definitions.html

<h2>Syntax</h2>
<emu-grammar>
  AsyncGeneratorMethod[Yield, Await] :
    `async` [no LineTerminator here] `*` PropertyName[?Yield, ?Await] `(` UniqueFormalParameters[+Yield, +Await] `)` `{` AsyncGeneratorBody `}`

  AsyncGeneratorDeclaration[Yield, Await, Default] :
    `async` [no LineTerminator here] `function` `*` BindingIdentifier[?Yield, ?Await] `(` FormalParameters[+Yield, +Await] `)` `{` AsyncGeneratorBody `}`
    [+Default] `async` [no LineTerminator here] `function` `*` `(` FormalParameters[+Yield, +Await] `)` `{` AsyncGeneratorBody `}`

  AsyncGeneratorExpression :
    `async` [no LineTerminator here] `function` `*` BindingIdentifier[+Yield, +Await]? `(` FormalParameters[+Yield, +Await] `)` `{` AsyncGeneratorBody `}`

  AsyncGeneratorBody :
    FunctionBody[+Yield, +Await]
</emu-grammar>
<emu-note>
  <p>|YieldExpression| and |AwaitExpression| cannot be used within the |FormalParameters| of an async generator function because any expressions that are part of |FormalParameters| are evaluated before the resulting async generator object is in a resumable state.</p>
</emu-note>
<emu-note>
  <p>Abstract operations relating to async generator objects are defined in <emu-xref href="#sec-asyncgenerator-abstract-operations"></emu-xref>.</p>
</emu-note>

<emu-clause id="sec-async-generator-function-definitions-static-semantics-early-errors">
  <h1>Static Semantics: Early Errors</h1>
  <emu-grammar>AsyncGeneratorMethod : `*` PropertyName `(` UniqueFormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <ul>
    <li>It is a Syntax Error if HasDirectSuper of |AsyncGeneratorMethod| is *true*.</li>
    <li>It is a Syntax Error if |UniqueFormalParameters| Contains |YieldExpression| is *true*.</li>
    <li>It is a Syntax Error if |UniqueFormalParameters| Contains |AwaitExpression| is *true*.</li>
    <li>It is a Syntax Error if ContainsUseStrict of |AsyncGeneratorBody| is *true* and IsSimpleParameterList of |UniqueFormalParameters| is *false*.</li>
    <li>It is a Syntax Error if any element of the BoundNames of |UniqueFormalParameters| also occurs in the LexicallyDeclaredNames of |AsyncGeneratorBody|.</li>
  </ul>
  <emu-grammar>
    AsyncGeneratorDeclaration : `function` `*` BindingIdentifier `(` FormalParameters `)` `{` AsyncGeneratorBody `}`

    AsyncGeneratorDeclaration : `function` `*` `(` FormalParameters `)` `{` AsyncGeneratorBody `}`

    AsyncGeneratorDeclaration : `function` `*` BindingIdentifier? `(` FormalParameters `)` `{` AsyncGeneratorBody `}`
  </emu-grammar>
  <ul>
    <li>If the source code matching this production is strict mode code, the Early Error rules for <emu-grammar>UniqueFormalParameters : FormalParameters</emu-grammar> are applied.</li>
    <li>If the source code matching this production is strict mode code, it is a Syntax Error if |BindingIdentifier| is the |IdentifierName| `eval` or the |IdentifierName| `arguments`.</li>
    <li>It is a Syntax Error if ContainsUseStrict of |AsyncGeneratorBody| is *true* and IsSimpleParameterList of |FormalParameters| is *false*.</li>
    <li>It is a Syntax Error if any element of the BoundNames of |FormalParameters| also occurs in the LexicallyDeclaredNames of |AsyncGeneratorBody|.</li>
    <li>It is a Syntax Error if |FormalParameters| Contains |YieldExpression| is *true*.</li>
    <li>It is a Syntax Error if |FormalParameters| Contains |AwaitExpression| is *true*.</li>
    <li>It is a Syntax Error if |FormalParameters| Contains |SuperProperty| is *true*.</li>
    <li>It is a Syntax Error if |AsyncGeneratorBody| Contains |SuperProperty| is *true*.</li>
    <li>It is a Syntax Error if |FormalParameters| Contains |SuperCall| is *true*.</li>
    <li>It is a Syntax Error if |AsyncGeneratorBody| Contains |SuperCall| is *true*.</li>
  </ul>
</emu-clause>

<emu-clause id="sec-generator-function-definitions-static-semantics-boundnames">
  <h1>Static Semantics: BoundNames</h1>
  <emu-see-also-para op="BoundNames"></emu-see-also-para>
  <emu-grammar>AsyncGeneratorDeclaration : `function` `*` BindingIdentifier `(` FormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <emu-alg>
    1. Return the BoundNames of |BindingIdentifier|.
  </emu-alg>
  <emu-grammar>AsyncGeneratorDeclaration : `function` `*` `(` FormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <emu-alg>
    1. Return &laquo; `"*default*"` &raquo;.
  </emu-alg>
  <emu-note>
    <p>`"*default*"` is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.</p>
  </emu-note>
</emu-clause>

<emu-clause id="sec-generator-function-definitions-static-semantics-computedpropertycontains">
  <h1>Static Semantics: ComputedPropertyContains</h1>
  <p>With parameter _symbol_.</p>
  <emu-see-also-para op="ComputedPropertyContains"></emu-see-also-para>
  <emu-grammar>AsyncGeneratorMethod : `*` PropertyName `(` UniqueFormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <emu-alg>
    1. Return the result of ComputedPropertyContains for |PropertyName| with argument _symbol_.
  </emu-alg>
</emu-clause>

<emu-clause id="sec-generator-function-definitions-static-semantics-contains">
  <h1>Static Semantics: Contains</h1>
  <p>With parameter _symbol_.</p>
  <emu-see-also-para op="Contains"></emu-see-also-para>
  <emu-grammar>
    AsyncGeneratorDeclaration : `function` `*` BindingIdentifier `(` FormalParameters `)` `{` AsyncGeneratorBody `}`

    AsyncGeneratorDeclaration : `function` `*` `(` FormalParameters `)` `{` AsyncGeneratorBody `}`

    AsyncGeneratorExpression : `function` `*` BindingIdentifier? `(` FormalParameters `)` `{` AsyncGeneratorBody `}`
  </emu-grammar>
  <emu-alg>
    1. Return *false*.
  </emu-alg>
  <emu-note>
    <p>Static semantic rules that depend upon substructure generally do not look into function definitions.</p>
  </emu-note>
</emu-clause>

<emu-clause id="sec-generator-function-definitions-static-semantics-hasdirectsuper">
  <h1>Static Semantics: HasDirectSuper</h1>
  <emu-see-also-para op="HasDirectSuper"></emu-see-also-para>
  <emu-grammar>AsyncGeneratorMethod : `*` PropertyName `(` UniqueFormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <emu-alg>
    1. If |UniqueFormalParameters| Contains |SuperCall| is *true*, return *true*.
    1. Return |AsyncGeneratorBody| Contains |SuperCall|.
  </emu-alg>
</emu-clause>

<emu-clause id="sec-generator-function-definitions-static-semantics-hasname">
  <h1>Static Semantics: HasName</h1>
  <emu-see-also-para op="HasName"></emu-see-also-para>
  <emu-grammar>AsyncGeneratorExpression : `function` `*` `(` FormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <emu-alg>
    1. Return *false*.
  </emu-alg>
  <emu-grammar>AsyncGeneratorExpression : `function` `*` BindingIdentifier `(` FormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <emu-alg>
    1. Return *true*.
  </emu-alg>
</emu-clause>

<emu-clause id="sec-generator-function-definitions-static-semantics-isconstantdeclaration">
  <h1>Static Semantics: IsConstantDeclaration</h1>
  <emu-see-also-para op="IsConstantDeclaration"></emu-see-also-para>
  <emu-grammar>
    AsyncGeneratorDeclaration : `function` `*` BindingIdentifier `(` FormalParameters `)` `{` AsyncGeneratorBody `}`

    AsyncGeneratorDeclaration : `function` `*` `(` FormalParameters `)` `{` AsyncGeneratorBody `}`
  </emu-grammar>
  <emu-alg>
    1. Return *false*.
  </emu-alg>
</emu-clause>

<emu-clause id="sec-generator-function-definitions-static-semantics-isfunctiondefinition">
  <h1>Static Semantics: IsFunctionDefinition</h1>
  <emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
  <emu-grammar>AsyncGeneratorExpression : `function` `*` BindingIdentifier? `(` FormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <emu-alg>
    1. Return *true*.
  </emu-alg>
</emu-clause>

<emu-clause id="sec-generator-function-definitions-static-semantics-propname">
  <h1>Static Semantics: PropName</h1>
  <emu-see-also-para op="PropName"></emu-see-also-para>
  <emu-grammar>AsyncGeneratorMethod : `*` PropertyName `(` UniqueFormalParameters `)` `{` AsyncGeneratorBody `}`</emu-grammar>
  <emu-alg>
    1. Return PropName of |PropertyName|.
  </emu-alg>
</emu-clause>

<emu-clause id="sec-asyncgenerator-definitions-evaluatebody">
  <h1>Runtime Semantics: EvaluateBody</h1>
  <p>With parameters _functionObject_ and List _argumentsList_.</p>
  <emu-grammar>
    AsyncGeneratorBody : FunctionBody
  </emu-grammar>
  <emu-alg>
    1. Perform ? FunctionDeclarationInstantiation(_functionObject_, _argumentsList_).
    1. Let _generator_ be ? OrdinaryCreateFromConstructor(_functionObject_, `"%AsyncGeneratorPrototype%"`, « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]] »).
    1. Perform ! AsyncGeneratorStart(_generator_, _FunctionBody_).
    1. Return Completion{[[Type]]: ~return~, [[Value]]: _generator_, [[Target]]: ~empty~}.
  </emu-alg>
</emu-clause>

<emu-clause id="sec-asyncgenerator-definitions-instantiatefunctionobject">
  <h1>Runtime Semantics: InstantiateFunctionObject</h1>
  <p>With parameter _scope_.</p>
  <emu-grammar>
    AsyncGeneratorDeclaration : `async` [no LineTerminator here] `function` `*` BindingIdentifier `(` FormalParameters `)` `{` AsyncGeneratorBody `}`
  </emu-grammar>
  <emu-alg>
    1. If the function code for |AsyncGeneratorDeclaration| is strict mode code, let _strict_ be *true*. Otherwise let _strict_ be *false*.
    1. Let _name_ be StringValue of |BindingIdentifier|.
    1. Let _F_ be ! AsyncGeneratorFunctionCreate(~Normal~, |FormalParameters|, |AsyncGeneratorBody|, _scope_, _strict_).
    1. Let _prototype_ be ! ObjectCreate(%AsyncGeneratorPrototype%).
    1. Perform ! DefinePropertyOrThrow(_F_, `"prototype"`, PropertyDescriptor{[[Value]]: _prototype_, [[Writable]]: *true*, [[Enumerable]]: *false*, [[Configurable]]: *false*}).
    1. Perform ! SetFunctionName(_F_, _name_).
    1. Return _F_.
  </emu-alg>

  <emu-grammar>
    AsyncGeneratorDeclaration : `async` [no LineTerminator here] `function` `*` `(` FormalParameters `)` `{` AsyncGeneratorBody `}`
  </emu-grammar>
  <emu-alg>
    1. If the function code for |AsyncGeneratorDeclaration| is strict mode code, let _strict_ be *true*. Otherwise let _strict_ be *false*.
    1. Let _F_ be AsyncGeneratorFunctionCreate(~Normal~, |FormalParameters|, |AsyncGeneratorBody|, _scope_, _strict_).
    1. Let _prototype_ be ObjectCreate(%AsyncGeneratorPrototype%).
    1. Perform DefinePropertyOrThrow(_F_, `"prototype"`, PropertyDescriptor{[[Value]]: _prototype_, [[Writable]]: *true*, [[Enumerable]]: *false*, [[Configurable]]: *false*}).
    1. Perform SetFunctionName(_F_, `"default"`).
    1. Return _F_.
  </emu-alg>
  <emu-note>
    <p>An anonymous |AsyncGeneratorDeclaration| can only occur as part of an `export default` declaration.</p>
  </emu-note>
</emu-clause>

<emu-clause id="sec-asyncgenerator-definitions-propertydefinitionevaluation">
  <h1>Runtime Semantics: PropertyDefinitionEvaluation</h1>
  <p>With parameter _object_ and _enumerable_.</p>
  <emu-grammar>
    AsyncGeneratorMethod : `async` [no LineTerminator here] `*` PropertyName `(` UniqueFormalParameters `)` `{` AsyncGeneratorBody `}`
  </emu-grammar>
  <emu-alg>
    1. Let _propKey_ be the result of evaluating |PropertyName|.
    1. ReturnIfAbrupt(_propKey_).
    1. If the function code for this |AsyncGeneratorMethod| is strict mode code, let _strict_ be *true*. Otherwise let _strict_ be *false*.
    1. Let _scope_ be the running execution context's LexicalEnvironment.
    1. Let _closure_ be ! AsyncGeneratorFunctionCreate(~Method~, |UniqueFormalParameters|, |AsyncGeneratorBody|, _scope_, _strict_).
    1. Perform ! MakeMethod(_closure_, _object_).
    1. Let _prototype_ be ! ObjectCreate(%AsyncGeneratorPrototype%).
    1. Perform ! DefinePropertyOrThrow(_closure_, `"prototype"`, PropertyDescriptor{[[Value]]: _prototype_, [[Writable]]: *true*, [[Enumerable]]: *false*, [[Configurable]]: *false*}).
    1. Perform ! SetFunctionName(_closure_, _propKey_).
    1. Let _desc_ be PropertyDescriptor{[[Value]]: _closure_, [[Writable]]: *true*, [[Enumerable]]: _enumerable_, [[Configurable]]: *true*}.
    1. Return ? DefinePropertyOrThrow(_object_, _propKey_, _desc_).
  </emu-alg>
</emu-clause>

<emu-clause id="sec-asyncgenerator-definitions-evaluation">
  <h1>Runtime Semantics: Evaluation</h1>

  <emu-grammar>
    AsyncGeneratorExpression : `async` [no LineTerminator here] `function` `*` `(` FormalParameters `)` `{` AsyncGeneratorBody `}`
  </emu-grammar>
  <emu-alg>
    1. If the function code for this |AsyncGeneratorExpression| is strict mode code, let _strict_ be *true*. Otherwise let _strict_ be *false*.
    1. Let _scope_ be the LexicalEnvironment of the running execution context.
    1. Let _closure_ be ! AsyncGeneratorFunctionCreate(~Normal~, |FormalParameters|, |AsyncGeneratorBody|, _scope_, _strict_).
    1. Let _prototype_ be ! ObjectCreate(%AsyncGeneratorPrototype%).
    1. Perform ! DefinePropertyOrThrow(_closure_, `"prototype"`, PropertyDescriptor{[[Value]]: _prototype_, [[Writable]]: *true*, [[Enumerable]]: *false*, [[Configurable]]: *false*}).
    1. Return _closure_.
  </emu-alg>

  <emu-grammar>
    AsyncGeneratorExpression : `async` [no LineTerminator here] `function` `*` BindingIdentifier `(` FormalParameters `)` `{` AsyncGeneratorBody `}`
  </emu-grammar>
  <emu-alg>
    1. If the function code for this |AsyncGeneratorExpression| is strict mode code, let _strict_ be *true*. Otherwise let _strict_ be *false*.
    1. Let _scope_ be the running execution context's LexicalEnvironment.
    1. Let _funcEnv_ be ! NewDeclarativeEnvironment(_scope_).
    1. Let _envRec_ be _funcEnv_'s EnvironmentRecord.
    1. Let _name_ be StringValue of |BindingIdentifier|.
    1. Perform ! _envRec_.CreateImmutableBinding(_name_).
    1. Let _closure_ be ! AsyncGeneratorFunctionCreate(~Normal~, |FormalParameters|, |AsyncGeneratorBody|, _funcEnv_, _strict_).
    1. Let _prototype_ be ! ObjectCreate(%AsyncGeneratorPrototype%).
    1. Perform ! DefinePropertyOrThrow(_closure_, `"prototype"`, PropertyDescriptor{[[Value]]: _prototype_, [[Writable]]: *true*, [[Enumerable]]: *false*, [[Configurable]]: *false*}).
    1. Perform ! SetFunctionName(_closure_, _name_).
    1. Perform ! _envRec_.InitializeBinding(_name_, _closure_).
    1. Return _closure_.
  </emu-alg>
  <emu-note>
    <p>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|.</p>
  </emu-note>
</emu-clause>