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

nameOptional: The function name. Can be omitted, in which case the function isanonymous. The name is only local to the function body.
paramNOptional: The name of a formal parameter for the function. For the parameters' syntax, see theFunctions reference.
statementsOptional: The 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

AnasyncIIFE 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 });  }})();