uTools-Manuals/docs/javascript/Reference/Statements/break.html

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<p><strong>break 语句</strong>中止当前循环，<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>语句或<a href="Reference/Statements/label" title="标记语句可以和 break 或 continue 语句一起使用。标记就是在一条语句前面加个可以引用的标识符。"><code>label</code></a> 语句，并把程序控制流转到紧接着被中止语句后面的语句。</p>
<h2 id="语法">语法</h2>
<pre><code class="language-javascript"><code>break [label];</code></code></pre>
<dl>
<dt><code>label</code></dt>
<dd>可选。与语句标签相关联的标识符。如果 break 语句不在一个循环或 <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> 语句中，则该项是必须的。</dd>
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<h2 id="描述">描述</h2>
<p><code>break</code>语句包含一个可选的标签，可允许程序摆脱一个被标记的语句。<code>break</code>语句需要内嵌在引用的标签中。被标记的语句可以是任何 <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>块</code></a>语句；不一定是循环语句。</p>
<h2 id="示例">示例</h2>
<p>下面的函数里有个 <code>break</code> 语句，当 <code>i</code> 为 3 时，会中止 <a href="Reference/Statements/while" title="while 语句可以在某个条件表达式为真的前提下，循环执行指定的一段代码，直到那个表达式不为真时结束循环。"><code>while</code></a> 循环，然后返回 3 * <code>x</code> 的值。</p>
<pre><code  class="language-javascript"><code class="language-js"><span class="keyword token">function</span> <span class="function token">testBreak</span><span class="punctuation token">(</span>x<span class="punctuation token">)</span> <span class="punctuation token">{</span>
  <span class="keyword token">var</span> i <span class="operator token">=</span> <span class="number token">0</span><span class="punctuation token">;</span>

  <span class="keyword token">while</span> <span class="punctuation token">(</span>i <span class="operator token">&lt;</span> <span class="number token">6</span><span class="punctuation token">)</span> <span class="punctuation token">{</span>
    <span class="keyword token">if</span> <span class="punctuation token">(</span>i <span class="operator token">==</span> <span class="number token">3</span><span class="punctuation token">)</span> <span class="punctuation token">{</span>
      <span class="keyword token">break</span><span class="punctuation token">;</span>
    <span class="punctuation token">}</span>
    i <span class="operator token">+</span><span class="operator token">=</span> <span class="number token">1</span><span class="punctuation token">;</span>
  <span class="punctuation token">}</span>

  <span class="keyword token">return</span> i <span class="operator token">*</span> x<span class="punctuation token">;</span>
<span class="punctuation token">}</span></code></code></pre>
<p>下面的代码中一起使用 <code>break</code> 语句和被标记的块语句。一个 <code>break</code> 语句必须内嵌在它引用的标记中。注意，<code>inner_block</code> 内嵌在 <code>outer_block</code> 中。</p>
<pre><code class="language-js;highlight:[1,3,5];">outer_block:{

  inner_block:{
    console.log ('1');
    break outer_block;      // breaks out of both inner_block and outer_block
    console.log (':-(');    // skipped
  }

  console.log ('2');        // skipped
}
</code></pre>
<p>下面的代码同样使用了 <code>break</code> 语句和被标记的块语句，但是产生了一个语法错误，因为它的 <code>break</code> 语句在 <code>block_1</code> 中，但是引用了 <code>block_2</code>。<code>break</code> 语句必须内嵌在它引用的标签中。</p>
<pre><code class="language-js;highlight:[1,3,6];">block_1:{
  console.log ('1');
  break block_2;            // SyntaxError: label not found
}

block_2:{
  console.log ('2');
}
</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://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf" hreflang="en" lang="en" rel="noopener" title="ECMAScript 1st Edition (ECMA-262)">ECMAScript 1st Edition (ECMA-262)</a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition. Unlabeled version.</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" hreflang="en" lang="en" rel="noopener" title="ECMAScript 3rd Edition (ECMA-262)">ECMAScript 3rd Edition (ECMA-262)</a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Labeled version added.</td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-12.8" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Break 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-262/6.0/#sec-break-statement" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Break statement</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-break-statement" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Break statement</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </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>break</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/continue" title="continue 语句结束当前（或标签）的循环语句的本次迭代，并继续执行循环的下一次迭代。"><code>continue</code></a></li>
<li><a href="Reference/Statements/label" title="标记语句可以和 break 或 continue 语句一起使用。标记就是在一条语句前面加个可以引用的标识符。"><code>label</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>
</ul>
</article>