---
title: Atomics.compareExchange()
slug: Web/JavaScript/Reference/Global_Objects/Atomics/compareExchange
tags:
  - Atomics
  - JavaScript
  - 共享内存
  - 方法
translation_of: Web/JavaScript/Reference/Global_Objects/Atomics/compareExchange
---
<div>{{JSRef}} {{SeeCompatTable}}</div>

<p><code><strong>Atomics</strong></code><strong><code>.compareExchange()</code></strong> 静态方法会在数组的值与期望值相等的时候，将给定的替换值替换掉数组上的值，然后返回旧值。此原子操作保证在写上修改的值之前不会发生其他写操作。</p>

<div>{{EmbedInteractiveExample("pages/js/atomics-compareexchange.html")}}</div>



<h2 id="语法">语法</h2>

<pre class="syntaxbox">Atomics.compareExchange(typedArray, index, expectedValue, replacementValue)
</pre>

<h3 id="参数">参数</h3>

<dl>
 <dt><code>typedArray</code></dt>
 <dd>一个共享的整型 typed array。例如 {{jsxref("Int8Array")}}，{{jsxref("Uint8Array")}}，{{jsxref("Int16Array")}}，{{jsxref("Uint16Array")}}，{{jsxref("Int32Array")}}，或 {{jsxref("Uint32Array")}}。</dd>
 <dt><code>index</code></dt>
 <dd><code>typedArray</code> 的索引。</dd>
 <dt><code>expectedValue</code></dt>
 <dd>用于比较的值。</dd>
 <dt>replacementValue</dt>
 <dd>将要替换上的值。</dd>
</dl>

<h3 id="返回值">返回值</h3>

<p>给定位置的旧值(<code>typedArray[index]</code>).</p>

<h3 id="错误">错误</h3>

<ul>
 <li>假如 <code>typedArray</code> 不是允许的整型之一，则抛出 {{jsxref("TypeError")}}。</li>
 <li><code><font face="Open Sans, Arial, sans-serif">假如 </font>typedArray</code> 不是一个贡献的 typed array，则抛出 {{jsxref("TypeError")}}。</li>
 <li>如果 <code>index</code> 超出了 <code>typedArray 的边界，则抛出</code> {{jsxref("RangeError")}}。</li>
</ul>

<h2 id="示例">示例</h2>

<pre class="brush: js">var sab = new SharedArrayBuffer(1024);
var ta = new Uint8Array(sab);
ta[0] = 7;

Atomics.compareExchange(ta, 0, 7, 12); // returns 7, the old value
Atomics.load(ta, 0); // 12</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('Shared Memory', '#Atomics.compareExchange', 'Atomics.compareExchange')}}</td>
   <td>{{Spec2('Shared Memory')}}</td>
   <td>Initial definition.</td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器支持">浏览器支持</h2>



<p>{{Compat("javascript.builtins.Atomics.compareExchange")}}</p>

<h2 id="相关">相关</h2>

<ul>
 <li>{{jsxref("Atomics")}}</li>
 <li>{{jsxref("Atomics.exchange()")}}</li>
</ul>