ECMAScript® 2021 Language Specification

Draft ECMA-262 / March 6, 2021

8.5 Miscellaneous

These operations are used in multiple places throughout the specification.

8.5.1 Runtime Semantics: InstantiateFunctionObject

With parameter scope.

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody }
  1. Return ? InstantiateOrdinaryFunctionObject of FunctionDeclaration with argument scope.
 GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody }
  1. Return ? InstantiateGeneratorFunctionObject of GeneratorDeclaration with argument scope.
 AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody }
  1. Return ? InstantiateAsyncGeneratorFunctionObject of AsyncGeneratorDeclaration with argument scope.
 AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody }
  1. Return ? InstantiateAsyncFunctionObject of AsyncFunctionDeclaration with argument scope.

8.5.2 Runtime Semantics: BindingInitialization

With parameters value and environment.

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

8.5.2.1 InitializeBoundName ( name, value, environment )

The abstract operation InitializeBoundName takes arguments name, value, and environment. It performs the following steps when called:

  1. Assert: Type(name) is String.
  2. If environment is not undefined, then
    1. Perform environment.InitializeBinding(name, value).
    2. Return NormalCompletion(undefined).
  3. Else,
    1. Let lhs be ResolveBinding(name).
    2. Return ? PutValue(lhs, value).

8.5.3 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

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.

 ArrayBindingPattern : [ ]
  1. Return NormalCompletion(empty).
 ArrayBindingPattern : [ Elision ]
  1. Return the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
 ArrayBindingPattern : [ Elisionopt BindingRestElement ]
  1. If Elision is present, then
    1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
  2. Return the result of performing IteratorBindingInitialization for BindingRestElement with iteratorRecord and environment as arguments.
 ArrayBindingPattern : [ BindingElementList , Elision ]
  1. Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
  2. Return the result of performing IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
 ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
  1. Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
  2. If Elision is present, then
    1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
  3. Return the result of performing IteratorBindingInitialization for BindingRestElement with iteratorRecord and environment as arguments.
 BindingElementList : BindingElementList , BindingElisionElement
  1. Perform ? IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
  2. Return the result of performing IteratorBindingInitialization for BindingElisionElement using iteratorRecord and environment as arguments.
 BindingElisionElement : Elision BindingElement
  1. Perform ? IteratorDestructuringAssignmentEvaluation of Elision with iteratorRecord as the argument.
  2. Return the result of performing IteratorBindingInitialization of BindingElement with iteratorRecord and environment as the arguments.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId, environment).
  3. If iteratorRecord.[[Done]] is false, then
    1. Let next be 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.
    5. Else,
      1. Let v be IteratorValue(next).
      2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(v).
  4. If iteratorRecord.[[Done]] is true, let v be undefined.
  5. If Initializer is present and v is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) is true, then
      1. Set v to the result of performing NamedEvaluation for Initializer with argument bindingId.
    2. Else,
      1. Let defaultValue be the result of evaluating 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. If iteratorRecord.[[Done]] is false, then
    1. Let next be 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.
    5. Else,
      1. Let v be IteratorValue(next).
      2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
      3. ReturnIfAbrupt(v).
  2. If iteratorRecord.[[Done]] is true, let v be undefined.
  3. If Initializer is present and v is undefined, then
    1. Let defaultValue be the result of evaluating Initializer.
    2. Set v to ? GetValue(defaultValue).
  4. Return the result of performing BindingInitialization of BindingPattern with v and environment as the arguments.
 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 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 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 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 the result of performing BindingInitialization of BindingPattern with A and environment as the arguments.
    3. Let nextValue be 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 NormalCompletion(empty).
 FormalParameters : FormalParameterList , FunctionRestParameter
  1. Perform ? IteratorBindingInitialization for FormalParameterList using iteratorRecord and environment as the arguments.
  2. Return the result of performing IteratorBindingInitialization for FunctionRestParameter using iteratorRecord and environment as the arguments.
 FormalParameterList : FormalParameterList , FormalParameter
  1. Perform ? IteratorBindingInitialization for FormalParameterList using iteratorRecord and environment as the arguments.
  2. Return the result of performing IteratorBindingInitialization for FormalParameter using iteratorRecord and environment as the arguments.
 ArrowParameters : BindingIdentifier
  1. Assert: iteratorRecord.[[Done]] is false.
  2. Let next be IteratorStep(iteratorRecord).
  3. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  4. ReturnIfAbrupt(next).
  5. If next is false, set iteratorRecord.[[Done]] to true.
  6. Else,
    1. Let v be IteratorValue(next).
    2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(v).
  7. If iteratorRecord.[[Done]] is true, let v be undefined.
  8. Return the result of performing BindingInitialization for BindingIdentifier using v and environment as the arguments.
 ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be CoveredFormalsList of CoverParenthesizedExpressionAndArrowParameterList.
  2. Return IteratorBindingInitialization of formals with arguments iteratorRecord and environment.
 AsyncArrowBindingIdentifier : BindingIdentifier
  1. Assert: iteratorRecord.[[Done]] is false.
  2. Let next be IteratorStep(iteratorRecord).
  3. If next is an abrupt completion, set iteratorRecord.[[Done]] to true.
  4. ReturnIfAbrupt(next).
  5. If next is false, set iteratorRecord.[[Done]] to true.
  6. Else,
    1. Let v be IteratorValue(next).
    2. If v is an abrupt completion, set iteratorRecord.[[Done]] to true.
    3. ReturnIfAbrupt(v).
  7. If iteratorRecord.[[Done]] is true, let v be undefined.
  8. Return the result of performing BindingInitialization for BindingIdentifier using v and environment as the arguments.

8.5.4 Static Semantics: AssignmentTargetType

IdentifierReference : Identifier
  1. If this IdentifierReference is contained in strict mode code and StringValue of Identifier is "eval" or "arguments", return invalid.
  2. Return simple.
 IdentifierReference : yield await CallExpression : CallExpression [ Expression ] CallExpression . IdentifierName MemberExpression : MemberExpression [ Expression ] MemberExpression . IdentifierName SuperProperty
  1. Return simple.
 PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be CoveredParenthesizedExpression of 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 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.

8.5.5 Static Semantics: PropName

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 : PropertyName ( UniqueFormalParameters ) { FunctionBody } get PropertyName ( ) { FunctionBody } set PropertyName ( PropertySetParameterList ) { FunctionBody }
  1. Return PropName of PropertyName.
 GeneratorMethod : * PropertyName ( UniqueFormalParameters ) { GeneratorBody }
  1. Return PropName of PropertyName.
 AsyncGeneratorMethod : async * PropertyName ( UniqueFormalParameters ) { AsyncGeneratorBody }
  1. Return PropName of PropertyName.
 ClassElement : ;
  1. Return empty.
 AsyncMethod : async PropertyName ( UniqueFormalParameters ) { AsyncFunctionBody }
  1. Return PropName of PropertyName.