---
title: if...else
slug: Web/JavaScript/Reference/Statements/if...else
tags:
  - JavaScript
  - Statement
translation_of: Web/JavaScript/Reference/Statements/if...else
---
<div>
<div> {{jsSidebar("Statements")}}</div>
</div>

<p><span style="line-height: 19.0909080505371px;">当指定条件为真，</span><strong>if 语句</strong>会执行一段语句。如果条件为假，则执行另一段语句。</p>

<p>{{EmbedInteractiveExample("pages/js/statement-ifelse.html")}}</p>

<div class="hidden">
<p>The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone <a href="https://github.com/mdn/interactive-examples">https://github.com/mdn/interactive-examples</a> and send us a pull request.</p>
</div>

<h2 id="Syntax" name="Syntax">语法</h2>

<pre class="syntaxbox">if (<em>condition</em>)
   <em>statement1</em>
[else
   <em>statement2</em>]
</pre>

<dl>
 <dt><code>condition</code></dt>
 <dd>值为真或假的<a href="/zh-CN/docs/Web/JavaScript/Guide/Expressions_and_Operators#Expressions">表达式</a></dd>
</dl>

<dl>
 <dt><code>statement1</code></dt>
 <dd>当<code>condition</code>为真时执行的语句。可为任意语句，包括更深层的内部<code>if</code>语句。要执行多条语句，使用<a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/block" title="en/JavaScript/Reference/Statements/block">块</a>语句（{ ... }）将这些语句分组；若不想执行语句，则使用<a href="/zh-CN/docs/Web/JavaScript/Reference/Statements/Empty">空</a>语句。 </dd>
</dl>

<dl>
 <dt><code>statement2</code></dt>
 <dd>如果<code>condition</code>为假且 <code>else</code>从句存在时执行的语句。<span style="line-height: 19.0909080505371px;">可为任意语句，包括块语句和</span>嵌套的<code>if</code>语句<span style="line-height: 19.0909080505371px;">。</span></dd>
</dl>

<h2 id="Description" name="Description">说明</h2>

<p>多层 <code>if...else</code> 语句可使用 <code>else if</code> 从句。注意：在 Javascript 中没有 <code>elseif</code> （一个单词）关键字。</p>

<pre class="eval">if (<em>condition1</em>)
   <em>statement1</em>
else if (<em>condition2</em>)
   <em>statement2</em>
else if (<em>condition3</em>)
   <em>statement3</em>
...
else
   <em>statementN</em>

</pre>

<p>要看看它如何工作，可以调整下嵌套的缩进：</p>

<pre class="eval">if (<em>condition1</em>)
   <em>statement1</em>
else
   if (<em>condition2</em>)
      <em>statement2</em>
   else
      if (<em>condition3</em>)
...
</pre>

<p>要在一个从句中执行多条语句，可使用语句块（<code>{ ... }</code>）。通常情况下，一直使用语句块是个好习惯，特别是在涉及嵌套<code>if</code>语句的代码中：</p>

<pre class="eval">if (<em>condition</em>) {
   <em>statements1</em>
} else {
   <em>statements2</em>
}
</pre>

<p>不要将原始布尔值的<code>true</code>和<code>false</code>与<a href="/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Boolean" title="en/JavaScript/Reference/Global_Objects/Boolean">Boolean</a>对象的真或假混淆。任何一个值，只要它不是 <code>undefined</code><font face="Open Sans, Arial, sans-serif">、</font><code>null</code>、 <code>0</code><font face="Open Sans, Arial, sans-serif">、</font><code>NaN</code>或空字符串（<code>""</code>），那么无论是任何对象，即使是值为假的Boolean对象，在条件语句中都为真。例如：</p>

<pre class="brush: js">var b = new Boolean(false);
if (b) //表达式的值为true
</pre>

<h2 id="Examples" name="Examples">示例</h2>

<h3 id="Example:_Using_if...else" name="Example:_Using_if...else">使用 <code>if...else</code></h3>

<pre class="brush: js">if (cipher_char === from_char) {
   result = result + to_char;
   x++;
} else {
   result = result + clear_char;
}
</pre>

<h3 id="使用_else_if">使用 <code>else if</code></h3>

<p>注意，Javascript中没有<code>elseif</code>语句。但可以使用<code>else</code>和<code>if</code>中间有空格的语句：</p>

<pre class="brush: js">if (x &gt; 5) {
 /* do the right thing */
} else if (x &gt; 50) {
 /* do the right thing */
} else {
 /* do the right thing */
}</pre>

<h3 id="Example:_Assignment_within_the_conditional_expression" name="Example:_Assignment_within_the_conditional_expression">条件表达式中的赋值运算</h3>

<p>建议不要在条件表达式中单纯的使用赋值运算，因为粗看下赋值运算的代码很容易让人误认为是等性比较。比如，不要使用下面示例的代码：</p>

<pre class="brush: js example-bad">if (x = y) {
   /* do the right thing */
}
</pre>

<p>如果你需要在条件表达式中使用赋值运算，用圆括号包裹赋值运算。例如：</p>

<pre class="brush: js example-good">if ((x = y)) {
   /* do the right thing */
}
</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('ESDraft', '#sec-if-statement', 'if statement')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-if-statement', 'if statement')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-12.5', 'if statement')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES3', '#sec-12.5', 'if statement')}}</td>
   <td>{{Spec2('ES3')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES1', '#sec-12.5', 'if statement')}}</td>
   <td>{{Spec2('ES1')}}</td>
   <td>Initial definition</td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器兼容">浏览器兼容</h2>

<p>{{Compat("javascript.statements.if_else")}}</p>

<h2 id="See_also" name="See_also">相关链接</h2>

<ul>
 <li>{{jsxref("Statements/block", "block")}}</li>
 <li>{{jsxref("Statements/switch", "switch")}}</li>
 <li>{{jsxref("Operators/conditional_operator", "条件运算符")}}</li>
</ul>