async function expression

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

Syntax

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

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);
  });
};

(async function(x) { // async function expression used as an IIFE
  var a = resolveAfter2Seconds(20);
  var b = resolveAfter2Seconds(30);
  return x + await a + await b;
})(10).then(v => {
  console.log(v);  // prints 60 after 2 seconds.
});

var add = async function(x) { // async function expression assigned to a variable
  var a = await resolveAfter2Seconds(20);
  var b = await resolveAfter2Seconds(30);
  return x + a + b;
};

add(10).then(v => {
  console.log(v);  // prints 60 after 4 seconds.
});

Specifications

Specification Status Comment
ECMAScript 2017 Draft (ECMA-262)
The definition of 'async function' in that specification.
Draft Initial definition in ES2017.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer  Edge Opera Safari (WebKit)
Basic support 55 52.0 (52.0) ? ? 42 ?
Feature Android Android Webview Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support ? ? 52.0 (52.0) ? 42 ? 55

See also

Document Tags and Contributors

 Contributors to this page: fscholz, jr-grenoble, DanielHerr, arai
 Last updated by: fscholz,