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+---
+title: while
+slug: Web/JavaScript/Reference/Statements/while
+tags:
+ - JavaScript
+ - Statement
+translation_of: Web/JavaScript/Reference/Statements/while
+---
+<div>{{jsSidebar("Statements")}}</div>
+
+<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>
+
+<div>{{EmbedInteractiveExample("pages/js/statement-while.html")}}</div>
+
+
+
+<h2 id="语法">语法</h2>
+
+<pre class="syntaxbox">while (<em>condition</em>)
+ <em>statement</em>
+</pre>
+
+<dl>
+ <dt><code>condition</code></dt>
+ <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>
+ <dt><code>statement</code></dt>
+ <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>
+ <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>
+</dl>
+
+<h2 id="示例">示例</h2>
+
+<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>
+
+<pre class="brush:js">var n = 0;
+var x = 0;
+
+while (n &lt; 3) {
+ n++;
+ x += n;
+}</pre>
+
+<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>
+
+<ul>
+ <li>第一轮后:<code>n</code> = 1,<code>x</code> = 1</li>
+ <li>第二轮后:<code>n</code> = 2,<code>x</code> = 3</li>
+ <li>第三轮后:<code>n</code> = 3,<code>x</code> = 6</li>
+</ul>
+
+<p>当完成第三轮循环后,条件表达式<code>n</code>&lt; 3 不再为真,因此循环终止。</p>
+
+<h2 id="规范">规范</h2>
+
+<table class="standard-table">
+ <tbody>
+ <tr>
+ <th scope="col">规范</th>
+ <th scope="col">状态</th>
+ <th scope="col">备注</th>
+ </tr>
+ <tr>
+ <td>{{SpecName('ESDraft', '#sec-while-statement', 'while statement')}}</td>
+ <td>{{Spec2('ESDraft')}}</td>
+ <td></td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES6', '#sec-while-statement', 'while statement')}}</td>
+ <td>{{Spec2('ES6')}}</td>
+ <td></td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES5.1', '#sec-12.6.2', 'while statement')}}</td>
+ <td>{{Spec2('ES5.1')}}</td>
+ <td></td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES3', '#sec-12.6.2', 'while statement')}}</td>
+ <td>{{Spec2('ES3')}}</td>
+ <td></td>
+ </tr>
+ <tr>
+ <td>{{SpecName('ES1', '#sec-12.6.1', 'while statement')}}</td>
+ <td>{{Spec2('ES1')}}</td>
+ <td>Initial definition</td>
+ </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</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 href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</div>
+
+<p>{{Compat("javascript.statements.while")}}</p>
+
+<h2 id="参见">参见</h2>
+
+<ul>
+ <li>{{jsxref("Statements/do...while", "do...while")}}</li>
+ <li>{{jsxref("Statements/for", "for")}}</li>
+</ul>