ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

15.9 Async Arrow Function Definitions

Syntax

AsyncArrowFunction[In, Yield, Await] : async [no LineTerminator here] AsyncArrowBindingIdentifier[?Yield] [no LineTerminator here] => AsyncConciseBody[?In] CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [no LineTerminator here] => AsyncConciseBody[?In] AsyncConciseBody[In] : [lookahead ≠ {] ExpressionBody[?In, +Await] { AsyncFunctionBody } AsyncArrowBindingIdentifier[Yield] : BindingIdentifier[?Yield, +Await] CoverCallExpressionAndAsyncArrowHead[Yield, Await] : MemberExpression[?Yield, ?Await] Arguments[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production
AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
the interpretation of CoverCallExpressionAndAsyncArrowHead is refined using the following grammar:

AsyncArrowHead : async [no LineTerminator here] ArrowFormalParameters[~Yield, +Await]

15.9.1 Static Semantics: Early Errors

AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody

15.9.2 Static Semantics: AsyncConciseBodyContainsUseStrict

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

AsyncConciseBody : ExpressionBody
  1. Return false.
 AsyncConciseBody : { AsyncFunctionBody }
  1. Return FunctionBodyContainsUseStrict of AsyncFunctionBody.

15.9.3 Runtime Semantics: EvaluateAsyncConciseBody

The syntax-directed operation EvaluateAsyncConciseBody 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:

AsyncConciseBody : ExpressionBody
  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, ExpressionBody).
  5. Return Completion Record { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.

15.9.4 Runtime Semantics: InstantiateAsyncArrowFunctionExpression

The syntax-directed operation InstantiateAsyncArrowFunctionExpression 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:

AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody
  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 AsyncArrowFunction.
  5. Let parameters be AsyncArrowBindingIdentifier.
  6. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, parameters, AsyncConciseBody, lexical-this, env, privateEnv).
  7. Perform SetFunctionName(closure, name).
  8. Return closure.
 AsyncArrowFunction : CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
  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 AsyncArrowFunction.
  5. Let head be the AsyncArrowHead that is covered by CoverCallExpressionAndAsyncArrowHead.
  6. Let parameters be the ArrowFormalParameters of head.
  7. Let closure be OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, parameters, AsyncConciseBody, lexical-this, env, privateEnv).
  8. Perform SetFunctionName(closure, name).
  9. Return closure.

15.9.5 Runtime Semantics: Evaluation

AsyncArrowFunction : async AsyncArrowBindingIdentifier => AsyncConciseBody CoverCallExpressionAndAsyncArrowHead => AsyncConciseBody
  1. Return InstantiateAsyncArrowFunctionExpression of AsyncArrowFunction.