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---
title: 自减 (--)
slug: Web/JavaScript/Reference/Operators/Decrement
tags:
- JavaScript
- 自减
- 语法特性
- 运算符
translation_of: Web/JavaScript/Reference/Operators/Decrement
original_slug: Web/JavaScript/Reference/Operators/自减
---
<div>{{jsSidebar("Operators")}}</div>
<p> 自减运算符(<code>--</code>) 将它的操作数减一，然后返回操作数.</p>
<div>{{EmbedInteractiveExample("pages/js/expressions-decrement.html")}}</div>
<div></div>
<h2 id="语法"><br>
语法</h2>
<pre class="syntaxbox notranslate"><strong>操作符:</strong> <var>x</var>-- or --<var>x</var>
</pre>
<h2 id="语法细节">语法细节</h2>
<p>如果使用后缀式，即将操作符放在操作数的后面 (如，<code><var>x</var>--</code>)，运算会减一，然后返回减一之前的值。</p>
<p>如果使用前缀式，即将操作符放在操作数的前面 (如，<code>--<var>x</var></code>)，运算会减一，然后返回减一之后的值。</p>
<h2 id="示例">示例</h2>
<h3 id="后缀式">后缀式</h3>
<pre class="brush: js notranslate">let x = 3;
y = x--;
// y = 3
// x = 2
</pre>
<h3 id="前缀式">前缀式</h3>
<pre class="brush: js notranslate">let a = 2;
b = --a;
// a = 1
// b = 1
</pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">规范</th>
</tr>
<tr>
<td>{{SpecName('ESDraft', '#sec-postfix-decrement-operator', '自减运算符')}}</td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性"><br>
浏览器兼容性</h2>
<p>{{Compat("javascript.operators.decrement")}}</p>
<h2 id="相关链接">相关链接</h2>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Addition">Addition operator</a></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Subtraction">Subtraction operator</a></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Division">Division operator</a></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Multiplication">Multiplication operator</a></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Remainder">Remainder operator</a></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation">Exponentiation operator</a></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Increment">Increment operator</a></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unary_negation">Unary negation operator</a></li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Unary_plus">Unary plus operator</a></li>
</ul>
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