---
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 =&gt; {
    setTimeout(() =&gt; {
      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>