async function expression
BaselineWidely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2017.
Theasync function
keywords can be used to define an async function inside an expression.
You can also define async functions using theasync function
declaration or thearrow syntax.
Syntax
async function (param0) { statements}async function (param0, param1) { statements}async function (param0, param1, /* …, */ paramN) { statements}async function name(param0) { statements}async function name(param0, param1) { statements}async function name(param0, param1, /* …, */ paramN) { statements}
Note:Anexpression statement cannot begin with the keywordsasync function
to avoid ambiguity with anasync function
declaration. Theasync function
keywords only begin an expression when they appear in a context that cannot accept statements.
Parameters
name
OptionalThe function name. Can be omitted, in which case the function isanonymous. The name is only local to the function body.
paramN
OptionalThe name of a formal parameter for the function. For the parameters' syntax, see theFunctions reference.
statements
OptionalThe statements which comprise the body of the function.
Description
Anasync function
expression is very similar to, and has almost the same syntax as, anasync function
declaration. The main difference between anasync function
expression and anasync function
declaration is thefunction name, which can be omitted inasync function
expressions to createanonymous functions. Anasync function
expression can be used as anIIFE (Immediately Invoked Function Expression) which runs as soon as it is defined, allowing you to mimictop-level await. See also the chapter aboutfunctions for more information.
Examples
Using async function expression
function resolveAfter2Seconds(x) { return new Promise((resolve) => { setTimeout(() => { resolve(x); }, 2000); });}// async function expression assigned to a variableconst add = async function (x) { const a = await resolveAfter2Seconds(20); const b = await resolveAfter2Seconds(30); return x + a + b;};add(10).then((v) => { console.log(v); // prints 60 after 4 seconds.});// async function expression used as an IIFE(async function (x) { const p1 = resolveAfter2Seconds(20); const p2 = resolveAfter2Seconds(30); return x + (await p1) + (await p2);})(10).then((v) => { console.log(v); // prints 60 after 2 seconds.});
Async IIFE
Anasync
IIFE allows you to useawait
andfor...await
in contexts wheretop-level await is not available. Here we use anarrow function to define the IIFE, butasync function
expressions can also be used.
const getFileStream = async (url) => { // implementation};(async () => { const stream = await getFileStream("https://domain.name/path/file.ext"); for await (const chunk of stream) { console.log({ chunk }); }})();
Specifications
Specification |
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ECMAScript® 2026 Language Specification # sec-async-function-definitions |