uTools-Manuals/docs/javascript/Reference/Statements/if...else.html

<article id="wikiArticle">
<div>
<div> </div>
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<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><iframe class="interactive interactive-js" frameborder="0" height="250" src="https://interactive-examples.mdn.mozilla.net/pages/js/statement-ifelse.html" width="100%"></iframe></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 class="external" href="https://github.com/mdn/interactive-examples" rel="noopener">https://github.com/mdn/interactive-examples</a> and send us a pull request.</p>
</div>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre><code class="language-javascript">if (<em>condition</em>)
   <em>statement1</em>
[else
   <em>statement2</em>]
</code></pre>
<dl>
<dt><code>condition</code></dt>
<dd>值为真或假的<a href="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="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="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>

</code></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>)
...
</code></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>
}
</code></pre>
<p>不要将原始布尔值的<code>true</code>和<code>false</code>与<a href="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><code  class="language-javascript">var b = new Boolean(false);
if (b) //表达式的值为true
</code></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><code  class="language-javascript">if (cipher_char === from_char) {
   result = result + to_char;
   x++;
} else {
   result = result + clear_char;
}
</code></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><code  class="language-javascript">if (x &gt; 5) {
 /* do the right thing */
} else if (x &gt; 50) {
 /* do the right thing */
} else {
 /* do the right thing */
}</code></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><code class="language-js example-bad">if (x = y) {
   /* do the right thing */
}
</code></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><code class="language-js example-good">if ((x = y)) {
   /* do the right thing */
}
</code></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><a class="external" href="https://tc39.github.io/ecma262/#sec-if-statement" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">if statement</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-if-statement" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">if statement</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-12.5" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">if statement</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%203rd%20edition,%20December%201999.pdf#sec-12.5" hreflang="en" lang="en" rel="noopener">ECMAScript 3rd Edition (ECMA-262)<br/><small lang="zh-CN">if statement</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf#sec-12.5" hreflang="en" lang="en" rel="noopener">ECMAScript 1st Edition (ECMA-262)<br/><small lang="zh-CN">if statement</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition</td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容">浏览器兼容</h2>
<div class="hidden">The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out <a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</div>
<p></p><div class="bc-data"><a class="bc-github-link external" href="https://github.com/mdn/browser-compat-data" rel="noopener">Update compatibility data on GitHub</a><table class="bc-table bc-table-js"><thead><tr class="bc-platforms"><td></td><th class="bc-platform-desktop" colspan="6"><span>Desktop</span></th><th class="bc-platform-mobile" colspan="7"><span>Mobile</span></th><th class="bc-platform-server" colspan="1"><span>Server</span></th></tr><tr class="bc-browsers"><td></td><th class="bc-browser-chrome"><span class="bc-head-txt-label bc-head-icon-chrome">Chrome</span></th><th class="bc-browser-edge"><span class="bc-head-txt-label bc-head-icon-edge">Edge</span></th><th class="bc-browser-firefox"><span class="bc-head-txt-label bc-head-icon-firefox">Firefox</span></th><th class="bc-browser-ie"><span class="bc-head-txt-label bc-head-icon-ie">Internet Explorer</span></th><th class="bc-browser-opera"><span class="bc-head-txt-label bc-head-icon-opera">Opera</span></th><th class="bc-browser-safari"><span class="bc-head-txt-label bc-head-icon-safari">Safari</span></th><th class="bc-browser-webview_android"><span class="bc-head-txt-label bc-head-icon-webview_android">Android webview</span></th><th class="bc-browser-chrome_android"><span class="bc-head-txt-label bc-head-icon-chrome_android">Chrome for Android</span></th><th class="bc-browser-edge_mobile"><span class="bc-head-txt-label bc-head-icon-edge_mobile">Edge Mobile</span></th><th class="bc-browser-firefox_android"><span class="bc-head-txt-label bc-head-icon-firefox_android">Firefox for Android</span></th><th class="bc-browser-opera_android"><span class="bc-head-txt-label bc-head-icon-opera_android">Opera for Android</span></th><th class="bc-browser-safari_ios"><span class="bc-head-txt-label bc-head-icon-safari_ios">Safari on iOS</span></th><th class="bc-browser-samsunginternet_android"><span class="bc-head-txt-label bc-head-icon-samsunginternet_android">Samsung Internet</span></th><th class="bc-browser-nodejs"><span class="bc-head-txt-label bc-head-icon-nodejs">Node.js</span></th></tr></thead><tbody><tr><th scope="row"><code>if...else</code></th><td class="bc-supports-yes bc-browser-chrome"><span class="bc-browser-name">Chrome</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-edge"><span class="bc-browser-name">Edge</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-firefox"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              1</td><td class="bc-supports-yes bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-opera"><span class="bc-browser-name">Opera</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-safari"><span class="bc-browser-name">Safari</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-webview_android"><span class="bc-browser-name">WebView Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-chrome_android"><span class="bc-browser-name">Chrome Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-edge_mobile"><span class="bc-browser-name">Edge Mobile</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-firefox_android"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              4</td><td class="bc-supports-yes bc-browser-opera_android"><span class="bc-browser-name">Opera Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-safari_ios"><span class="bc-browser-name">Safari iOS</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-samsunginternet_android"><span class="bc-browser-name">Samsung Internet Android</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td><td class="bc-supports-yes bc-browser-nodejs"><span class="bc-browser-name">nodejs</span><abbr class="bc-level-yes only-icon" title="Full support">
<span>Full support</span>
</abbr>
              Yes</td></tr></tbody></table><section class="bc-legend" id="sect1"><h3 class="offscreen" id="Legend">Legend</h3><dl><dt><span class="bc-supports-yes bc-supports">
<abbr class="bc-level bc-level-yes only-icon" title="Full support">
<span>Full support</span>
                  
                </abbr></span></dt><dd>Full support</dd></dl></section></div><p></p>
<h2 id="See_also" name="See_also">相关链接</h2>
<ul>
<li><a href="Reference/Statements/block" title="块语句（或其他语言的复合语句）用于组合零个或多个语句。该块由一对大括号界定，可以是labelled："><code>block</code></a></li>
<li><a href="Reference/Statements/switch" title="switch 语句评估一个表达式，将表达式的值与case子句匹配，并执行与该情况相关联的语句。"><code>switch</code></a></li>
<li><a href="Reference/Operators/conditional_operator" title="条件（三元）运算符是 JavaScript 仅有的使用三个操作数的运算符。本运算符经常作为if语句的简短形式来使用。"><code>条件运算符</code></a></li>
</ul>
</article>