name) is Symbol, then
    1. Let description be name’s [[Description]] value.
    2. If description is undefined, then let name be the empty String.
    3. Else, let name be the concatenation of "[", description, and "]".
  4. Let name be the concatenation of prefix, Unicode code point U+0020 (Space) , and name.
  5. Return DefinePropertyOrThrow(F, "name", PropertyDescriptor{[[Value]]: name, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true}.

9.2.12 CloneMethod(function, newHome, newName) Abstract Operation

The abstract operation CloneMethod is called with a function object function, an object newHome, and a property key newName as its argument. It performs the following steps:

  1. Assert: function is an ECMAScript function object or an exotic Built-in function object.
  2. Assert: Type(newHome) is Object.
  3. Assert: Type(newName) is one of Undefined, String, or Symbol.
  4. If function is an ECMAScript function, then
    1. Let new be a new ECMAScript function object that has all of the same internal methods and internal slots as function.
  5. Else
    1. Assert: function is an exotic Built-in function object.
    2. Let new be a new exotic Built-in function object that has all of the same internal methods and internal slots as function.
  6. Set the value of each of new’s internal slots, except for [[Extensible]], [[HomeObject]] and [[MethodName]] to the value of function’s corresponding internal slot.
  7. Set new’s [[Extensible]] internal slot to true.
  8. If the value of function’s [[NeedsSuper]] internal slot is true, then
    1. Set the value of new’s [[HomeObject]] internal slot to newHome.
    2. If newName is not undefined, then
      1. Set the value of new’s [[MethodName]] internal slot to newName.
    3. Else,
      1. Set the value of new’s [[MethodName]] internal slot to the value of function’s [[MethodName]] internal slot.
  9. If function is an exotic Built-in function object or if function’s [[Strict]] internal slot is true, then
    1. Let realm be GetFunctionRealm(new).
    2. Let status be AddRestrictedFunctionProperties(new, realm).
    3. ReturnIfAbrupt(status).
  10. Return new.

NOTE The purpose of this abstract operation is to create a new function object that is identical to the argument object in all always except for its identity and the value of its [[HomeObject]] internal slot. However, properties of the function object, except for the restricted function properties, are not created or copied.

9.2.13 FunctionDeclarationInstantiation(func, argumentsList, env ) Abstract Operation

NOTE When an execution context is established for evaluating an ECMAScript function a new Declarative Environment Record is created and bindings for each formal parameter are instantiated in that environment record. Each declaration in the function body is also instantiated. If the function’s formal parameters do not include any default value initializers then the body declarations are instantiated in the same environment record as the parameters. If default value parameter initializers exist, a second environment record is created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.

FunctionDeclarationInstantiation is performed as follows using arguments func, argumentsList, and env. func is the function object that for which the execution context is being established. env is the declarative environment record in which formal parameter bindings are to be created.

  1. Let code be the value of the [[Code]] internal slot of func.
  2. Let strict be the value of the [[Strict]] internal slot of func.
  3. Let formals be the value of the [[FormalParameters]] internal slot of func.
  4. Let parameterNames be the BoundNames of formals.
  5. If parameterNames has any duplicate entries, let hasDuplicates be true. Otherwise, let hasDuplicates be false.
  6. Let simpleParameterList be IsSimpleParameterList of formals.
  7. Let hasParameterExpressions be ContainsExpresson of formals.
  8. Let varNames be the VarDeclaredNames of code.
  9. Let varDeclarations be the VarScopedDeclarations of code.
  10. Let lexicalNames be the LexicallyDeclaredNames of code.
  11. Let functionNames be an empty List.
  12. Let functionsToInitialize be an empty List.
  13. For each d in varDeclarations, in reverse list order do
    1. If d is neither a VariableDeclaration or a ForBinding, then
      1. Assert: d is either a FunctionDeclaration or a GeneratorDeclaration.
      2. Let fn be the sole element of the BoundNames of d.
      3. If fn is not an element of functionNames, then
        1. Insert fn as the first element of functionNames.
        2. NOTE If there are multiple FunctionDeclarations or GeneratorDeclarations for the same name, the last declaration is used.
        3. Insert d as the first element of functionsToInitialize.
  14. Let argumentsObjectNeeded be true.
  15. If the value of the [[ThisMode]] internal slot of func is lexical, then
    1. NOTE Arrow functions never have an arguments objects.
    2. Let argumentsObjectNeeded be false.
  16. Else if "arguments" is an element of parameterNames, then
    1. Let argumentsObjectNeeded be false.
  17. Else if hasParameterExpressions is false, then
    1. If "arguments" is an element of functionNames or if "arguments" is an element of lexicalNames, then
      1. Let argumentsObjectNeeded be false.
  18. For each String paramName in parameterNames, do
    1. Let alreadyDeclared be the result of calling env’s HasBinding concrete method passing paramName as the argument.
    2. NOTE Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
    3. If alreadyDeclared is false, then
      1. Let status be the result of calling env’s CreateMutableBinding concrete method passing paramName as the argument.
      2. If hasDuplicates is true, then
        1. Let status be the result of calling env’s InitializeBinding concrete method passing paramName and undefined as the argument.
      3. Assert: status is never an abrupt completion for either of the above operations.
  19. If argumentsObjectNeeded is true, then
    1. If strict is true or if simpleParameterList is false, then
      1. Let ao be CreateUnmappedArgumentsObject(argumentsList).
    2. Else,
      1. NOTE mapped argument object is only provided for non-strict functions that don’t have a rest parameter, any parameter default value initializers, or any destructured parameters .
      2. Let ao be CreateMappedArgumentsObject(func, formals, argumentsList, env).
    3. ReturnIfAbrupt(ao).
    4. If strict is true, then
      1. Let status be the result of calling env’s CreateImmutableBinding concrete method passing "arguments" as the argument.
    5. Else,
      1. Let status be the result of calling env’s CreateMutableBinding concrete method passing "arguments" as the argument.
    6. Assert: status is never an abrupt completion.
    7. Call env’s InitializeBinding concrete method passing "arguments" and ao as arguments.
    8. Append "arguments" to parameterNames.
  20. If hasDuplicates is true, then
    1. Let formalStatus be the result of performing IteratorBindingInitialization for formals with CreateListIterator(argumentsList) and undefined as arguments.
  21. Else,
    1. Let formalStatus be the result of performing IteratorBindingInitialization for formals with CreateListIterator(argumentsList) and env as arguments.
  22. ReturnIfAbrupt(formalStatus).
  23. If hasParameterExpressions is false, then
    1. NOTE Only a single environment record is needed.
    2. Let bodyEnv be env.
    3. Let instantiatedVarNames be a copy of the List parameterNames.
    4. For each n in varNames, do
      1. If n is not an element of instantiatedVarNames, then
        1. Append n to instantiatedVarNames.
        2. Let status be the result of calling bodyEnv’s CreateMutableBinding concrete method passing n as the argument.
        3. Assert: status is never an abrupt completion.
        4. Call bodyEnv’s InitializeBinding concrete method passing n and undefined as arguments.
  24. Else,
    1. NOTE A separate environment record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
    2. Let bodyEnv be NewDeclarativeEnvironment(env).
    3. Let calleeContext be the running execution context.
    4. Set the LexicalEnvironment of calleeContext to bodyEnv.
    5. Set the VariableEnvironment of calleeContext to bodyEnv.
    6. Let instantiatedVarNames be a new emptyList.
    7. For each n in varNames, do
      1. If n is not an element of instantiatedVarNames, then
        1. Append n to instantiatedVarNames.
        2. Let status be the result of calling bodyEnv’s CreateMutableBinding concrete method passing n as the argument.
        3. Assert: status is never an abrupt completion.
        4. If n is not an element of parameterNames or if n is an element of functionNames, then let initialValue be undefined.
        5. else,
          1. Let initialValue be the result of calling env’s GetBindingValue concrete method passing n and false as the arguments.
          2. ReturnIfAbrupt(initialValue).
        6. Call bodyEnv’s InitializeBinding concrete method passing n and initialValue as arguments.
        7. NOTE vars whose names are the same as a formal parameter, initially have the same value as the corresponding initialized parameter.
  25. Let lexDeclarations be the LexicallyScopedDeclarations of code.
  26. For each element d in lexDeclarations do
    1. NOTE A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of the BoundNames of d do
      1. If IsConstantDeclaration of d is true, then
        1. Let status be the result of calling bodyEnv’s CreateImmutableBinding concrete method passing dn as the argument.
      2. Else,
        1. Let status be the result of calling bodyEnv’s CreateMutableBinding concrete method passing dn and false as the arguments.
    3. Assert: status is never an abrupt completion.
  27. For each production f in functionsToInitialize, do
    1. Let fn be the sole element of the BoundNames of f.
    2. Let fo be the result of performing InstantiateFunctionObject for f with argument bodyEnv.
    3. Let status be the result of calling bodyEnv’s SetMutableBinding concrete method passing fn, fo and false as the arguments..
    4. Assert: status is never an abrupt completion.
  28. Return NormalCompletion(empty).

NOTE B.3.2 provides an extension to the above algorithm that is necessary for backwards compatability with web browser implementations of ECAMScript that predate the sixth edition of ECMA-262.

9.3 Built-in Function Objects

The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (9.2) whose behaviour is provided using ECMAScript code or as implementation provided exotic function objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications.

If a built-in function object is implemented as an exotic object it must have the ordinary object behaviour specified in 9.1 except [[GetOwnProperty]] which must be as specified in 9.2.1. All such exotic function objects also have [[Prototype]] and [[Extensible]] internal slots.

Unless otherwise specified every built-in function object initially has the %FunctionPrototype% object (19.2.3) as the initial value of its [[Prototype]] internal slot.

The behaviour specified for each built-in function via algorithm steps or other means is the specification of the [[Call]] behaviour for that function with the [[Call]] thisArgument providing the this value and the [[Call]] argumentsList providing the named parameters for each built-in function. If the built-in function is implemented as an ECMAScript function object then this specified behaviour must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function objects must be strict mode functions.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-in constructor is called as part of a new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-in constructor’s named parameters.

Built-in functions that are not constructors do not have a prototype property unless otherwise specified in the description of a particular function.

If a built-in function object is not implemented as an ECMAScript function it must have a [[Realm]] internal slot. It must also have a [[Call]] internal method that conforms to the following definition:

9.3.1 [[Call]] ( thisArgument, argumentsList)

The [[Call]] internal method for a built-in function object F is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

  1. Let callerContext be the running execution context.
  2. If callerContext is not already suspended, then Suspend callerContext.
  3. Let calleeContext be a new execution context.
  4. Set the Function of calleeContext to F.
  5. Let calleeRealm be the value of F’s [[Realm]] internal slot.
  6. Set the Realm of calleeContext to calleeRealm.
  7. Perform any necessary implementation defined initialization of calleeContext.
  8. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
  9. Let result be the Completion Record that is the result of evaluating F in an implementation defined manner that conforms to this specification of F.
  10. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  11. Return result.

NOTE 1 When calleeContext is removed from the execution context stack it must not be destroyed because it may have been suspended and retained by a generator object for later resumption.

9.3.2 CreateBuiltinFunction(realm, steps, prototype, internalSlotsList) Abstract Operation

The abstract operation CreateBuiltinFunction takes arguments realm, prototype, and steps. The optional argument internalSlotsList is a List of the names of additional internal slot that must be defined as part of the object. If the list is not provided, an empty List is used. CreateBuiltinFunction returns a built-in function object created by the following steps:

  1. Assert: realm is a Realm Record.
  2. Assert: steps is either a set of algorithm steps or other definition of a functions behaviour provided in this specification.
  3. Let func be a new built-in function object that when called performs the action described by steps. The new function object has internal slots whose names are the elements of internalSlotsList. The initial value of each of those internal slots is undefined.
  4. Set the [[Realm]] internal slot of func to realm.
  5. Call the [[SetPrototypeOf]] internal method of func with argument prototype.
  6. Perform the AddRestrictedFunctionProperties(func, realm).
  7. Return func.

9.4 Built-in Exotic Object Internal Methods and Data Fields

This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:

9.4.1 Bound Function Exotic Objects

A bound function is an exotic object that wrappers another function object. A bound function is callable (it has a [[Call]] internal method and may have a [[Construct]] internal method). Calling a bound function generally results in a call of its wrapped function.

Bound function objects do not have the internal slots of ECMAScript function objects defined in Table 26. Instead they have the internal slots defined in Table 27.

Table 27 — Internal Slots of Exotic Bound Function Objects
Internal Slot Type Description
[[BoundTargetFunction]] Callable Object The wrappered function object.
[[BoundThis]] Any The value that is always passed as the this value when calling the wrappered function.
[[BoundArguments]] List of Any A list of values that whose elements are used as the first arguments to any call to the wrappered function.

Unlike ECMAScript function objects, bound function objects do not use alternative definitions of the [[Get]] and [[GetOwnProperty]] internal methods. Bound function objects provide all of the essential internal methods as specified in 9.1. However, they use the following definitions for the essential internal methods of function objects.

9.4.1.1 [[Call]]

When the [[Call]] internal method of an exotic bound function object, F, which was created using the bind function is called with parameters thisArgument and argumentsList, a List of ECMAScript language values, the following steps are taken:

  1. Let boundArgs be the value of F’s [[BoundArguments]] internal slot.
  2. Let boundThis be the value of F’s [[BoundThis]] internal slot.
  3. Let target be the value of F’s [[BoundTargetFunction]] internal slot.
  4. Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list argumentsList in the same order.
  5. Return the result of calling the [[Call]] internal method of target providing boundThis as thisArgument and providing args as argumentsList.

9.4.1.2 [[Construct]]

When the [[Construct]] internal method of an exotic bound function object, F that was created using the bind function is called with a list of arguments ExtraArgs, the following steps are taken:

  1. Let target be the value of F’s [[BoundTargetFunction]] internal slot.
  2. Assert: target has a [[Construct]] internal method.
  3. Let boundArgs be the value of F’s [[BoundArguments]] internal slot.
  4. Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list ExtraArgs in the same order.
  5. Return the result of calling the [[Construct]] internal method of target providing args as the arguments.

9.4.1.3 BoundFunctionCreate (targetFunction, boundThis, boundArgs) Abstract Operation

The abstract operation BoundFunctionCreate with arguments targetFunction, boundThis and boundArgs is used to specify the creation of new Bound Function exotic objects. It performs the following steps:

  1. Let proto be the intrinsic %FunctionPrototype%.
  2. Let obj be a newly created object.
  3. Set obj’s essential internal methods to the default ordinary object definitions specified in 9.1.
  4. Set the [[Call]] internal method of obj as described in 9.4.1.1.
  5. If targetFunction has a [[Construct]] internal method, then
    1. Set the [[Construct]] internal method of obj as described in 9.4.1.2.
  6. Set the [[Prototype]] internal slot of obj to proto.
  7. Set the [[Extensible]] internal slot of obj to true.
  8. Set the [[BoundTargetFunction]] internal slot of obj to targetFunction.
  9. Set the [[BoundThis]] internal slot of obj to the value of boundThis.
  10. Set the [[BoundArguments]] internal slot of obj to boundArgs.
  11. Return obj.

9.4.1.4 BoundFunctionClone ( function ) Abstract Operation

The abstract operation BoundFunctionClone is called with argument function it performs the following steps:

  1. Assert: function is a Bound Function exotic object.
  2. Let new be a new Bound Function exotic object that has all of the same internal methods and internal slots as function.
  3. Set the value of each of new’s internal slots, except for [[Extensible]] to the value of function’s corresponding internal slot.
  4. Set new’s [[Extensible]] internal slot to true.
  5. Let realm be GetFunctionRealm(new).
  6. Let status be AddRestrictedFunctionProperties(new, realm).
  7. ReturnIfAbrupt(status).
  8. Return new.

9.4.2 Array Exotic Objects

An Array object is an exotic object that gives special treatment to array index property keys (see 6.1.7). A property whose property name is an array index is also called an element. Every Array object has a length property whose value is always a nonnegative integer less than 232. The value of the length property is numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted. This constraint applies only to own properties of an Array object and is unaffected by length or array index properties that may be inherited from its prototypes.

NOTE A String property name P is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 232−1.

Exotic Array objects have the same internal slots as ordinary objects. They also have an [[ArrayInitializationState]] internal slot.

Exotic Array objects always have a non-configurable property named "length".

Exotic Array objects provide an alternative definition for the [[DefineOwnProperty]] internal method. Except for that internal method, exotic Array objects provide all of the other essential internal methods as specified in 9.1.

9.4.2.1 [[DefineOwnProperty]] ( P, Desc)

When the [[DefineOwnProperty]] internal method of an exotic Array object A is called with property key P, and Property Descriptor Desc the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. If P is "length", then
    1. Return ArraySetLength(A, Desc).
  3. Else if P is an array index, then
    1. Let oldLenDesc be OrdinaryGetOwnProperty(A, "length").
    2. Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
    3. Let oldLen be oldLenDesc.[[Value]].
    4. Let index be ToUint32(P).
    5. Assert: index will never be an abrupt completion.
    6. If indexoldLen and oldLenDesc.[[Writable]] is false, then return false.
    7. Let succeeded be the result of calling OrdinaryDefineOwnProperty passing A, P, and Desc as arguments.
    8. ReturnIfAbrupt(succeeded).
    9. If succeeded is false, then return false.
    10. If indexoldLen
      1. Set oldLenDesc.[[Value]] to index + 1.
      2. Let succeeded be OrdinaryDefineOwnProperty(A, "length", oldLenDesc).
      3. ReturnIfAbrupt(succeeded).
    11. Return true.
  4. Return OrdinaryDefineOwnProperty(A, P, Desc).

9.4.2.2 ArrayCreate(length, proto) Abstract Operation

The abstract operation ArrayCreate with argument length (a positive integer or undefined) and optional argument proto is used to specify the creation of new exotic Array objects. It performs the following steps:

  1. Assert: length is either undefined or a integer Number ≥ 0.
  2. If length is −0, then let length be +0.
  3. If the proto argument was not passed, then let proto be the intrinsic object %ArrayPrototype%.
  4. Let A be a newly created Array exotic object.
  5. Set A’s essential internal methods except for [[DefineOwnProperty]] to the default ordinary object definitions specified in 9.1.
  6. Set the [[DefineOwnProperty]] internal method of A as specified in 9.4.2.1.
  7. Set the [[Prototype]] internal slot of A to proto.
  8. Set the [[Extensible]] internal slot of A to true.
  9. If length is not undefined, then
    1. Set the [[ArrayInitializationState]] internal slot of A to true.
  10. Else
    1. Set the [[ArrayInitializationState]] internal slot of A to false.
    2. Let length be +0.
  11. If length>232-1, then throw a RangeError exception.
  12. Call OrdinaryDefineOwnProperty with arguments A, "length" and PropertyDescriptor{[[Value]]: length, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false}.
  13. Return A.

9.4.2.3 ArraySetLength(A, Desc) Abstract Operation

When the abstract operation ArraySetLength is called with an exotic Array object A, and Property Descriptor Desc the following steps are taken:

  1. If the [[Value]] field of Desc is absent, then
    1. Return OrdinaryDefineOwnProperty(A, "length", Desc).
  2. Let newLenDesc be a copy of Desc.
  3. Let newLen be ToUint32(Desc.[[Value]]).
  4. If newLen is not equal to ToNumber( Desc.[[Value]]), throw a RangeError exception.
  5. Set newLenDesc.[[Value]] to newLen.
  6. Let oldLenDesc be the result of calling the [[GetOwnProperty]] internal method of A passing "length" as the argument.
  7. ReturnIfAbrupt(oldLenDesc).
  8. Assert: oldLenDesc will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
  9. Let oldLen be oldLenDesc.[[Value]].
  10. If newLenoldLen, then
    1. Return OrdinaryDefineOwnProperty(A, "length", newLenDesc).
  11. If oldLenDesc.[[Writable]] is false, then return false.
  12. If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.
  13. Else,
    1. Need to defer setting the [[Writable]] attribute to false in case any elements cannot be deleted.
    2. Let newWritable be false.
    3. Set newLenDesc.[[Writable]] to true.
  14. Let succeeded be OrdinaryDefineOwnProperty(A, "length", newLenDesc).
  15. ReturnIfAbrupt(succeeded).
  16. If succeeded is false, return false.
  17. While newLen < oldLen repeat,
    1. Set oldLen to oldLen – 1.
    2. Let deleteSucceeded be the result of calling the [[Delete]] internal method of A passing ToString(oldLen).
    3. ReturnIfAbrupt(succeeded).
    4. If deleteSucceeded is false, then
      1. Set newLenDesc.[[Value]] to oldLen+1.
      2. If newWritable is false, set newLenDesc.[[Writable]] to false.
      3. Let succeeded be OrdinaryDefineOwnProperty(A, "length", newLenDesc).
      4. ReturnIfAbrupt(succeeded).
      5. Return false.
  18. If newWritable is false, then
    1. Call OrdinaryDefineOwnProperty passing A, "length", and PropertyDescriptor{[[Writable]]: false} as arguments. This call will always return true.
  19. Return true.

NOTE In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2nd Edition of this specification.

9.4.3 String Exotic Objects

A String object is an exotic object that encapsulates a String value and exposes virtual integer indexed data properties corresponding to the individual code unit elements of the string value. Exotic String objects always have a data property named "length" whose value is the number of code unit elements in the encapsulated String value. Both the code unit data properties and the "length" property are non-writable and non-configurable.

Exotic String objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.

Exotic String objects provide alternative definitions for the following internal methods. All of the other exotic String object essential internal methods that are not defined below are as specified in 9.1.

9.4.3.1 [[GetOwnProperty]] ( P )

When the [[GetOwnProperty]] internal method of an exotic String object S is called with property key P the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let desc be OrdinaryGetOwnProperty(S, P).
  3. If desc is not undefined return desc.
  4. If Type(P) is not String, then return undefined.
  5. Let index be CanonicalNumericIndexString (P).
  6. Assert: index is not an abrupt completion.
  7. If index is undefined, then return undefined.
  8. If IsInteger(index) is false, then return undefined.
  9. If index = −0, then return undefined.
  10. Let str be the String value of the [[StringData]] internal slot of S, if the value of [[StringData]] is undefined the empty string is used as its value.
  11. Let len be the number of elements in str.
  12. If index < 0 or lenindex, return undefined.
  13. Let resultStr be a String value of length 1, containing one code unit from str, specifically the code unit at position index, where the first (leftmost) element in str is considered to be at position 0, the next one at position 1, and so on.
  14. Return a PropertyDescriptor{ [[Value]]: resultStr, [[Enumerable]]: true, [[Writable]]: false, [[Configurable]]: false }.

9.4.3.2 [[Enumerate]] ()

When the [[Enumerate]] internal method of an exotic String object O is called the following steps are taken:

  1. Let indexKeys be a new empty List.
  2. Let str be the String value of the [[StringData]] internal slot of O, if the value of [[StringData]] is undefined the empty string is used as its value.
  3. Let len be the number of elements in str.
  4. For each integer i starting with 0 such that i < len, in ascending order,
    1. Add ToString(i) as the last element of indexKeys
  5. Let ordinary be the result of calling the default ordinary object [[Enumerate]] internal method (9.1.11) on O.
  6. ReturnIfAbrupt(ordinary).
  7. Return CreateCompoundIterator(CreateListIterator(indexKeys), ordinary).

9.4.3.3 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of a String exotic object O is called the following steps are taken:

  1. Let keys be a new empty List.
  2. Let str be the String value of the [[StringData]] internal slot of O, if the value of [[StringData]] is undefined the empty string is used as its value.
  3. Let len be the number of elements in str.
  4. For each integer i starting with 0 such that i < len, in ascending order,
    1. Add ToString(i) as the last element of keys
  5. For each own property key P of O such that P is an integer index and ToInteger(P) ≥ len, in ascending numeric index order,
    1. Add P as the last element of keys.
  6. For each own property key P of O such that Type(P) is String and P is not an integer index, in property creation order,
    1. Add P as the last element of keys.
  7. For each own property key P of O such that Type(P) is Symbol, in property creation order,
    1. Add P as the last element of keys.
  8. Return CreateArrayFromList(keys).

9.4.3.4 StringCreate Abstract Operation

The abstract operation StringCreate with argument prototype is used to specify the creation of new exotic String objects. It performs the following steps:

  1. Let A be a newly created String exotic object.
  2. Set A’s essential internal methods to the default ordinary object definitions specified in 9.1.
  3. Set the [[GetOwnProperty]] internal method of A as specified in 9.4.3.1.
  4. Set the [[Enumerate]] internal method of A as specified in 9.4.3.2.
  5. Set the [[OwnPropertyKeys]] internal method of A as specified in 9.4.3.3.
  6. Set the [[Prototype]] internal slot of A to prototype.
  7. Set the [[Extensible]] internal slot of A to true.
  8. Return A.

9.4.4 Arguments Exotic Objects

Most ECMAScript functions make an arguments objects available to their code. Depending upon the characteristics of the function definition, its argument object is either an ordinary object or an arguments exotic object. An aguments exotic object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.

Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinany argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (19.1.3.6) to identify them as such.

Arguments exotic objects provide alternative definitions for the following internal methods. All of the other exotic arguments object essential internal methods that are not defined below are as specified in 9.1

NOTE 1 For non-strict mode functions the integer indexed data properties of an arguments object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

NOTE 2 The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

NOTE 3 Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and "callee" which throw a TypeError exception on access. The "callee" property has a more specific meaning for non-strict mode functions and a "caller" property has historically been provided as an implementation-defined extension by some ECMAScript implementations. The strict mode definition of these properties exists to ensure that neither of them is defined in any other manner by conforming ECMAScript implementations.

9.4.4.1 [[GetOwnProperty]] (P)

The [[GetOwnProperty]] internal method of an arguments exotic object when called with a property name P performs the following steps:

  1. Let args be the arguments object.
  2. Let desc be OrdinaryGetOwnProperty(args, P).
  3. If desc is undefined then return desc.
  4. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
  5. Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as the argument.
  6. Assert: isMapped is never an abrupt completion.
  7. If the value of isMapped is true, then
    1. Set desc.[[Value]] to Get(map, P).
  8. If IsDataDescriptor(desc) is true and P is "caller" and desc.[[Value]] is a strict mode Function object, throw a TypeError exception.
  9. Return desc.

If an implementation does not provide a built-in caller property for argument exotic objects then step 8 of this algorithm is must be skipped.

9.4.4.2 [[DefineOwnProperty]] (P, Desc)

The [[DefineOwnProperty]] internal method of an arguments exotic object when called with a property name P and Property Descriptor Desc performs the following steps:

  1. Let args be the arguments object.
  2. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
  3. Let isMapped be HasOwnProperty(map, P).
  4. Let allowed be OrdinaryDefineOwnProperty(args, P, Desc).
  5. ReturnIfAbrupt(allowed).
  6. If allowed is false, then return false.
  7. If the value of isMapped is true, then
    1. If IsAccessorDescriptor(Desc) is true, then
      1. Call the [[Delete]] internal method of map passing P as the argument.
    2. Else
      1. If Desc.[[Value]] is present, then
        1. Let putStatus be Put(map, P, Desc.[[Value]], false).
        2. Assert: putStatus is true because formal parameters mapped by argument objects are always writable.
      2. If Desc.[[Writable]] is present and its value is false, then
        1. Call the [[Delete]] internal method of map passing P as the argument.
  8. Return true.

9.4.4.3 [[Get]] (P, Receiver)

The [[Get]] internal method of an arguments exotic object when called with a property name P and ECMAScript language value Receiver performs the following steps:

  1. Let args be the arguments object.
  2. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
  3. Let isMapped be HasOwnProperty(map, P).
  4. Assert: isMapped is not an abrupt completion.
  5. If the value of isMapped is false, then
    1. Let v be the result of calling the default ordinary object [[Get]] internal method (9.1.8) on args passing P and Receiver as the arguments.
  6. Else map contains a formal parameter mapping for P,
    1. Let v be Get(map, P).
  7. ReturnIfAbrupt(v).
  8. If P is "caller" and v is a strict mode Function object, throw a TypeError exception.
  9. Return v.

If an implementation does not provide a built-in caller property for argument exotic objects then step 8 of this algorithm must be skipped.

9.4.4.4 [[Set]] ( P, V, Receiver)

The [[Set]] internal method of an arguments exotic object when called with property key P, value V, and ECMAScript language value Receiver performs the following steps:

  1. Let args be the arguments object.
  2. If SameValue(args, Receiver) is false, then
    1. Let isMapped be undefined.
  3. Else,
    1. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
    2. Let isMapped be HasOwnProperty(map, P).
    3. Assert: isMapped is not an abrupt completion.
  4. If the value of isMapped is false, then
    1. Return the result of calling the default ordinary object [[Set]] internal method (9.1.8) on args passing P, V and Receiver as the arguments.
  5. Else map contains a formal parameter mapping for P,
    1. Return Put(map, P, V, false).

9.4.4.5 [[Delete]] (P)

The [[Delete]] internal method of an arguments exotic object when called with a property key P performs the following steps:

  1. Let map be the value of the [[ParameterMap]] internal slot of the arguments object.
  2. Let isMapped be HasOwnProperty(map, P).
  3. Assert: isMapped is not an abrupt completion.
  4. Let result be the result of calling the default [[Delete]] internal method for ordinary objects (9.1.10) on the arguments object passing P as the argument.
  5. ReturnIfAbrupt(result).
  6. If result is true and the value of isMapped is true, then
    1. Call the [[Delete]] internal method of map passing P as the argument.
  7. Return result.

NOTE 1 For non-strict mode functions with simple parameter lists, those integer indexed data properties of an arguments object whose numeric name values are less than the number of formal parameters of the function initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

NOTE 2 The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly accessible from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

NOTE 3 Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and "callee" which throw a TypeError exception on access. The "callee" property has a more specific meaning for non-strict mode functions and a "caller" property has historically been provided as an implementation-defined extension by some ECMAScript implementations. The strict mode definition of these properties exists to ensure that neither of them is defined in any other manner by conforming ECMAScript implementations.

9.4.4.6 CreateUnmappedArgumentsObject(argumentsList) Abstract Operation

The abstract operation CreateUnmappedArgumentsObject called with an argument argumentsList performs the following steps:

  1. Let len be the number of elements in argumentsList.
  2. Let obj be ObjectCreate(%ObjectPrototype%, ([[ParameterMap]])).
  3. Set obj’s [[ParameterMap]] internal slot to undefined.
  4. Perform DefinePropertyOrThrow(obj, "length", PropertyDescriptor{[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
  5. Let index be 0.
  6. Repeat while index < len,
    1. Let val be the element of argumentsList at 0-origined list position index.
    2. Perform CreateDataProperty(obj, ToString(index), val).
    3. Let index be index + 1
  7. Perform DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor {[[Value]]:%ArrayProto_values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
  8. Perform DefinePropertyOrThrow(obj, "caller", PropertyDescriptor {[[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false}).
  9. Perform DefinePropertyOrThrow(obj, "callee", PropertyDescriptor {[[Get]]: %ThrowTypeError%, [[Set]]: %ThrowTypeError%, [[Enumerable]]: false, [[Configurable]]: false}).
  10. Assert: the above property definitions will not produce an abrupt completion.
  11. Return obj

9.4.4.7 CreateMappedArgumentsObject ( func, formals, argumentsList, env ) Abstract Operation

The abstract operation CreateMappedArgumentsObject is called with object func, grammar production formals, List argumentsList, and environment record env. The following steps are performed:

  1. Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
  2. Let len be the number of elements in argumentsList.
  3. Let obj be a newly created arguments exotic object with a [[ParameterMap]] internal slot.
  4. Set the [[GetOwnProperty]] internal method of obj as specified in 9.4.4.1.
  5. Set the [[DefineOwnProperty]] internal method of obj as specified in 0.
  6. Set the [[Get]] internal method of obj as specified in 9.4.4.3.
  7. Set the [[Set]] internal method of obj as specified in 0.
  8. Set the [[Delete]] internal method of obj as specified in 9.4.4.5.
  9. Set the remainder of obj’s essential internal methods to the default ordinary object definitions specified in 9.1.
  10. Set the [[Prototype]] internal slot of obj to %ObjectPrototype%.
  11. Set the [[Extensible]] internal slot of obj to true.
  12. Let parameterNames be the BoundNames of formals.
  13. Let numberOfParameters be the number of elements in parameterNames
  14. Let index be 0.
  15. Repeat while index < len ,
    1. Let val be the element of argumentsList at 0-origined list position index.
    2. Perform CreateDataProperty(obj, ToString(index), val).
    3. Let index be index + 1
  16. Perform DefinePropertyOrThrow(obj, "length", PropertyDescriptor{[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
  17. Let map be ObjectCreate(null).
  18. Let mappedNames be an empty List.
  19. Let index be numberOfParameters − 1.
  20. Repeat while index ≥ 0 ,
    1. Let name be the element of parameterNames at 0-origined list position index.
    2. If name is not an element of mappedNames, then
      1. Add name as an element of the list mappedNames.
      2. If index < len, then
        1. Let g be MakeArgGetter(name, env).
        2. Let p be MakeArgSetter(name, env).
        3. Call the [[DefineOwnProperty]] internal method of map passing ToString(index) and the PropertyDescriptor{[[Set]]: p, [[Get]]: g,[[Enumerable]]: false, [[Configurable]]: true} as arguments.
    3. Let index be index − 1
  21. Set the [[ParameterMap]] internal slot of obj to map.
  22. Perform DefinePropertyOrThrow(obj, @@iterator, PropertyDescriptor {[[Value]]:%ArrayProto_values%, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
  23. Perform DefinePropertyOrThrow(obj, "callee", PropertyDescriptor {[[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}).
  24. Assert: the above property definitions will not produce an abrupt completion.
  25. Return obj

9.4.4.7.1 MakeArgGetter ( name, env) Abstract Operation

The abstract operation MakeArgGetter called with String name and environment record env creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps:

  1. Let realm be the current Realm.
  2. Let steps be the steps of a ArgGetter function as specified below.
  3. Let getter be CreateBuiltinFunction(realm, steps, %FunctionPrototype%, ([[name]], [[env]]) ).
  4. Set getter’s [[name]] internal slot to name.
  5. Set getter’s [[env]] internal slot to env.
  6. Return getter.

An ArgGetter function is an anonymous built-in function with [[name]] and [[env]] internal slots. When an ArgGetter function f that expects no arguments is called it performs the following steps:

  1. Let name be the value of f’s [[name]] internal slot.
  2. Let env be the value of f’s [[env]] internal slot
  3. Return the result of calling the GetBindingValue concrete method of env with arguments name and false.

NOTE ArgGetter functions are never directly accessible to ECMAScript code.

9.4.4.7.2 MakeArgSetter ( name, env) Abstract Operation

The abstract operation MakeArgSetter called with String name and environment record env creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps:

  1. Let realm be the current Realm.
  2. Let steps be the steps of a ArgSetter function as specified below.
  3. Let setter be CreateBuiltinFunction(realm, steps, %FunctionPrototype%, ([[name]], [[env]]) ).
  4. Set setter’s [[name]] internal slot to name.
  5. Set setter’s [[env]] internal slot to env.
  6. Return setter.

An ArgSetter function is an anonymous built-in function with [[name]] and [[env]] internal slots. When an ArgSetter function f is called with argument value it performs the following steps:

  1. Let name be the value of f’s [[name]] internal slot.
  2. Let env be the value of f’s [[env]] internal slot
  3. Return the result of calling the SetMutableBinding concrete method of env with arguments name, value, and false.

NOTE ArgSetter functions are never directly accessible to ECMAScript code.

9.4.5 Integer Indexed Exotic Objects

An Integer Indexed object is an exotic object that performs special handling of integer index property keys.

Integer Indexed exotic objects have the same internal slots as ordinary objects additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.

Integer Indexed Exotic objects provide alternative definitions for the following internal methods. All of the other Integer Indexed exotic object essential internal methods that are not defined below are as specified in 9.1.

9.4.5.1 [[GetOwnProperty]] ( P )

When the [[GetOwnProperty]] internal method of an Integer Indexed exotic object O is called with property key P the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
  3. If Type(P) is String, then
    1. Let numericIndex be CanonicalNumericIndexString(P).
    2. Assert: numericIndex is not an abrupt completion.
    3. If numericIndex is not undefined, then
      1. Let value be IntegerIndexedElementGet (O, numericIndex).
      2. ReturnIfAbrupt(value).
      3. If value is undefined, then return undefined.
      4. Return a PropertyDescriptor{ [[Value]]: value, [[Enumerable]]: true, [[Writable]]: true, [[Configurable]]: false }.
  4. Return OrdinaryGetOwnProperty(O, P).

9.4.5.2 [[DefineOwnProperty]] ( P, Desc)

When the [[DefineOwnProperty]] internal method of an Integer Indexed exotic object O is called with property key P, and Property Descriptor Desc the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
  3. If Type(P) is String, then
    1. Let numericIndex be CanonicalNumericIndexString (P).
    2. Assert: numericIndex is not an abrupt completion.
    3. If numericIndex is not undefined, then
      1. If the value of O’s [[ViewedArrayBuffer]] is undefined, then throw a TypeError exception.
      2. If IsInteger(numericIndex) is false then return false
      3. Let intIndex be numericIndex.
      4. If intIndex = −0, then return false.
      5. If intIndex < 0, then return false.
      6. Let length be the value of O’s [[ArrayLength]] internal slot.
      7. If intIndexlength, then return false.
      8. If IsAccessorDescriptor(Desc) is true, then return false.
      9. If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is true, then return false.
      10. If Desc has an [[Enumerable]] field and if Desc.[[Enumerable]] is false, then return false.
      11. If Desc has a [[Writable]] field and if Desc.[[Writable]] is false, then return false.
      12. If Desc has a [[Value]] field, then
        1. Let value be Desc.[[Value]].
        2. Let status be IntegerIndexedElementSet (O, intIndex, value).
        3. ReturnIfAbrupt(status).
      13. Return true.
  4. Return OrdinaryDefineOwnProperty(O, P, Desc).

9.4.5.3 [[Get]] (P, Receiver)

When the [[Get]] internal method of an Integer Indexed exotic object O is called with property key P and ECMAScript language value Receiver the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. If Type(P) is String and if SameValue(O, Receiver) is true, then
    1. Let numericIndex be CanonicalNumericIndexString (P).
    2. Assert: numericIndex is not an abrupt completion.
    3. If numericIndex is not undefined, then
      1. Return IntegerIndexedElementGet (O, numericIndex).
  3. Return the result of calling the default ordinary object [[Get]] internal method (9.1.8) on O passing P and Receiver as arguments.

9.4.5.4 [[Set]] ( P, V, Receiver)

When the [[Set]] internal method of an Integer Indexed exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. If Type(P) is String and if SameValue(O, Receiver) is true, then
    1. Let numericIndex be CanonicalNumericIndexString (P).
    2. Assert: numericIndex is not an abrupt completion.
    3. If numericIndex is not undefined, then
      1. Return ToBoolean(IntegerIndexedElementSet (O, numericIndex, V)).
  3. Return the result of calling the default ordinary object [[Set]] internal method (9.1.8) on O passing P, V, and Receiver as arguments.

9.4.5.5 [[Enumerate]] ()

When the [[Enumerate]] internal method of an Integer Indexed exotic object O is called the following steps are taken:

  1. Let indexKeys be a new empty List.
  2. Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
  3. If the value of O’s [[ViewedArrayBuffer]] is undefined, then throw a TypeError exception.
  4. Let len be the value of O’s [[ArrayLength]] internal slot.
  5. For each integer i starting with 0 such that i < len, in ascending order,
    1. Add ToString(i) as the last element of indexKeys.
  6. Let ordinary be the result of calling the default ordinary object [[Enumerate]] internal method (9.1.11) on O.
  7. ReturnIfAbrupt(ordinary).
  8. Return CreateCompoundIterator(CreateListIterator(indexKeys), ordinary).

9.4.5.6 [[OwnPropertyKeys]] ()

When the [[OwnPropertyKeys]] internal method of an Integer Indexed exotic object O is called the following steps are taken:

  1. Let keys be a new empty List.
  2. Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
  3. If the value of O’s [[ViewedArrayBuffer]] is undefined, then throw a TypeError exception.
  4. Let len be the value of O’s [[ArrayLength]] internal slot.
  5. For each integer i starting with 0 such that i < len, in ascending order,
    1. Add ToString(i) as the last element of keys.
  6. For each own property key P of O such that P is an integer index and ToInteger(P) ≥ len, in ascending numeric index order
    1. Add P as the last element of keys.
  7. For each own property key P of O such that Type(P) is String and P is not an integer index, in property creation order
    1. Add P as the last element of keys.
  8. For each own property key P of O such that Type(P) is Symbol, in property creation order
    1. Add P as the last element of keys.
  9. Return CreateArrayFromList(keys).

9.4.5.7 IntegerIndexedObjectCreate (prototype, internalSlotsList) Abstract Operation

The abstract operation IntegerIndexedObjectCreate with arguments prototype and internalSlotsList is used to specify the creation of new Integer Indexed exotic objects. The argument internalSlotsList is a List of the names of additional internal slots that must be defined as part of the object. IntegerIndexedObjectCreate performs the following steps:

  1. Let A be a newly created object with an internal slot for each name in internalSlotsList.
  2. Set A’s essential internal methods to the default ordinary object definitions specified in 9.1.
  3. Set the [[GetOwnProperty]] internal method of A as specified in 9.4.5.1.
  4. Set the [[DefineOwnProperty]] internal method of A as specified in 9.4.5.2.
  5. Set the [[Get]] internal method of A as specified in 9.4.5.3.
  6. Set the [[Set]] internal method of A as specified in 9.4.5.4.
  7. Set the [[Enumerate]] internal method of A as specified in 9.4.5.5.
  8. Set the [[OwnPropertyKeys]] internal method of A as specified in 9.4.5.6.
  9. Set the [[Prototype]] internal slot of A to prototype.
  10. Set the [[Extensible]] internal slot of A to true.
  11. Return A.

9.4.5.8 IntegerIndexedElementGet ( O, index ) Abstract Operation

The abstract operation IntegerIndexedElementGet with arguments O and index performs the following steps:

  1. Assert: Type(index) is Number.
  2. Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
  3. Let buffer be the value of O’s [[ViewedArrayBuffer]] internal slot.
  4. If buffer is undefined, then throw a TypeError exception.
  5. If IsNeuteredBuffer(buffer) is true, then throw a TypeError exception.
  6. If IsInteger(index) is false then return undefined
  7. If index = −0, then return undefined.
  8. Let length be the value of O’s [[ArrayLength]] internal slot.
  9. If index < 0 or indexlength, then return undefined.
  10. Let offset be the value of O’s [[ByteOffset]] internal slot.
  11. Let arrayTypeName be the string value of O’s [[TypedArrayName]] internal slot.
  12. Let elementSize be the Number value of the Element Size value specified in Table 45 for arrayTypeName.
  13. Let indexedPosition = (index × elementSize) + offset.
  14. Let elementType be the string value of the Element Type value in Table 45 for arrayTypeName.
  15. Return GetValueFromBuffer(buffer, indexedPosition, elementType).

9.4.5.9 IntegerIndexedElementSet ( O, index, value ) Abstract Operation

The abstract operation IntegerIndexedElementSet with arguments O, index, and value performs the following steps:

  1. Assert: Type(index) is Number.
  2. Assert: O is an Object that has [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], and [[TypedArrayName]] internal slots.
  3. Let numValue be ToNumber(value).
  4. ReturnIfAbrupt(numValue).
  5. Let buffer be the value of O’s [[ViewedArrayBuffer]] internal slot.
  6. If buffer is undefined, then throw a TypeError exception.
  7. If IsNeuteredBuffer(buffer) is true, then throw a TypeError exception.
  8. If IsInteger(index) is false then return false
  9. If index = −0, then return false.
  10. Let length be the value of O’s [[ArrayLength]] internal slot.
  11. If index < 0 or indexlength, then return false.
  12. Let offset be the value of O’s [[ByteOffset]] internal slot.
  13. Let arrayTypeName be the string value of O’s [[TypedArrayName]] internal slot.
  14. Let elementSize be the Number value of the Element Size value specified in Table 45 for arrayTypeName.
  15. Let indexedPosition = (index × elementSize) + offset.
  16. Let elementType be the string value of the Element Type value in Table 45 for arrayTypeName.
  17. Let status be SetValueInBuffer(buffer, indexedPosition, elementType, numValue).
  18. ReturnIfAbrupt(status).
  19. Return true.

9.4.6 Module Exotic Objects

A module object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 15.1.9). There is a one-to-one correspondence between the own properties of a module exotic object and the ExportedBindings of the Module. Each own property name is the StringValue of the corresponding exported binding. These are the only properties of a module exotic object. Each such property has the attributes {[[Configurable]]: false, [[Enumerable]]: true}. Module objects are not extensible.

Module objects have the internal slots defined in Table 28.

Table 28 — Internal Slots of Module Exotic Objects
Internal Slot Type Description
[[ModuleEnvironment]] Environment The Declarative Environment Record that contains all of the declared top-level bindings for the corresponding module.
[[Exports]] List of String A List containing the bound names exposed as own properties of this object. The list is ordered as if an Array of the same values had been sorted using Array.prototype.sort using SortCompare as comparefn.

Module exotic objects provide alternative definitions for all of the internal methods.

9.4.6.1 [[GetPrototypeOf]] ( )

When the [[GetPrototypeOf]] internal method of a module exotic object O is called the following steps are taken:

  1. Return null.

9.4.6.2 [[SetPrototypeOf]] (V)

When the [[SetPrototypeOf]] internal method of a module exotic object O is called with argument V the following steps are taken:

  1. Assert: Either Type(V) is Object or Type(V) is Null.
  2. Return false.

9.4.6.3 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of a module exotic object O is called the following steps are taken:

  1. Return false.

9.4.6.4 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of a module exotic object O is called the following steps are taken:

  1. Return true.

9.4.6.5 [[GetOwnProperty]] (P)

When the [[GetOwnProperty]] internal method of a module exotic object O is called with property key P, the following steps are taken:

  1. Throw a TypeError exception.

9.4.6.6 [[DefineOwnProperty]] (P, Desc)

When the [[DefineOwnProperty]] internal method of a module exotic object O is called with property key P and Property Descriptor Desc, the following steps are taken:

  1. Return false.

9.4.6.7 [[HasProperty]] (P)

When the [[HasProperty]] internal method of a module exotic object O is called with property key P, the following steps are taken:

  1. Let exports be the value of O’s [[Exports]] internal slot.
  2. If P is an element of exports, then return true.
  3. Return false.

9.4.6.8 [[Get]] (P, Receiver)

When the [[Get]] internal method of a module exotic object O is called with property key P and ECMAScript language value Receiver the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let exports be the value of O’s [[Exports]] internal slot.
  3. If P is not an element of exports, then return undefined.
  4. Let env be the value of O’s [[ModuleEnvironment]] internal slot.
  5. Return the result of calling the GetBindingValue concrete method of env with arguments P and true.

NOTE Attempting to [[Get]] the value of a module export that has not yet been initialized will throw a ReferenceError exception.

9.4.6.9 [[Set]] ( P, V, Receiver)

When the [[Set]] internal method of a module exotic object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

  1. Return false.

9.4.6.10 [[Delete]] (P)

When the [[Delete]] internal method of a module exotic object O is called with property key P the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let exports be the value of O’s [[Exports]] internal slot.
  3. If P is an element of exports, then return false.
  4. Return true.

9.4.6.11 [[Enumerate]] ()

When the [[Enumerate]] internal method of a module exotic object O is called the following steps are taken:

  1. Let exports be the value of O’s [[Exports]] internal slot.
  2. Return CreateListIterator(exports).

9.4.6.12 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of a module exotic object O is called the following steps are taken:

  1. Let exports be the value of O’s [[Exports]] internal slot.
  2. Return CreateArrayFromList (exports).

9.4.6.13 ModuleObjectCreate (environment, exports)

  1. Assert: environment is a Declarative Environment Record.
  2. Assert: exports is a List of string values.
  3. Let M be a newly created object.
  4. Set M’s essential internal methods to the definitions specified in 9.4.6.
  5. Set M’s [[ModuleEnvironment]] internal slot to environment.
  6. Set M’s [[Exports]] internal slot to exports.
  7. Return M.

9.5 Proxy Object Internal Methods and Internal Slots

A proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every proxy objects has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is always an object, called the proxy’s handler object. Methods (see Table 29) of a handler object may be used to augment the implementation for one or more of the proxy object’s internal methods. Every proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy’s target object.

Table 29 — Proxy Handler Methods
Internal Method Handler Method
[[GetPrototypeOf]] getPrototypeOf
[[SetPrototypeOf]] setPrototypeOf
[[IsExtensible]] isExtensible
[[PreventExtensions]] preventExtensions
[[GetOwnProperty]] getOwnPropertyDescriptor
[[HasProperty]] has
[[Get]] get
[[Set]] set
[[Delete]] deleteProperty
[[DefineOwnProperty]] defineProperty
[[Enumerate]] enumerate
[[OwnPropertyKeys]] ownKeys
[[Call]] apply
[[Construct]] construct

When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy’s target object as a parameter. A proxy’s handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy’s target object if the handler object does not have a method corresponding to the internal trap.

The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHander]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that proxy obeject to throw a TypeError exception.

Because proxy permit arbitrary ECMAScript code to be used to in the implementation of internal methods, it is possible to define a proxy object whose handler methods violates the invariants defined in 6.1.7.3. Some of the internal method invariants defined in 6.1.7.3 are essential integrity invariants. These invariants are explicitly enforced by the proxy internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.

In the following algorithm descriptions, assume O is an ECMAScript proxy object, P is a property key value, V is any ECMAScript language value, Desc is a Property Descriptor record, and B is a Boolean flag.

9.5.1 [[GetPrototypeOf]] ( )

When the [[GetPrototypeOf]] internal method of an exotic Proxy object O is called the following steps are taken:

  1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  2. If handler is null, then throw a TypeError exception.
  3. Let target be the value of the [[ProxyTarget]] internal slot of O.
  4. Let trap be GetMethod(handler, "getPrototypeOf").
  5. ReturnIfAbrupt(trap).
  6. If trap is undefined, then
    1. Return the result of calling the [[GetPrototypeOf]] internal method of target.
  7. Let handlerProto be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
  8. ReturnIfAbrupt(handlerProto).
  9. If Type(handlerProto) is neither Object nor Null, then throw a TypeError exception.
  10. Let extensibleTarget be IsExtensible(target).
  11. ReturnIfAbrupt(extensibleTarget).
  12. If extensibleTarget is true, then return handlerProto.
  13. Let targetProto be the result of calling the [[GetPrototypeOf]] internal method of target.
  14. ReturnIfAbrupt(targetProto).
  15. If SameValue(handlerProto, targetProto) is false, then throw a TypeError exception.
  16. Return handlerProto.

NOTE [[GetPrototypeOf]] for proxy objects enforces the following invariant:

  • The result of [[GetPrototypeOf]] must be either an Object or null.

  • If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf] applied to the proxy object’s target object.

9.5.2 [[SetPrototypeOf]] (V)

When the [[SetPrototypeOf]] internal method of an exotic Proxy object O is called with argument V the following steps are taken:

  1. Assert: Either Type(V) is Object or Type(V) is Null.
  2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  3. If handler is null, then throw a TypeError exception.
  4. Let target be the value of the [[ProxyTarget]] internal slot of O.
  5. Let trap be GetMethod(handler, "setPrototypeOf").
  6. ReturnIfAbrupt(trap).
  7. If trap is undefined, then
    1. Return the result of calling the [[SetPrototypeOf]] internal method of target with argument V.
  8. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and V.
  9. Let booleanTrapResult be ToBoolean(trapResult).
  10. ReturnIfAbrupt(booleanTrapResult).
  11. Let extensibleTarget be IsExtensible(target).
  12. ReturnIfAbrupt(extensibleTarget).
  13. If extensibleTarget is true, then return booleanTrapResult.
  14. Let targetProto be the result of calling the [[GetPrototypeOf]] internal method of target.
  15. ReturnIfAbrupt(targetProto).
  16. If booleanTrapResult is true and SameValue(V, targetProto) is false, then throw a TypeError exception.
  17. Return booleanTrapResult.

NOTE [[SetPrototypeOf]] for proxy objects enforces the following invariant:

  • If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.

9.5.3 [[IsExtensible]] ( )

When the [[IsExtensible]] internal method of an exotic Proxy object O is called the following steps are taken:

  1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  2. If handler is null, then throw a TypeError exception.
  3. Let target be the value of the [[ProxyTarget]] internal slot of O.
  4. Let trap be GetMethod(handler, "isExtensible").
  5. ReturnIfAbrupt(trap).
  6. If trap is undefined, then
    1. Return the result of calling the [[IsExtensible]] internal method of target.
  7. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
  8. Let booleanTrapResult be ToBoolean(trapResult).
  9. ReturnIfAbrupt(booleanTrapResult).
  10. Let targetResult be the result of calling the [[IsExtensible]] internal method of target.
  11. ReturnIfAbrupt(targetResult).
  12. If SameValue(booleanTrapResult, targetResult) is false, then throw a TypeError exception.
  13. Return booleanTrapResult.

NOTE [[IsExtensible]] for proxy objects enforces the following invariant:

  • [[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object’s target object with the same argument.

9.5.4 [[PreventExtensions]] ( )

When the [[PreventExtensions]] internal method of an exotic Proxy object O is called the following steps are taken:

  1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  2. If handler is null, then throw a TypeError exception.
  3. Let target be the value of the [[ProxyTarget]] internal slot of O.
  4. Let trap be GetMethod(handler, "preventExtensions").
  5. ReturnIfAbrupt(trap).
  6. If trap is undefined, then
    1. Return the result of calling the [[PreventExtensions]] internal method of target.
  7. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
  8. Let booleanTrapResult be ToBoolean(trapResult)
  9. ReturnIfAbrupt(booleanTrapResult).
  10. If booleanTrapResult is true, then
    1. Let targetIsExtensible be the result of calling the [[IsExtensible]] internal method of target.
    2. ReturnIfAbrupt(targetIsExtensible).
    3. If targetIsExtensible is true, then throw a TypeError exception.
  11. Return booleanTrapResult.

NOTE [[PreventExtensions]] for proxy objects enforces the following invariant:

  • [[PreventExtensions]] applied to the proxy object only returns true if [[IsExtensible]] applied to the proxy object’s target object is false.

9.5.5 [[GetOwnProperty]] (P)

When the [[GetOwnProperty]] internal method of an exotic Proxy object O is called with property key P, the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  3. If handler is null, then throw a TypeError exception.
  4. Let target be the value of the [[ProxyTarget]] internal slot of O.
  5. Let trap be GetMethod(handler, "getOwnPropertyDescriptor").
  6. ReturnIfAbrupt(trap).
  7. If trap is undefined, then
    1. Return the result of calling the [[GetOwnProperty]] internal method of target with argument P.
  8. Let trapResultObj be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and P.
  9. ReturnIfAbrupt(trapResultObj).
  10. If Type(trapResultObj) is neither Object nor Undefined, then throw a TypeError exception.
  11. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
  12. ReturnIfAbrupt(targetDesc).
  13. If trapResultObj is undefined, then
    1. If targetDesc is undefined, then return undefined.
    2. If targetDesc.[[Configurable]] is false, then throw a TypeError exception.
    3. Let extensibleTarget be IsExtensible(target).
    4. ReturnIfAbrupt(extensibleTarget).
    5. If ToBoolean(extensibleTarget) is false, then throw a TypeError exception.
    6. Return undefined.
  14. Let extensibleTarget be IsExtensible(target).
  15. ReturnIfAbrupt(extensibleTarget).
  16. Let resultDesc be ToPropertyDescriptor(trapResultObj).
  17. ReturnIfAbrupt(resultDesc).
  18. Call CompletePropertyDescriptor(resultDesc).
  19. Let valid be IsCompatiblePropertyDescriptor (extensibleTarget, resultDesc, targetDesc).
  20. If valid is false, then throw a TypeError exception.
  21. If resultDesc.[[Configurable]] is false, then
    1. If targetDesc is undefined or targetDesc.[[Configurable]] is true, then
      1. Throw a TypeError exception.
  22. Return resultDesc.

NOTE [[GetOwnProperty]] for proxy objects enforces the following invariants:

  • The result of [[GetOwnProperty]] must be either an Object or undefined.

  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.

  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.

  • A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object is not extensible.

  • A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.

  • The result of [[GetOwnProperty]] can be applied to the target object using [[DefineOwnProperty]] and will not throw an exception.

9.5.6 [[DefineOwnProperty]] (P, Desc)

When the [[DefineOwnProperty]] internal method of an exotic Proxy object O is called with property key P and Property Descriptor Desc, the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  3. If handler is null, then throw a TypeError exception.
  4. Let target be the value of the [[ProxyTarget]] internal slot of O.
  5. Let trap be GetMethod(handler, "defineProperty").
  6. ReturnIfAbrupt(trap).
  7. If trap is undefined, then
    1. Return the result of calling the [[DefineOwnProperty]] internal method of target with arguments P and Desc.
  8. Let descObj be FromPropertyDescriptor(Desc).
  9. NOTE If Desc was originally generated from an object using ToPropertyDescriptor, then descObj will be that original object.
  10. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target, P, and descObj.
  11. Let booleanTrapResult be ToBoolean(trapResult).
  12. ReturnIfAbrupt(booleanTrapResult).
  13. If booleanTrapResult is false, then return false.
  14. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
  15. ReturnIfAbrupt(targetDesc).
  16. Let extensibleTarget be IsExtensible(target).
  17. ReturnIfAbrupt(extensibleTarget).
  18. If Desc has a [[Configurable]] field and if Desc.[[Configurable]] is false, then
    1. Let settingConfigFalse be true.
  19. Else let settingConfigFalse be false.
  20. If targetDesc is undefined, then
    1. If extensibleTarget is false, then throw a TypeError exception.
    2. If settingConfigFalse is true, then throw a TypeError exception.
  21. Else targetDesc is not undefined,
    1. If IsCompatiblePropertyDescriptor(extensibleTarget, Desc , targetDesc) is false, then throw a TypeError exception.
    2. If settingConfigFalse is true and targetDesc.[[Configurable]] is true, then throw a TypeError exception.
  22. Return true.

NOTE [[DefineOwnProperty]] for proxy objects enforces the following invariants:

  • A property cannot be added, if the target object is not extensible.

  • A property cannot be added as or modified to be non-configurable, if it does not exists as a non-configurable own property of the target object.

  • A property may not be non-configurable, if is corresponding configurable property of the target object exists.

  • If a property has a corresponding target object property then apply the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.

9.5.7 [[HasProperty]] (P)

When the [[HasProperty]] internal method of an exotic Proxy object O is called with property key P, the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  3. If handler is null, then throw a TypeError exception.
  4. Let target be the value of the [[ProxyTarget]] internal slot of O.
  5. Let trap be GetMethod(handler, "has").
  6. ReturnIfAbrupt(trap).
  7. If trap is undefined, then
    1. Return the result of calling the [[HasProperty]] internal method of target with argument P.
  8. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and P.
  9. Let booleanTrapResult be ToBoolean(trapResult).
  10. ReturnIfAbrupt(booleanTrapResult).
  11. If booleanTrapResult is false, then
    1. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
    2. ReturnIfAbrupt(targetDesc).
    3. If targetDesc is not undefined, then
      1. If targetDesc.[[Configurable]] is false, then throw a TypeError exception.
      2. Let extensibleTarget be IsExtensible(target).
      3. ReturnIfAbrupt(extensibleTarget).
      4. If extensibleTarget is false, then throw a TypeError exception.
  12. Return booleanTrapResult.

NOTE [[HasProperty]] for proxy objects enforces the following invariants:

  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.

  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.

9.5.8 [[Get]] (P, Receiver)

When the [[Get]] internal method of an exotic Proxy object O is called with property key P and ECMAScript language value Receiver the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  3. If handler is null, then throw a TypeError exception.
  4. Let target be the value of the [[ProxyTarget]] internal slot of O.
  5. Let trap be GetMethod(handler, "get").
  6. ReturnIfAbrupt(trap).
  7. If trap is undefined, then
    1. Return the result of calling the [[Get]] internal method of target with arguments P and Receiver.
  8. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target, P, and Receiver.
  9. ReturnIfAbrupt(trapResult).
  10. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
  11. ReturnIfAbrupt(targetDesc).
  12. If targetDesc is not undefined, then
    1. If IsDataDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
      1. If SameValue(trapResult, targetDesc.[[Value]]) is false, then throw a TypeError exception.
    2. If IsAccessorDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Get]] is undefined, then
      1. If trapResult is not undefined, then throw a TypeError exception.
  13. Return trapResult.

NOTE [[Get]] for proxy objects enforces the following invariants:

  • The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable data property.

  • The value reported for a property must be undefined if the corresponding corresponding target object property is non-configurable accessor property that has undefined as its [[Get]] attribute.

9.5.9 [[Set]] ( P, V, Receiver)

When the [[Set]] internal method of an exotic Proxy object O is called with property key P, value V, and ECMAScript language value Receiver, the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  3. If handler is null, then throw a TypeError exception.
  4. Let target be the value of the [[ProxyTarget]] internal slot of O.
  5. Let trap be GetMethod(handler, "set").
  6. ReturnIfAbrupt(trap).
  7. If trap is undefined, then
    1. Return the result of calling the [[Set]] internal method of target with arguments P, V, and Receiver.
  8. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target, P, V, and Receiver.
  9. Let booleanTrapResult be ToBoolean(trapResult).
  10. ReturnIfAbrupt(booleanTrapResult).
  11. If booleanTrapResult is false, then return false.
  12. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
  13. ReturnIfAbrupt(targetDesc).
  14. If targetDesc is not undefined, then
    1. If IsDataDescriptor(targetDesc) and targetDesc.[[Configurable]] is false and targetDesc.[[Writable]] is false, then
      1. If SameValue(V, targetDesc.[[Value]]) is false, then throw a TypeError exception.
    2. If IsAccessorDescriptor(targetDesc) and targetDesc.[[Configurable]] is false, then
      1. If targetDesc.[[Set]] is undefined, then throw a TypeError exception.
  15. Return true.

NOTE [[Set]] for proxy objects enforces the following invariants:

  • Cannnot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable data property.

  • Cannot set the value of a property if the corresponding corresponding target object property is a non-configurable accessor property that has undefined as its [[Set]] attribute.

9.5.10 [[Delete]] (P)

When the [[Delete]] internal method of an exotic Proxy object O is called with property name P the following steps are taken:

  1. Assert: IsPropertyKey(P) is true.
  2. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  3. If handler is null, then throw a TypeError exception.
  4. Let target be the value of the [[ProxyTarget]] internal slot of O.
  5. Let trap be GetMethod(handler, "deleteProperty").
  6. ReturnIfAbrupt(trap).
  7. If trap is undefined, then
    1. Return the result of calling the [[Delete]] internal method of target with argument P.
  8. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target and P.
  9. Let booleanTrapResult be ToBoolean(trapResult).
  10. ReturnIfAbrupt(booleanTrapResult).
  11. If booleanTrapResult is false, then return false.
  12. Let targetDesc be the result of calling the [[GetOwnProperty]] internal method of target with argument P.
  13. ReturnIfAbrupt(targetDesc).
  14. If targetDesc is undefined, then return true.
  15. If targetDesc.[[Configurable]] is false, then throw a TypeError exception.
  16. Return true.

NOTE [[Delete]] for proxy objects enforces the following invariant:

  • A property cannot be deleted, if it exists as a non-configurable own property of the target object.

9.5.11 [[Enumerate]] ()

When the [[Enumerate]] internal method of an exotic Proxy object O is called the following steps are taken:

  1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  2. If handler is null, then throw a TypeError exception.
  3. Let target be the value of the [[ProxyTarget]] internal slot of O.
  4. Let trap be GetMethod(handler, "enumerate").
  5. ReturnIfAbrupt(trap).
  6. If trap is undefined, then
    1. Return the result of calling the [[Enumerate]] internal method of target.
  7. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
  8. ReturnIfAbrupt(trapResult).
  9. If Type(trapResult) is not Object, then throw a TypeError exception.
  10. Return trapResult.

NOTE [[Enumerate]] for proxy objects enforces the following invariants:

  • The result of [[Enumerate]] must be an Object.

9.5.12 [[OwnPropertyKeys]] ( )

When the [[OwnPropertyKeys]] internal method of an exotic Proxy object O is called the following steps are taken:

  1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  2. If handler is null, then throw a TypeError exception.
  3. Let target be the value of the [[ProxyTarget]] internal slot of O.
  4. Let trap be GetMethod(handler, "ownKeys").
  5. ReturnIfAbrupt(trap).
  6. If trap is undefined, then
    1. Return the result of calling the [[OwnPropertyKeys]] internal method of target.
  7. Let trapResult be the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target.
  8. ReturnIfAbrupt(trapResult).
  9. If Type(trapResult) is not Object, then throw a TypeError exception.
  10. If trapResult is not an Array exotic object, then throw a TypeError exception.
  11. Let extensibleTarget be IsExtensible(target).
  12. ReturnIfAbrupt(extensibleTarget).
  13. Let targetKeys be the result of calling the [[OwnPropertyKeys]] internal method of target.
  14. ReturnIfAbrupt(targetKeys).
  15. Let targetLength be ToLength(Get(targetKeys, "length").
  16. ReturnIfAbrupt(targetLength).
  17. Let targetConfigurableKeys be an empty List.
  18. Let targetNonconfigurableKeys be an empty List.
  19. Let n be 0.
  20. Repeat, while n<targetLength,
    1. Let key be Get(targetKeys, ToString(n)).
    2. ReturnIfAbrupt(key).
    3. Let desc the result of calling the [[GetOwnProperty]] internal method of target with argument key.
    4. ReturnIfAbrupt(desc).
    5. If desc is not undefined and desc.[[Configurable]] is false, then
      1. Append key as an element of targetNonconfurableKeys.
    6. Else,
      1. Append key as an element of targetConfigurableKeys.
    7. Let n be n + 1.
  21. If extensibleTarget is true and targetNonconfigurableKeys is empty, then
    1. Return trapResult.
  22. Let resultLength be ToLength(Get(trapResult, "length").
  23. ReturnIfAbrupt(resultLength).
  24. Let uncheckedResultKeys be an empty List.
  25. Let n be 0.
  26. Repeat, while n< resultLength,
    1. Let key be Get(trapResult, ToString(n)).
    2. ReturnIfAbrupt(key).
    3. Append key as an element of uncheckedResultKeys.
    4. Let n be n + 1.
  27. Repeat, for each key that is an element of targetNonconfigurableKeys,
    1. If key is not an element of uncheckedResultKeys, then throw a TypeError exception.
    2. Remove key from uncheckedResultKeys
  28. If extensibleTarget is true, then return trapResult.
  29. Repeat, for each key that is an element of targetConfigurableKeys,
    1. If key is not an element of uncheckedResultKeys, then throw a TypeError exception.
    2. Remove key from uncheckedResultKeys
  30. If uncheckedResultKeys is not empty, then throw a TypeError exception.
  31. Return trapResult.

NOTE [[OwnPropertyKeys]] for proxy objects enforces the following invariants:

  • The result of [[OwnPropertyKeys]] must be an exotic Array Object.

  • The result array must contain the keys of all non-configurable own properties of the target object.

  • If the target object is not extensible, then the result array must contain all the keys of the own properties of the target object and no other values.

9.5.13 [[Call]] (thisArgument, argumentsList)

The [[Call]] internal method of an exotic Proxy object O is called with parameters thisArgument and argumentsList, a List of ECMAScript language values. The following steps are taken:

  1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  2. If handler is null, then throw a TypeError exception.
  3. Let target be the value of the [[ProxyTarget]] internal slot of O.
  4. Let trap be GetMethod(handler, "apply").
  5. ReturnIfAbrupt(trap).
  6. If trap is undefined, then
    1. Return the result of calling the [[Call]] internal method of target with arguments thisArgument and argumentsList.
  7. Let argArray be CreateArrayFromList(argumentsList).
  8. Return the result of calling the [[Call]] internal method of trap with handler as the this value and a new List containing target, thisArgument, and argArray.

NOTE A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.

9.5.14 [[Construct]] Internal Method

The [[Construct]] internal method of an exotic Proxy object O is called with a single parameter argumentsList which is a possibly empty List of ECMAScript language values. The following steps are taken:

  1. Let handler be the value of the [[ProxyHandler]] internal slot of O.
  2. If handler is null, then throw a TypeError exception.
  3. Let target be the value of the [[ProxyTarget]] internal slot of O.
  4. Let trap be GetMethod(handler, "construct").
  5. ReturnIfAbrupt(trap).
  6. If trap is undefined, then
    1. If target does not have a [[Construct]] internal method, then throw a TypeError exception.
    2. Return the result of calling the [[Construct]] internal method of target with argument argumentsList.
  7. Let argArray be CreateArrayFromList(argumentsList).
  8. Let newObj be the result of calling trap with handler as the this value and a new List containing target and argArray.
  9. ReturnIfAbrupt(newObj).
  10. If Type(newObj) is not Object, then throw a TypeError exception.
  11. Return newObj.

NOTE 1 A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.

NOTE 2 [[Construct]]] for proxy objects enforces the following invariants:

  • The result of [[Construct]] must be an Object.

9.5.15 ProxyCreate(target, handler) Abstract Operation

The abstract operation ProxyCreate with arguments target and handler is used to specify the creation of new Proxy exotic objects. It performs the following steps:

  1. If Type(target) is not Object, throw a TypeError Exception.
  2. If Type(handler) is not Object, throw a TypeError Exception.
  3. Let P be a newly created object.
  4. Set P’s essential internal methods (except for [[Call]] nad [[Construct]]) to the definitions specified in 9.5.
  5. If IsCallable(target) is true, then
    1. Set the [[Call]] internal method of P as specified in 9.5.13.
    2. If target has a [[Construct]] internal method, then
      1. Set the [[Construct]] internal method of P as specified in 9.5.14.
  6. Set the [[ProxyTarget]] internal slot of P to target.
  7. Set the [[ProxyHandler]] internal slot of P to handler.
  8. Return P.

10 ECMAScript Language: Source Code

10.1 Source Text

Syntax

SourceCharacter ::
any Unicode code point

The ECMAScript code is expressed using Unicode, version 5.1 or later. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values. Each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of text, or behave as though they were performing normalization of text.

The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.

NOTE In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point’s numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is line feed) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a line feed to be part of the string value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a quote mark that might terminate the string literal.

10.1.1 Static Semantics: UTF-16Encoding

The UTF-16Encoding of a numeric code point value, cp, is determined as follows:

  1. Assert: 0 ≤ cp ≤ 0x10FFFF.
  2. If cp ≤ 65535, then return cp.
  3. Let cu1 be floor((cp – 65536) / 1024) + 0xD800.
  4. Let cu2 be ((cp – 65536) modulo 1024) + 0xDC00.
  5. Return the code unit sequence consisting of cu1 followed by cu2.

10.1.2 Static Semantics: UTF16Decode(lead, trail)

Two code units, lead and trail, that form a UTF-16 surrogate pair are converted to a code point by performing the following steps:

  1. Assert: 0xD800 ≤ lead ≤ 0xDBFF and 0xDC00 ≤ trail ≤ 0xDFFF.
  2. Let cp be (lead–0xD800)×1024+( trail–0xDC00)+0x10000.
  3. Return the code point cp.

10.2 Types of Source Code

There are four types of ECMAScript code:

  • Global code is source text that is treated as an ECMAScript Script. The global code of a particular Script does not include any source text that is parsed as part of a FunctionBody, GeneratorBody, ConciseBody, ClassBody, or ModuleBody.

  • Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Script. The eval code for a particular invocation of eval is the global code portion of that Script.

  • Function code is source text that is parsed to supply the value of the [[Code]] internal slot (see 9.1.14) of function and generator objects. It includes the code that defines and initializes the formal parameters of the function. The function code of a particular function or generator does not include any source text that is parsed as the function code of a nested FunctionBody, GeneratorBody, ConciseBody, or ClassBody.

  • Module code is source text that is code that is provided as a ModuleBody. It is the code that is directly evaluated when a module is initialized. The module code of a particular module does not include any source text that is parsed as part of a nested FunctionBody, GeneratorBody, ConciseBody, ClassBody, or ModuleBody.

NOTE Function code is generally provided as the bodies of Function Definitions (14.1), Arrow Function Definitions (14.2), Method Definitions (14.3) and Generator Definitions (14.4). Function code is also derived from the last argument to the Function constructor (19.2.1.1) and the GeneratorFunction constructor (25.2.1.1).

10.2.1 Strict Mode Code

An ECMAScript Script syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. When processed using strict mode the four types of ECMAScript code are referred to as module code, strict global code, strict eval code, and strict function code. Code is interpreted as strict mode code in the following situations:

  • Global code is strict global code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1.1).

  • Module code is always strict code.

  • All parts of a ClassDeclaration or a ClassExpression are strict code.

  • Eval code is strict eval code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct call (see 18.2.1.1) to the eval function that is contained in strict mode code.

  • Function code that is part of a FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is strict function code if its GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is contained in strict mode code or if its FunctionBody begins with a Directive Prologue that contains a Use Strict Directive.

  • Function code that is supplied as the last argument to the built-in Function constructor is strict function code if the last argument is a String that when processed as a FunctionBody begins with a Directive Prologue that contains a Use Strict Directive.

10.2.2 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of exotic function objects whose evaluative behaviour is expressed in some implementation defined form of executable code other than via ECMAScript code. Whether a function object is an ECMAScript code function or a non-ECMAScript function is not semantically observable from the perspective of an ECMAScript code function that calls or is called by such a non-ECMAScript function.

11 ECMAScript Language: Lexical Grammar

The source text of an ECMAScript script is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code units as the next input element.

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The InputElementDiv goal symbol is the default goal symbol and is used in those syntactic grammar contexts where a leading division (/) or division-assignment (/=) operator is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted. The InputElementTemplateTail goal is used in syntactic grammar contexts where a TemplateLiteral logically continues after a substitution element.

NOTE There are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 11.9); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment code point after a LineTerminator is slash (/) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:

a = b / hi / g.exec(c).map(d);

Syntax

InputElementDiv ::
WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
RightBracePunctuator
InputElementRegExp ::
WhiteSpace
LineTerminator
Comment
Token
RightBracePunctuator
RegularExpressionLiteral
InputElementTemplateTail ::
WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
TemplateSubstitutionTail

11.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as left-to-right mark or right-to-left mark) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.

U+200C (Zero width non-joiner) and U+200D (Zero width joiner) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in an IdentifierName (see 11.6.1) after the first character.

U+FEFF (Byte Order Mark) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <BOM> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <BOM> code points are treated as white space characters (see 11.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 30.

Table 30 — Format-Control Code Point Usage
Code Point Name Abbreviation Usage
U+200C Zero width non-joiner <ZWNJ> IdentifierPart
U+200D Zero width joiner <ZWJ> IdentifierPart
U+FEFF Byte Order Mark <BOM> Whitespace

11.2 White Space

White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space code points are listed in Table 31.

Table 31 — Whitespace Code Point
Code Point Name Abbreviation
U+0009 Character Tabulation <TAB>
U+000B LINE TABULATION <VT>
U+000C Form Feed <FF>
U+0020 Space <SP>
U+00A0 No-break space <NBSP>
U+FEFF Byte Order Mark <BOM>
Other category “Zs” Any other Unicode “Separator, space” code point <USP>

ECMAScript implementations must recognize as Whitespace code points listed in the “Separator, space” (Zs) category by Unicode 5.1. ECMAScript implementations may also recognize as Whitespace additional category Zs code points from subsequent editions of the Unicode Standard.

NOTE Other than for the code points listed in Table 31, ECMAScript Whitespace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Zs”.

Syntax

WhiteSpace ::
<TAB>
<VT>
<FF>
<SP>
<NBSP>
<BOM>
<USP>

11.3 Line Terminators

Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (11.9). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. Line terminators may only occur within a StringLiteral token as part of a LineContinuation.

A line terminator can occur within a MultiLineComment (11.4) but cannot occur within a SingleLineComment.

Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.

The ECMAScript line terminator code points are listed in Table 32.

Table 32 — Line Terminator Code Points
Code Point Unicode Name Abbreviation
U+000A Line Feed <LF>
U+000D Carriage Return <CR>
U+2028 Line separator <LS>
U+2029 Paragraph separator <PS>

Only the Unicode code points in Table 32 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 31. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.

Syntax

LineTerminator ::
<LF>
<CR>
<LS>
<PS>
LineTerminatorSequence ::
<LF>
<CR> [lookahead ∉ <LF> ]
<LS>
<PS>
<CR> <LF>

11.4 Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 11.9).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

Comment ::
MultiLineComment
SingleLineComment
MultiLineComment ::
/* MultiLineCommentCharsopt */
MultiLineCommentChars ::
MultiLineNotAsteriskChar MultiLineCommentCharsopt
* PostAsteriskCommentCharsopt
PostAsteriskCommentChars ::
MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt
* PostAsteriskCommentCharsopt
MultiLineNotAsteriskChar ::
SourceCharacter but not *
MultiLineNotForwardSlashOrAsteriskChar ::
SourceCharacter but not one of / or *
SingleLineComment ::
// SingleLineCommentCharsopt
SingleLineCommentChars ::
SingleLineCommentChar SingleLineCommentCharsopt
SingleLineCommentChar ::
SourceCharacter but not LineTerminator

11.5 Tokens

Syntax

Token ::
IdentifierName
Punctuator
NumericLiteral
StringLiteral
Template

NOTE The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions define tokens, but are not included in the Token production.

11.6 Names and Keywords

IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammer defines Identifier as an IdentifierName that is not a ReservedWord (see 11.6.2). The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in version 5.1.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.

NOTE 1 This standard specifies specific code point additions: U+0024 (dollar sign) and U+005F (LOW LINE) are permitted anywhere in an IdentifierName, and the characters U+200C (zero-width non-joiner) and U+200D (zero-width joiner) are permitted anywhere after the first code unit of an IdentifierName.

Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point to the IdentifierName. The code point is expressed by the HexDigits of the UnicodeEscapeSequence (see 11.8.4). The \ preceding the UnicodeEscapeSequence and the u and { } code units, if they appear, do not contribute code points to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a code point into an IdentifierName that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.

Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unless they are represented by the exact same sequence of code points (in other words, conforming ECMAScript implementations are only required to do bitwise comparison on IdentifierName values).

Syntax

IdentifierName ::
IdentifierStart
IdentifierName IdentifierPart
IdentifierStart ::
UnicodeIDStart
$
_
\ UnicodeEscapeSequence
IdentifierPart ::
UnicodeIDContinue
$
_
\ UnicodeEscapeSequence
<ZWNJ>
<ZWJ>
UnicodeIDStart ::
any Unicode code point with the Unicode property “ID_Start” or “Other_ID_Start”
UnicodeIDContinue ::
any Unicode code point with the Unicode property “ID_Continue” or “Other_ID_ Continue”

The definitions of the nonterminal UnicodeEscapeSequence is given in 11.8.4.

11.6.1 Identifier Names

11.6.1.1 Static Semantics: Early Errors

IdentifierStart :: \ UnicodeEscapeSequence
  • It is a Syntax Error if SV(UnicodeEscapeSequence) is neither the UTF-16Encoding (10.1.1) of a single Unicode code point with the Unicode property “ID_Start” nor "$" or "_".

IdentifierPart :: \ UnicodeEscapeSequence
  • It is a Syntax Error if SV(UnicodeEscapeSequence) is neither the UTF-16Encoding (10.1.1) of a single Unicode code point with the Unicode property “ID_Continue” nor "$" or "_" nor the UTF-16Encoding of either <ZWNJ> or <ZWJ>.

11.6.1.2 Static Semantics: StringValue

See also: 11.8.4.2, 12.1.3.

IdentifierName ::
IdentifierStart
IdentifierName IdentifierPart
  1. Return the String value consisting of the sequence of code units corresponding to IdentifierName. In determining the sequence any occurrences of \ UnicodeEscapeSequence are first replaced with the code point represented by the UnicodeEscapeSequence and then the code points of the entire IdentifierName are converted to code units by UTF-16Encoding (10.1.1) each code point.

11.6.2 Reserved Words

A reserved word is an IdentifierName that cannot be used as an Identifier.

Syntax

ReservedWord ::
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral

NOTE The ReservedWord definitions are specified as literal sequences of specific SourceCharacter elements. A code point in a ReservedWord cannot be expressed by a \ UnicodeEscapeSequence.

11.6.2.1 Keywords

The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.

Syntax

Keyword :: one of
break do in typeof
case else instanceof var
catch export new void
class extends return while
const finally super with
continue for switch yield
debugger function this
default if throw
delete import try

NOTE In some contexts yield is given the semantics of an Identifier. See 12.1.1. In strict mode code, let is treated as a keyword through static semantic restrictions (see 12.1.1, 12.2.4.2.1, 13.2.1.1, 13.6.4.1, and 14.5.1) rather than the lexical grammar.

11.6.2.2 Future Reserved Words

The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility of future adoption of those extensions.

Syntax

FutureReservedWord ::
enum
await

await is only treated as a FutureReservedWord when Module is the goal symbol of the syntactic grammar.

NOTE Use of the following tokens within strict mode code (see 10.2.1) is also reserved. That usage is restricted using static semantic restrictions (see 12.1.1) rather than the lexical grammar:

implements package protected static
interface private public

11.7 Punctuators

Syntax

Punctuator :: one of
{ ( ) [ ] .
... ; , < > <=
>= == != === !==
+ - * % ++ --
<< >> >>> & | ^
! ~ && || ? :
= += -= *= %= <<=
>>= >>>= &= |= ^= =>
DivPunctuator :: one of
/ /=
RightBracePunctuator ::
}

11.8 Literals

11.8.1 Null Literals

Syntax

NullLiteral ::
null

11.8.2 Boolean Literals

Syntax

BooleanLiteral ::
true
false

11.8.3 Numeric Literals

Syntax

NumericLiteral ::
DecimalLiteral
BinaryIntegerLiteral
OctalIntegerLiteral
HexIntegerLiteral
DecimalLiteral ::
DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
DecimalIntegerLiteral ExponentPartopt
DecimalIntegerLiteral ::
0
NonZeroDigit DecimalDigitsopt
DecimalDigits ::
DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit :: one of
0 1 2 3 4 5 6 7 8 9
NonZeroDigit :: one of
1 2 3 4 5 6 7 8 9
ExponentPart ::
ExponentIndicator SignedInteger
ExponentIndicator :: one of
e E
SignedInteger ::
DecimalDigits
+ DecimalDigits
- DecimalDigits
BinaryIntegerLiteral ::
0b BinaryDigits
0B BinaryDigits
BinaryDigits ::
BinaryDigit
BinaryDigits BinaryDigit
BinaryDigit :: one of
0 1
OctalIntegerLiteral ::
0o OctalDigits
0O OctalDigits
OctalDigits ::
OctalDigit
OctalDigits OctalDigit
OctalDigit :: one of
0 1 2 3 4 5 6 7
HexIntegerLiteral ::
0x HexDigits
0X HexDigits
HexDigits ::
HexDigit
HexDigits HexDigit
HexDigit :: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

The SourceCharacter immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

NOTE For example:

3in

is an error and not the two input elements 3 and in.

A conforming implementation, when processing strict mode code (see 10.2.1), must not extend the syntax of NumericLiteral to include LegacyOctalIntegerLiteral as described in B.1.1.

11.8.3.1 Static Semantics: MV’s

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.

  • The MV of NumericLiteral :: DecimalLiteral is the MV of DecimalLiteral.

  • The MV of NumericLiteral :: BinaryIntegerLiteral is the MV of BinaryIntegerLiteral.

  • The MV of NumericLiteral :: OctalIntegerLiteral is the MV of OctalIntegerLiteral.

  • The MV of NumericLiteral :: HexIntegerLiteral is the MV of HexIntegerLiteral.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral . is the MV of DecimalIntegerLiteral.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits is the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10n), where n is the number of code points in DecimalDigits.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of DecimalIntegerLiteral × 10e, where e is the MV of ExponentPart.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits ExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits × 10n)) × 10e, where n is the number of code points in DecimalDigits and e is the MV of ExponentPart.

  • The MV of DecimalLiteral :: . DecimalDigits is the MV of DecimalDigits × 10n, where n is the number of code points in DecimalDigits.

  • The MV of DecimalLiteral :: . DecimalDigits ExponentPart is the MV of DecimalDigits × 10en, where n is the number of code points in DecimalDigits and e is the MV of ExponentPart.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.

  • The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral × 10e, where e is the MV of ExponentPart.

  • The MV of DecimalIntegerLiteral :: 0 is 0.

  • The MV of DecimalIntegerLiteral :: NonZeroDigit is the MV of NonZeroDigit.

  • The MV of DecimalIntegerLiteral :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit × 10n) plus the MV of DecimalDigits, where n is the number of code points in DecimalDigits.

  • The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.

  • The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits × 10) plus the MV of DecimalDigit.

  • The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.

  • The MV of SignedInteger :: DecimalDigits is the MV of DecimalDigits.

  • The MV of SignedInteger :: + DecimalDigits is the MV of DecimalDigits.

  • The MV of SignedInteger :: - DecimalDigits is the negative of the MV of DecimalDigits.

  • The MV of DecimalDigit :: 0 or of HexDigit :: 0 or of OctalDigit :: 0 or of BinaryDigit :: 0 is 0.

  • The MV of DecimalDigit :: 1 or of NonZeroDigit :: 1 or of HexDigit :: 1 or of OctalDigit :: 1 or
    of BinaryDigit :: 1 is 1.

  • The MV of DecimalDigit :: 2 or of NonZeroDigit :: 2 or of HexDigit :: 2 or of OctalDigit :: 2 is 2.

  • The MV of DecimalDigit :: 3 or of NonZeroDigit :: 3 or of HexDigit :: 3 or of OctalDigit :: 3 is 3.

  • The MV of DecimalDigit :: 4 or of NonZeroDigit :: 4 or of HexDigit :: 4 or of OctalDigit :: 4 is 4.

  • The MV of DecimalDigit :: 5 or of NonZeroDigit :: 5 or of HexDigit :: 5 or of OctalDigit :: 5 is 5.

  • The MV of DecimalDigit :: 6 or of NonZeroDigit :: 6 or of HexDigit :: 6 or of OctalDigit :: 6 is 6.

  • The MV of DecimalDigit :: 7 or of NonZeroDigit :: 7 or of HexDigit :: 7 or of OctalDigit :: 7 is 7.

  • The MV of DecimalDigit :: 8 or of NonZeroDigit :: 8 or of HexDigit :: 8 is 8.

  • The MV of DecimalDigit :: 9 or of NonZeroDigit :: 9 or of HexDigit :: 9 is 9.

  • The MV of HexDigit :: a or of HexDigit :: A is 10.

  • The MV of HexDigit :: b or of HexDigit :: B is 11.

  • The MV of HexDigit :: c or of HexDigit :: C is 12.

  • The MV of HexDigit :: d or of HexDigit :: D is 13.

  • The MV of HexDigit :: e or of HexDigit :: E is 14.

  • The MV of HexDigit :: f or of HexDigit :: F is 15.

  • The MV of BinaryIntegerLiteral :: 0b BinaryDigits is the MV of BinaryDigits.

  • The MV of BinaryIntegerLiteral :: 0B BinaryDigits is the MV of BinaryDigits.

  • The MV of BinaryDigits :: BinaryDigit is the MV of BinaryDigit.

  • The MV of BinaryDigits :: BinaryDigits BinaryDigit is (the MV of BinaryDigits × 2) plus the MV of BinaryDigit.

  • The MV of OctalIntegerLiteral :: 0o OctalDigits is the MV of OctalDigits.

  • The MV of OctalIntegerLiteral :: 0O OctalDigits is the MV of OctalDigits.

  • The MV of OctalDigits :: OctalDigit is the MV of OctalDigit.

  • The MV of OctalDigits :: OctalDigits OctalDigit is (the MV of OctalDigits × 8) plus the MV of OctalDigit.

  • The MV of HexIntegerLiteral :: 0x HexDigits is the MV of HexDigits.

  • The MV of HexIntegerLiteral :: 0X HexDigits is the MV of HexDigits.

  • The MV of HexDigits :: HexDigit is the MV of HexDigit.

  • The MV of HexDigits :: HexDigits HexDigit is (the MV of HexDigits × 16) plus the MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the MV (as specified in 6.1.6), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

  • it is not 0; or
  • there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

11.8.4 String Literals

NOTE A string literal is zero or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, backslash, carriage return, line separator, paragraph separator, and line feed. Any code points may appear in the form of an escape sequence. String literals evaluate to ECAMScript String values. When generating these string values Unicode code points are UTF-16 encoded as defined in 10.1.1. Code points belonging to Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.

Syntax

StringLiteral ::
" DoubleStringCharactersopt "
' SingleStringCharactersopt '
DoubleStringCharacters ::
DoubleStringCharacter DoubleStringCharactersopt
SingleStringCharacters ::
SingleStringCharacter SingleStringCharactersopt
DoubleStringCharacter ::
SourceCharacter but not one of " or \ or LineTerminator
\ EscapeSequence
LineContinuation
SingleStringCharacter ::
SourceCharacter but not one of ' or \ or LineTerminator
\ EscapeSequence
LineContinuation
LineContinuation ::
\ LineTerminatorSequence
EscapeSequence ::
CharacterEscapeSequence
0 [lookahead ∉ DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence

A conforming implementation, when processing strict mode code (see 10.2.1), must not extend the syntax of EscapeSequence to include LegacyOctalEscapeSequence as described in B.1.1.

CharacterEscapeSequence ::
SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter :: one of
' " \ b f n r t v
NonEscapeCharacter ::
SourceCharacter but not one of EscapeCharacter or LineTerminator
EscapeCharacter ::
SingleEscapeCharacter
DecimalDigit
x
u
HexEscapeSequence ::
x HexDigit HexDigit
UnicodeEscapeSequence ::
u Hex4Digits
u{ HexDigits }
Hex4Digits ::
HexDigit HexDigit HexDigit HexDigit

The definition of the nonterminal HexDigit is given in 11.8.3. SourceCharacter is defined in 10.1.

NOTE A line terminator code point cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The correct way to cause a line terminator code points to be part of the String value of a string literal is to use an escape sequence such as \n or \u000A.

11.8.4.1 Static Semantics: Early Errors

UnicodeEscapeSequence :: u{ HexDigits }
  • It is a Syntax Error if the MV of HexDigits > 1114111.

11.8.4.2 Static Semantics: StringValue

See also: 11.6.1.2, 12.1.3.

StringLiteral ::
" DoubleStringCharactersopt "
' SingleStringCharactersopt '
  1. Return the String value whose elements are the SV of this StringLiteral.

11.8.4.3 Static Semantics: SV’s and CV’s

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of code unit values (CV) contributed by the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value (MV), as described below or in 11.8.3.

  • The SV of StringLiteral :: "" is the empty code unit sequence.

  • The SV of StringLiteral :: '' is the empty code unit sequence.

  • The SV of StringLiteral :: " DoubleStringCharacters " is the SV of DoubleStringCharacters.

  • The SV of StringLiteral :: ' SingleStringCharacters ' is the SV of SingleStringCharacters.

  • The SV of DoubleStringCharacters :: DoubleStringCharacter is a sequence of one or two code units that is the CV of DoubleStringCharacter.

  • The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence of one or two code units that is the CV of DoubleStringCharacter followed by all the code units in the SV of DoubleStringCharacters in order.

  • The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one or two code units that is the CV of SingleStringCharacter.

  • The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence of one or two code units that is the CV of SingleStringCharacter followed by all the code units in the SV of SingleStringCharacters in order.

  • The CV of DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator is the UTF-16Encoding (10.1.1) of the code point value of SourceCharacter.

  • The CV of DoubleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.

  • The CV of DoubleStringCharacter :: LineContinuation is the empty code unit sequence.

  • The CV of SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator is the UTF-16Encoding (10.1.1) of the code point value of SourceCharacter.

  • The CV of SingleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.

  • The CV of SingleStringCharacter :: LineContinuation is the empty code unit sequence.

  • The CV of EscapeSequence :: CharacterEscapeSequence is the CV of the CharacterEscapeSequence.

  • The CV of EscapeSequence :: 0 is the code unit value 0.

  • The CV of EscapeSequence :: HexEscapeSequence is the CV of the HexEscapeSequence.

  • The CV of EscapeSequence :: UnicodeEscapeSequence is the CV of the UnicodeEscapeSequence.

  • The CV of CharacterEscapeSequence :: SingleEscapeCharacter is the code unit whose value is determined by the SingleEscapeCharacter according to Table 33.

Table 33 — String Single Character Escape Sequences
Escape Sequence Code Unit Value Unicode Character Name Symbol
\b 0x0008 BACKSPACE <BS>
\t 0x0009 CHARACTER TABULATION <HT>
\n 0x000A line feed <LF>
\v 0x000B LINE TABULATION <VT>
\f 0x000C form feed <FF>
\r 0x000D carriage return <CR>
\" 0x0022 quotation Mark "
\' 0x0027 apostrophe '
\\ 0x005C REverse Solidus \
  • The CV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.

  • The CV of NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator is the UTF-16Encoding (10.1.1) of the code point value of SourceCharacter.

  • The CV of HexEscapeSequence :: x HexDigit HexDigit is the code unit value that is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.

  • The CV of UnicodeEscapeSequence :: u Hex4Digits is the CV of Hex4Digits.

  • The CV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the code unit value that is (4096 times the MV of the first HexDigit) plus (256 times the MV of the second HexDigit) plus (16 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.

  • The CV of UnicodeEscapeSequence :: u{ HexDigits } is the UTF-16Encoding (10.1.1) of the MV of HexDigits.

11.8.5 Regular Expression Literals

NOTE A regular expression literal is an input element that is converted to a RegExp object (see 21.1.5) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp (see 21.2.3.2) or calling the RegExp constructor as a function (21.2.3.1).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source code comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed using the more stringent ECMAScript Regular Expression grammar (21.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 21.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

Syntax

RegularExpressionLiteral ::
/ RegularExpressionBody / RegularExpressionFlags
RegularExpressionBody ::
RegularExpressionFirstChar RegularExpressionChars
RegularExpressionChars ::
[empty]
RegularExpressionChars RegularExpressionChar
RegularExpressionFirstChar ::
RegularExpressionNonTerminator but not one of * or \ or / or [
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar ::
RegularExpressionNonTerminator but not one of \ or / or [
RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence ::
\ RegularExpressionNonTerminator
RegularExpressionNonTerminator ::
SourceCharacter but not LineTerminator
RegularExpressionClass ::
[ RegularExpressionClassChars ]
RegularExpressionClassChars ::
[empty]
RegularExpressionClassChars RegularExpressionClassChar
RegularExpressionClassChar ::
RegularExpressionNonTerminator but not one of ] or \
RegularExpressionBackslashSequence
RegularExpressionFlags ::
[empty]
RegularExpressionFlags IdentifierPart

NOTE Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.

11.8.5.1 Static Semantics: Early Errors

RegularExpressionFlags :: RegularExpressionFlags IdentifierPart
  • It is a Syntax Error if IdentifierPart contains a Unicode escape sequence.

11.8.5.2 Static Semantics: BodyText

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
  1. Return the source code that was recognized as RegularExpressionBody.

11.8.5.3 Static Semantics: FlagText

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
  1. Return the source code that was recognized as RegularExpressionFlags.

11.8.6 Template Literal Lexical Components

Syntax

Template ::
NoSubstitutionTemplate
TemplateHead
NoSubstitutionTemplate ::
` TemplateCharactersopt `
TemplateHead ::
` TemplateCharactersopt ${
TemplateSubstitutionTail ::
TemplateMiddle
TemplateTail
TemplateMiddle ::
} TemplateCharactersopt ${
TemplateTail ::
} TemplateCharactersopt `
TemplateCharacters ::
TemplateCharacter TemplateCharactersopt
TemplateCharacter ::
$ [lookahead ≠ { ]
\ EscapeSequence
LineContinuation
LineTerminatorSequence
SourceCharacter but not one of ` or \ or $ or LineTerminatorSequence

A conforming implementation must not use the extended definition of EscapeSequence described in B.1.2 when parsing a TemplateCharacter.

NOTE TemplateSubstitutionTail is used by the InputElementTemplateTail alternative lexical goal.

11.8.6.1 Static Semantics: TV’s and TRV’s

A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of code unit values (CV, 11.8.4) contributed by the various parts of the template literal component. As part of this process, some Unicode code points within the template component are interpreted as having a mathematical value (MV, 11.8.3). In determining a TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence. The Template Raw Value (TRV) is similar to a Template Value with the difference that in TRVs escape sequences are interpreted literally.

  • The TV and TRV of NoSubstitutionTemplate :: `` is the empty code unit sequence.

  • The TV and TRV of TemplateHead :: `${ is the empty code unit sequence.

  • The TV and TRV of TemplateMiddle :: }${ is the empty code unit sequence.

  • The TV and TRV of TemplateTail :: }` is the empty code unit sequence.

  • The TV of NoSubstitutionTemplate :: ` TemplateCharacters ` is the TV of TemplateCharacters.

  • The TV of TemplateHead :: ` TemplateCharacters ${ is the TV of TemplateCharacters.

  • The TV of TemplateMiddle :: } TemplateCharacters ${ is the TV of TemplateCharacters.

  • The TV of TemplateTail :: } TemplateCharacters ` is the TV of TemplateCharacters.

  • The TV of TemplateCharacters :: TemplateCharacter is the TV of TemplateCharacter.

  • The TV of TemplateCharacters :: TemplateCharacter TemplateCharacters is a sequence consisting of the code units in the TV of TemplateCharacter followed by all the code units in the TV of TemplateCharacters in order.

  • The TV of TemplateCharacter :: SourceCharacter but not one of ` or \ or $ or LineTerminatorSequence is the UTF-16Encoding (10.1.1) of the code point value of SourceCharacter.

  • The TV of TemplateCharacter :: $ is the code unit value 0x0024.

  • The TV of TemplateCharacter :: \ EscapeSequence is the CV of EscapeSequence.

  • The TV of TemplateCharacter :: LineContinuation is the TV of LineContinuation.

  • The TV of TemplateCharacter :: LineTerminatorSequence is the TRV of LineTerminatorSequence.

  • The TV of LineContinuation :: \ LineTerminatorSequence is the empty code unit sequence.

  • The TRV of NoSubstitutionTemplate :: ` TemplateCharacters ` is the TRV of TemplateCharacters.

  • The TRV of TemplateHead :: ` TemplateCharacters ${ is the TRV of TemplateCharacters.

  • The TRV of TemplateMiddle :: } TemplateCharacters ${ is the TRV of TemplateCharacters.

  • The TRV of TemplateTail :: } TemplateCharacters ` is the TRV of TemplateCharacters.

  • The TRV of TemplateCharacters :: TemplateCharacter is the TRV of TemplateCharacter.

  • The TRV of TemplateCharacters :: TemplateCharacter TemplateCharacters is a sequence consisting of the code units in the TRV of TemplateCharacter followed by all the code units in the TRV of TemplateCharacters, in order.

  • The TRV of TemplateCharacter :: SourceCharacter but not one of ` or \ or $ or LineTerminatorSequence is the UTF-16Encoding (10.1.1) of the code point value of SourceCharacter.

  • The TRV of TemplateCharacter :: $ is the code unit value 0x0024.

  • The TRV of TemplateCharacter :: \ EscapeSequence is the sequence consisting of the code unit value 0x005C followed by the code units of TRV of EscapeSequence.

  • The TRV of TemplateCharacter :: LineContinuation is the TRV of LineContinuation.

  • The TRV of TemplateCharacter :: LineTerminatorSequence is the TRV of LineTerminatorSequence.

  • The TRV of EscapeSequence :: CharacterEscapeSequence is the TRV of the CharacterEscapeSequence.

  • The TRV of EscapeSequence :: 0 is the code unit value 0x0030.

  • The TRV of EscapeSequence :: HexEscapeSequence is the TRV of the HexEscapeSequence.

  • The TRV of EscapeSequence :: UnicodeEscapeSequence is the TRV of the UnicodeEscapeSequence.

  • The TRV of CharacterEscapeSequence :: SingleEscapeCharacter is the TRV of the SingleEscapeCharacter.

  • The TRV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.

  • The TRV of SingleEscapeCharacter :: one of ' " \ b f n r t v is the CV of the SourceCharacter that is that single code point.

  • The TRV of HexEscapeSequence :: x HexDigit HexDigit is the sequence consisting of code unit value 0x0078 followed by TRV of the first HexDigit followed by the TRV of the second HexDigit.

  • The TRV of UnicodeEscapeSequence :: u Hex4Digits is the sequence consisting of code unit value 0x0075 followed by TRV of Hex4Digits.

  • The TRV of UnicodeEscapeSequence :: u{ HexDigits } is the sequence consisting of code unit value 0x0075 followed by code unit value 0x007B followed by TRV of HexDigits followed by code unit value 0x007D.

  • The TRV of Hex4Digits :: HexDigit HexDigit HexDigit HexDigit is the sequence consisting of the TRV of the first HexDigit followed by the TRV of the second HexDigit followed by the TRV of the third HexDigit followed by the TRV of the fourth HexDigit.

  • The TRV of HexDigits :: HexDigit is the TRV of HexDigit.

  • The TRV of HexDigits :: HexDigits HexDigit is the sequence consisting of TRV of HexDigits followed by TRV of HexDigit.

  • The TRV of a HexDigit is the CV of the SourceCharacter that is that HexDigit.

  • The TRV of LineContinuation :: \ LineTerminatorSequence is the sequence consisting of the code unit value 0x005C followed by the code units of TRV of LineTerminatorSequence.

  • The TRV of LineTerminatorSequence :: <LF> is the code unit value 0x000A.

  • The TRV of LineTerminatorSequence :: <CR> is the code unit value 0x000A.

  • The TRV of LineTerminatorSequence :: <LS> is the code unit value 0x2028.

  • The TRV of LineTerminatorSequence :: <PS> is the code unit value 0x2029.

  • The TRV of LineTerminatorSequence :: <CR><LF> is the sequence consisting of the code unit value 0x000A.

NOTE TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit EscapeSequence is needed to include a <CR> or <CR><LF> sequence.

11.9 Automatic Semicolon Insertion

Certain ECMAScript statements (empty statement, let, const, import, export, and module declarations, variable statement, expression statement, debugger statement, continue statement, break statement, return statement, and throw statement) must be terminated with semicolons. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

11.9.1 Rules of Automatic Semicolon Insertion

There are three basic rules of semicolon insertion:

  1. When, as the script is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
    • The offending token is separated from the previous token by at least one LineTerminator.
    • The offending token is }.
  2. When, as the script is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single complete ECMAScript script, then a semicolon is automatically inserted at the end of the input stream.
  3. When, as the script is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation [no LineTerminator here] within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 13.6.3).

NOTE The following are the only restricted productions in the grammar:

PostfixExpression[Yield] :
LeftHandSideExpression[?Yield] [no LineTerminator here] ++
LeftHandSideExpression[?Yield] [no LineTerminator here] --
ContinueStatement[Yield] :
continue;
continue [no LineTerminator here] LabelIdentifier[?Yield] ;
BreakStatement[Yield] :
break ;
break [no LineTerminator here] LabelIdentifier[?Yield] ;
ReturnStatement[Yield] :
return [no LineTerminator here] Expression ;
return [no LineTerminator here] Expression[In, ?Yield] ;
ThrowStatement[Yield] :
throw [no LineTerminator here] Expression[In, ?Yield] ;
ArrowFunction[In, Yield] :
ArrowParameters[?Yield] [no LineTerminator here] => ConciseBody[?In]
YieldExpression[In] :
yield [no LineTerminator here] * [Lexical goal InputElementRegExp] AssignmentExpression[?In, Yield]
yield [no LineTerminator here] [Lexical goal InputElementRegExp] AssignmentExpression[?In, Yield]
ModuleImport :
module [no LineTerminator here] ImportedBinding FromClause ;

The practical effect of these restricted productions is as follows:

When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.

When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.

The resulting practical advice to ECMAScript programmers is:

A postfix ++ or -- operator should appear on the same line as its operand.

An Expression in a return or throw statement or an AssignmentExpression in a yield expression should start on the same line as the return, throw, or yield token.

An IdentifierReference in a break or continue statement should be on the same line as the break or continue token.

11.9.2 Examples of Automatic Semicolon Insertion

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;

NOTE The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;

NOTE The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.

The source

if (a > b)
else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

a = b + c
(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

12 ECMAScript Language: Expressions

12.1 Identifiers

Syntax

IdentifierReference[Yield] :
Identifier
[~Yield] yield

BindingIdentifier[Default, Yield] :

[+Default] default
[~Yield] yield
Identifier

LabelIdentifier[Yield] :
Identifier
[~Yield] yield
Identifier :
IdentifierName but not ReservedWord

12.1.1 Static Semantics: Early Errors

BindingIdentifier : Identifier
  • It is a Syntax Error if this production is contained in strict code and the StringValue of Identifier is "arguments" or "eval".

IdentifierReference[Yield] : Identifier

BindingIdentifier[Default, Yield] : Identifier

LabelIdentifier [Yield] : Identifier

  • It is a Syntax Error if this production has a [Yield] parameter and the StringValue of Identifier is "yield".

IdentifierReference : yield

BindingIdentifier : yield

LabelIdentifier : yield
  • It is a Syntax Error if this production is contained in strict code.

  • It is a Syntax Error if this production is within the FunctionBody of a GeneratorMethod, GeneratorDeclaration, or GeneratorExpression.

Identifier :: IdentifierName but not ReservedWord
  • It is a Syntax Error if this production is contained in strict code and the StringValue of IdentifierName is: "implements", "interface", "let", "package", "private", "protected", "public", or "static".

  • It is a Syntax Error if this production is contained in strict code and the StringValue of IdentifierName is "yield".

  • It is a Syntax Error if StringValue of IdentifierName is the same string value as the StringValue of any ReservedWord except for yield.

NOTE StringValue of IdentifierName normalizes any Unicode escape sequences in IdentifierName hence such escapes cannot be used to write an Identifier whose code point sequence is the same as a ReservedWord.

12.1.2 Static Semantics: BoundNames

See also: 13.2.1.2, 13.2.2.1, 13.6.4.2, 14.1.3, 14.2.2, 14.4.2, 14.5.2, 15.2.1.2, 15.2.2.1.

BindingIdentifier : Identifier
  1. Return a new List containing the StringValue of Identifier.
BindingIdentifier : yield
  1. Return a new List containing "yield".
BindingIdentifier : default
  1. Return a new List containing "default".

12.1.3 Static Semantics: StringValue

See also: 11.6.1.2, 11.8.4.2.

IdentifierReference : yield

BindingIdentifier : yield

LabelIdentifier : yield
  1. Return "yield".
BindingIdentifier : default
  1. Return "default".
Identifier : IdentifierName but not ReservedWord
  1. Return the StringValue of IdentifierName.

12.1.4 Runtime Semantics: BindingInitialization

With arguments value and environment.

See also: 12.2.4.2.2, 13.2.2.4, 13.2.3.5, 13.14.4.

NOTE undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See 9.2.13). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

BindingIdentifier : Identifier
  1. Let name be StringValue of Identifier.
  2. Return InitializeBoundName( name, value, environment).
BindingIdentifier : default
  1. Return InitializeBoundName("default", value, environment).
BindingIdentifier : yield
  1. Return InitializeBoundName("yield", value, environment).

12.1.4.1 Runtime Semantics: InitializeBoundName(name, value, environment)

  1. Assert: Type(name) is String.
  2. If environment is not undefined, then
    1. Let env be the environment record component of environment.
    2. Call the InitializeBinding concrete method of env passing name and value as the arguments.
    3. Return NormalCompletion(undefined).
  3. Else
    1. Let lhs be ResolveBinding(name).
    2. Return PutValue(lhs, value).

12.1.5 Runtime Semantics: Evaluation

IdentifierReference : Identifier
  1. Return ResolveBinding(StringValue(Identifier)).
IdentifierReference : yield
  1. Return ResolveBinding("yield").

NOTE 1: The result of evaluating an IdentifierReference is always a value of type Reference.

NOTE 2: In non-strict code, the keyword yield may be used as an identifier. Evaluating the IdentifierReference production resolves the binding of yield as if it was an Identifier. Early Error restriction ensures that such an evaluation only can occur for non-strict code. See 13.2.1 for the handling of yield in binding creation contexts.

12.2 Primary Expression

Syntax

PrimaryExpression[Yield] :
this
IdentifierReference[?Yield]
Literal
ArrayInitializer[?Yield]
ObjectLiteral[?Yield]
FunctionExpression
ClassExpression
GeneratorExpression
GeneratorComprehension[?Yield]
RegularExpressionLiteral
TemplateLiteral[?Yield]
CoverParenthesizedExpressionAndArrowParameterList[?Yield]
CoverParenthesizedExpressionAndArrowParameterList[Yield] :
( Expression[In, ?Yield] )
( )
( ... BindingIdentifier[?Yield] )
( Expression[In, ?Yield] , ... BindingIdentifier[?Yield] )

Supplemental Syntax

When processing the production

PrimaryExpression[Yield] : CoverParenthesizedExpressionAndArrowParameterList[?Yield]
the interpretation of CoverParenthesizedExpressionAndArrowParameterList is refined using the following grammar:

ParenthesizedExpression[Yield] :
( Expression[In, ?Yield] )

12.2.0 Semantics

12.2.0.1 Static Semantics: CoveredParenthesizedExpression

CoverParenthesizedExpressionAndArrowParameterList[Yield] : ( Expression[In, ?Yield] )
  1. Return the result of parsing the lexical token stream matched by CoverParenthesizedExpressionAndArrowParameterList[Yield] using either ParenthesizedExpression or ParenthesizedExpression[Yield] as the goal symbol depending upon whether the [Yield] grammar parameter was present when CoverParenthesizedExpressionAndArrowParameterList was matched.

12.2.0.2 Static Semantics: IsFunctionDefinition

See also: 12.2.10.2, 12.3.1.2, 12.4.2, 12.5.2, 12.6.1, 12.7.1, 12.8.1, 12.9.1, 12.10.1, 12.11.1, 12.12.1, 12.13.1, 12.14.2, 12.15.1, 14.1.11, 14.4.8, 14.5.8.

PrimaryExpression :
this
IdentifierReference
Literal
ArrayInitializer
ObjectLiteral
GeneratorComprehension
RegularExpressionLiteral
TemplateLiteral
  1. Return false.
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
  2. Return IsFunctionDefinition of expr.

12.2.0.3 Static Semantics: IsIdentifierRef

See also: 12.3.1.3.

PrimaryExpression :
IdentifierReference
  1. Return true.
PrimaryExpression :
this
Literal
ArrayInitializer
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
GeneratorComprehension
RegularExpressionLiteral
TemplateLiteral
CoverParenthesizedExpressionAndArrowParameterList
  1. Return false.

12.2.0.4 Static Semantics: IsValidSimpleAssignmentTarget

See also: 12.2.10.3, 12.3.1.3, 12.4.3, 12.5.3, 12.6.2, 12.7.2, 12.8.2, 12.9.2, 12.10.2, 12.11.2, 12.12.2, 12.13.2, 12.14.3, 12.15.2.

PrimaryExpression :
this
Literal
ArrayInitializer
ObjectLiteral
FunctionExpression
ClassExpression
GeneratorExpression
GeneratorComprehension
RegularExpressionLiteral
TemplateLiteral
  1. Return false.
PrimaryExpression : IdentifierReference
  1. If this PrimaryExpression is contained in strict code and StringValue of IdentifierReference is "eval" or "arguments", then return false.
  2. Return true.
PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be CoveredParenthesizedExpression of CoverParenthesizedExpressionAndArrowParameterList.
  2. Return IsValidSimpleAssignmentTarget of expr.

12.2.1 The this Keyword

12.2.1.1 Runtime Semantics: Evaluation

PrimaryExpression : this
  1. Return ResolveThisBinding( ) .

12.2.2 Identifier Reference

See 12.1 for PrimaryExpression : IdentifierReference .

12.2.3 Literals

Syntax

Literal :
NullLiteral
ValueLiteral
ValueLiteral :
BooleanLiteral
NumericLiteral
StringLiteral

12.2.3.1 Runtime Semantics: Evaluation

Literal : NullLiteral
  1. Return null.
ValueLiteral : BooleanLiteral
  1. Return false if BooleanLiteral is the token false.
  2. Return true if BooleanLiteral is the token true.
ValueLiteral : NumericLiteral
  1. Return the number whose value is MV of NumericLiteral as defined in 11.8.3.
ValueLiteral : StringLiteral
  1. Return the StringValue of StringLiteral as defined in 11.8.4.2.

12.2.4 Array Initializer

Syntax

ArrayInitializer[Yield] :
ArrayLiteral[?Yield]
ArrayComprehension[?Yield]

12.2.4.1 Array Literal

NOTE An ArrayLiteral is an expression describing the initialization of an Array object, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

Syntax

ArrayLiteral[Yield] :
[ Elisionopt ]
[ ElementList[?Yield] ]
[ ElementList[?Yield] , Elisionopt ]
ElementList[Yield] :
Elisionopt AssignmentExpression[In, ?Yield]
Elisionopt SpreadElement[?Yield]
ElementList[?Yield] , Elisionopt AssignmentExpression[In, ?Yield]
ElementList[?Yield] , Elisionopt SpreadElement[?Yield]
Elision :
,
Elision ,
SpreadElement[Yield] :
... AssignmentExpression[In, ?Yield]

12.2.4.1.1 Static Semantics: ElisionWidth

Elision : ,
  1. Return the numeric value 1.
Elision : Elision ,
  1. Let preceding be the ElisionWidth of Elision.
  2. Return preceding+1.

12.2.4.1.2 Runtime Semantics: ArrayAccumulation

With parameters array and nextIndex.

ElementList : Elisionopt AssignmentExpression
  1. Let padding be the ElisionWidth of Elision; if Elision is not present, use the numeric value zero.
  2. Let initResult be the result of evaluating AssignmentExpression.
  3. Let initValue be GetValue(initResult).
  4. ReturnIfAbrupt(initValue).
  5. Let created be the result of calling the [[DefineOwnProperty]] internal method of array with arguments ToString(ToUint32(nextIndex+padding)) and