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---
title: WeakMap.prototype.get()
slug: Web/JavaScript/Reference/Global_Objects/WeakMap/get
translation_of: Web/JavaScript/Reference/Global_Objects/WeakMap/get
---
<div>{{JSRef}}</div>
<p><code><strong>get()</strong></code> 方法返回 <code>WeakMap</code> 指定的元素。</p>
<h2 id="语法">语法</h2>
<pre class="syntaxbox"><code><em>wm</em>.get(key);</code></pre>
<h3 id="参数">参数</h3>
<dl>
<dt>key</dt>
<dd>必须。 想要从 <code>WeakMap </code>获取的元素的键。</dd>
</dl>
<h3 id="返回值">返回值</h3>
<h3 id="返回与指定键相关联的值，如果_WeakMap_对象找不到这个键则返回_undefined。">返回与指定键相关联的值，如果 <code style="font-size: 14px; font-style: normal; font-weight: normal; line-height: 1.5;">WeakMap</code> 对象找不到这个键则返回 <code style="font-size: 14px; font-style: normal; font-weight: normal; line-height: 1.5;">undefined</code>。</h3>
<h2 id="例子">例子</h2>
<h3 id="使用_get_方法">使用 <code>get </code>方法<code> </code></h3>
<pre class="brush: js">var wm = new WeakMap();
wm.set(window, "foo");
wm.get(window); // 返回 "foo".
wm.get("baz"); // 返回 undefined.
</pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td>{{SpecName('ES6', '#sec-weakmap.prototype.get', 'WeakMap.prototype.get')}}</td>
<td>{{Spec2('ES6')}}</td>
<td>Initial definition.</td>
</tr>
<tr>
<td>{{SpecName('ESDraft', '#sec-weakmap.prototype.get', 'WeakMap.prototype.get')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p>{{CompatibilityTable}}</p>
<div>
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
</tr>
<tr>
<td>Basic support</td>
<td>36</td>
<td>{{CompatGeckoDesktop("6.0")}}</td>
<td>11</td>
<td>23</td>
<td>7.1</td>
</tr>
</tbody>
</table>
</div>
<div>
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
</tr>
<tr>
<td>Basic support</td>
<td>{{CompatNo}}</td>
<td>{{CompatGeckoMobile("6.0")}}</td>
<td>{{CompatNo}}</td>
<td>{{CompatNo}}</td>
<td>8</td>
</tr>
</tbody>
</table>
</div>
<h2 id="Firefox_中的注意事项">Firefox 中的注意事项</h2>
<ul>
<li>Prior to SpiderMonkey 38 {{geckoRelease(38)}}, this method threw a {{jsxref("TypeError")}} when the key parameter was not an object. However, the latest ES6 standard specifies to return <code>undefined</code> instead. Furthermore, <code>WeakMap.prototype.get</code> accepted an optional second argument as a fallback value, which is not part of the standard. Both non-standard behaviors are removed in version 38 and higher ({{bug(1127827)}}).</li>
</ul>
<h2 id="相关链接">相关链接</h2>
<ul>
<li>{{jsxref("WeakMap")}}</li>
<li>{{jsxref("WeakMap.set()")}}</li>
<li>{{jsxref("WeakMap.has()")}}</li>
</ul>
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