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 AsyncGenerator is in a resumable state.

Note 2

Abstract operations relating to AsyncGenerators are defined in 27.6.3.

15.6.1 Static Semantics: Early Errors

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

BindingIdentifier

opt

(

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.

15.6.2 Runtime Semantics: EvaluateAsyncGeneratorBody

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

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

15.6.3 Runtime Semantics: InstantiateAsyncGeneratorFunctionObject

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

}

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

AsyncGeneratorDeclaration

async

function

(

FormalParameters

)

{

AsyncGeneratorBody

}

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

Note

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

15.6.4 Runtime Semantics: InstantiateAsyncGeneratorFunctionExpression

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

}

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

AsyncGeneratorExpression

async

function

BindingIdentifier

(

FormalParameters

)

{

AsyncGeneratorBody

}

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