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authorPeter Bengtsson <mail@peterbe.com>2020-12-08 14:40:17 -0500
committerPeter Bengtsson <mail@peterbe.com>2020-12-08 14:40:17 -0500
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+---
+title: 逗号操作符
+slug: Web/JavaScript/Reference/Operators/Comma_Operator
+tags:
+ - comma operator
+ - 逗号操作符
+translation_of: Web/JavaScript/Reference/Operators/Comma_Operator
+---
+<div>
+<div>{{jsSidebar("Operators")}}</div>
+</div>
+
+<p><strong>逗号操作符 </strong> 对它的每个操作数求值(从左到右),并返回最后一个操作数的值。</p>
+
+<p>{{EmbedInteractiveExample("pages/js/expressions-commaoperators.html")}}</p>
+
+<h2 id="语法">语法</h2>
+
+<pre class="syntaxbox"><em>expr1</em>, <em>expr2, expr3...</em></pre>
+
+<h2 id="参数">参数</h2>
+
+<dl>
+ <dt><code>expr1</code>, <code>expr2, expr3...</code></dt>
+ <dd>任一表达式。</dd>
+</dl>
+
+<h2 id="描述">描述</h2>
+
+<p>当你想要在期望一个表达式的位置包含多个表达式时,可以使用逗号操作符。这个操作符最常用的一种情况是:<code>for</code> 循环中提供多个参数。</p>
+
+<h2 id="示例">示例</h2>
+
+<p>假设 <code>a</code> 是一个二维数组,每一维度包含10个元素,则下面的代码使用逗号操作符一次递增/递减两个变量。需要注意的是,<code>var</code> 语句中的逗号<em><strong>不是</strong></em>逗号操作符,因为它不是存在于一个表达式中。尽管从实际效果来看,那个逗号同逗号运算符的表现很相似。但确切地说,它是 <code style="font-style: normal;">var</code> 语句中的一个特殊符号,用于把多个变量声明结合成一个。下面的代码打印一个二维数组中斜线方向的元素:</p>
+
+<pre class="brush:js;highlight:[1]">for (var i = 0, j = 9; i &lt;= 9; i++, j--)
+ document.writeln("a[" + i + "][" + j + "] = " + a[i][j]);</pre>
+
+<h3 id="处理后返回">处理后返回</h3>
+
+<p>另一个使用逗号操作符的例子是在返回值前处理一些操作。如同下面的代码,只有最后一个表达式被返回,其他的都只是被求值。</p>
+
+<pre>function myFunc () {
+ var x = 0;
+
+ return (x += 1, x); // the same of return ++x;
+}</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>ECMAScript 1st Edition.</td>
+ <td>Standard</td>
+ <td>Initial definition.</td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES5.1', '#sec-11.14', 'Comma operator')}}</td>
+ <td>{{Spec2('ES5.1')}}</td>
+ <td> </td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES6', '#sec-comma-operator', 'Comma operator')}}</td>
+ <td>{{Spec2('ES6')}}</td>
+ <td> </td>
+ </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+<p>{{Compat("javascript.operators.comma")}}</p>
+
+<h2 id="See_also" name="See_also">参见</h2>
+
+<ul>
+ <li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for">for loop</a></li>
+</ul>