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---
title: await
slug: Web/JavaScript/Reference/Operators/await
tags:
- Experimental
- JavaScript
- 運算子
translation_of: Web/JavaScript/Reference/Operators/await
---
<div>{{jsSidebar("Operators")}}</div>
<div>await運算子可被用來等待 {{jsxref("Promise")}}，只能在 {{jsxref("Statements/async_function", "async function")}}內使用。</div>
<h2 id="語法">語法</h2>
<pre class="syntaxbox">[<em>rv</em>] = await <em>expression</em>;</pre>
<dl>
<dt><code>expression</code></dt>
<dd>等待解析的 {{jsxref("Promise")}} 物件或任何值。</dd>
<dt><font face="Consolas, Liberation Mono, Courier, monospace">rv = 回傳值</font></dt>
<dd>
<p>回傳 Promise 物件的 resolved 值，或當該值不是 Promise 物件時，回傳該值本身。</p>
</dd>
</dl>
<h2 id="描述">描述</h2>
<p>此 await 表示法會暫停 async 函式執行，等待 Promise 物件的解析，並在 promise 物件的值被 resolve 時回復 async 函式的執行。await 接著回傳這個被 resolve 的值。如果回傳值不是一個 Promise 物件，則會被轉換為 resolved 狀態的 Promise 物件。</p>
<p>如果 Promise 物件被 rejected，則 await 會丟出 rejected 的值。</p>
<h2 id="範例">範例</h2>
<p>若將 Promise 物件傳給 await 運算式，它會等待 Promise 解析並回傳 resolve 後的值。</p>
<pre class="brush: js">function resolveAfter2Seconds(x) {
return new Promise(resolve => {
setTimeout(() => {
resolve(x);
}, 2000);
});
}
async function f1() {
var x = await resolveAfter2Seconds(10);
console.log(x); // 10
}
f1();
</pre>
<p>若傳給 await 的值並非一個 Promise 物件，它會將該值轉換為 resolved Promise，並等待之。</p>
<pre class="brush: js">async function f2() {
var y = await 20;
console.log(y); // 20
}
f2();</pre>
<p>若 Promise 被 reject，則丟出 reject 後的異常值。</p>
<pre class="brush: js">async function f3() {
try {
var z = await Promise.reject(30);
} catch(e) {
console.log(e); // 30
}
}
f3();</pre>
<h2 id="規範">規範</h2>
<table class="standard-table">
<thead>
<tr>
<th scope="col">規範</th>
<th scope="col">狀態</th>
<th scope="col">註解</th>
</tr>
</thead>
<tbody>
<tr>
<td>{{SpecName('ESDraft', '#sec-async-function-definitions', 'async functions')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td>ES2017中初始定義</td>
</tr>
</tbody>
</table>
<h2 id="瀏覽器相容性">瀏覽器相容性</h2>
<div>{{CompatibilityTable}}</div>
<div id="compat-desktop">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Edge</th>
<th>Opera</th>
<th>Safari (WebKit)</th>
</tr>
<tr>
<td>基本支援</td>
<td>{{CompatChrome(55)}}</td>
<td>{{CompatGeckoDesktop("52.0")}}</td>
<td>{{CompatUnknown}}</td>
<td>{{CompatUnknown}}</td>
<td>{{CompatOpera(42)}}</td>
<td>10.1</td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Android Webview</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
<th>Chrome for Android</th>
</tr>
<tr>
<td>基本支援</td>
<td>{{CompatNo}}</td>
<td>{{CompatNo}}</td>
<td>{{CompatGeckoMobile("52.0")}}</td>
<td>{{CompatUnknown}}</td>
<td>{{CompatOpera(42)}}</td>
<td>10.1</td>
<td>{{CompatChrome(55)}}</td>
</tr>
</tbody>
</table>
</div>
<h2 id="See_also">See also</h2>
<ul>
<li>{{jsxref("Statements/async_function", "async function")}}</li>
<li>{{jsxref("Operators/async_function", "async function expression")}}</li>
<li>{{jsxref("AsyncFunction")}} object</li>
</ul>
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