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---
title: async function expression
slug: Web/JavaScript/Reference/Operators/async_function
translation_of: Web/JavaScript/Reference/Operators/async_function
---
<div>{{jsSidebar("Operators")}}</div>
<p>關鍵字 <strong><code>async function</code></strong> 可以用來定義陳述式中的 <code>async</code> 函式。</p>
<p>你也可以使用 <a href="https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Reference/Statements/async_function" title="The async function keyword can be used to define async functions inside expressions.">async function statement</a> 來定義一個非同步函式</p>
<h2 id="Syntax">Syntax</h2>
<pre class="syntaxbox">async function [<em>name</em>]([<em>param1</em>[, <em>param2[</em>, ..., <em>paramN</em>]]]) {
<em>statements</em>
}</pre>
<p>As of ES2015, you can also use <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions">arrow functions</a>.</p>
<h3 id="Parameters">Parameters</h3>
<dl>
<dt><code>name</code></dt>
<dd>The function name. Can be omitted, in which case the function is <em>anonymous</em>. The name is only local to the function body.</dd>
<dt><code>paramN</code></dt>
<dd>The name of an argument to be passed to the function.</dd>
<dt><code>statements</code></dt>
<dd>The statements which comprise the body of the function.</dd>
</dl>
<h2 id="Description">Description</h2>
<p>An <code>async function</code> 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 <code>function</code> expression and an async <code>function</code> statement is the <em>function name,</em> which can be omitted in <code>async function</code> expressions to create <em>anonymous</em> functions. An <code>async function</code> 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 <a href="/en-US/docs/Web/JavaScript/Reference/Functions">functions</a> for more information.</p>
<h2 id="Examples">Examples</h2>
<h3 id="Simple_example">Simple example</h3>
<pre class="brush: js">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.
});
</pre>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<thead>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
</thead>
<tbody>
<tr>
<td>{{SpecName('ESDraft', '#sec-async-function-definitions', 'async function')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES2018', '#sec-async-function-definitions', 'async function')}}</td>
<td>{{Spec2('ES2018')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES2017', '#sec-async-function-definitions', 'async function')}}</td>
<td>{{Spec2('ES2017')}}</td>
<td>Initial definition.</td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<div>
<p>{{Compat("javascript.operators.async_function_expression")}}</p>
</div>
<h2 id="See_also">See also</h2>
<ul>
<li>{{jsxref("Statements/async_function", "async function")}}</li>
<li>{{jsxref("AsyncFunction")}} object</li>
<li>{{jsxref("Operators/await", "await")}}</li>
</ul>
|
