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+---
+title: await
+slug: Web/JavaScript/Reference/Operators/await
+tags:
+  - JavaScript
+  - Promise
+  - await
+  - 实验性
+  - 操作符
+translation_of: Web/JavaScript/Reference/Operators/await
+---
+<div>{{jsSidebar("Operators")}}</div>
+
+<p><code>await</code>  操作符用于等待一个{{jsxref("Promise")}} 对象。它只能在异步函数 {{jsxref("Statements/async_function", "async function")}} 中使用。</p>
+
+<h2 id="语法">语法</h2>
+
+<pre class="syntaxbox notranslate">[<font face="Consolas, Liberation Mono, Courier, monospace">返回值</font>] = await <font face="Consolas, Liberation Mono, Courier, monospace">表达式</font>;</pre>
+
+<dl>
+ <dt><font face="Consolas, Liberation Mono, Courier, monospace">表达式</font></dt>
+ <dd>一个 {{jsxref("Promise")}} 对象或者任何要等待的值。</dd>
+ <dt><font face="Consolas, Liberation Mono, Courier, monospace">返回值</font></dt>
+ <dd>
+ <p>返回 Promise 对象的处理结果。如果等待的不是 Promise 对象，则返回该值本身。</p>
+ </dd>
+</dl>
+
+<h2 id="描述">描述</h2>
+
+<p>await 表达式会暂停当前 {{jsxref("Statements/async_function", "async function")}} 的执行，等待 Promise 处理完成。若 Promise 正常处理(fulfilled)，其回调的resolve函数参数作为 await 表达式的值，继续执行 {{jsxref("Statements/async_function", "async function")}}。</p>
+
+<p>若 Promise 处理异常(rejected)，await 表达式会把 Promise 的异常原因抛出。</p>
+
+<p>另外，如果 await 操作符后的表达式的值不是一个 Promise，则返回该值本身。</p>
+
+<h2 id="例子">例子</h2>
+
+<p>如果一个 Promise 被传递给一个 await 操作符，await 将等待 Promise 正常处理完成并返回其处理结果。</p>
+
+<pre class="brush: js notranslate">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>如果该值不是一个 Promise，await 会把该值转换为已正常处理的Promise，然后等待其处理结果。</p>
+
+<pre class="brush: js notranslate">async function f2() {
+  var y = await 20;
+  console.log(y); // 20
+}
+f2();
+</pre>
+
+<p>如果 Promise 处理异常，则异常值被抛出。</p>
+
+<pre class="brush: js notranslate">async function f3() {
+  try {
+    var z = await Promise.reject(30);
+  } catch (e) {
+    console.log(e); // 30
+  }
+}
+f3();</pre>
+
+<h2 id="规范">规范</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('Async Function', '#async-function-definitions', 'async function')}}</td>
+   <td>{{Spec2('Async Function')}}</td>
+   <td>提案</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</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>基本支持</td>
+   <td>{{CompatChrome(55)}}</td>
+   <td>{{CompatGeckoDesktop("52.0")}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatOpera(42)}}</td>
+   <td>{{CompatUnknown}}</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> 基本支持</td>
+   <td>{{CompatNo}}</td>
+   <td>{{CompatNo}}</td>
+   <td>{{CompatGeckoMobile("52.0")}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatOpera(42)}}</td>
+   <td>{{CompatUnknown}}</td>
+   <td>{{CompatChrome(55)}}</td>
+  </tr>
+ </tbody>
+</table>
+</div>
+
+<h2 id="查看更多">查看更多</h2>
+
+<ul>
+ <li>{{jsxref("Statements/async_function", "async 函数")}}</li>
+ <li>{{jsxref("Operators/async_function", "async 函数表达式")}}</li>
+ <li>{{jsxref("AsyncFunction")}} object</li>
+</ul>