ECMAScript® 2024 Language Specification

Draft ECMA-262 / December 6, 2023

8.6 Miscellaneous

These operations are used in multiple places throughout the specification.

8.6.1 Runtime Semantics: InstantiateFunctionObject

The syntax-directed operation InstantiateFunctionObject takes arguments env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and returns an ECMAScript function object. It is defined piecewise over the following productions:

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody }
  1. Return InstantiateOrdinaryFunctionObject of FunctionDeclaration with arguments env and privateEnv.
 GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody }
  1. Return InstantiateGeneratorFunctionObject of GeneratorDeclaration with arguments env and privateEnv.
 AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody }
  1. Return InstantiateAsyncGeneratorFunctionObject of AsyncGeneratorDeclaration with arguments env and privateEnv.
 AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody }
  1. Return InstantiateAsyncFunctionObject of AsyncFunctionDeclaration with arguments env and privateEnv.

8.6.2 Runtime Semantics: BindingInitialization

The syntax-directed operation BindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

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 10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

It is defined piecewise over the following productions:

BindingIdentifier : Identifier
  1. Let name be StringValue of Identifier.
  2. Return ? InitializeBoundName(name, value, environment).
 BindingIdentifier : yield
  1. Return ? InitializeBoundName("yield", value, environment).
 BindingIdentifier : await
  1. Return ? InitializeBoundName("await", value, environment).
 BindingPattern : ObjectBindingPattern
  1. Perform ? RequireObjectCoercible(value).
  2. Return ? BindingInitialization of ObjectBindingPattern with arguments value and environment.
 BindingPattern : ArrayBindingPattern
  1. Let iteratorRecord be ? GetIterator(value, sync).
  2. Let result be Completion(IteratorBindingInitialization of ArrayBindingPattern with arguments iteratorRecord and environment).
  3. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
  4. Return ? result.
 ObjectBindingPattern : { }
  1. Return unused.
 ObjectBindingPattern : { BindingPropertyList } { BindingPropertyList , }
  1. Perform ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Return unused.
 ObjectBindingPattern : { BindingRestProperty }
  1. Let excludedNames be a new empty List.
  2. Return ? RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.
 ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
  1. Let excludedNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Return ? RestBindingInitialization of BindingRestProperty with arguments value, environment, and excludedNames.

8.6.2.1 InitializeBoundName ( name, value, environment )

The abstract operation InitializeBoundName takes arguments name (a String), value (an ECMAScript language value), and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion. It performs the following steps when called:

  1. If environment is not undefined, then
    1. Perform ! environment.InitializeBinding(name, value).
    2. Return unused.
  2. Else,
    1. Let lhs be ? ResolveBinding(name).
    2. Return ? PutValue(lhs, value).

8.6.3 Runtime Semantics: IteratorBindingInitialization

The syntax-directed operation IteratorBindingInitialization takes arguments iteratorRecord (an Iterator Record) and environment (an Environment Record or undefined) and returns either a normal completion containing unused or an abrupt completion.

Note

When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

It is defined piecewise over the following productions:

ArrayBindingPattern : [ ]
  1. Return unused.
 ArrayBindingPattern : [ Elision ]
  1. Return ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
 ArrayBindingPattern : [ Elisionopt BindingRestElement ]
  1. If Elision is present, then
    1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  2. Return ? IteratorBindingInitialization of BindingRestElement with arguments iteratorRecord and environment.
 ArrayBindingPattern : [ BindingElementList , Elision ]
  1. Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
  2. Return ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
 ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
  1. Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
  2. If Elision is present, then
    1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  3. Return ? IteratorBindingInitialization of BindingRestElement with arguments iteratorRecord and environment.
 BindingElementList : BindingElementList , BindingElisionElement
  1. Perform ? IteratorBindingInitialization of BindingElementList with arguments iteratorRecord and environment.
  2. Return ? IteratorBindingInitialization of BindingElisionElement with arguments iteratorRecord and environment.
 BindingElisionElement : Elision BindingElement
  1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with argument iteratorRecord.
  2. Return ? IteratorBindingInitialization of BindingElement with arguments iteratorRecord and environment.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId, environment).
  3. Let v be undefined.
  4. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
    5. Else,
      1. Set v to Completion(IteratorValue(next)).
      2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(v).
  5. If Initializer is present and v is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) is true, then
      1. Set v to ? NamedEvaluation of Initializer with argument bindingId.
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Set v to ? GetValue(defaultValue).
  6. If environment is undefined, return ? PutValue(lhs, v).
  7. Return ? InitializeReferencedBinding(lhs, v).
 BindingElement : BindingPattern Initializeropt
  1. Let v be undefined.
  2. If iteratorRecord.[[Done]] is false, then
    1. Let next be Completion(IteratorStep(iteratorRecord)).
    2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(next).
    4. If next is false, then
      1. Set iteratorRecord.[[Done]] to true.
    5. Else,
      1. Set v to Completion(IteratorValue(next)).
      2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(v).
  3. If Initializer is present and v is undefined, then
    1. Let defaultValue be ? Evaluation of Initializer.
    2. Set v to ? GetValue(defaultValue).
  4. Return ? BindingInitialization of BindingPattern with arguments v and environment.
 BindingRestElement : ... BindingIdentifier
  1. Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
  2. Let A be ! ArrayCreate(0).
  3. Let n be 0.
  4. Repeat,
    1. If iteratorRecord.[[Done]] is false, then
      1. Let next be Completion(IteratorStep(iteratorRecord)).
      2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(next).
      4. If next is false, set iteratorRecord.[[Done]] to true.
    2. If iteratorRecord.[[Done]] is true, then
      1. If environment is undefined, return ? PutValue(lhs, A).
      2. Return ? InitializeReferencedBinding(lhs, A).
    3. Let nextValue be Completion(IteratorValue(next)).
    4. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    5. ReturnIfAbrupt(nextValue).
    6. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), nextValue).
    7. Set n to n + 1.
 BindingRestElement : ... BindingPattern
  1. Let A be ! ArrayCreate(0).
  2. Let n be 0.
  3. Repeat,
    1. If iteratorRecord.[[Done]] is false, then
      1. Let next be Completion(IteratorStep(iteratorRecord)).
      2. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(next).
      4. If next is false, set iteratorRecord.[[Done]] to true.
    2. If iteratorRecord.[[Done]] is true, then
      1. Return ? BindingInitialization of BindingPattern with arguments A and environment.
    3. Let nextValue be Completion(IteratorValue(next)).
    4. If nextValue is an abrupt completion, set iteratorRecord.[[Done]] to true.
    5. ReturnIfAbrupt(nextValue).
    6. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), nextValue).
    7. Set n to n + 1.
 FormalParameters : [empty]
  1. Return unused.
 FormalParameters : FormalParameterList , FunctionRestParameter
  1. Perform ? IteratorBindingInitialization of FormalParameterList with arguments iteratorRecord and environment.
  2. Return ? IteratorBindingInitialization of FunctionRestParameter with arguments iteratorRecord and environment.
 FormalParameterList : FormalParameterList , FormalParameter
  1. Perform ? IteratorBindingInitialization of FormalParameterList with arguments iteratorRecord and environment.
  2. Return ? IteratorBindingInitialization of FormalParameter with arguments iteratorRecord and environment.
 ArrowParameters : BindingIdentifier
  1. Let v be undefined.
  2. Assert: iteratorRecord.[[Done]] is false.
  3. Let next be Completion(IteratorStep(iteratorRecord)).
  4. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  5. ReturnIfAbrupt(next).
  6. If next is false, then
    1. Set iteratorRecord.[[Done]] to true.
  7. Else,
    1. Set v to Completion(IteratorValue(next)).
    2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(v).
  8. Return ? BindingInitialization of BindingIdentifier with arguments v and environment.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return ? IteratorBindingInitialization of formals with arguments iteratorRecord and environment.
 AsyncArrowBindingIdentifier : BindingIdentifier
  1. Let v be undefined.
  2. Assert: iteratorRecord.[[Done]] is false.
  3. Let next be Completion(IteratorStep(iteratorRecord)).
  4. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  5. ReturnIfAbrupt(next).
  6. If next is false, then
    1. Set iteratorRecord.[[Done]] to true.
  7. Else,
    1. Set v to Completion(IteratorValue(next)).
    2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(v).
  8. Return ? BindingInitialization of BindingIdentifier with arguments v and environment.

8.6.4 Static Semantics: AssignmentTargetType

The syntax-directed operation AssignmentTargetType takes no arguments and returns simple or invalid. It is defined piecewise over the following productions:

IdentifierReference : Identifier
  1. If this IdentifierReference is contained in strict mode code and StringValue of Identifier is either "eval" or "arguments", return invalid.
  2. Return simple.
 IdentifierReference : yield await CallExpression : CallExpression [ Expression ] CallExpression . IdentifierName CallExpression . PrivateIdentifier MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName SuperProperty MemberExpression . PrivateIdentifier
  1. Return simple.
 PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be the ParenthesizedExpression that is covered by CoverParenthesizedExpressionAndArrowParameterList.
  2. Return AssignmentTargetType of expr.
 PrimaryExpression : this Literal ArrayLiteral ObjectLiteral FunctionExpression ClassExpression GeneratorExpression AsyncFunctionExpression AsyncGeneratorExpression RegularExpressionLiteral TemplateLiteral CallExpression : CoverCallExpressionAndAsyncArrowHead SuperCall ImportCall CallExpression Arguments CallExpression TemplateLiteral NewExpression : new NewExpression MemberExpression : MemberExpression TemplateLiteral new MemberExpression Arguments NewTarget : new . target ImportMeta : import . meta LeftHandSideExpression : OptionalExpression UpdateExpression : LeftHandSideExpression ++ LeftHandSideExpression -- ++ UnaryExpression -- UnaryExpression UnaryExpression : delete UnaryExpression void UnaryExpression typeof UnaryExpression + UnaryExpression - UnaryExpression ~ UnaryExpression ! UnaryExpression AwaitExpression ExponentiationExpression : UpdateExpression ** ExponentiationExpression MultiplicativeExpression : MultiplicativeExpression MultiplicativeOperator ExponentiationExpression AdditiveExpression : AdditiveExpression + MultiplicativeExpression AdditiveExpression - MultiplicativeExpression ShiftExpression : ShiftExpression << AdditiveExpression ShiftExpression >> AdditiveExpression ShiftExpression >>> AdditiveExpression RelationalExpression : RelationalExpression < ShiftExpression RelationalExpression > ShiftExpression RelationalExpression <= ShiftExpression RelationalExpression >= ShiftExpression RelationalExpression instanceof ShiftExpression RelationalExpression in ShiftExpression PrivateIdentifier in ShiftExpression EqualityExpression : EqualityExpression == RelationalExpression EqualityExpression != RelationalExpression EqualityExpression === RelationalExpression EqualityExpression !== RelationalExpression BitwiseANDExpression : BitwiseANDExpression & EqualityExpression BitwiseXORExpression : BitwiseXORExpression ^ BitwiseANDExpression BitwiseORExpression : BitwiseORExpression | BitwiseXORExpression LogicalANDExpression : LogicalANDExpression && BitwiseORExpression LogicalORExpression : LogicalORExpression || LogicalANDExpression CoalesceExpression : CoalesceExpressionHead ?? BitwiseORExpression ConditionalExpression : ShortCircuitExpression ? AssignmentExpression : AssignmentExpression AssignmentExpression : YieldExpression ArrowFunction AsyncArrowFunction LeftHandSideExpression = AssignmentExpression LeftHandSideExpression AssignmentOperator AssignmentExpression LeftHandSideExpression &&= AssignmentExpression LeftHandSideExpression ||= AssignmentExpression LeftHandSideExpression ??= AssignmentExpression Expression : Expression , AssignmentExpression
  1. Return invalid.

8.6.5 Static Semantics: PropName

The syntax-directed operation PropName takes no arguments and returns a String or empty. It is defined piecewise over the following productions:

PropertyDefinition : IdentifierReference
  1. Return StringValue of IdentifierReference.
 PropertyDefinition : ... AssignmentExpression
  1. Return empty.
 PropertyDefinition : PropertyName : AssignmentExpression
  1. Return PropName of PropertyName.
 LiteralPropertyName : IdentifierName
  1. Return StringValue of IdentifierName.
 LiteralPropertyName : StringLiteral
  1. Return the SV of StringLiteral.
 LiteralPropertyName : NumericLiteral
  1. Let nbr be the NumericValue of NumericLiteral.
  2. Return ! ToString(nbr).
 ComputedPropertyName : [ AssignmentExpression ]
  1. Return empty.
 MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody } get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody }
  1. Return PropName of ClassElementName.
 GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody }
  1. Return PropName of ClassElementName.
 AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }
  1. Return PropName of ClassElementName.
 ClassElement : ClassStaticBlock
  1. Return empty.
 ClassElement : ;
  1. Return empty.
 AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }
  1. Return PropName of ClassElementName.
 FieldDefinition : ClassElementName Initializeropt
  1. Return PropName of ClassElementName.
 ClassElementName : PrivateIdentifier
  1. Return empty.