ECMAScript® 2024 Language Specification

Draft ECMA-262 / December 6, 2023

15.8 Async Function Definitions

Syntax

AsyncFunctionDeclaration[Yield, Await, Default] : async [no LineTerminator here] function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, +Await] ) { AsyncFunctionBody } [+Default] async [no LineTerminator here] function ( FormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncFunctionExpression : async [no LineTerminator here] function BindingIdentifier[~Yield, +Await]opt ( FormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncMethod[Yield, Await] : async [no LineTerminator here] ClassElementName[?Yield, ?Await] ( UniqueFormalParameters[~Yield, +Await] ) { AsyncFunctionBody } AsyncFunctionBody : FunctionBody[~Yield, +Await] AwaitExpression[Yield] : await UnaryExpression[?Yield, +Await] Note 1

await is parsed as a keyword of an AwaitExpression when the [Await] parameter is present. The [Await] parameter is present in the top level of the following contexts, although the parameter may be absent in some contexts depending on the nonterminals, such as FunctionBody:

When Script is the syntactic goal symbol, await may be parsed as an identifier when the [Await] parameter is absent. This includes the following contexts:

Note 2

Unlike YieldExpression, it is a Syntax Error to omit the operand of an AwaitExpression. You must await something.

15.8.1 Static Semantics: Early Errors

AsyncMethod : async ClassElementName ( UniqueFormalParameters ) { AsyncFunctionBody } AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody } async function ( FormalParameters ) { AsyncFunctionBody } AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody }

15.8.2 Runtime Semantics: InstantiateAsyncFunctionObject

The syntax-directed operation InstantiateAsyncFunctionObject 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:

AsyncFunctionDeclaration : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody }
  1. Let name be StringValue of BindingIdentifier.
  2. Let sourceText be the source text matched by AsyncFunctionDeclaration.
  3. Let F be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
  4. Perform SetFunctionName(F, name).
  5. Return F.
 AsyncFunctionDeclaration : async function ( FormalParameters ) { AsyncFunctionBody }
  1. Let sourceText be the source text matched by AsyncFunctionDeclaration.
  2. Let F be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
  3. Perform SetFunctionName(F, "default").
  4. Return F.

15.8.3 Runtime Semantics: InstantiateAsyncFunctionExpression

The syntax-directed operation InstantiateAsyncFunctionExpression 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:

AsyncFunctionExpression : async function ( FormalParameters ) { AsyncFunctionBody }
  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 AsyncFunctionExpression.
  5. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, env, privateEnv).
  6. Perform SetFunctionName(closure, name).
  7. Return closure.
 AsyncFunctionExpression : async function BindingIdentifier ( FormalParameters ) { AsyncFunctionBody }
  1. Assert: name is not present.
  2. Set name to StringValue of BindingIdentifier.
  3. Let outerEnv be the LexicalEnvironment of the running execution context.
  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 AsyncFunctionExpression.
  8. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, FormalParameters, AsyncFunctionBody, non-lexical-this, funcEnv, privateEnv).
  9. Perform SetFunctionName(closure, name).
  10. Perform ! funcEnv.InitializeBinding(name, closure).
  11. Return closure.
 Note

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

15.8.4 Runtime Semantics: EvaluateAsyncFunctionBody

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

AsyncFunctionBody : FunctionBody
  1. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  2. Let declResult be Completion(FunctionDeclarationInstantiation(functionObject, argumentsList)).
  3. If declResult is an abrupt completion, then
    1. Perform ! Call(promiseCapability.[[Reject]], undefined, « declResult.[[Value]] »).
  4. Else,
    1. Perform AsyncFunctionStart(promiseCapability, FunctionBody).
  5. Return Completion Record { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

15.8.5 Runtime Semantics: Evaluation

AsyncFunctionExpression : async function BindingIdentifieropt ( FormalParameters ) { AsyncFunctionBody }
  1. Return InstantiateAsyncFunctionExpression of AsyncFunctionExpression.
 AwaitExpression : await UnaryExpression
  1. Let exprRef be ? Evaluation of UnaryExpression.
  2. Let value be ? GetValue(exprRef).
  3. Return ? Await(value).