async function expression

The async function keyword can be used to define async functions inside expressions.

You can also define async functions using an async function statement.


async function (param0) {
async function (param0, param1) {
async function (param0, param1, /* … ,*/ paramN) {

async function name(param0) {
async function name(param0, param1) {
async function name(param0, param1, /* … ,*/ paramN) {

As of ES2015, you can also use arrow functions.


name Optional

The function name. Can be omitted, in which case the function is anonymous. The name is only local to the function body.

paramN Optional

The name of an argument to be passed to the function.

statements Optional

The statements which comprise the body of the function.


An async function expression is very similar to, and has almost the same syntax as, an async function statement. The main difference between an async function expression and an async function statement is the function name, which can be omitted in async function expressions to create anonymous functions. An async function expression can be used as an IIFE (Immediately Invoked Function Expression) which runs as soon as it is defined. See also the chapter about functions for more information.


Simple example

function resolveAfter2Seconds(x) {
  return new Promise((resolve) => {
    setTimeout(() => {
    }, 2000);

const add = async function (x) { // async function expression assigned to a variable
  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 (x) { // async function expression used as an IIFE
  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.


ECMAScript Language Specification
# sec-async-function-definitions

Browser compatibility

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See also