UniqueFormalParameters : FormalParameters

• It is a Syntax Error if BoundNames of FormalParameters contains any duplicate elements.

FormalParameters : FormalParameterList

• It is a Syntax Error if IsSimpleParameterList of FormalParameterList is false and BoundNames of FormalParameterList contains any duplicate elements.

Note

Multiple occurrences of the same BindingIdentifier in a FormalParameterList is only allowed for functions which have simple parameter lists and which are not defined in strict mode code.

The syntax-directed operation ContainsExpression takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

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.

FormalParameters : [empty]

1. Return false.

FormalParameters : FormalParameterList , FunctionRestParameter

1. If ContainsExpression of FormalParameterList is true, return true.
2. Return ContainsExpression of FunctionRestParameter.

FormalParameterList : FormalParameterList , FormalParameter

1. If ContainsExpression of FormalParameterList is true, return true.
2. Return ContainsExpression of FormalParameter.

ArrowParameters : BindingIdentifier

1. Return false.

ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList

1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
2. Return ContainsExpression of formals.

AsyncArrowBindingIdentifier : BindingIdentifier

1. Return false.

The syntax-directed operation IsSimpleParameterList takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

BindingElement : BindingPattern

1. Return false.

BindingElement : BindingPattern Initializer

1. Return false.

SingleNameBinding : BindingIdentifier

1. Return true.

SingleNameBinding : BindingIdentifier Initializer

1. Return false.

FormalParameters : [empty]

1. Return true.

FormalParameters : FunctionRestParameter

1. Return false.

FormalParameters : FormalParameterList , FunctionRestParameter

1. Return false.

FormalParameterList : FormalParameterList , FormalParameter

1. If IsSimpleParameterList of FormalParameterList is false, return false.
2. Return IsSimpleParameterList of FormalParameter.

FormalParameter : BindingElement

1. Return IsSimpleParameterList of BindingElement.

ArrowParameters : BindingIdentifier

1. Return true.

ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList

1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
2. Return IsSimpleParameterList of formals.

AsyncArrowBindingIdentifier : BindingIdentifier

1. Return true.

CoverCallExpressionAndAsyncArrowHead : MemberExpression Arguments

1. Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
2. Return IsSimpleParameterList of head.

The syntax-directed operation HasInitializer takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

BindingElement : BindingPattern

1. Return false.

BindingElement : BindingPattern Initializer

1. Return true.

SingleNameBinding : BindingIdentifier

1. Return false.

SingleNameBinding : BindingIdentifier Initializer

1. Return true.

FormalParameterList : FormalParameterList , FormalParameter

1. If HasInitializer of FormalParameterList is true, return true.
2. Return HasInitializer of FormalParameter.

The syntax-directed operation ExpectedArgumentCount takes no arguments and returns an integer. It is defined piecewise over the following productions:

FormalParameters : [empty] FunctionRestParameter

1. Return 0.

FormalParameters : FormalParameterList , FunctionRestParameter

1. Return ExpectedArgumentCount of FormalParameterList.

Note

The ExpectedArgumentCount of a FormalParameterList is the number of FormalParameters to the left of either the rest parameter or the first FormalParameter with an Initializer. A FormalParameter without an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional with undefined as their default value.

FormalParameterList : FormalParameter

1. If HasInitializer of FormalParameter is true, return 0.
2. Return 1.

FormalParameterList : FormalParameterList , FormalParameter

1. Let count be ExpectedArgumentCount of FormalParameterList.
2. If HasInitializer of FormalParameterList is true or HasInitializer of FormalParameter is true, return count.
3. Return count + 1.

ArrowParameters : BindingIdentifier

1. Return 1.

ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList

1. Let formals be the ArrowFormalParameters that is covered by CoverParenthesizedExpressionAndArrowParameterList.
2. Return ExpectedArgumentCount of formals.

PropertySetParameterList : FormalParameter

1. If HasInitializer of FormalParameter is true, return 0.
2. Return 1.

AsyncArrowBindingIdentifier : BindingIdentifier

1. Return 1.

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody }

• If the source text matched by FormalParameters is strict mode code, the Early Error rules for UniqueFormalParameters : FormalParameters are applied.
• If BindingIdentifier is present and the source text matched by BindingIdentifier is strict mode code, it is a Syntax Error if the StringValue of BindingIdentifier is either "eval" or "arguments".
• It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true and IsSimpleParameterList of FormalParameters is false.
• It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of FunctionBody.
• It is a Syntax Error if FormalParameters Contains SuperProperty is true.
• It is a Syntax Error if FunctionBody Contains SuperProperty is true.
• It is a Syntax Error if FormalParameters Contains SuperCall is true.
• It is a Syntax Error if FunctionBody Contains SuperCall is true.

Note

The LexicallyDeclaredNames of a FunctionBody does not include identifiers bound using var or function declarations.

FunctionBody : FunctionStatementList

• It is a Syntax Error if the LexicallyDeclaredNames of FunctionStatementList contains any duplicate entries.
• It is a Syntax Error if any element of the LexicallyDeclaredNames of FunctionStatementList also occurs in the VarDeclaredNames of FunctionStatementList.
• It is a Syntax Error if ContainsDuplicateLabels of FunctionStatementList with argument « » is true.
• It is a Syntax Error if ContainsUndefinedBreakTarget of FunctionStatementList with argument « » is true.
• It is a Syntax Error if ContainsUndefinedContinueTarget of FunctionStatementList with arguments « » and « » is true.

The syntax-directed operation FunctionBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

FunctionBody : FunctionStatementList

1. If the Directive Prologue of FunctionBody contains a Use Strict Directive, return true; otherwise, return false.

The syntax-directed operation EvaluateFunctionBody takes arguments functionObject (an ECMAScript function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

FunctionBody : FunctionStatementList

1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
2. Return ? Evaluation of FunctionStatementList.

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

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }

1. Let name be StringValue of BindingIdentifier.
2. Let sourceText be the source text matched by FunctionDeclaration.
3. Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
4. Perform SetFunctionName(F, name).
5. Perform MakeConstructor(F).
6. Return F.

FunctionDeclaration : function ( FormalParameters ) { FunctionBody }

1. Let sourceText be the source text matched by FunctionDeclaration.
2. Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
3. Perform SetFunctionName(F, "default").
4. Perform MakeConstructor(F).
5. Return F.

Note

An anonymous FunctionDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

The syntax-directed operation InstantiateOrdinaryFunctionExpression takes optional argument name (a property key or a Private Name) and returns an ECMAScript function object. It is defined piecewise over the following productions:

FunctionExpression : function ( FormalParameters ) { FunctionBody }

1. If name is not present, set name to "".
2. Let env be the LexicalEnvironment of the running execution context.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by FunctionExpression.
5. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
6. Perform SetFunctionName(closure, name).
7. Perform MakeConstructor(closure).
8. Return closure.

FunctionExpression : function BindingIdentifier ( FormalParameters ) { FunctionBody }

1. Assert: name is not present.
2. Set name to StringValue of BindingIdentifier.
3. Let outerEnv be the running execution context's LexicalEnvironment.
4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
5. Perform ! funcEnv.CreateImmutableBinding(name, false).
6. Let privateEnv be the running execution context's PrivateEnvironment.
7. Let sourceText be the source text matched by FunctionExpression.
8. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, funcEnv, privateEnv).
9. Perform SetFunctionName(closure, name).
10. Perform MakeConstructor(closure).
11. Perform ! funcEnv.InitializeBinding(name, closure).
12. Return closure.

Note

The BindingIdentifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the BindingIdentifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }

1. Return empty.

Note 1

An alternative semantics is provided in B.3.2.

FunctionDeclaration : function ( FormalParameters ) { FunctionBody }

1. Return empty.

FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody }

1. Return InstantiateOrdinaryFunctionExpression of FunctionExpression.

Note 2

A "prototype" property is automatically created for every function defined using a FunctionDeclaration or FunctionExpression, to allow for the possibility that the function will be used as a constructor.

FunctionStatementList : [empty]

1. Return undefined.

ArrowFunction : ArrowParameters => ConciseBody

• It is a Syntax Error if ArrowParameters Contains YieldExpression is true.
• It is a Syntax Error if ArrowParameters Contains AwaitExpression is true.
• It is a Syntax Error if ConciseBodyContainsUseStrict of ConciseBody is true and IsSimpleParameterList of ArrowParameters is false.
• It is a Syntax Error if any element of the BoundNames of ArrowParameters also occurs in the LexicallyDeclaredNames of ConciseBody.

ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList

• CoverParenthesizedExpressionAndArrowParameterList must cover an ArrowFormalParameters.

The syntax-directed operation ConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

ConciseBody : ExpressionBody

1. Return false.

ConciseBody : { FunctionBody }

1. Return FunctionBodyContainsUseStrict of FunctionBody.

The syntax-directed operation EvaluateConciseBody takes arguments functionObject (an ECMAScript function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

ConciseBody : ExpressionBody

1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
2. Return ? Evaluation of ExpressionBody.

The syntax-directed operation InstantiateArrowFunctionExpression takes optional argument name (a property key or a Private Name) and returns an ECMAScript function object. It is defined piecewise over the following productions:

ArrowFunction : ArrowParameters => ConciseBody

1. If name is not present, set name to "".
2. Let env be the LexicalEnvironment of the running execution context.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by ArrowFunction.
5. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, ArrowParameters, ConciseBody, lexical-this, env, privateEnv).
6. Perform SetFunctionName(closure, name).
7. Return closure.

Note

An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain references to super, the function object created in step 5 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the env that is captured by the function object of the ArrowFunction.

ArrowFunction : ArrowParameters => ConciseBody

1. Return InstantiateArrowFunctionExpression of ArrowFunction.

ExpressionBody : AssignmentExpression

1. Let exprRef be ? Evaluation of AssignmentExpression.
2. Let exprValue be ? GetValue(exprRef).
3. Return Completion Record { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }

• It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
• It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of FunctionBody.

MethodDefinition : set ClassElementName ( PropertySetParameterList ) { FunctionBody }

• It is a Syntax Error if BoundNames of PropertySetParameterList contains any duplicate elements.
• It is a Syntax Error if FunctionBodyContainsUseStrict of FunctionBody is true and IsSimpleParameterList of PropertySetParameterList is false.
• It is a Syntax Error if any element of the BoundNames of PropertySetParameterList also occurs in the LexicallyDeclaredNames of FunctionBody.

The syntax-directed operation HasDirectSuper takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }

1. If UniqueFormalParameters Contains SuperCall is true, return true.
2. Return FunctionBody Contains SuperCall.

MethodDefinition : get ClassElementName ( ) { FunctionBody }

1. Return FunctionBody Contains SuperCall.

MethodDefinition : set ClassElementName ( PropertySetParameterList ) { FunctionBody }

1. If PropertySetParameterList Contains SuperCall is true, return true.
2. Return FunctionBody Contains SuperCall.

GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody }

1. If UniqueFormalParameters Contains SuperCall is true, return true.
2. Return GeneratorBody Contains SuperCall.

AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }

1. If UniqueFormalParameters Contains SuperCall is true, return true.
2. Return AsyncGeneratorBody Contains SuperCall.

AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }

1. If UniqueFormalParameters Contains SuperCall is true, return true.
2. Return AsyncFunctionBody Contains SuperCall.

The syntax-directed operation SpecialMethod takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }

1. Return false.

MethodDefinition : GeneratorMethod AsyncMethod AsyncGeneratorMethod get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody }

1. Return true.

The syntax-directed operation DefineMethod takes argument object (an Object) and optional argument functionPrototype (an Object) and returns either a normal completion containing a Record with fields [[Key]] (a property key) and [[Closure]] (an ECMAScript function object) or an abrupt completion. It is defined piecewise over the following productions:

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }

1. Let propKey be ? Evaluation of ClassElementName.
2. Let env be the running execution context's LexicalEnvironment.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. If functionPrototype is present, then
   1. Let prototype be functionPrototype.
5. Else,
   1. Let prototype be %Function.prototype%.
6. Let sourceText be the source text matched by MethodDefinition.
7. Let closure be OrdinaryFunctionCreate(prototype, sourceText, UniqueFormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
8. Perform MakeMethod(closure, object).
9. Return the Record { [[Key]]: propKey, [[Closure]]: closure }.

The syntax-directed operation MethodDefinitionEvaluation takes arguments object (an Object) and enumerable (a Boolean) and returns either a normal completion containing either a PrivateElement or unused, or an abrupt completion. It is defined piecewise over the following productions:

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody }

1. Let methodDef be ? DefineMethod of MethodDefinition with argument object.
2. Perform SetFunctionName(methodDef.[[Closure]], methodDef.[[Key]]).
3. Return ? DefineMethodProperty(object, methodDef.[[Key]], methodDef.[[Closure]], enumerable).

MethodDefinition : get ClassElementName ( ) { FunctionBody }

1. Let propKey be ? Evaluation of ClassElementName.
2. Let env be the running execution context's LexicalEnvironment.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by MethodDefinition.
5. Let formalParameterList be an instance of the production FormalParameters : [empty].
6. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameterList, FunctionBody, non-lexical-this, env, privateEnv).
7. Perform MakeMethod(closure, object).
8. Perform SetFunctionName(closure, propKey, "get").
9. If propKey is a Private Name, then
   1. Return PrivateElement { [[Key]]: propKey, [[Kind]]: accessor, [[Get]]: closure, [[Set]]: undefined }.
10. Else,
    1. Let desc be the PropertyDescriptor { [[Get]]: closure, [[Enumerable]]: enumerable, [[Configurable]]: true }.
    2. Perform ? DefinePropertyOrThrow(object, propKey, desc).
    3. Return unused.

MethodDefinition : set ClassElementName ( PropertySetParameterList ) { FunctionBody }

1. Let propKey be ? Evaluation of ClassElementName.
2. Let env be the running execution context's LexicalEnvironment.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by MethodDefinition.
5. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, PropertySetParameterList, FunctionBody, non-lexical-this, env, privateEnv).
6. Perform MakeMethod(closure, object).
7. Perform SetFunctionName(closure, propKey, "set").
8. If propKey is a Private Name, then
   1. Return PrivateElement { [[Key]]: propKey, [[Kind]]: accessor, [[Get]]: undefined, [[Set]]: closure }.
9. Else,
   1. Let desc be the PropertyDescriptor { [[Set]]: closure, [[Enumerable]]: enumerable, [[Configurable]]: true }.
   2. Perform ? DefinePropertyOrThrow(object, propKey, desc).
   3. Return unused.

GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody }

1. Let propKey be ? Evaluation of ClassElementName.
2. Let env be the running execution context's LexicalEnvironment.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by GeneratorMethod.
5. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, UniqueFormalParameters, GeneratorBody, non-lexical-this, env, privateEnv).
6. Perform MakeMethod(closure, object).
7. Perform SetFunctionName(closure, propKey).
8. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
9. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
10. Return ? DefineMethodProperty(object, propKey, closure, enumerable).

AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }

1. Let propKey be ? Evaluation of ClassElementName.
2. Let env be the running execution context's LexicalEnvironment.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by AsyncGeneratorMethod.
5. Let closure be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, UniqueFormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
6. Perform MakeMethod(closure, object).
7. Perform SetFunctionName(closure, propKey).
8. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
9. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
10. Return ? DefineMethodProperty(object, propKey, closure, enumerable).

AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }

1. Let propKey be ? Evaluation of ClassElementName.
2. Let env be the LexicalEnvironment of the running execution context.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by AsyncMethod.
5. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, UniqueFormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
6. Perform MakeMethod(closure, object).
7. Perform SetFunctionName(closure, propKey).
8. Return ? DefineMethodProperty(object, propKey, closure, enumerable).

GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody }

• It is a Syntax Error if HasDirectSuper of GeneratorMethod is true.
• It is a Syntax Error if UniqueFormalParameters Contains YieldExpression is true.
• It is a Syntax Error if FunctionBodyContainsUseStrict of GeneratorBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
• It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of GeneratorBody.

GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody } GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody }

• If the source text matched by FormalParameters is strict mode code, the Early Error rules for UniqueFormalParameters : FormalParameters are applied.
• If BindingIdentifier is present and the source text matched by BindingIdentifier is strict mode code, it is a Syntax Error if the StringValue of BindingIdentifier is either "eval" or "arguments".
• It is a Syntax Error if FunctionBodyContainsUseStrict of GeneratorBody is true and IsSimpleParameterList of FormalParameters is false.
• It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of GeneratorBody.
• It is a Syntax Error if FormalParameters Contains YieldExpression is true.
• It is a Syntax Error if FormalParameters Contains SuperProperty is true.
• It is a Syntax Error if GeneratorBody Contains SuperProperty is true.
• It is a Syntax Error if FormalParameters Contains SuperCall is true.
• It is a Syntax Error if GeneratorBody Contains SuperCall is true.

The syntax-directed operation EvaluateGeneratorBody takes arguments functionObject (an ECMAScript function object) and argumentsList (a List of ECMAScript language values) and returns a throw completion or a return completion. It is defined piecewise over the following productions:

GeneratorBody : FunctionBody

1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
2. Let G be ? OrdinaryCreateFromConstructor(functionObject, "%GeneratorFunction.prototype.prototype%", « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] »).
3. Set G.[[GeneratorBrand]] to empty.
4. Perform GeneratorStart(G, FunctionBody).
5. Return Completion Record { [[Type]]: return, [[Value]]: G, [[Target]]: empty }.

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

GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody }

1. Let name be StringValue of BindingIdentifier.
2. Let sourceText be the source text matched by GeneratorDeclaration.
3. Let F be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, env, privateEnv).
4. Perform SetFunctionName(F, name).
5. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
6. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
7. Return F.

GeneratorDeclaration : function * ( FormalParameters ) { GeneratorBody }

1. Let sourceText be the source text matched by GeneratorDeclaration.
2. Let F be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, env, privateEnv).
3. Perform SetFunctionName(F, "default").
4. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
5. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
6. Return F.

Note

An anonymous GeneratorDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

The syntax-directed operation InstantiateGeneratorFunctionExpression takes optional argument name (a property key or a Private Name) and returns an ECMAScript function object. It is defined piecewise over the following productions:

GeneratorExpression : function * ( FormalParameters ) { GeneratorBody }

1. If name is not present, set name to "".
2. Let env be the LexicalEnvironment of the running execution context.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by GeneratorExpression.
5. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, env, privateEnv).
6. Perform SetFunctionName(closure, name).
7. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
8. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
9. Return closure.

GeneratorExpression : function * BindingIdentifier ( FormalParameters ) { GeneratorBody }

1. Assert: name is not present.
2. Set name to StringValue of BindingIdentifier.
3. Let outerEnv be the running execution context's LexicalEnvironment.
4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
5. Perform ! funcEnv.CreateImmutableBinding(name, false).
6. Let privateEnv be the running execution context's PrivateEnvironment.
7. Let sourceText be the source text matched by GeneratorExpression.
8. Let closure be OrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText, FormalParameters, GeneratorBody, non-lexical-this, funcEnv, privateEnv).
9. Perform SetFunctionName(closure, name).
10. Let prototype be OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
11. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
12. Perform ! funcEnv.InitializeBinding(name, closure).
13. Return closure.

Note

The BindingIdentifier in a GeneratorExpression can be referenced from inside the GeneratorExpression's FunctionBody to allow the generator code to call itself recursively. However, unlike in a GeneratorDeclaration, the BindingIdentifier in a GeneratorExpression cannot be referenced from and does not affect the scope enclosing the GeneratorExpression.

GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody }

1. Return InstantiateGeneratorFunctionExpression of GeneratorExpression.

YieldExpression : yield

1. Return ? Yield(undefined).

YieldExpression : yield AssignmentExpression

1. Let exprRef be ? Evaluation of AssignmentExpression.
2. Let value be ? GetValue(exprRef).
3. Return ? Yield(value).

YieldExpression : yield * AssignmentExpression

1. Let generatorKind be GetGeneratorKind().
2. Let exprRef be ? Evaluation of AssignmentExpression.
3. Let value be ? GetValue(exprRef).
4. Let iteratorRecord be ? GetIterator(value, generatorKind).
5. Let iterator be iteratorRecord.[[Iterator]].
6. Let received be NormalCompletion(undefined).
7. Repeat,
   1. If received is a normal completion, then
      1. Let innerResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « received.[[Value]] »).
      2. If generatorKind is async, set innerResult to ? Await(innerResult).
      3. If innerResult is not an Object, throw a TypeError exception.
      4. Let done be ? IteratorComplete(innerResult).
      5. If done is true, then
         1. Return ? IteratorValue(innerResult).
      6. If generatorKind is async, set received to Completion(AsyncGeneratorYield(? IteratorValue(innerResult))).
      7. Else, set received to Completion(GeneratorYield(innerResult)).
   2. Else if received is a throw completion, then
      1. Let throw be ? GetMethod(iterator, "throw").
      2. If throw is not undefined, then
         1. Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
         2. If generatorKind is async, set innerResult to ? Await(innerResult).
         3. NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
         4. If innerResult is not an Object, throw a TypeError exception.
         5. Let done be ? IteratorComplete(innerResult).
         6. If done is true, then
            1. Return ? IteratorValue(innerResult).
         7. If generatorKind is async, set received to Completion(AsyncGeneratorYield(? IteratorValue(innerResult))).
         8. Else, set received to Completion(GeneratorYield(innerResult)).
      3. Else,
         1. NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
         2. Let closeCompletion be Completion Record { [[Type]]: normal, [[Value]]: empty, [[Target]]: empty }.
         3. If generatorKind is async, perform ? AsyncIteratorClose(iteratorRecord, closeCompletion).
         4. Else, perform ? IteratorClose(iteratorRecord, closeCompletion).
         5. NOTE: The next step throws a TypeError to indicate that there was a yield* protocol violation: iterator does not have a throw method.
         6. Throw a TypeError exception.
   3. Else,
      1. Assert: received is a return completion.
      2. Let return be ? GetMethod(iterator, "return").
      3. If return is undefined, then
         1. Set value to received.[[Value]].
         2. If generatorKind is async, then
            1. Set value to ? Await(value).
         3. Return Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
      4. Let innerReturnResult be ? Call(return, iterator, « received.[[Value]] »).
      5. If generatorKind is async, set innerReturnResult to ? Await(innerReturnResult).
      6. If innerReturnResult is not an Object, throw a TypeError exception.
      7. Let done be ? IteratorComplete(innerReturnResult).
      8. If done is true, then
         1. Set value to ? IteratorValue(innerReturnResult).
         2. Return Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
      9. If generatorKind is async, set received to Completion(AsyncGeneratorYield(? IteratorValue(innerReturnResult))).
      10. Else, set received to Completion(GeneratorYield(innerReturnResult)).

AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }

• It is a Syntax Error if HasDirectSuper of AsyncGeneratorMethod is true.
• It is a Syntax Error if UniqueFormalParameters Contains YieldExpression is true.
• It is a Syntax Error if UniqueFormalParameters Contains AwaitExpression is true.
• It is a Syntax Error if FunctionBodyContainsUseStrict of AsyncGeneratorBody is true and IsSimpleParameterList of UniqueFormalParameters is false.
• It is a Syntax Error if any element of the BoundNames of UniqueFormalParameters also occurs in the LexicallyDeclaredNames of AsyncGeneratorBody.

AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody }

• If the source text matched by FormalParameters is strict mode code, the Early Error rules for UniqueFormalParameters : FormalParameters are applied.
• If BindingIdentifier is present and the source text matched by BindingIdentifier is strict mode code, it is a Syntax Error if the StringValue of BindingIdentifier is either "eval" or "arguments".
• It is a Syntax Error if FunctionBodyContainsUseStrict of AsyncGeneratorBody is true and IsSimpleParameterList of FormalParameters is false.
• It is a Syntax Error if any element of the BoundNames of FormalParameters also occurs in the LexicallyDeclaredNames of AsyncGeneratorBody.
• It is a Syntax Error if FormalParameters Contains YieldExpression is true.
• It is a Syntax Error if FormalParameters Contains AwaitExpression is true.
• It is a Syntax Error if FormalParameters Contains SuperProperty is true.
• It is a Syntax Error if AsyncGeneratorBody Contains SuperProperty is true.
• It is a Syntax Error if FormalParameters Contains SuperCall is true.
• It is a Syntax Error if AsyncGeneratorBody Contains SuperCall is true.

The syntax-directed operation EvaluateAsyncGeneratorBody takes arguments functionObject (an ECMAScript function object) and argumentsList (a List of ECMAScript language values) and returns a throw completion or a return completion. It is defined piecewise over the following productions:

AsyncGeneratorBody : FunctionBody

1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
2. Let generator be ? OrdinaryCreateFromConstructor(functionObject, "%AsyncGeneratorFunction.prototype.prototype%", « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] »).
3. Set generator.[[GeneratorBrand]] to empty.
4. Perform AsyncGeneratorStart(generator, FunctionBody).
5. Return Completion Record { [[Type]]: return, [[Value]]: generator, [[Target]]: empty }.

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

AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody }

1. Let name be StringValue of BindingIdentifier.
2. Let sourceText be the source text matched by AsyncGeneratorDeclaration.
3. Let F be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
4. Perform SetFunctionName(F, name).
5. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
6. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
7. Return F.

AsyncGeneratorDeclaration : async function * ( FormalParameters ) { AsyncGeneratorBody }

1. Let sourceText be the source text matched by AsyncGeneratorDeclaration.
2. Let F be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
3. Perform SetFunctionName(F, "default").
4. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
5. Perform ! DefinePropertyOrThrow(F, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
6. Return F.

Note

An anonymous AsyncGeneratorDeclaration can only occur as part of an export default declaration.

The syntax-directed operation InstantiateAsyncGeneratorFunctionExpression takes optional argument name (a property key or a Private Name) and returns an ECMAScript function object. It is defined piecewise over the following productions:

AsyncGeneratorExpression : async function * ( FormalParameters ) { AsyncGeneratorBody }

1. If name is not present, set name to "".
2. Let env be the LexicalEnvironment of the running execution context.
3. Let privateEnv be the running execution context's PrivateEnvironment.
4. Let sourceText be the source text matched by AsyncGeneratorExpression.
5. Let closure be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, env, privateEnv).
6. Perform SetFunctionName(closure, name).
7. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
8. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
9. Return closure.

AsyncGeneratorExpression : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody }

1. Assert: name is not present.
2. Set name to StringValue of BindingIdentifier.
3. Let outerEnv be the running execution context's LexicalEnvironment.
4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
5. Perform ! funcEnv.CreateImmutableBinding(name, false).
6. Let privateEnv be the running execution context's PrivateEnvironment.
7. Let sourceText be the source text matched by AsyncGeneratorExpression.
8. Let closure be OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText, FormalParameters, AsyncGeneratorBody, non-lexical-this, funcEnv, privateEnv).
9. Perform SetFunctionName(closure, name).
10. Let prototype be OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
11. Perform ! DefinePropertyOrThrow(closure, "prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }).
12. Perform ! funcEnv.InitializeBinding(name, closure).
13. Return closure.

Note

The BindingIdentifier in an AsyncGeneratorExpression can be referenced from inside the AsyncGeneratorExpression's AsyncGeneratorBody to allow the generator code to call itself recursively. However, unlike in an AsyncGeneratorDeclaration, the BindingIdentifier in an AsyncGeneratorExpression cannot be referenced from and does not affect the scope enclosing the AsyncGeneratorExpression.

AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody }

1. Return InstantiateAsyncGeneratorFunctionExpression of AsyncGeneratorExpression.

ClassTail : ClassHeritageopt { ClassBody }

• It is a Syntax Error if ClassHeritage is not present and the following algorithm returns true:

  1. Let constructor be ConstructorMethod of ClassBody.
  2. If constructor is empty, return false.
  3. Return HasDirectSuper of constructor.

ClassBody : ClassElementList

• It is a Syntax Error if PrototypePropertyNameList of ClassElementList contains more than one occurrence of "constructor".
• It is a Syntax Error if PrivateBoundIdentifiers of ClassElementList contains any duplicate entries, unless the name is used once for a getter and once for a setter and in no other entries, and the getter and setter are either both static or both non-static.

ClassElement : MethodDefinition

• It is a Syntax Error if PropName of MethodDefinition is not "constructor" and HasDirectSuper of MethodDefinition is true.
• It is a Syntax Error if PropName of MethodDefinition is "constructor" and SpecialMethod of MethodDefinition is true.

ClassElement : static MethodDefinition

• It is a Syntax Error if HasDirectSuper of MethodDefinition is true.
• It is a Syntax Error if PropName of MethodDefinition is "prototype".

ClassElement : FieldDefinition ;

• It is a Syntax Error if PropName of FieldDefinition is "constructor".

ClassElement : static FieldDefinition ;

• It is a Syntax Error if PropName of FieldDefinition is either "prototype" or "constructor".

FieldDefinition : ClassElementName Initializeropt

• It is a Syntax Error if Initializer is present and ContainsArguments of Initializer is true.
• It is a Syntax Error if Initializer is present and Initializer Contains SuperCall is true.

ClassElementName : PrivateIdentifier

• It is a Syntax Error if StringValue of PrivateIdentifier is "#constructor".

ClassStaticBlockBody : ClassStaticBlockStatementList

• It is a Syntax Error if the LexicallyDeclaredNames of ClassStaticBlockStatementList contains any duplicate entries.
• It is a Syntax Error if any element of the LexicallyDeclaredNames of ClassStaticBlockStatementList also occurs in the VarDeclaredNames of ClassStaticBlockStatementList.
• It is a Syntax Error if ContainsDuplicateLabels of ClassStaticBlockStatementList with argument « » is true.
• It is a Syntax Error if ContainsUndefinedBreakTarget of ClassStaticBlockStatementList with argument « » is true.
• It is a Syntax Error if ContainsUndefinedContinueTarget of ClassStaticBlockStatementList with arguments « » and « » is true.
• It is a Syntax Error if ContainsArguments of ClassStaticBlockStatementList is true.
• It is a Syntax Error if ClassStaticBlockStatementList Contains SuperCall is true.
• It is a Syntax Error if ClassStaticBlockStatementList Contains await is true.

The syntax-directed operation ClassElementKind takes no arguments and returns constructor-method, non-constructor-method, or empty. It is defined piecewise over the following productions:

ClassElement : MethodDefinition

1. If PropName of MethodDefinition is "constructor", return constructor-method.
2. Return non-constructor-method.

ClassElement : static MethodDefinition FieldDefinition ; static FieldDefinition ;

1. Return non-constructor-method.

ClassElement : ClassStaticBlock

1. Return non-constructor-method.

ClassElement : ;

1. Return empty.

The syntax-directed operation ConstructorMethod takes no arguments and returns a ClassElement Parse Node or empty. It is defined piecewise over the following productions:

ClassElementList : ClassElement

1. If ClassElementKind of ClassElement is constructor-method, return ClassElement.
2. Return empty.

ClassElementList : ClassElementList ClassElement

1. Let head be ConstructorMethod of ClassElementList.
2. If head is not empty, return head.
3. If ClassElementKind of ClassElement is constructor-method, return ClassElement.
4. Return empty.

Note

Early Error rules ensure that there is only one method definition named "constructor" and that it is not an accessor property or generator definition.

The syntax-directed operation IsStatic takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

ClassElement : MethodDefinition

1. Return false.

ClassElement : static MethodDefinition

1. Return true.

ClassElement : FieldDefinition ;

1. Return false.

ClassElement : static FieldDefinition ;

1. Return true.

ClassElement : ClassStaticBlock

1. Return true.

ClassElement : ;

1. Return false.

The syntax-directed operation NonConstructorElements takes no arguments and returns a List of ClassElement Parse Nodes. It is defined piecewise over the following productions:

ClassElementList : ClassElement

1. If ClassElementKind of ClassElement is non-constructor-method, then
   1. Return « ClassElement ».
2. Return a new empty List.

ClassElementList : ClassElementList ClassElement

1. Let list be NonConstructorElements of ClassElementList.
2. If ClassElementKind of ClassElement is non-constructor-method, then
   1. Append ClassElement to the end of list.
3. Return list.

The syntax-directed operation PrototypePropertyNameList takes no arguments and returns a List of property keys. It is defined piecewise over the following productions:

ClassElementList : ClassElement

1. Let propName be PropName of ClassElement.
2. If propName is empty, return a new empty List.
3. If IsStatic of ClassElement is true, return a new empty List.
4. Return « propName ».

ClassElementList : ClassElementList ClassElement

1. Let list be PrototypePropertyNameList of ClassElementList.
2. Let propName be PropName of ClassElement.
3. If propName is empty, return list.
4. If IsStatic of ClassElement is true, return list.
5. Return the list-concatenation of list and « propName ».

The syntax-directed operation AllPrivateIdentifiersValid takes argument names (a List of Strings) and returns a Boolean.

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of AllPrivateIdentifiersValid:

1. For each child node child of this Parse Node, do
   1. If child is an instance of a nonterminal, then
      1. If AllPrivateIdentifiersValid of child with argument names is false, return false.
2. Return true.

MemberExpression : MemberExpression . PrivateIdentifier

1. If names contains the StringValue of PrivateIdentifier, then
   1. Return AllPrivateIdentifiersValid of MemberExpression with argument names.
2. Return false.

CallExpression : CallExpression . PrivateIdentifier

1. If names contains the StringValue of PrivateIdentifier, then
   1. Return AllPrivateIdentifiersValid of CallExpression with argument names.
2. Return false.

OptionalChain : ?. PrivateIdentifier

1. If names contains the StringValue of PrivateIdentifier, return true.
2. Return false.

OptionalChain : OptionalChain . PrivateIdentifier

1. If names contains the StringValue of PrivateIdentifier, then
   1. Return AllPrivateIdentifiersValid of OptionalChain with argument names.
2. Return false.

ClassBody : ClassElementList

1. Let newNames be the list-concatenation of names and PrivateBoundIdentifiers of ClassBody.
2. Return AllPrivateIdentifiersValid of ClassElementList with argument newNames.

RelationalExpression : PrivateIdentifier in ShiftExpression

1. If names contains the StringValue of PrivateIdentifier, then
   1. Return AllPrivateIdentifiersValid of ShiftExpression with argument names.
2. Return false.

The syntax-directed operation PrivateBoundIdentifiers takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:

FieldDefinition : ClassElementName Initializeropt

1. Return PrivateBoundIdentifiers of ClassElementName.

ClassElementName : PrivateIdentifier

1. Return a List whose sole element is the StringValue of PrivateIdentifier.

ClassElementName : PropertyName ClassElement : ClassStaticBlock ;

1. Return a new empty List.

ClassElementList : ClassElementList ClassElement

1. Let names1 be PrivateBoundIdentifiers of ClassElementList.
2. Let names2 be PrivateBoundIdentifiers of ClassElement.
3. Return the list-concatenation of names1 and names2.

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody } get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody } GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody } AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody } AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody }

1. Return PrivateBoundIdentifiers of ClassElementName.

The syntax-directed operation ContainsArguments takes no arguments and returns a Boolean.

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of ContainsArguments:

1. For each child node child of this Parse Node, do
   1. If child is an instance of a nonterminal, then
      1. If ContainsArguments of child is true, return true.
2. Return false.

IdentifierReference : Identifier

1. If the StringValue of Identifier is "arguments", return true.
2. Return false.

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody } GeneratorDeclaration : function * BindingIdentifier ( FormalParameters ) { GeneratorBody } function * ( FormalParameters ) { GeneratorBody } GeneratorExpression : function * BindingIdentifieropt ( FormalParameters ) { GeneratorBody } AsyncGeneratorDeclaration : async function * BindingIdentifier ( FormalParameters ) { AsyncGeneratorBody } async function * ( FormalParameters ) { AsyncGeneratorBody } AsyncGeneratorExpression : async function * BindingIdentifieropt ( FormalParameters ) { AsyncGeneratorBody } AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody } AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody }

1. Return false.

MethodDefinition : ClassElementName ( UniqueFormalParameters ) { FunctionBody } get ClassElementName ( ) { FunctionBody } set ClassElementName ( PropertySetParameterList ) { FunctionBody } GeneratorMethod : * ClassElementName ( UniqueFormalParameters ) { GeneratorBody } AsyncGeneratorMethod : async * ClassElementName ( UniqueFormalParameters ) { AsyncGeneratorBody } AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody }

1. Return ContainsArguments of ClassElementName.

The syntax-directed operation ClassFieldDefinitionEvaluation takes argument homeObject (an Object) and returns either a normal completion containing a ClassFieldDefinition Record or an abrupt completion. It is defined piecewise over the following productions:

FieldDefinition : ClassElementName Initializeropt

1. Let name be ? Evaluation of ClassElementName.
2. If Initializer is present, then
   1. Let formalParameterList be an instance of the production FormalParameters : [empty].
   2. Let env be the LexicalEnvironment of the running execution context.
   3. Let privateEnv be the running execution context's PrivateEnvironment.
   4. Let sourceText be the empty sequence of Unicode code points.
   5. Let initializer be OrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameterList, Initializer, non-lexical-this, env, privateEnv).
   6. Perform MakeMethod(initializer, homeObject).
   7. Set initializer.[[ClassFieldInitializerName]] to name.
3. Else,
   1. Let initializer be empty.
4. Return the ClassFieldDefinition Record { [[Name]]: name, [[Initializer]]: initializer }.

Note

The function created for initializer is never directly accessible to ECMAScript code.

The syntax-directed operation ClassStaticBlockDefinitionEvaluation takes argument homeObject (an Object) and returns a ClassStaticBlockDefinition Record. It is defined piecewise over the following productions:

ClassStaticBlock : static { ClassStaticBlockBody }

1. Let lex be the running execution context's LexicalEnvironment.
2. Let privateEnv be the running execution context's PrivateEnvironment.
3. Let sourceText be the empty sequence of Unicode code points.
4. Let formalParameters be an instance of the production FormalParameters : [empty].
5. Let bodyFunction be OrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameters, ClassStaticBlockBody, non-lexical-this, lex, privateEnv).
6. Perform MakeMethod(bodyFunction, homeObject).
7. Return the ClassStaticBlockDefinition Record { [[BodyFunction]]: bodyFunction }.

Note

The function bodyFunction is never directly accessible to ECMAScript code.

The syntax-directed operation EvaluateClassStaticBlockBody takes argument functionObject (an ECMAScript function object) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

ClassStaticBlockBody : ClassStaticBlockStatementList

1. Assert: functionObject is a synthetic function created by ClassStaticBlockDefinitionEvaluation step 5.
2. Perform ! FunctionDeclarationInstantiation(functionObject, « »).
3. Return ? Evaluation of ClassStaticBlockStatementList.

The syntax-directed operation ClassElementEvaluation takes argument object (an Object) and returns either a normal completion containing either a ClassFieldDefinition Record, a ClassStaticBlockDefinition Record, a PrivateElement, or unused, or an abrupt completion. It is defined piecewise over the following productions:

ClassElement : FieldDefinition ; static FieldDefinition ;

1. Return ? ClassFieldDefinitionEvaluation of FieldDefinition with argument object.

ClassElement : MethodDefinition static MethodDefinition

1. Return ? MethodDefinitionEvaluation of MethodDefinition with arguments object and false.

ClassElement : ClassStaticBlock

1. Return ClassStaticBlockDefinitionEvaluation of ClassStaticBlock with argument object.

ClassElement : ;

1. Return unused.

The syntax-directed operation ClassDefinitionEvaluation takes arguments classBinding (a String or undefined) and className (a property key or a Private Name) and returns either a normal completion containing a function object or an abrupt completion.

Note

For ease of specification, private methods and accessors are included alongside private fields in the [[PrivateElements]] slot of class instances. However, any given object has either all or none of the private methods and accessors defined by a given class. This feature has been designed so that implementations may choose to implement private methods and accessors using a strategy which does not require tracking each method or accessor individually.

For example, an implementation could directly associate instance private methods with their corresponding Private Name and track, for each object, which class constructors have run with that object as their this value. Looking up an instance private method on an object then consists of checking that the class constructor which defines the method has been used to initialize the object, then returning the method associated with the Private Name.

This differs from private fields: because field initializers can throw during class instantiation, an individual object may have some proper subset of the private fields of a given class, and so private fields must in general be tracked individually.

It is defined piecewise over the following productions:

ClassTail : ClassHeritageopt { ClassBodyopt }

1. Let env be the LexicalEnvironment of the running execution context.
2. Let classEnv be NewDeclarativeEnvironment(env).
3. If classBinding is not undefined, then
   1. Perform ! classEnv.CreateImmutableBinding(classBinding, true).
4. Let outerPrivateEnvironment be the running execution context's PrivateEnvironment.
5. Let classPrivateEnvironment be NewPrivateEnvironment(outerPrivateEnvironment).
6. If ClassBody is present, then
   1. For each String dn of the PrivateBoundIdentifiers of ClassBody, do
      1. If classPrivateEnvironment.[[Names]] contains a Private Name pn such that pn.[[Description]] is dn, then
         1. Assert: This is only possible for getter/setter pairs.
      2. Else,
         1. Let name be a new Private Name whose [[Description]] is dn.
         2. Append name to classPrivateEnvironment.[[Names]].
7. If ClassHeritage is not present, then
   1. Let protoParent be %Object.prototype%.
   2. Let constructorParent be %Function.prototype%.
8. Else,
   1. Set the running execution context's LexicalEnvironment to classEnv.
   2. NOTE: The running execution context's PrivateEnvironment is outerPrivateEnvironment when evaluating ClassHeritage.
   3. Let superclassRef be Completion(Evaluation of ClassHeritage).
   4. Set the running execution context's LexicalEnvironment to env.
   5. Let superclass be ? GetValue(? superclassRef).
   6. If superclass is null, then
      1. Let protoParent be null.
      2. Let constructorParent be %Function.prototype%.
   7. Else if IsConstructor(superclass) is false, then
      1. Throw a TypeError exception.
   8. Else,
      1. Let protoParent be ? Get(superclass, "prototype").
      2. If protoParent is not an Object and protoParent is not null, throw a TypeError exception.
      3. Let constructorParent be superclass.
9. Let proto be OrdinaryObjectCreate(protoParent).
10. If ClassBody is not present, let constructor be empty.
11. Else, let constructor be ConstructorMethod of ClassBody.
12. Set the running execution context's LexicalEnvironment to classEnv.
13. Set the running execution context's PrivateEnvironment to classPrivateEnvironment.
14. If constructor is empty, then
    1. Let defaultConstructor be a new Abstract Closure with no parameters that captures nothing and performs the following steps when called:
       1. Let args be the List of arguments that was passed to this function by [[Call]] or [[Construct]].
       2. If NewTarget is undefined, throw a TypeError exception.
       3. Let F be the active function object.
       4. If F.[[ConstructorKind]] is derived, then
          1. NOTE: This branch behaves similarly to constructor(...args) { super(...args); }. The most notable distinction is that while the aforementioned ECMAScript source text observably calls the @@iterator method on %Array.prototype%, this function does not.
          2. Let func be ! F.[[GetPrototypeOf]]().
          3. If IsConstructor(func) is false, throw a TypeError exception.
          4. Let result be ? Construct(func, args, NewTarget).
       5. Else,
          1. NOTE: This branch behaves similarly to constructor() {}.
          2. Let result be ? OrdinaryCreateFromConstructor(NewTarget, "%Object.prototype%").
       6. Perform ? InitializeInstanceElements(result, F).
       7. Return result.
    2. Let F be CreateBuiltinFunction(defaultConstructor, 0, className, « [[ConstructorKind]], [[SourceText]] », the current Realm Record, constructorParent).
15. Else,
    1. Let constructorInfo be ! DefineMethod of constructor with arguments proto and constructorParent.
    2. Let F be constructorInfo.[[Closure]].
    3. Perform MakeClassConstructor(F).
    4. Perform SetFunctionName(F, className).
16. Perform MakeConstructor(F, false, proto).
17. If ClassHeritage is present, set F.[[ConstructorKind]] to derived.
18. Perform ! DefineMethodProperty(proto, "constructor", F, false).
19. If ClassBody is not present, let elements be a new empty List.
20. Else, let elements be NonConstructorElements of ClassBody.
21. Let instancePrivateMethods be a new empty List.
22. Let staticPrivateMethods be a new empty List.
23. Let instanceFields be a new empty List.
24. Let staticElements be a new empty List.
25. For each ClassElement e of elements, do
    1. If IsStatic of e is false, then
       1. Let element be Completion(ClassElementEvaluation of e with argument proto).
    2. Else,
       1. Let element be Completion(ClassElementEvaluation of e with argument F).
    3. If element is an abrupt completion, then
       1. Set the running execution context's LexicalEnvironment to env.
       2. Set the running execution context's PrivateEnvironment to outerPrivateEnvironment.
       3. Return ? element.
    4. Set element to ! element.
    5. If element is a PrivateElement, then
       1. Assert: element.[[Kind]] is either method or accessor.
       2. If IsStatic of e is false, let container be instancePrivateMethods.
       3. Else, let container be staticPrivateMethods.
       4. If container contains a PrivateElement pe such that pe.[[Key]] is element.[[Key]], then
          1. Assert: element.[[Kind]] and pe.[[Kind]] are both accessor.
          2. If element.[[Get]] is undefined, then
             1. Let combined be PrivateElement { [[Key]]: element.[[Key]], [[Kind]]: accessor, [[Get]]: pe.[[Get]], [[Set]]: element.[[Set]] }.
          3. Else,
             1. Let combined be PrivateElement { [[Key]]: element.[[Key]], [[Kind]]: accessor, [[Get]]: element.[[Get]], [[Set]]: pe.[[Set]] }.
          4. Replace pe in container with combined.
       5. Else,
          1. Append element to container.
    6. Else if element is a ClassFieldDefinition Record, then
       1. If IsStatic of e is false, append element to instanceFields.
       2. Else, append element to staticElements.
    7. Else if element is a ClassStaticBlockDefinition Record, then
       1. Append element to staticElements.
26. Set the running execution context's LexicalEnvironment to env.
27. If classBinding is not undefined, then
    1. Perform ! classEnv.InitializeBinding(classBinding, F).
28. Set F.[[PrivateMethods]] to instancePrivateMethods.
29. Set F.[[Fields]] to instanceFields.
30. For each PrivateElement method of staticPrivateMethods, do
    1. Perform ! PrivateMethodOrAccessorAdd(F, method).
31. For each element elementRecord of staticElements, do
    1. If elementRecord is a ClassFieldDefinition Record, then
       1. Let result be Completion(DefineField(F, elementRecord)).
    2. Else,
       1. Assert: elementRecord is a ClassStaticBlockDefinition Record.
       2. Let result be Completion(Call(elementRecord.[[BodyFunction]], F)).
    3. If result is an abrupt completion, then
       1. Set the running execution context's PrivateEnvironment to outerPrivateEnvironment.
       2. Return ? result.
32. Set the running execution context's PrivateEnvironment to outerPrivateEnvironment.
33. Return F.

The syntax-directed operation BindingClassDeclarationEvaluation takes no arguments and returns either a normal completion containing a function object or an abrupt completion. It is defined piecewise over the following productions:

ClassDeclaration : class BindingIdentifier ClassTail

1. Let className be StringValue of BindingIdentifier.
2. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments className and className.
3. Set value.[[SourceText]] to the source text matched by ClassDeclaration.
4. Let env be the running execution context's LexicalEnvironment.
5. Perform ? InitializeBoundName(className, value, env).
6. Return value.

ClassDeclaration : class ClassTail

1. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments undefined and "default".
2. Set value.[[SourceText]] to the source text matched by ClassDeclaration.
3. Return value.

Note

ClassDeclaration : class ClassTail only occurs as part of an ExportDeclaration and establishing its binding is handled as part of the evaluation action for that production. See 16.2.3.7.

ClassDeclaration : class BindingIdentifier ClassTail

1. Perform ? BindingClassDeclarationEvaluation of this ClassDeclaration.
2. Return empty.

Note

ClassDeclaration : class ClassTail only occurs as part of an ExportDeclaration and is never directly evaluated.

ClassExpression : class ClassTail

1. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments undefined and "".
2. Set value.[[SourceText]] to the source text matched by ClassExpression.
3. Return value.

ClassExpression : class BindingIdentifier ClassTail

1. Let className be StringValue of BindingIdentifier.
2. Let value be ? ClassDefinitionEvaluation of ClassTail with arguments className and className.
3. Set value.[[SourceText]] to the source text matched by ClassExpression.
4. Return value.

ClassElementName : PrivateIdentifier

1. Let privateIdentifier be StringValue of PrivateIdentifier.
2. Let privateEnvRec be the running execution context's PrivateEnvironment.
3. Let names be privateEnvRec.[[Names]].
4. Assert: Exactly one element of names is a Private Name whose [[Description]] is privateIdentifier.
5. Let privateName be the Private Name in names whose [[Description]] is privateIdentifier.
6. Return privateName.

ClassStaticBlockStatementList : [empty]

1. Return undefined.