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13 ECMAScript Language: Statements and Declarations

Syntax

Statement[Yield, Await, Return] : BlockStatement[?Yield, ?Await, ?Return] VariableStatement[?Yield, ?Await] EmptyStatement ExpressionStatement[?Yield, ?Await] IfStatement[?Yield, ?Await, ?Return] BreakableStatement[?Yield, ?Await, ?Return] ContinueStatement[?Yield, ?Await] BreakStatement[?Yield, ?Await] [+Return] ReturnStatement[?Yield, ?Await] WithStatement[?Yield, ?Await, ?Return] LabelledStatement[?Yield, ?Await, ?Return] ThrowStatement[?Yield, ?Await] TryStatement[?Yield, ?Await, ?Return] DebuggerStatement Declaration[Yield, Await] : HoistableDeclaration[?Yield, ?Await, ~Default] ClassDeclaration[?Yield, ?Await, ~Default] LexicalDeclaration[+In, ?Yield, ?Await] HoistableDeclaration[Yield, Await, Default] : FunctionDeclaration[?Yield, ?Await, ?Default] GeneratorDeclaration[?Yield, ?Await, ?Default] AsyncFunctionDeclaration[?Yield, ?Await, ?Default] AsyncGeneratorDeclaration[?Yield, ?Await, ?Default] BreakableStatement[Yield, Await, Return] : IterationStatement[?Yield, ?Await, ?Return] SwitchStatement[?Yield, ?Await, ?Return]

13.1 Statement Semantics

13.1.1 Static Semantics: ContainsDuplicateLabels

13.1.2 Static Semantics: ContainsUndefinedBreakTarget

13.1.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Statement : VariableStatement EmptyStatement ExpressionStatement BreakStatement ReturnStatement ThrowStatement DebuggerStatement
  1. Return false.
 BreakableStatement : IterationStatement
  1. Let newIterationSet be a copy of iterationSet with all the elements of labelSet appended.
  2. Return ContainsUndefinedContinueTarget of IterationStatement with arguments newIterationSet and « ».

13.1.4 Static Semantics: DeclarationPart

13.1.5 Static Semantics: VarDeclaredNames

13.1.6 Static Semantics: VarScopedDeclarations

13.1.7 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

BreakableStatement : IterationStatement
  1. Let stmtResult be LabelledEvaluation of IterationStatement with argument labelSet.
  2. If stmtResult.[[Type]] is break, then
    1. If stmtResult.[[Target]] is empty, then
      1. If stmtResult.[[Value]] is empty, set stmtResult to NormalCompletion(undefined).
      2. Else, set stmtResult to NormalCompletion(stmtResult.[[Value]]).
  3. Return Completion(stmtResult).
 BreakableStatement : SwitchStatement
  1. Let stmtResult be the result of evaluating SwitchStatement.
  2. If stmtResult.[[Type]] is break, then
    1. If stmtResult.[[Target]] is empty, then
      1. If stmtResult.[[Value]] is empty, set stmtResult to NormalCompletion(undefined).
      2. Else, set stmtResult to NormalCompletion(stmtResult.[[Value]]).
  3. Return Completion(stmtResult).
 Note

A BreakableStatement is one that can be exited via an unlabelled BreakStatement.

13.1.8 Runtime Semantics: Evaluation

HoistableDeclaration : GeneratorDeclaration AsyncFunctionDeclaration AsyncGeneratorDeclaration
  1. Return NormalCompletion(empty).
 HoistableDeclaration : FunctionDeclaration
  1. Return the result of evaluating FunctionDeclaration.
 BreakableStatement : IterationStatement SwitchStatement
  1. Let newLabelSet be a new empty List.
  2. Return the result of performing LabelledEvaluation of this BreakableStatement with argument newLabelSet.

13.2 Block

Syntax

BlockStatement[Yield, Await, Return] : Block[?Yield, ?Await, ?Return] Block[Yield, Await, Return] : { StatementList[?Yield, ?Await, ?Return]opt } StatementList[Yield, Await, Return] : StatementListItem[?Yield, ?Await, ?Return] StatementList[?Yield, ?Await, ?Return] StatementListItem[?Yield, ?Await, ?Return] StatementListItem[Yield, Await, Return] : Statement[?Yield, ?Await, ?Return] Declaration[?Yield, ?Await]

13.2.1 Static Semantics: Early Errors

Block : { StatementList }
  • It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate entries.
  • It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList also occurs in the VarDeclaredNames of StatementList.

13.2.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

Block : { }
  1. Return false.
 StatementList : StatementList StatementListItem
  1. Let hasDuplicates be ContainsDuplicateLabels of StatementList with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of StatementListItem with argument labelSet.
 StatementListItem : Declaration
  1. Return false.

13.2.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

Block : { }
  1. Return false.
 StatementList : StatementList StatementListItem
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of StatementList with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of StatementListItem with argument labelSet.
 StatementListItem : Declaration
  1. Return false.

13.2.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Block : { }
  1. Return false.
 StatementList : StatementList StatementListItem
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of StatementListItem with arguments iterationSet and « ».
 StatementListItem : Declaration
  1. Return false.

13.2.5 Static Semantics: LexicallyDeclaredNames

Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let names be LexicallyDeclaredNames of StatementList.
  2. Append to names the elements of the LexicallyDeclaredNames of StatementListItem.
  3. Return names.
 StatementListItem : Statement
  1. If Statement is Statement : LabelledStatement , return LexicallyDeclaredNames of LabelledStatement.
  2. Return a new empty List.
 StatementListItem : Declaration
  1. Return the BoundNames of Declaration.

13.2.6 Static Semantics: LexicallyScopedDeclarations

StatementList : StatementList StatementListItem
  1. Let declarations be LexicallyScopedDeclarations of StatementList.
  2. Append to declarations the elements of the LexicallyScopedDeclarations of StatementListItem.
  3. Return declarations.
 StatementListItem : Statement
  1. If Statement is Statement : LabelledStatement , return LexicallyScopedDeclarations of LabelledStatement.
  2. Return a new empty List.
 StatementListItem : Declaration
  1. Return a new List containing DeclarationPart of Declaration.

13.2.7 Static Semantics: TopLevelLexicallyDeclaredNames

StatementList : StatementList StatementListItem
  1. Let names be TopLevelLexicallyDeclaredNames of StatementList.
  2. Append to names the elements of the TopLevelLexicallyDeclaredNames of StatementListItem.
  3. Return names.
 StatementListItem : Statement
  1. Return a new empty List.
 StatementListItem : Declaration
  1. If Declaration is Declaration : HoistableDeclaration , then
    1. Return « ».
  2. Return the BoundNames of Declaration.
 Note

At the top level of a function, or script, function declarations are treated like var declarations rather than like lexical declarations.

13.2.8 Static Semantics: TopLevelLexicallyScopedDeclarations

Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let declarations be TopLevelLexicallyScopedDeclarations of StatementList.
  2. Append to declarations the elements of the TopLevelLexicallyScopedDeclarations of StatementListItem.
  3. Return declarations.
 StatementListItem : Statement
  1. Return a new empty List.
 StatementListItem : Declaration
  1. If Declaration is Declaration : HoistableDeclaration , then
    1. Return « ».
  2. Return a new List containing Declaration.

13.2.9 Static Semantics: TopLevelVarDeclaredNames

Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let names be TopLevelVarDeclaredNames of StatementList.
  2. Append to names the elements of the TopLevelVarDeclaredNames of StatementListItem.
  3. Return names.
 StatementListItem : Declaration
  1. If Declaration is Declaration : HoistableDeclaration , then
    1. Return the BoundNames of HoistableDeclaration.
  2. Return a new empty List.
 StatementListItem : Statement
  1. If Statement is Statement : LabelledStatement , return TopLevelVarDeclaredNames of Statement.
  2. Return VarDeclaredNames of Statement.
 Note

At the top level of a function or script, inner function declarations are treated like var declarations.

13.2.10 Static Semantics: TopLevelVarScopedDeclarations

Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let declarations be TopLevelVarScopedDeclarations of StatementList.
  2. Append to declarations the elements of the TopLevelVarScopedDeclarations of StatementListItem.
  3. Return declarations.
 StatementListItem : Statement
  1. If Statement is Statement : LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
  2. Return VarScopedDeclarations of Statement.
 StatementListItem : Declaration
  1. If Declaration is Declaration : HoistableDeclaration , then
    1. Let declaration be DeclarationPart of HoistableDeclaration.
    2. Return « declaration ».
  2. Return a new empty List.

13.2.11 Static Semantics: VarDeclaredNames

Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let names be VarDeclaredNames of StatementList.
  2. Append to names the elements of the VarDeclaredNames of StatementListItem.
  3. Return names.
 StatementListItem : Declaration
  1. Return a new empty List.

13.2.12 Static Semantics: VarScopedDeclarations

Block : { }
  1. Return a new empty List.
 StatementList : StatementList StatementListItem
  1. Let declarations be VarScopedDeclarations of StatementList.
  2. Append to declarations the elements of the VarScopedDeclarations of StatementListItem.
  3. Return declarations.
 StatementListItem : Declaration
  1. Return a new empty List.

13.2.13 Runtime Semantics: Evaluation

Block : { }
  1. Return NormalCompletion(empty).
 Block : { StatementList }
  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. Let blockEnv be NewDeclarativeEnvironment(oldEnv).
  3. Perform BlockDeclarationInstantiation(StatementList, blockEnv).
  4. Set the running execution context's LexicalEnvironment to blockEnv.
  5. Let blockValue be the result of evaluating StatementList.
  6. Set the running execution context's LexicalEnvironment to oldEnv.
  7. Return blockValue.
 Note 1

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

 StatementList : StatementList StatementListItem
  1. Let sl be the result of evaluating StatementList.
  2. ReturnIfAbrupt(sl).
  3. Let s be the result of evaluating StatementListItem.
  4. Return Completion(UpdateEmpty(s, sl)).
 Note 2

The value of a StatementList is the value of the last value-producing item in the StatementList. For example, the following calls to the eval function all return the value 1:


          eval("1;;;;;")
          eval("1;{}")
          eval("1;var a;")

13.2.14 Runtime Semantics: BlockDeclarationInstantiation ( code, env )

Note

When a Block or CaseBlock is evaluated a new declarative Environment Record is created and bindings for each block scoped variable, constant, function, or class declared in the block are instantiated in the Environment Record.

BlockDeclarationInstantiation is performed as follows using arguments code and env. code is the Parse Node corresponding to the body of the block. env is the Lexical Environment in which bindings are to be created.

  1. Let envRec be env's EnvironmentRecord.
  2. Assert: envRec is a declarative Environment Record.
  3. Let declarations be the LexicallyScopedDeclarations of code.
  4. For each element d in declarations, do
    1. For each element dn of the BoundNames of d, do
      1. If IsConstantDeclaration of d is true, then
        1. Perform ! envRec.CreateImmutableBinding(dn, true).
      2. Else,
        1. Perform ! envRec.CreateMutableBinding(dn, false).
    2. If d is a FunctionDeclaration, a GeneratorDeclaration, an AsyncFunctionDeclaration, or an AsyncGeneratorDeclaration, then
      1. Let fn be the sole element of the BoundNames of d.
      2. Let fo be InstantiateFunctionObject of d with argument env.
      3. Perform envRec.InitializeBinding(fn, fo).

13.3 Declarations and the Variable Statement

13.3.1 Let and Const Declarations

Note

let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Lexical Environment is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created. If a LexicalBinding in a let declaration does not have an Initializer the variable is assigned the value undefined when the LexicalBinding is evaluated.

Syntax

LexicalDeclaration[In, Yield, Await] : LetOrConst BindingList[?In, ?Yield, ?Await] ; LetOrConst : let const BindingList[In, Yield, Await] : LexicalBinding[?In, ?Yield, ?Await] BindingList[?In, ?Yield, ?Await] , LexicalBinding[?In, ?Yield, ?Await] LexicalBinding[In, Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]

13.3.1.1 Static Semantics: Early Errors

LexicalDeclaration : LetOrConst BindingList ;
  • It is a Syntax Error if the BoundNames of BindingList contains "let".
  • It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.
LexicalBinding : BindingIdentifier Initializeropt

13.3.1.2 Static Semantics: BoundNames

LexicalDeclaration : LetOrConst BindingList ;
  1. Return the BoundNames of BindingList.
 BindingList : BindingList , LexicalBinding
  1. Let names be the BoundNames of BindingList.
  2. Append to names the elements of the BoundNames of LexicalBinding.
  3. Return names.
 LexicalBinding : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 LexicalBinding : BindingPattern Initializer
  1. Return the BoundNames of BindingPattern.

13.3.1.3 Static Semantics: IsConstantDeclaration

LexicalDeclaration : LetOrConst BindingList ;
  1. Return IsConstantDeclaration of LetOrConst.
 LetOrConst : let
  1. Return false.
 LetOrConst : const
  1. Return true.

13.3.1.4 Runtime Semantics: Evaluation

LexicalDeclaration : LetOrConst BindingList ;
  1. Let next be the result of evaluating BindingList.
  2. ReturnIfAbrupt(next).
  3. Return NormalCompletion(empty).
 BindingList : BindingList , LexicalBinding
  1. Let next be the result of evaluating BindingList.
  2. ReturnIfAbrupt(next).
  3. Return the result of evaluating LexicalBinding.
 LexicalBinding : BindingIdentifier
  1. Let lhs be ResolveBinding(StringValue of BindingIdentifier).
  2. Return InitializeReferencedBinding(lhs, undefined).
 Note

A static semantics rule ensures that this form of LexicalBinding never occurs in a const declaration.

 LexicalBinding : BindingIdentifier Initializer
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be the result of evaluating Initializer.
    2. Let value be ? GetValue(rhs).
  5. Return InitializeReferencedBinding(lhs, value).
 LexicalBinding : BindingPattern Initializer
  1. Let rhs be the result of evaluating Initializer.
  2. Let value be ? GetValue(rhs).
  3. Let env be the running execution context's LexicalEnvironment.
  4. Return the result of performing BindingInitialization for BindingPattern using value and env as the arguments.

13.3.2 Variable Statement

Note

A var statement declares variables that are scoped to the running execution context's VariableEnvironment. Var variables are created when their containing Lexical Environment is instantiated and are initialized to undefined when created. Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collectively define only one variable. A variable defined by a VariableDeclaration with an Initializer is assigned the value of its Initializer's AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

Syntax

VariableStatement[Yield, Await] : var VariableDeclarationList[+In, ?Yield, ?Await] ; VariableDeclarationList[In, Yield, Await] : VariableDeclaration[?In, ?Yield, ?Await] VariableDeclarationList[?In, ?Yield, ?Await] , VariableDeclaration[?In, ?Yield, ?Await] VariableDeclaration[In, Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]

13.3.2.1 Static Semantics: BoundNames

VariableDeclarationList : VariableDeclarationList , VariableDeclaration
  1. Let names be BoundNames of VariableDeclarationList.
  2. Append to names the elements of BoundNames of VariableDeclaration.
  3. Return names.
 VariableDeclaration : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 VariableDeclaration : BindingPattern Initializer
  1. Return the BoundNames of BindingPattern.

13.3.2.2 Static Semantics: VarDeclaredNames

13.3.2.3 Static Semantics: VarScopedDeclarations

VariableDeclarationList : VariableDeclaration
  1. Return a new List containing VariableDeclaration.
 VariableDeclarationList : VariableDeclarationList , VariableDeclaration
  1. Let declarations be VarScopedDeclarations of VariableDeclarationList.
  2. Append VariableDeclaration to declarations.
  3. Return declarations.

13.3.2.4 Runtime Semantics: Evaluation

VariableStatement : var VariableDeclarationList ;
  1. Let next be the result of evaluating VariableDeclarationList.
  2. ReturnIfAbrupt(next).
  3. Return NormalCompletion(empty).
 VariableDeclarationList : VariableDeclarationList , VariableDeclaration
  1. Let next be the result of evaluating VariableDeclarationList.
  2. ReturnIfAbrupt(next).
  3. Return the result of evaluating VariableDeclaration.
 VariableDeclaration : BindingIdentifier
  1. Return NormalCompletion(empty).
 VariableDeclaration : BindingIdentifier Initializer
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be the result of evaluating Initializer.
    2. Let value be ? GetValue(rhs).
  5. Return ? PutValue(lhs, value).
 Note

If a VariableDeclaration is nested within a with statement and the BindingIdentifier in the VariableDeclaration is the same as a property name of the binding object of the with statement's object Environment Record, then step 5 will assign value to the property instead of assigning to the VariableEnvironment binding of the Identifier.

 VariableDeclaration : BindingPattern Initializer
  1. Let rhs be the result of evaluating Initializer.
  2. Let rval be ? GetValue(rhs).
  3. Return the result of performing BindingInitialization for BindingPattern passing rval and undefined as arguments.

13.3.3 Destructuring Binding Patterns

Syntax

BindingPattern[Yield, Await] : ObjectBindingPattern[?Yield, ?Await] ArrayBindingPattern[?Yield, ?Await] ObjectBindingPattern[Yield, Await] : { } { BindingRestProperty[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] , BindingRestProperty[?Yield, ?Await]opt } ArrayBindingPattern[Yield, Await] : [ Elisionopt BindingRestElement[?Yield, ?Await]opt ] [ BindingElementList[?Yield, ?Await] ] [ BindingElementList[?Yield, ?Await] , Elisionopt BindingRestElement[?Yield, ?Await]opt ] BindingRestProperty[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] BindingPropertyList[Yield, Await] : BindingProperty[?Yield, ?Await] BindingPropertyList[?Yield, ?Await] , BindingProperty[?Yield, ?Await] BindingElementList[Yield, Await] : BindingElisionElement[?Yield, ?Await] BindingElementList[?Yield, ?Await] , BindingElisionElement[?Yield, ?Await] BindingElisionElement[Yield, Await] : Elisionopt BindingElement[?Yield, ?Await] BindingProperty[Yield, Await] : SingleNameBinding[?Yield, ?Await] PropertyName[?Yield, ?Await] : BindingElement[?Yield, ?Await] BindingElement[Yield, Await] : SingleNameBinding[?Yield, ?Await] BindingPattern[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt SingleNameBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt BindingRestElement[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] ... BindingPattern[?Yield, ?Await]

13.3.3.1 Static Semantics: BoundNames

ObjectBindingPattern : { }
  1. Return a new empty List.
 ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
  1. Let names be BoundNames of BindingPropertyList.
  2. Append to names the elements of BoundNames of BindingRestProperty.
  3. Return names.
 ArrayBindingPattern : [ Elisionopt ]
  1. Return a new empty List.
 ArrayBindingPattern : [ Elisionopt BindingRestElement ]
  1. Return the BoundNames of BindingRestElement.
 ArrayBindingPattern : [ BindingElementList , Elisionopt ]
  1. Return the BoundNames of BindingElementList.
 ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
  1. Let names be BoundNames of BindingElementList.
  2. Append to names the elements of BoundNames of BindingRestElement.
  3. Return names.
 BindingPropertyList : BindingPropertyList , BindingProperty
  1. Let names be BoundNames of BindingPropertyList.
  2. Append to names the elements of BoundNames of BindingProperty.
  3. Return names.
 BindingElementList : BindingElementList , BindingElisionElement
  1. Let names be BoundNames of BindingElementList.
  2. Append to names the elements of BoundNames of BindingElisionElement.
  3. Return names.
 BindingElisionElement : Elisionopt BindingElement
  1. Return BoundNames of BindingElement.
 BindingProperty : PropertyName : BindingElement
  1. Return the BoundNames of BindingElement.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Return the BoundNames of BindingIdentifier.
 BindingElement : BindingPattern Initializeropt
  1. Return the BoundNames of BindingPattern.

13.3.3.2 Static Semantics: ContainsExpression

ObjectBindingPattern : { } { BindingRestProperty }
  1. Return false.
 ObjectBindingPattern : { BindingPropertyList , BindingRestProperty }
  1. Return ContainsExpression of BindingPropertyList.
 ArrayBindingPattern : [ Elisionopt ]
  1. Return false.
 ArrayBindingPattern : [ Elisionopt BindingRestElement ]
  1. Return ContainsExpression of BindingRestElement.
 ArrayBindingPattern : [ BindingElementList , Elisionopt ]
  1. Return ContainsExpression of BindingElementList.
 ArrayBindingPattern : [ BindingElementList , Elisionopt BindingRestElement ]
  1. Let has be ContainsExpression of BindingElementList.
  2. If has is true, return true.
  3. Return ContainsExpression of BindingRestElement.
 BindingPropertyList : BindingPropertyList , BindingProperty
  1. Let has be ContainsExpression of BindingPropertyList.
  2. If has is true, return true.
  3. Return ContainsExpression of BindingProperty.
 BindingElementList : BindingElementList , BindingElisionElement
  1. Let has be ContainsExpression of BindingElementList.
  2. If has is true, return true.
  3. Return ContainsExpression of BindingElisionElement.
 BindingElisionElement : Elisionopt BindingElement
  1. Return ContainsExpression of BindingElement.
 BindingProperty : PropertyName : BindingElement
  1. Let has be IsComputedPropertyKey of PropertyName.
  2. If has is true, return true.
  3. Return ContainsExpression of BindingElement.
 BindingElement : BindingPattern Initializer
  1. Return true.
 SingleNameBinding : BindingIdentifier
  1. Return false.
 SingleNameBinding : BindingIdentifier Initializer
  1. Return true.
 BindingRestElement : ... BindingIdentifier
  1. Return false.
 BindingRestElement : ... BindingPattern
  1. Return ContainsExpression of BindingPattern.

13.3.3.3 Static Semantics: HasInitializer

13.3.3.4 Static Semantics: IsSimpleParameterList

13.3.3.5 Runtime Semantics: BindingInitialization

With parameters value 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.

 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.

13.3.3.6 Runtime Semantics: PropertyBindingInitialization

With parameters value and environment.

Note
These collect a list of all bound property names rather than just empty completion.
 BindingPropertyList : BindingPropertyList , BindingProperty
  1. Let boundNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Let nextNames be ? PropertyBindingInitialization of BindingProperty with arguments value and environment.
  3. Append each item in nextNames to the end of boundNames.
  4. Return boundNames.
 BindingProperty : SingleNameBinding
  1. Let name be the string that is the only element of BoundNames of SingleNameBinding.
  2. Perform ? KeyedBindingInitialization for SingleNameBinding using value, environment, and name as the arguments.
  3. Return a new List containing name.
 BindingProperty : PropertyName : BindingElement
  1. Let P be the result of evaluating PropertyName.
  2. ReturnIfAbrupt(P).
  3. Perform ? KeyedBindingInitialization of BindingElement with value, environment, and P as the arguments.
  4. Return a new List containing P.

13.3.3.7 Runtime Semantics: RestBindingInitialization

With parameters value, environment, and excludedNames.

BindingRestProperty : ... BindingIdentifier
  1. Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
  2. Let restObj be OrdinaryObjectCreate(%Object.prototype%).
  3. Perform ? CopyDataProperties(restObj, value, excludedNames).
  4. If environment is undefined, return PutValue(lhs, restObj).
  5. Return InitializeReferencedBinding(lhs, restObj).

13.3.3.8 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 ]
  1. Return the result of performing IteratorBindingInitialization for BindingElementList with iteratorRecord and environment as arguments.
 ArrayBindingPattern : [ BindingElementList , ]
  1. Return the result of performing IteratorBindingInitialization for BindingElementList 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 : BindingElisionElement
  1. Return the result of performing IteratorBindingInitialization for BindingElisionElement 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 : BindingElement
  1. Return the result of performing IteratorBindingInitialization of BindingElement with iteratorRecord and environment as the 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.
 BindingElement : SingleNameBinding
  1. Return the result of performing IteratorBindingInitialization for SingleNameBinding 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(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(n), nextValue).
    7. Set n to n + 1.

13.3.3.9 Runtime Semantics: KeyedBindingInitialization

With parameters value, environment, and propertyName.

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.

 BindingElement : BindingPattern Initializeropt
  1. Let v be ? GetV(value, propertyName).
  2. If Initializer is present and v is undefined, then
    1. Let defaultValue be the result of evaluating Initializer.
    2. Set v to ? GetValue(defaultValue).
  3. Return the result of performing BindingInitialization for BindingPattern passing v and environment as arguments.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId, environment).
  3. Let v be ? GetV(value, propertyName).
  4. 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).
  5. If environment is undefined, return ? PutValue(lhs, v).
  6. Return InitializeReferencedBinding(lhs, v).

13.4 Empty Statement

Syntax

EmptyStatement : ;

13.4.1 Runtime Semantics: Evaluation

EmptyStatement : ;
  1. Return NormalCompletion(empty).

13.5 Expression Statement

Syntax

ExpressionStatement[Yield, Await] : [lookahead ∉ {{, function, async [no |LineTerminator| here] function, class, let [}] Expression[+In, ?Yield, ?Await] ; Note

An ExpressionStatement cannot start with a U+007B (LEFT CURLY BRACKET) because that might make it ambiguous with a Block. An ExpressionStatement cannot start with the function or class keywords because that would make it ambiguous with a FunctionDeclaration, a GeneratorDeclaration, or a ClassDeclaration. An ExpressionStatement cannot start with async function because that would make it ambiguous with an AsyncFunctionDeclaration or a AsyncGeneratorDeclaration. An ExpressionStatement cannot start with the two token sequence let [ because that would make it ambiguous with a let LexicalDeclaration whose first LexicalBinding was an ArrayBindingPattern.

13.5.1 Runtime Semantics: Evaluation

ExpressionStatement : Expression ;
  1. Let exprRef be the result of evaluating Expression.
  2. Return ? GetValue(exprRef).

13.6 The if Statement

Syntax

IfStatement[Yield, Await, Return] : if ( Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] else Statement[?Yield, ?Await, ?Return] if ( Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]

Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible if that would otherwise have no corresponding else.

13.6.1 Static Semantics: Early Errors

IfStatement : if ( Expression ) Statement else Statement if ( Expression ) Statement Note

It is only necessary to apply this rule if the extension specified in B.3.2 is implemented.

13.6.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

IfStatement : if ( Expression ) Statement else Statement
  1. Let hasDuplicate be ContainsDuplicateLabels of the first Statement with argument labelSet.
  2. If hasDuplicate is true, return true.
  3. Return ContainsDuplicateLabels of the second Statement with argument labelSet.
 IfStatement : if ( Expression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.

13.6.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

IfStatement : if ( Expression ) Statement else Statement
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first Statement with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of the second Statement with argument labelSet.
 IfStatement : if ( Expression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.6.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

IfStatement : if ( Expression ) Statement else Statement
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first Statement with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of the second Statement with arguments iterationSet and « ».
 IfStatement : if ( Expression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.6.5 Static Semantics: VarDeclaredNames

IfStatement : if ( Expression ) Statement else Statement
  1. Let names be VarDeclaredNames of the first Statement.
  2. Append to names the elements of the VarDeclaredNames of the second Statement.
  3. Return names.
 IfStatement : if ( Expression ) Statement
  1. Return the VarDeclaredNames of Statement.

13.6.6 Static Semantics: VarScopedDeclarations

IfStatement : if ( Expression ) Statement else Statement
  1. Let declarations be VarScopedDeclarations of the first Statement.
  2. Append to declarations the elements of the VarScopedDeclarations of the second Statement.
  3. Return declarations.
 IfStatement : if ( Expression ) Statement
  1. Return the VarScopedDeclarations of Statement.

13.6.7 Runtime Semantics: Evaluation

IfStatement : if ( Expression ) Statement else Statement
  1. Let exprRef be the result of evaluating Expression.
  2. Let exprValue be ! ToBoolean(? GetValue(exprRef)).
  3. If exprValue is true, then
    1. Let stmtCompletion be the result of evaluating the first Statement.
  4. Else,
    1. Let stmtCompletion be the result of evaluating the second Statement.
  5. Return Completion(UpdateEmpty(stmtCompletion, undefined)).
 IfStatement : if ( Expression ) Statement
  1. Let exprRef be the result of evaluating Expression.
  2. Let exprValue be ! ToBoolean(? GetValue(exprRef)).
  3. If exprValue is false, then
    1. Return NormalCompletion(undefined).
  4. Else,
    1. Let stmtCompletion be the result of evaluating Statement.
    2. Return Completion(UpdateEmpty(stmtCompletion, undefined)).

13.7 Iteration Statements

Syntax

IterationStatement[Yield, Await, Return] : do Statement[?Yield, ?Await, ?Return] while ( Expression[+In, ?Yield, ?Await] ) ; while ( Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( [lookahead ≠ let [] Expression[~In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] for ( var VariableDeclarationList[~In, ?Yield, ?Await] ; Expression[+In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] for ( LexicalDeclaration[~In, ?Yield, ?Await] Expression[+In, ?Yield, ?Await]opt ; Expression[+In, ?Yield, ?Await]opt ) Statement[?Yield, ?Await, ?Return] for ( [lookahead ≠ let [] LeftHandSideExpression[?Yield, ?Await] in Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( var ForBinding[?Yield, ?Await] in Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( ForDeclaration[?Yield, ?Await] in Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( [lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( var ForBinding[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] for ( ForDeclaration[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [+Await] for await ( [lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [+Await] for await ( var ForBinding[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [+Await] for await ( ForDeclaration[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] ForDeclaration[Yield, Await] : LetOrConst ForBinding[?Yield, ?Await] ForBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] Note

This section is extended by Annex B.3.6.

13.7.1 Semantics

13.7.1.1 Static Semantics: Early Errors

IterationStatement : do Statement while ( Expression ) ; while ( Expression ) Statement for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement Note

It is only necessary to apply this rule if the extension specified in B.3.2 is implemented.

13.7.1.2 Runtime Semantics: LoopContinues ( completion, labelSet )

The abstract operation LoopContinues with arguments completion and labelSet is defined by the following steps:

  1. If completion.[[Type]] is normal, return true.
  2. If completion.[[Type]] is not continue, return false.
  3. If completion.[[Target]] is empty, return true.
  4. If completion.[[Target]] is an element of labelSet, return true.
  5. Return false.
 Note

Within the Statement part of an IterationStatement a ContinueStatement may be used to begin a new iteration.

13.7.2 The do-while Statement

13.7.2.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

IterationStatement : do Statement while ( Expression ) ;
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.2.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

IterationStatement : do Statement while ( Expression ) ;
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.2.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

IterationStatement : do Statement while ( Expression ) ;
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.2.4 Static Semantics: VarDeclaredNames

IterationStatement : do Statement while ( Expression ) ;
  1. Return the VarDeclaredNames of Statement.

13.7.2.5 Static Semantics: VarScopedDeclarations

IterationStatement : do Statement while ( Expression ) ;
  1. Return the VarScopedDeclarations of Statement.

13.7.2.6 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

IterationStatement : do Statement while ( Expression ) ;
  1. Let V be undefined.
  2. Repeat,
    1. Let stmtResult be the result of evaluating Statement.
    2. If LoopContinues(stmtResult, labelSet) is false, return Completion(UpdateEmpty(stmtResult, V)).
    3. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].
    4. Let exprRef be the result of evaluating Expression.
    5. Let exprValue be ? GetValue(exprRef).
    6. If ! ToBoolean(exprValue) is false, return NormalCompletion(V).

13.7.3 The while Statement

13.7.3.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

IterationStatement : while ( Expression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.3.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

IterationStatement : while ( Expression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.3.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

IterationStatement : while ( Expression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.3.4 Static Semantics: VarDeclaredNames

IterationStatement : while ( Expression ) Statement
  1. Return the VarDeclaredNames of Statement.

13.7.3.5 Static Semantics: VarScopedDeclarations

IterationStatement : while ( Expression ) Statement
  1. Return the VarScopedDeclarations of Statement.

13.7.3.6 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

IterationStatement : while ( Expression ) Statement
  1. Let V be undefined.
  2. Repeat,
    1. Let exprRef be the result of evaluating Expression.
    2. Let exprValue be ? GetValue(exprRef).
    3. If ! ToBoolean(exprValue) is false, return NormalCompletion(V).
    4. Let stmtResult be the result of evaluating Statement.
    5. If LoopContinues(stmtResult, labelSet) is false, return Completion(UpdateEmpty(stmtResult, V)).
    6. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].

13.7.4 The for Statement

13.7.4.1 Static Semantics: Early Errors

IterationStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement

13.7.4.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

IterationStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.

13.7.4.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

IterationStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.7.4.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

IterationStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.7.4.5 Static Semantics: VarDeclaredNames

IterationStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
  1. Return the VarDeclaredNames of Statement.
 IterationStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
  1. Let names be BoundNames of VariableDeclarationList.
  2. Append to names the elements of the VarDeclaredNames of Statement.
  3. Return names.
 IterationStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return the VarDeclaredNames of Statement.

13.7.4.6 Static Semantics: VarScopedDeclarations

IterationStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
  1. Return the VarScopedDeclarations of Statement.
 IterationStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
  1. Let declarations be VarScopedDeclarations of VariableDeclarationList.
  2. Append to declarations the elements of the VarScopedDeclarations of Statement.
  3. Return declarations.
 IterationStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Return the VarScopedDeclarations of Statement.

13.7.4.7 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

IterationStatement : for ( Expressionopt ; Expressionopt ; Expressionopt ) Statement
  1. If the first Expression is present, then
    1. Let exprRef be the result of evaluating the first Expression.
    2. Perform ? GetValue(exprRef).
  2. Return ? ForBodyEvaluation(the second Expression, the third Expression, Statement, « », labelSet).
 IterationStatement : for ( var VariableDeclarationList ; Expressionopt ; Expressionopt ) Statement
  1. Let varDcl be the result of evaluating VariableDeclarationList.
  2. ReturnIfAbrupt(varDcl).
  3. Return ? ForBodyEvaluation(the first Expression, the second Expression, Statement, « », labelSet).
 IterationStatement : for ( LexicalDeclaration Expressionopt ; Expressionopt ) Statement
  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. Let loopEnv be NewDeclarativeEnvironment(oldEnv).
  3. Let loopEnvRec be loopEnv's EnvironmentRecord.
  4. Let isConst be IsConstantDeclaration of LexicalDeclaration.
  5. Let boundNames be the BoundNames of LexicalDeclaration.
  6. For each element dn of boundNames, do
    1. If isConst is true, then
      1. Perform ! loopEnvRec.CreateImmutableBinding(dn, true).
    2. Else,
      1. Perform ! loopEnvRec.CreateMutableBinding(dn, false).
  7. Set the running execution context's LexicalEnvironment to loopEnv.
  8. Let forDcl be the result of evaluating LexicalDeclaration.
  9. If forDcl is an abrupt completion, then
    1. Set the running execution context's LexicalEnvironment to oldEnv.
    2. Return Completion(forDcl).
  10. If isConst is false, let perIterationLets be boundNames; otherwise let perIterationLets be « ».
  11. Let bodyResult be ForBodyEvaluation(the first Expression, the second Expression, Statement, perIterationLets, labelSet).
  12. Set the running execution context's LexicalEnvironment to oldEnv.
  13. Return Completion(bodyResult).

13.7.4.8 Runtime Semantics: ForBodyEvaluation ( test, increment, stmt, perIterationBindings, labelSet )

The abstract operation ForBodyEvaluation with arguments test, increment, stmt, perIterationBindings, and labelSet is performed as follows:

  1. Let V be undefined.
  2. Perform ? CreatePerIterationEnvironment(perIterationBindings).
  3. Repeat,
    1. If test is not [empty], then
      1. Let testRef be the result of evaluating test.
      2. Let testValue be ? GetValue(testRef).
      3. If ! ToBoolean(testValue) is false, return NormalCompletion(V).
    2. Let result be the result of evaluating stmt.
    3. If LoopContinues(result, labelSet) is false, return Completion(UpdateEmpty(result, V)).
    4. If result.[[Value]] is not empty, set V to result.[[Value]].
    5. Perform ? CreatePerIterationEnvironment(perIterationBindings).
    6. If increment is not [empty], then
      1. Let incRef be the result of evaluating increment.
      2. Perform ? GetValue(incRef).

13.7.4.9 Runtime Semantics: CreatePerIterationEnvironment ( perIterationBindings )

The abstract operation CreatePerIterationEnvironment with argument perIterationBindings is performed as follows:

  1. If perIterationBindings has any elements, then
    1. Let lastIterationEnv be the running execution context's LexicalEnvironment.
    2. Let lastIterationEnvRec be lastIterationEnv's EnvironmentRecord.
    3. Let outer be lastIterationEnv's outer environment reference.
    4. Assert: outer is not null.
    5. Let thisIterationEnv be NewDeclarativeEnvironment(outer).
    6. Let thisIterationEnvRec be thisIterationEnv's EnvironmentRecord.
    7. For each element bn of perIterationBindings, do
      1. Perform ! thisIterationEnvRec.CreateMutableBinding(bn, false).
      2. Let lastValue be ? lastIterationEnvRec.GetBindingValue(bn, true).
      3. Perform thisIterationEnvRec.InitializeBinding(bn, lastValue).
    8. Set the running execution context's LexicalEnvironment to thisIterationEnv.
  2. Return undefined.

13.7.5 The for-in, for-of, and for-await-of Statements

13.7.5.1 Static Semantics: Early Errors

IterationStatement : for ( LeftHandSideExpression in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement

If LeftHandSideExpression is either an ObjectLiteral or an ArrayLiteral, the following Early Error rules are applied:

If LeftHandSideExpression is neither an ObjectLiteral nor an ArrayLiteral, the following Early Error rule is applied:

IterationStatement : for ( ForDeclaration in Expression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  • It is a Syntax Error if the BoundNames of ForDeclaration contains "let".
  • It is a Syntax Error if any element of the BoundNames of ForDeclaration also occurs in the VarDeclaredNames of Statement.
  • It is a Syntax Error if the BoundNames of ForDeclaration contains any duplicate entries.

13.7.5.2 Static Semantics: BoundNames

ForDeclaration : LetOrConst ForBinding
  1. Return the BoundNames of ForBinding.

13.7.5.3 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

IterationStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.
 Note

This section is extended by Annex B.3.6.

13.7.5.4 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

IterationStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.
 Note

This section is extended by Annex B.3.6.

13.7.5.5 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

IterationStatement : for ( LeftHandSideExpression in Expression ) Statement for ( var ForBinding in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».
 Note

This section is extended by Annex B.3.6.

13.7.5.6 Static Semantics: IsDestructuring

ForDeclaration : LetOrConst ForBinding
  1. Return IsDestructuring of ForBinding.
 ForBinding : BindingIdentifier
  1. Return false.
 ForBinding : BindingPattern
  1. Return true.
 Note

This section is extended by Annex B.3.6.

13.7.5.7 Static Semantics: VarDeclaredNames

IterationStatement : for ( LeftHandSideExpression in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return the VarDeclaredNames of Statement.
 IterationStatement : for ( var ForBinding in Expression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement
  1. Let names be the BoundNames of ForBinding.
  2. Append to names the elements of the VarDeclaredNames of Statement.
  3. Return names.
 Note

This section is extended by Annex B.3.6.

13.7.5.8 Static Semantics: VarScopedDeclarations

IterationStatement : for ( LeftHandSideExpression in Expression ) Statement for ( ForDeclaration in Expression ) Statement for ( LeftHandSideExpression of AssignmentExpression ) Statement for ( ForDeclaration of AssignmentExpression ) Statement for await ( LeftHandSideExpression of AssignmentExpression ) Statement for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Return the VarScopedDeclarations of Statement.
 IterationStatement : for ( var ForBinding in Expression ) Statement for ( var ForBinding of AssignmentExpression ) Statement for await ( var ForBinding of AssignmentExpression ) Statement
  1. Let declarations be a List containing ForBinding.
  2. Append to declarations the elements of the VarScopedDeclarations of Statement.
  3. Return declarations.
 Note

This section is extended by Annex B.3.6.

13.7.5.9 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 the formal parameter lists of some non-strict functions (see 9.2.10). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

 ForDeclaration : LetOrConst ForBinding
  1. Return the result of performing BindingInitialization for ForBinding passing value and environment as the arguments.

13.7.5.10 Runtime Semantics: BindingInstantiation

With parameter environment.

ForDeclaration : LetOrConst ForBinding
  1. Let envRec be environment's EnvironmentRecord.
  2. Assert: envRec is a declarative Environment Record.
  3. For each element name of the BoundNames of ForBinding, do
    1. If IsConstantDeclaration of LetOrConst is true, then
      1. Perform ! envRec.CreateImmutableBinding(name, true).
    2. Else,
      1. Perform ! envRec.CreateMutableBinding(name, false).

13.7.5.11 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

IterationStatement : for ( LeftHandSideExpression in Expression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
  2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, enumerate, assignment, labelSet).
 IterationStatement : for ( var ForBinding in Expression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
  2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, enumerate, varBinding, labelSet).
 IterationStatement : for ( ForDeclaration in Expression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, Expression, enumerate).
  2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, enumerate, lexicalBinding, labelSet).
 IterationStatement : for ( LeftHandSideExpression of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
  2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet).
 IterationStatement : for ( var ForBinding of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
  2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, varBinding, labelSet).
 IterationStatement : for ( ForDeclaration of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, iterate).
  2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet).
 IterationStatement : for await ( LeftHandSideExpression of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
  2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet, async).
 IterationStatement : for await ( var ForBinding of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, async-iterate).
  2. Return ? ForIn/OfBodyEvaluation(ForBinding, Statement, keyResult, iterate, varBinding, labelSet, async).
 IterationStatement : for await ( ForDeclaration of AssignmentExpression ) Statement
  1. Let keyResult be ? ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, async-iterate).
  2. Return ? ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet, async).
 Note

This section is extended by Annex B.3.6.

13.7.5.12 Runtime Semantics: ForIn/OfHeadEvaluation ( uninitializedBoundNames, expr, iterationKind )

The abstract operation ForIn/OfHeadEvaluation is called with arguments uninitializedBoundNames, expr, and iterationKind. The value of iterationKind is either enumerate, iterate, or async-iterate.

  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. If uninitializedBoundNames is not an empty List, then
    1. Assert: uninitializedBoundNames has no duplicate entries.
    2. Let newEnv be NewDeclarativeEnvironment(oldEnv).
    3. Let newEnvRec be newEnv's EnvironmentRecord.
    4. For each string name in uninitializedBoundNames, do
      1. Perform ! newEnvRec.CreateMutableBinding(name, false).
    5. Set the running execution context's LexicalEnvironment to newEnv.
  3. Let exprRef be the result of evaluating expr.
  4. Set the running execution context's LexicalEnvironment to oldEnv.
  5. Let exprValue be ? GetValue(exprRef).
  6. If iterationKind is enumerate, then
    1. If exprValue is undefined or null, then
      1. Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.
    2. Let obj be ! ToObject(exprValue).
    3. Return ? EnumerateObjectProperties(obj).
  7. Else,
    1. Assert: iterationKind is iterate or async-iterate.
    2. If iterationKind is async-iterate, let iteratorHint be async.
    3. Else, let iteratorHint be sync.
    4. Return ? GetIterator(exprValue, iteratorHint).

13.7.5.13 Runtime Semantics: ForIn/OfBodyEvaluation ( lhs, stmt, iteratorRecord, iterationKind, lhsKind, labelSet [ , iteratorKind ] )

The abstract operation ForIn/OfBodyEvaluation is called with arguments lhs, stmt, iteratorRecord, iterationKind, lhsKind, labelSet, and optional argument iteratorKind. The value of lhsKind is either assignment, varBinding or lexicalBinding. The value of iteratorKind is either sync or async.

  1. If iteratorKind is not present, set iteratorKind to sync.
  2. Let oldEnv be the running execution context's LexicalEnvironment.
  3. Let V be undefined.
  4. Let destructuring be IsDestructuring of lhs.
  5. If destructuring is true and if lhsKind is assignment, then
    1. Assert: lhs is a LeftHandSideExpression.
    2. Let assignmentPattern be the AssignmentPattern that is covered by lhs.
  6. Repeat,
    1. Let nextResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
    2. If iteratorKind is async, then set nextResult to ? Await(nextResult).
    3. If Type(nextResult) is not Object, throw a TypeError exception.
    4. Let done be ? IteratorComplete(nextResult).
    5. If done is true, return NormalCompletion(V).
    6. Let nextValue be ? IteratorValue(nextResult).
    7. If lhsKind is either assignment or varBinding, then
      1. If destructuring is false, then
        1. Let lhsRef be the result of evaluating lhs. (It may be evaluated repeatedly.)
    8. Else,
      1. Assert: lhsKind is lexicalBinding.
      2. Assert: lhs is a ForDeclaration.
      3. Let iterationEnv be NewDeclarativeEnvironment(oldEnv).
      4. Perform BindingInstantiation for lhs passing iterationEnv as the argument.
      5. Set the running execution context's LexicalEnvironment to iterationEnv.
      6. If destructuring is false, then
        1. Assert: lhs binds a single name.
        2. Let lhsName be the sole element of BoundNames of lhs.
        3. Let lhsRef be ! ResolveBinding(lhsName).
    9. If destructuring is false, then
      1. If lhsRef is an abrupt completion, then
        1. Let status be lhsRef.
      2. Else if lhsKind is lexicalBinding, then
        1. Let status be InitializeReferencedBinding(lhsRef, nextValue).
      3. Else,
        1. Let status be PutValue(lhsRef, nextValue).
    10. Else,
      1. If lhsKind is assignment, then
        1. Let status be DestructuringAssignmentEvaluation of assignmentPattern with argument nextValue.
      2. Else if lhsKind is varBinding, then
        1. Assert: lhs is a ForBinding.
        2. Let status be BindingInitialization of lhs with arguments nextValue and undefined.
      3. Else,
        1. Assert: lhsKind is lexicalBinding.
        2. Assert: lhs is a ForDeclaration.
        3. Let status be BindingInitialization of lhs with arguments nextValue and iterationEnv.
    11. If status is an abrupt completion, then
      1. Set the running execution context's LexicalEnvironment to oldEnv.
      2. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
      3. If iterationKind is enumerate, then
        1. Return status.
      4. Else,
        1. Assert: iterationKind is iterate.
        2. Return ? IteratorClose(iteratorRecord, status).
    12. Let result be the result of evaluating stmt.
    13. Set the running execution context's LexicalEnvironment to oldEnv.
    14. If LoopContinues(result, labelSet) is false, then
      1. If iterationKind is enumerate, then
        1. Return Completion(UpdateEmpty(result, V)).
      2. Else,
        1. Assert: iterationKind is iterate.
        2. Set status to UpdateEmpty(result, V).
        3. If iteratorKind is async, return ? AsyncIteratorClose(iteratorRecord, status).
        4. Return ? IteratorClose(iteratorRecord, status).
    15. If result.[[Value]] is not empty, set V to result.[[Value]].

13.7.5.14 Runtime Semantics: Evaluation

ForBinding : BindingIdentifier
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Return ? ResolveBinding(bindingId).

13.7.5.15 EnumerateObjectProperties ( O )

When the abstract operation EnumerateObjectProperties is called with argument O, the following steps are taken:

  1. Assert: Type(O) is Object.
  2. Return an Iterator object (25.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object 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 target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it 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 enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.

In addition, if neither O nor any object in its prototype chain is a Proxy exotic object, Integer-Indexed exotic object, module namespace exotic object, or implementation provided exotic object, then the iterator must behave as would the iterator given by CreateForInIterator(O) until one of the following occurs:

  • the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
  • a property is removed from O or an object in its prototype chain,
  • a property is added to an object in O's prototype chain, or
  • the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.
Note 1

Hosts are not required to implement the algorithm in 13.7.5.16.2.1 directly. They may choose any implementation whose behaviour will not deviate from that algorithm unless one of the constraints in the previous paragraph is violated.

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


            function* EnumerateObjectProperties(obj) {
              const visited = new Set();
              for (const key of Reflect.ownKeys(obj)) {
                if (typeof key === "symbol") continue;
                const desc = Reflect.getOwnPropertyDescriptor(obj, key);
                if (desc) {
                  visited.add(key);
                  if (desc.enumerable) yield key;
                }
              }
              const proto = Reflect.getPrototypeOf(obj);
              if (proto === null) return;
              for (const protoKey of EnumerateObjectProperties(proto)) {
                if (!visited.has(protoKey)) yield protoKey;
              }
            }
Note 2
The list of exotic objects for which implementations are not required to match CreateForInIterator was chosen because implementations historically differed in behaviour for those cases, and agreed in all others.

13.7.5.16 For-In Iterator Objects

A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour of EnumerateObjectProperties.

13.7.5.16.1 CreateForInIterator ( object )

The abstract operation CreateForInIterator with argument object is used to create a For-In Iterator object which iterates over the own and inherited enumerable string properties of object in a specific order. It performs the following steps:

  1. Assert: Type(object) is Object.
  2. Let iterator be OrdinaryObjectCreate(%ForInIteratorPrototype%, « [[Object]], [[ObjectWasVisited]], [[VisitedKeys]], [[RemainingKeys]] »).
  3. Set iterator.[[Object]] to object.
  4. Set iterator.[[ObjectWasVisited]] to false.
  5. Set iterator.[[VisitedKeys]] to a new empty List.
  6. Set iterator.[[RemainingKeys]] to a new empty List.
  7. Return iterator.

13.7.5.16.2 The %ForInIteratorPrototype% Object

The %ForInIteratorPrototype% object:

  • has properties that are inherited by all For-In Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is the intrinsic object %IteratorPrototype%.
  • is never directly accessible to ECMAScript code.
  • has the following properties:

13.7.5.16.2.1 %ForInIteratorPrototype%.next ( )

  1. Let O be the this value.
  2. Assert: Type(O) is Object.
  3. Assert: O has all of the internal slots of a For-In Iterator Instance (13.7.5.16.3).
  4. Let object be O.[[Object]].
  5. Let visited be O.[[VisitedKeys]].
  6. Let remaining be O.[[RemainingKeys]].
  7. Repeat,
    1. If O.[[ObjectWasVisited]] is false, then
      1. Let keys be ? object.[[OwnPropertyKeys]]().
      2. For each key of keys in List order, do
        1. If Type(key) is String, then
          1. Append key to remaining.
      3. Set O.[[ObjectWasVisited]] to true.
    2. Repeat, while remaining is not empty,
      1. Remove the first element from remaining and let r be the value of the element.
      2. If there does not exist an element v of visited such that SameValue(r, v) is true, then
        1. Let desc be ? object.[[GetOwnProperty]](r).
        2. If desc is not undefined, then
          1. Append r to visited.
          2. If desc.[[Enumerable]] is true, return CreateIterResultObject(r, false).
    3. Set object to ? object.[[GetPrototypeOf]]().
    4. Set O.[[Object]] to object.
    5. Set O.[[ObjectWasVisited]] to false.
    6. If object is null, return CreateIterResultObject(undefined, true).

13.7.5.16.3 Properties of For-In Iterator Instances

For-In Iterator instances are ordinary objects that inherit properties from the %ForInIteratorPrototype% intrinsic object. For-In Iterator instances are initially created with the internal slots listed in Table 36.

Internal Slot Description
[[Object]] The Object value whose properties are being iterated.
[[ObjectWasVisited]] true if the iterator has invoked [[OwnPropertyKeys]] on [[Object]], false otherwise.
[[VisitedKeys]] A list of String values which have been emitted by this iterator thus far.
[[RemainingKeys]] A list of String values remaining to be emitted for the current object, before iterating the properties of its prototype (if its prototype is not null).

13.8 The continue Statement

Syntax

ContinueStatement[Yield, Await] : continue ; continue [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ;

13.8.1 Static Semantics: Early Errors

ContinueStatement : continue ; ContinueStatement : continue LabelIdentifier ;

13.8.2 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

ContinueStatement : continue ;
  1. Return false.
 ContinueStatement : continue LabelIdentifier ;
  1. If the StringValue of LabelIdentifier is not an element of iterationSet, return true.
  2. Return false.

13.8.3 Runtime Semantics: Evaluation

ContinueStatement : continue ;
  1. Return Completion { [[Type]]: continue, [[Value]]: empty, [[Target]]: empty }.
 ContinueStatement : continue LabelIdentifier ;
  1. Let label be the StringValue of LabelIdentifier.
  2. Return Completion { [[Type]]: continue, [[Value]]: empty, [[Target]]: label }.

13.9 The break Statement

Syntax

BreakStatement[Yield, Await] : break ; break [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ;

13.9.1 Static Semantics: Early Errors

BreakStatement : break ;

13.9.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

BreakStatement : break ;
  1. Return false.
 BreakStatement : break LabelIdentifier ;
  1. If the StringValue of LabelIdentifier is not an element of labelSet, return true.
  2. Return false.

13.9.3 Runtime Semantics: Evaluation

BreakStatement : break ;
  1. Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: empty }.
 BreakStatement : break LabelIdentifier ;
  1. Let label be the StringValue of LabelIdentifier.
  2. Return Completion { [[Type]]: break, [[Value]]: empty, [[Target]]: label }.

13.10 The return Statement

Syntax

ReturnStatement[Yield, Await] : return ; return [no LineTerminator here] Expression[+In, ?Yield, ?Await] ; Note

A return statement causes a function to cease execution and, in most cases, returns a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression. A return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's completion record may be replaced with another completion record during evaluation of the finally block.

13.10.1 Runtime Semantics: Evaluation

ReturnStatement : return ;
  1. Return Completion { [[Type]]: return, [[Value]]: undefined, [[Target]]: empty }.
 ReturnStatement : return Expression ;
  1. Let exprRef be the result of evaluating Expression.
  2. Let exprValue be ? GetValue(exprRef).
  3. If ! GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
  4. Return Completion { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.

13.11 The with Statement

Syntax

WithStatement[Yield, Await, Return] : with ( Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] Note

The with statement adds an object Environment Record for a computed object to the lexical environment of the running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.

13.11.1 Static Semantics: Early Errors

WithStatement : with ( Expression ) Statement Note

It is only necessary to apply the second rule if the extension specified in B.3.2 is implemented.

13.11.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

WithStatement : with ( Expression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.

13.11.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

WithStatement : with ( Expression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

13.11.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

WithStatement : with ( Expression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

13.11.5 Static Semantics: VarDeclaredNames

WithStatement : with ( Expression ) Statement
  1. Return the VarDeclaredNames of Statement.

13.11.6 Static Semantics: VarScopedDeclarations

WithStatement : with ( Expression ) Statement
  1. Return the VarScopedDeclarations of Statement.

13.11.7 Runtime Semantics: Evaluation

WithStatement : with ( Expression ) Statement
  1. Let val be the result of evaluating Expression.
  2. Let obj be ? ToObject(? GetValue(val)).
  3. Let oldEnv be the running execution context's LexicalEnvironment.
  4. Let newEnv be NewObjectEnvironment(obj, oldEnv).
  5. Set the withEnvironment flag of newEnv's EnvironmentRecord to true.
  6. Set the running execution context's LexicalEnvironment to newEnv.
  7. Let C be the result of evaluating Statement.
  8. Set the running execution context's LexicalEnvironment to oldEnv.
  9. Return Completion(UpdateEmpty(C, undefined)).
 Note

No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.

13.12 The switch Statement

Syntax

SwitchStatement[Yield, Await, Return] : switch ( Expression[+In, ?Yield, ?Await] ) CaseBlock[?Yield, ?Await, ?Return] CaseBlock[Yield, Await, Return] : { CaseClauses[?Yield, ?Await, ?Return]opt } { CaseClauses[?Yield, ?Await, ?Return]opt DefaultClause[?Yield, ?Await, ?Return] CaseClauses[?Yield, ?Await, ?Return]opt } CaseClauses[Yield, Await, Return] : CaseClause[?Yield, ?Await, ?Return] CaseClauses[?Yield, ?Await, ?Return] CaseClause[?Yield, ?Await, ?Return] CaseClause[Yield, Await, Return] : case Expression[+In, ?Yield, ?Await] : StatementList[?Yield, ?Await, ?Return]opt DefaultClause[Yield, Await, Return] : default : StatementList[?Yield, ?Await, ?Return]opt

13.12.1 Static Semantics: Early Errors

SwitchStatement : switch ( Expression ) CaseBlock
  • It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any duplicate entries.
  • It is a Syntax Error if any element of the LexicallyDeclaredNames of CaseBlock also occurs in the VarDeclaredNames of CaseBlock.

13.12.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

SwitchStatement : switch ( Expression ) CaseBlock
  1. Return ContainsDuplicateLabels of CaseBlock with argument labelSet.
 CaseBlock : { }
  1. Return false.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, then
    1. Let hasDuplicates be ContainsDuplicateLabels of the first CaseClauses with argument labelSet.
    2. If hasDuplicates is true, return true.
  2. Let hasDuplicates be ContainsDuplicateLabels of DefaultClause with argument labelSet.
  3. If hasDuplicates is true, return true.
  4. If the second CaseClauses is not present, return false.
  5. Return ContainsDuplicateLabels of the second CaseClauses with argument labelSet.
 CaseClauses : CaseClauses CaseClause
  1. Let hasDuplicates be ContainsDuplicateLabels of CaseClauses with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of CaseClause with argument labelSet.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
  2. Return false.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return ContainsDuplicateLabels of StatementList with argument labelSet.
  2. Return false.

13.12.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

SwitchStatement : switch ( Expression ) CaseBlock
  1. Return ContainsUndefinedBreakTarget of CaseBlock with argument labelSet.
 CaseBlock : { }
  1. Return false.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, then
    1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of the first CaseClauses with argument labelSet.
    2. If hasUndefinedLabels is true, return true.
  2. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of DefaultClause with argument labelSet.
  3. If hasUndefinedLabels is true, return true.
  4. If the second CaseClauses is not present, return false.
  5. Return ContainsUndefinedBreakTarget of the second CaseClauses with argument labelSet.
 CaseClauses : CaseClauses CaseClause
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of CaseClauses with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of CaseClause with argument labelSet.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
  2. Return false.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return ContainsUndefinedBreakTarget of StatementList with argument labelSet.
  2. Return false.

13.12.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

SwitchStatement : switch ( Expression ) CaseBlock
  1. Return ContainsUndefinedContinueTarget of CaseBlock with arguments iterationSet and « ».
 CaseBlock : { }
  1. Return false.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, then
    1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of the first CaseClauses with arguments iterationSet and « ».
    2. If hasUndefinedLabels is true, return true.
  2. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of DefaultClause with arguments iterationSet and « ».
  3. If hasUndefinedLabels is true, return true.
  4. If the second CaseClauses is not present, return false.
  5. Return ContainsUndefinedContinueTarget of the second CaseClauses with arguments iterationSet and « ».
 CaseClauses : CaseClauses CaseClause
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of CaseClauses with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of CaseClause with arguments iterationSet and « ».
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
  2. Return false.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return ContainsUndefinedContinueTarget of StatementList with arguments iterationSet and « ».
  2. Return false.

13.12.5 Static Semantics: LexicallyDeclaredNames

CaseBlock : { }
  1. Return a new empty List.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, let names be the LexicallyDeclaredNames of the first CaseClauses.
  2. Else, let names be a new empty List.
  3. Append to names the elements of the LexicallyDeclaredNames of DefaultClause.
  4. If the second CaseClauses is not present, return names.
  5. Return the result of appending to names the elements of the LexicallyDeclaredNames of the second CaseClauses.
 CaseClauses : CaseClauses CaseClause
  1. Let names be LexicallyDeclaredNames of CaseClauses.
  2. Append to names the elements of the LexicallyDeclaredNames of CaseClause.
  3. Return names.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
  2. Return a new empty List.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return the LexicallyDeclaredNames of StatementList.
  2. Return a new empty List.

13.12.6 Static Semantics: LexicallyScopedDeclarations

CaseBlock : { }
  1. Return a new empty List.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, let declarations be the LexicallyScopedDeclarations of the first CaseClauses.
  2. Else, let declarations be a new empty List.
  3. Append to declarations the elements of the LexicallyScopedDeclarations of DefaultClause.
  4. If the second CaseClauses is not present, return declarations.
  5. Return the result of appending to declarations the elements of the LexicallyScopedDeclarations of the second CaseClauses.
 CaseClauses : CaseClauses CaseClause
  1. Let declarations be LexicallyScopedDeclarations of CaseClauses.
  2. Append to declarations the elements of the LexicallyScopedDeclarations of CaseClause.
  3. Return declarations.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
  2. Return a new empty List.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return the LexicallyScopedDeclarations of StatementList.
  2. Return a new empty List.

13.12.7 Static Semantics: VarDeclaredNames

SwitchStatement : switch ( Expression ) CaseBlock
  1. Return the VarDeclaredNames of CaseBlock.
 CaseBlock : { }
  1. Return a new empty List.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, let names be the VarDeclaredNames of the first CaseClauses.
  2. Else, let names be a new empty List.
  3. Append to names the elements of the VarDeclaredNames of DefaultClause.
  4. If the second CaseClauses is not present, return names.
  5. Return the result of appending to names the elements of the VarDeclaredNames of the second CaseClauses.
 CaseClauses : CaseClauses CaseClause
  1. Let names be VarDeclaredNames of CaseClauses.
  2. Append to names the elements of the VarDeclaredNames of CaseClause.
  3. Return names.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return the VarDeclaredNames of StatementList.
  2. Return a new empty List.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return the VarDeclaredNames of StatementList.
  2. Return a new empty List.

13.12.8 Static Semantics: VarScopedDeclarations

SwitchStatement : switch ( Expression ) CaseBlock
  1. Return the VarScopedDeclarations of CaseBlock.
 CaseBlock : { }
  1. Return a new empty List.
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. If the first CaseClauses is present, let declarations be the VarScopedDeclarations of the first CaseClauses.
  2. Else, let declarations be a new empty List.
  3. Append to declarations the elements of the VarScopedDeclarations of DefaultClause.
  4. If the second CaseClauses is not present, return declarations.
  5. Return the result of appending to declarations the elements of the VarScopedDeclarations of the second CaseClauses.
 CaseClauses : CaseClauses CaseClause
  1. Let declarations be VarScopedDeclarations of CaseClauses.
  2. Append to declarations the elements of the VarScopedDeclarations of CaseClause.
  3. Return declarations.
 CaseClause : case Expression : StatementListopt
  1. If the StatementList is present, return the VarScopedDeclarations of StatementList.
  2. Return a new empty List.
 DefaultClause : default : StatementListopt
  1. If the StatementList is present, return the VarScopedDeclarations of StatementList.
  2. Return a new empty List.

13.12.9 Runtime Semantics: CaseBlockEvaluation

With parameter input.

CaseBlock : { }
  1. Return NormalCompletion(undefined).
 CaseBlock : { CaseClauses }
  1. Let V be undefined.
  2. Let A be the List of CaseClause items in CaseClauses, in source text order.
  3. Let found be false.
  4. For each CaseClause C in A, do
    1. If found is false, then
      1. Set found to ? CaseClauseIsSelected(C, input).
    2. If found is true, then
      1. Let R be the result of evaluating C.
      2. If R.[[Value]] is not empty, set V to R.[[Value]].
      3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
  5. Return NormalCompletion(V).
 CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt }
  1. Let V be undefined.
  2. If the first CaseClauses is present, then
    1. Let A be the List of CaseClause items in the first CaseClauses, in source text order.
  3. Else,
    1. Let A be « ».
  4. Let found be false.
  5. For each CaseClause C in A, do
    1. If found is false, then
      1. Set found to ? CaseClauseIsSelected(C, input).
    2. If found is true, then
      1. Let R be the result of evaluating C.
      2. If R.[[Value]] is not empty, set V to R.[[Value]].
      3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
  6. Let foundInB be false.
  7. If the second CaseClauses is present, then
    1. Let B be the List of CaseClause items in the second CaseClauses, in source text order.
  8. Else,
    1. Let B be « ».
  9. If found is false, then
    1. For each CaseClause C in B, do
      1. If foundInB is false, then
        1. Set foundInB to ? CaseClauseIsSelected(C, input).
      2. If foundInB is true, then
        1. Let R be the result of evaluating CaseClause C.
        2. If R.[[Value]] is not empty, set V to R.[[Value]].
        3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
  10. If foundInB is true, return NormalCompletion(V).
  11. Let R be the result of evaluating DefaultClause.
  12. If R.[[Value]] is not empty, set V to R.[[Value]].
  13. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
  14. NOTE: The following is another complete iteration of the second CaseClauses.
  15. For each CaseClause C in B, do
    1. Let R be the result of evaluating CaseClause C.
    2. If R.[[Value]] is not empty, set V to R.[[Value]].
    3. If R is an abrupt completion, return Completion(UpdateEmpty(R, V)).
  16. Return NormalCompletion(V).

13.12.10 Runtime Semantics: CaseClauseIsSelected ( C, input )

The abstract operation CaseClauseIsSelected, given CaseClause C and value input, determines whether C matches input.

  1. Assert: C is an instance of the production CaseClause : case Expression : StatementListopt.
  2. Let exprRef be the result of evaluating the Expression of C.
  3. Let clauseSelector be ? GetValue(exprRef).
  4. Return the result of performing Strict Equality Comparison input === clauseSelector.
 Note

This operation does not execute C's StatementList (if any). The CaseBlock algorithm uses its return value to determine which StatementList to start executing.

13.12.11 Runtime Semantics: Evaluation

SwitchStatement : switch ( Expression ) CaseBlock
  1. Let exprRef be the result of evaluating Expression.
  2. Let switchValue be ? GetValue(exprRef).
  3. Let oldEnv be the running execution context's LexicalEnvironment.
  4. Let blockEnv be NewDeclarativeEnvironment(oldEnv).
  5. Perform BlockDeclarationInstantiation(CaseBlock, blockEnv).
  6. Set the running execution context's LexicalEnvironment to blockEnv.
  7. Let R be CaseBlockEvaluation of CaseBlock with argument switchValue.
  8. Set the running execution context's LexicalEnvironment to oldEnv.
  9. Return R.
 Note

No matter how control leaves the SwitchStatement the LexicalEnvironment is always restored to its former state.

 CaseClause : case Expression :
  1. Return NormalCompletion(empty).
 CaseClause : case Expression : StatementList
  1. Return the result of evaluating StatementList.
 DefaultClause : default :
  1. Return NormalCompletion(empty).
 DefaultClause : default : StatementList
  1. Return the result of evaluating StatementList.

13.13 Labelled Statements

Syntax

LabelledStatement[Yield, Await, Return] : LabelIdentifier[?Yield, ?Await] : LabelledItem[?Yield, ?Await, ?Return] LabelledItem[Yield, Await, Return] : Statement[?Yield, ?Await, ?Return] FunctionDeclaration[?Yield, ?Await, ~Default] Note

A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.

13.13.1 Static Semantics: Early Errors

LabelledItem : FunctionDeclaration
  • It is a Syntax Error if any source text matches this rule.
Note

An alternative definition for this rule is provided in B.3.2.

13.13.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

LabelledStatement : LabelIdentifier : LabelledItem
  1. Let label be the StringValue of LabelIdentifier.
  2. If label is an element of labelSet, return true.
  3. Let newLabelSet be a copy of labelSet with label appended.
  4. Return ContainsDuplicateLabels of LabelledItem with argument newLabelSet.
 LabelledItem : FunctionDeclaration
  1. Return false.

13.13.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

LabelledStatement : LabelIdentifier : LabelledItem
  1. Let label be the StringValue of LabelIdentifier.
  2. Let newLabelSet be a copy of labelSet with label appended.
  3. Return ContainsUndefinedBreakTarget of LabelledItem with argument newLabelSet.
 LabelledItem : FunctionDeclaration
  1. Return false.

13.13.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

LabelledStatement : LabelIdentifier : LabelledItem
  1. Let label be the StringValue of LabelIdentifier.
  2. Let newLabelSet be a copy of labelSet with label appended.
  3. Return ContainsUndefinedContinueTarget of LabelledItem with arguments iterationSet and newLabelSet.
 LabelledItem : FunctionDeclaration
  1. Return false.

13.13.5 Static Semantics: IsLabelledFunction ( stmt )

The abstract operation IsLabelledFunction with argument stmt performs the following steps:

  1. If stmt is not a LabelledStatement, return false.
  2. Let item be the LabelledItem of stmt.
  3. If item is LabelledItem : FunctionDeclaration , return true.
  4. Let subStmt be the Statement of item.
  5. Return IsLabelledFunction(subStmt).

13.13.6 Static Semantics: LexicallyDeclaredNames

LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the LexicallyDeclaredNames of LabelledItem.
 LabelledItem : Statement
  1. Return a new empty List.
 LabelledItem : FunctionDeclaration
  1. Return BoundNames of FunctionDeclaration.

13.13.7 Static Semantics: LexicallyScopedDeclarations

LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the LexicallyScopedDeclarations of LabelledItem.
 LabelledItem : Statement
  1. Return a new empty List.
 LabelledItem : FunctionDeclaration
  1. Return a new List containing FunctionDeclaration.

13.13.8 Static Semantics: TopLevelLexicallyDeclaredNames

13.13.9 Static Semantics: TopLevelLexicallyScopedDeclarations

13.13.10 Static Semantics: TopLevelVarDeclaredNames

LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the TopLevelVarDeclaredNames of LabelledItem.
 LabelledItem : Statement
  1. If Statement is Statement : LabelledStatement , return TopLevelVarDeclaredNames of Statement.
  2. Return VarDeclaredNames of Statement.
 LabelledItem : FunctionDeclaration
  1. Return BoundNames of FunctionDeclaration.

13.13.11 Static Semantics: TopLevelVarScopedDeclarations

LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the TopLevelVarScopedDeclarations of LabelledItem.
 LabelledItem : Statement
  1. If Statement is Statement : LabelledStatement , return TopLevelVarScopedDeclarations of Statement.
  2. Return VarScopedDeclarations of Statement.
 LabelledItem : FunctionDeclaration
  1. Return a new List containing FunctionDeclaration.

13.13.12 Static Semantics: VarDeclaredNames

LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the VarDeclaredNames of LabelledItem.
 LabelledItem : FunctionDeclaration
  1. Return a new empty List.

13.13.13 Static Semantics: VarScopedDeclarations

LabelledStatement : LabelIdentifier : LabelledItem
  1. Return the VarScopedDeclarations of LabelledItem.
 LabelledItem : FunctionDeclaration
  1. Return a new empty List.

13.13.14 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

LabelledStatement : LabelIdentifier : LabelledItem
  1. Let label be the StringValue of LabelIdentifier.
  2. Append label as an element of labelSet.
  3. Let stmtResult be LabelledEvaluation of LabelledItem with argument labelSet.
  4. If stmtResult.[[Type]] is break and SameValue(stmtResult.[[Target]], label) is true, then
    1. Set stmtResult to NormalCompletion(stmtResult.[[Value]]).
  5. Return Completion(stmtResult).
 LabelledItem : Statement
  1. If Statement is either a LabelledStatement or a BreakableStatement, then
    1. Return LabelledEvaluation of Statement with argument labelSet.
  2. Else,
    1. Return the result of evaluating Statement.
 LabelledItem : FunctionDeclaration
  1. Return the result of evaluating FunctionDeclaration.

13.13.15 Runtime Semantics: Evaluation

LabelledStatement : LabelIdentifier : LabelledItem
  1. Let newLabelSet be a new empty List.
  2. Return LabelledEvaluation of this LabelledStatement with argument newLabelSet.

13.14 The throw Statement

Syntax

ThrowStatement[Yield, Await] : throw [no LineTerminator here] Expression[+In, ?Yield, ?Await] ;

13.14.1 Runtime Semantics: Evaluation

ThrowStatement : throw Expression ;
  1. Let exprRef be the result of evaluating Expression.
  2. Let exprValue be ? GetValue(exprRef).
  3. Return ThrowCompletion(exprValue).

13.15 The try Statement

Syntax

TryStatement[Yield, Await, Return] : try Block[?Yield, ?Await, ?Return] Catch[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return] Finally[?Yield, ?Await, ?Return] try Block[?Yield, ?Await, ?Return] Catch[?Yield, ?Await, ?Return] Finally[?Yield, ?Await, ?Return] Catch[Yield, Await, Return] : catch ( CatchParameter[?Yield, ?Await] ) Block[?Yield, ?Await, ?Return] catch Block[?Yield, ?Await, ?Return] Finally[Yield, Await, Return] : finally Block[?Yield, ?Await, ?Return] CatchParameter[Yield, Await] : BindingIdentifier[?Yield, ?Await] BindingPattern[?Yield, ?Await] Note

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.

13.15.1 Static Semantics: Early Errors

Catch : catch ( CatchParameter ) Block
  • It is a Syntax Error if BoundNames of CatchParameter contains any duplicate elements.
  • It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the LexicallyDeclaredNames of Block.
  • It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the VarDeclaredNames of Block.
Note

An alternative static semantics for this production is given in B.3.5.

13.15.2 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

TryStatement : try Block Catch
  1. Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of Catch with argument labelSet.
 TryStatement : try Block Finally
  1. Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Return ContainsDuplicateLabels of Finally with argument labelSet.
 TryStatement : try Block Catch Finally
  1. Let hasDuplicates be ContainsDuplicateLabels of Block with argument labelSet.
  2. If hasDuplicates is true, return true.
  3. Let hasDuplicates be ContainsDuplicateLabels of Catch with argument labelSet.
  4. If hasDuplicates is true, return true.
  5. Return ContainsDuplicateLabels of Finally with argument labelSet.
 Catch : catch ( CatchParameter ) Block
  1. Return ContainsDuplicateLabels of Block with argument labelSet.

13.15.3 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

TryStatement : try Block Catch
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of Catch with argument labelSet.
 TryStatement : try Block Finally
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedBreakTarget of Finally with argument labelSet.
 TryStatement : try Block Catch Finally
  1. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Block with argument labelSet.
  2. If hasUndefinedLabels is true, return true.
  3. Let hasUndefinedLabels be ContainsUndefinedBreakTarget of Catch with argument labelSet.
  4. If hasUndefinedLabels is true, return true.
  5. Return ContainsUndefinedBreakTarget of Finally with argument labelSet.
 Catch : catch ( CatchParameter ) Block
  1. Return ContainsUndefinedBreakTarget of Block with argument labelSet.

13.15.4 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

TryStatement : try Block Catch
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « ».
 TryStatement : try Block Finally
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».
 TryStatement : try Block Catch Finally
  1. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».
  2. If hasUndefinedLabels is true, return true.
  3. Let hasUndefinedLabels be ContainsUndefinedContinueTarget of Catch with arguments iterationSet and « ».
  4. If hasUndefinedLabels is true, return true.
  5. Return ContainsUndefinedContinueTarget of Finally with arguments iterationSet and « ».
 Catch : catch ( CatchParameter ) Block
  1. Return ContainsUndefinedContinueTarget of Block with arguments iterationSet and « ».

13.15.5 Static Semantics: VarDeclaredNames

TryStatement : try Block Catch
  1. Let names be VarDeclaredNames of Block.
  2. Append to names the elements of the VarDeclaredNames of Catch.
  3. Return names.
 TryStatement : try Block Finally
  1. Let names be VarDeclaredNames of Block.
  2. Append to names the elements of the VarDeclaredNames of Finally.
  3. Return names.
 TryStatement : try Block Catch Finally
  1. Let names be VarDeclaredNames of Block.
  2. Append to names the elements of the VarDeclaredNames of Catch.
  3. Append to names the elements of the VarDeclaredNames of Finally.
  4. Return names.
 Catch : catch ( CatchParameter ) Block
  1. Return the VarDeclaredNames of Block.

13.15.6 Static Semantics: VarScopedDeclarations

TryStatement : try Block Catch
  1. Let declarations be VarScopedDeclarations of Block.
  2. Append to declarations the elements of the VarScopedDeclarations of Catch.
  3. Return declarations.
 TryStatement : try Block Finally
  1. Let declarations be VarScopedDeclarations of Block.
  2. Append to declarations the elements of the VarScopedDeclarations of Finally.
  3. Return declarations.
 TryStatement : try Block Catch Finally
  1. Let declarations be VarScopedDeclarations of Block.
  2. Append to declarations the elements of the VarScopedDeclarations of Catch.
  3. Append to declarations the elements of the VarScopedDeclarations of Finally.
  4. Return declarations.
 Catch : catch ( CatchParameter ) Block
  1. Return the VarScopedDeclarations of Block.

13.15.7 Runtime Semantics: CatchClauseEvaluation

With parameter thrownValue.

Catch : catch ( CatchParameter ) Block
  1. Let oldEnv be the running execution context's LexicalEnvironment.
  2. Let catchEnv be NewDeclarativeEnvironment(oldEnv).
  3. Let catchEnvRec be catchEnv's EnvironmentRecord.
  4. For each element argName of the BoundNames of CatchParameter, do
    1. Perform ! catchEnvRec.CreateMutableBinding(argName, false).
  5. Set the running execution context's LexicalEnvironment to catchEnv.
  6. Let status be BindingInitialization of CatchParameter with arguments thrownValue and catchEnv.
  7. If status is an abrupt completion, then
    1. Set the running execution context's LexicalEnvironment to oldEnv.
    2. Return Completion(status).
  8. Let B be the result of evaluating Block.
  9. Set the running execution context's LexicalEnvironment to oldEnv.
  10. Return Completion(B).
 Catch : catch Block
  1. Return the result of evaluating Block.
 Note

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

13.15.8 Runtime Semantics: Evaluation

TryStatement : try Block Catch
  1. Let B be the result of evaluating Block.
  2. If B.[[Type]] is throw, let C be CatchClauseEvaluation of Catch with argument B.[[Value]].
  3. Else, let C be B.
  4. Return Completion(UpdateEmpty(C, undefined)).
 TryStatement : try Block Finally
  1. Let B be the result of evaluating Block.
  2. Let F be the result of evaluating Finally.
  3. If F.[[Type]] is normal, set F to B.
  4. Return Completion(UpdateEmpty(F, undefined)).
 TryStatement : try Block Catch Finally
  1. Let B be the result of evaluating Block.
  2. If B.[[Type]] is throw, let C be CatchClauseEvaluation of Catch with argument B.[[Value]].
  3. Else, let C be B.
  4. Let F be the result of evaluating Finally.
  5. If F.[[Type]] is normal, set F to C.
  6. Return Completion(UpdateEmpty(F, undefined)).

13.16 The debugger Statement

Syntax

DebuggerStatement : debugger ;

13.16.1 Runtime Semantics: Evaluation

Note

Evaluating a DebuggerStatement may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.

 DebuggerStatement : debugger ;
  1. If an implementation-defined debugging facility is available and enabled, then
    1. Perform an implementation-defined debugging action.
    2. Let result be an implementation-defined Completion value.
  2. Else,
    1. Let result be NormalCompletion(empty).
  3. Return result.