javascript / latest / operators / async_function.html /

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.

Syntax

async function [name]([param1[, param2[, ..., paramN]]]) {
  statements
}

As of ES2015, you can also use arrow functions.

Parameters

name

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

paramN

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

statements

The statements which comprise the body of the function.

Description

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.

Examples

Simple example

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

const add = async function(x) { // async function expression assigned to a variable
  let a = await resolveAfter2Seconds(20);
  let 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
  let p_a = resolveAfter2Seconds(20);
  let p_b = resolveAfter2Seconds(30);
  return x + await p_a + await p_b;
})(10).then(v => {
  console.log(v);  // prints 60 after 2 seconds.
});

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
async_function
55
15
52
No
42
10.1
55
55
52
42
10.3
6.0
1.0
7.6.0
7.0.0

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function