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120 lines
9.7 KiB
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<article id="wikiArticle">
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<div></div>
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<p><strong>while 语句</strong>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。</p>
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<h2 id="Syntax" name="Syntax">语法</h2>
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<pre><code class="language-javascript">while (<em>condition</em>) {
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<em>statement</em>
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}</code></pre>
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<dl>
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<dt><code>condition</code></dt>
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<dd>条件表达式，在每次循环前被求值。如果求值为真，<code>statement</code>就会被执行。如果求值为假，则跳出<code>while</code>循环执行后面的语句。</dd>
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<dt><code>statement</code></dt>
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<dd>只要条件表达式求值为真，该语句就会一直被执行。要在循环中执行多条语句，可以使用块语句（<code>{ ... }</code>）包住多条语句。</dd>
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<dd>注意：使用<code>break</code>语句在<code>condition</code>计算结果为真之前停止循环。</dd>
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</dl>
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<h2 id="Examples" name="Examples">示例</h2>
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<p>下面的<code>while</code>循环会一直循环若干次直到 <code>n</code> 等于 <code>3</code>。</p>
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<pre><code class="language-js">var n = 0;
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var x = 0;
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while (n < 3) {
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n++;
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x += n;
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}</code></pre>
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<p>在每次循环中，<code>n</code> 都会自增 <code>1</code>，然后再把 <code>n</code> 加到 <code>x</code> 上。因此，在每轮循环结束后，<code>x</code> 和 <code>n</code> 的值分别是：</p>
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<ul>
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<li>第一轮后：<code>n</code> = 1，<code>x</code> = 1</li>
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<li>第二轮后：<code>n</code> = 2，<code>x</code> = 3</li>
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<li>第三轮后：<code>n</code> = 3，<code>x</code> = 6</li>
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</ul>
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<p>当完成第三轮循环后，条件表达式<code>n</code>< 3 不再为真，因此循环终止。</p>
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<h2 id="规范">规范</h2>
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<table class="standard-table">
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<tbody>
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<tr>
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<th scope="col">Specification</th>
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<th scope="col">Status</th>
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<th scope="col">Comment</th>
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</tr>
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<tr>
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<td><a class="external" href="https://tc39.github.io/ecma262/#sec-while-statement" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">while statement</small></a></td>
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<td><span class="spec-Draft">Draft</span></td>
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<td> </td>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-while-statement" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">while statement</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td> </td>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-12.6.2" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">while statement</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td> </td>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%203rd%20edition,%20December%201999.pdf#sec-12.6.2" hreflang="en" lang="en" rel="noopener">ECMAScript 3rd Edition (ECMA-262)<br/><small lang="zh-CN">while statement</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td> </td>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf#sec-12.6.1" hreflang="en" lang="en" rel="noopener">ECMAScript 1st Edition (ECMA-262)<br/><small lang="zh-CN">while statement</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td>Initial definition</td>
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</tr>
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</tbody>
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</table>
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<h2 id="浏览器兼容">浏览器兼容</h2>
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<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>
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<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>while</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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<span>Full support</span>
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</abbr>
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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">
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<abbr class="bc-level bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr></span></dt><dd>Full support</dd></dl></section></div><p></p>
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<h2 id="See_also" name="See_also">相关链接</h2>
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<ul>
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<li><a href="Reference/Statements/do...while"><code>do...while</code></a></li>
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<li><a href="Reference/Statements/for" title="for 语句用于创建一个循环，它包含了三个可选的表达式，三个可选的表达式包围在圆括号中并由分号分隔， 后跟一个在循环中执行的语句（通常是一个块语句）。"><code>for</code></a></li>
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</ul>
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</article> |