--- title: async function expression slug: Web/JavaScript/Reference/Operators/async_function translation_of: Web/JavaScript/Reference/Operators/async_function ---

關鍵字 async function 可以用來定義陳述式中的 async 函式。

你也可以使用 async function statement 來定義一個非同步函式

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 {{jsxref('Statements/async_function', '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 {{Glossary("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);
  });
};


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.
});

Specifications

Specification	Status	Comment
{{SpecName('ESDraft', '#sec-async-function-definitions', 'async function')}}	{{Spec2('ESDraft')}}
{{SpecName('ES2018', '#sec-async-function-definitions', 'async function')}}	{{Spec2('ES2018')}}
{{SpecName('ES2017', '#sec-async-function-definitions', 'async function')}}	{{Spec2('ES2017')}}	Initial definition.