Web/JavaScript/Reference/Operators/async function

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

Specification
ECMAScript (ECMA-262)The definition of 'async function' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`async function` expression	Chrome Full support 55	Edge Full support 15	Firefox Full support 52	IE No support No	Opera Full support 42	Safari Full support 10.1	WebView Android Full support 55	Chrome Android Full support 55	Firefox Android Full support 52	Opera Android Full support 42	Safari iOS Full support 10.3	Samsung Internet Android Full support 6.0	nodejs Full support 7.6.0 Full support 7.6.0 Full support 7.0.0 Disabled' Disabled' From version 7.0.0: this feature is behind the `--harmony` runtime flag.

Legend

Full support: Full support
No support: No support
User must explicitly enable this feature.': User must explicitly enable this feature.