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 [name]([param1[, param2[, ..., paramN]]]) { statements }
As of ES2015, you can also use arrow functions.
name
paramN
statements
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.
function resolveAfter2Seconds(x) { return new Promise(resolve => { setTimeout(() => { resolve(x); }, 2000); }); }; 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. }); (async function(x) { // async function expression used as an IIFE var p_a = resolveAfter2Seconds(20); var p_b = resolveAfter2Seconds(30); return x + await p_a + await p_b; })(10).then(v => { console.log(v); // prints 60 after 2 seconds. });
Specification | Status | Comment |
---|---|---|
ECMAScript Latest Draft (ECMA-262) The definition of 'async function' in that specification. | Living Standard | Initial definition in ES2017. |
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 55 | ? | 52 | No | 42 | 10.1 |
Feature | Android webview | Chrome for Android | Edge mobile | Firefox for Android | IE mobile | Opera Android | iOS Safari |
---|---|---|---|---|---|---|---|
Basic support | 55 | 55 | ? | 52 | No | 42 | 10.1 |
async function
AsyncFunction
objectawait
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function