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

<article id="wikiArticle">
<div></div>
<p><strong><code>for 语句</code></strong>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， <span class="short_text" id="result_box" lang="zh-CN"><span>后跟一个在循环中执行的语句（通常是一个<a href="Reference/Statements/block">块语句</a>）。</span></span></p>
<h2 id="语法">语法</h2>
<pre><code class="language-javascript">for ([<em>initialization</em>]; [<em>condition</em>]; [<em>final-expression</em>])
   <em>statement</em>
</code></pre>
<dl>
<dt><code>initialization</code></dt>
<dd>一个表达式 (包含赋值语句) 或者变量声明。典型地被用于初始化一个计数器。该表达式可以使用<code>var</code>关键字声明新的变量。初始化中的变量不是该循环的局部变量，而是与<code>for</code>循环处在同样的作用域中。该表达式的结果无意义。</dd>
<dt><code>condition</code></dt>
<dd>一个条件表达式被用于确定每一次循环是否能被执行。如果该表达式的结果为true， <code>statement</code> 将被执行。 这个表达式是可选的。如果被忽略，那么就被认为永远为真。如果计算结果为假，那么执行流程将被跳到<code>for</code>语句结构后面的第一条语句。</dd>
<dt><code>final-expression</code></dt>
<dd>每次循环的最后都要执行的表达式。执行时机是在下一次<code>condition</code>的计算之前。通常被用于更新或者递增计数器变量。</dd>
<dt><code>statement</code></dt>
<dd>只要<code>condition</code>的结果为true就会被执行的语句。 要在循环体内执行多条语句，使用一个<span class="short_text" id="result_box" lang="zh-CN"><span><a href="Reference/Statements/block">块语句</a></span></span>（<code>{ ... }</code>）来包含要执行的语句。没有任何语句要执行，使用一个<a href="Reference/Statements/Empty">空语句</a>（<code>;</code>）。</dd>
</dl>
<h2 id="示例">示例</h2>
<h3 id="使用_for">使用 <code>for</code></h3>
<p>以下例子声明了变量<code>i</code>并被初始赋值为<code>0</code>，<code>for</code>语句检查<code>i</code>的值是否小于9，如果小于9，则执行语句块内的语句，并且最后将<code>i</code>的值增加 1。</p>
<pre><code  class="language-javascript">for (var i = 0; i &lt; 9; i++) {
   console.log(i);
   // more statements
}
</code></pre>
<h3 id="可选的for表达式">可选的<code>for</code>表达式</h3>
<p><code>for</code>语句的所有的表达式都是可选的</p>
<p>例如，<span class="short_text" id="result_box" lang="zh-CN"><span>初始化块</span></span>中的表达式没有被指定：</p>
<pre><code  class="language-javascript">var i = 0;
for (; i &lt; 9; i++) {
    console.log(i);
    // more statements
}
</code></pre>
<p><span class="short_text" id="result_box" lang="zh-CN"><span>像初始化块一样，条件块也是可选的。如果省略此表达式，则必须确保在循环</span></span><span class="short_text" lang="zh-CN"><span>体内跳出，以防创建死循环。</span></span></p>
<pre><code  class="language-javascript">for (var i = 0;; i++) {
   console.log(i);
   if (i &gt; 3) break;
   // more statements
}</code></pre>
<p>你当然可以忽略所有的表达式。同样的，确保使用了<code><a href="Reference/Statements/break">break</a></code>语句来跳出循环<span class="short_text" id="result_box" lang="zh-CN"><span>并且还要修改（增加）一个变量，使得break 语句的条件在某个时候是真的。</span></span></p>
<pre><code  class="language-javascript">var i = 0;

for (;;) {
  if (i &gt; 3) break;
  console.log(i);
  i++;
}
</code></pre>
<h3 id="使用无语句的for">使用无语句的<code>for</code></h3>
<p>以下 <code>for</code> 循环<span id="result_box" lang="zh-CN"><span>计算 [final-expression] 部分中节点的偏移位置</span></span> ，它不需要执行一个 <code>statement</code> 或者一组 <code><a href="Reference/Statements/block" title="JavaScript/Reference/Statements/block">block</a> statement</code> ，因此使用了空语句。</p>
<pre><code  class="language-javascript"><code class="language-js"><span class="keyword token">function</span> <span class="function token">showOffsetPos</span><span class="punctuation token">(</span>sId<span class="punctuation token">)</span> <span class="punctuation token">{</span>

  <span class="keyword token">var</span> nLeft <span class="operator token">=</span> <span class="number token">0</span><span class="punctuation token">,</span> nTop <span class="operator token">=</span> <span class="number token">0</span><span class="punctuation token">;</span>

  <span class="keyword token">for</span> <span class="punctuation token">(</span>

    <span class="keyword token">var</span> oItNode <span class="operator token">=</span> document<span class="punctuation token">.</span><span class="function token">getElementById</span><span class="punctuation token">(</span>sId<span class="punctuation token">)</span><span class="punctuation token">;</span> <span class="comment token">/* initialization */</span>

      oItNode<span class="punctuation token">;</span> <span class="comment token">/* condition */</span>

    nLeft <span class="operator token">+</span><span class="operator token">=</span> oItNode<span class="punctuation token">.</span>offsetLeft<span class="punctuation token">,</span> nTop <span class="operator token">+</span><span class="operator token">=</span> oItNode<span class="punctuation token">.</span>offsetTop<span class="punctuation token">,</span> oItNode <span class="operator token">=</span> oItNode<span class="punctuation token">.</span>offsetParent <span class="comment token">/* final-expression */</span>

  <span class="punctuation token">)</span><span class="punctuation token">;</span> <span class="comment token">/* semicolon */</span> 

  console<span class="punctuation token">.</span><span class="function token">log</span><span class="punctuation token">(</span><span class="string token">'Offset position of \''</span> <span class="operator token">+</span> sId <span class="operator token">+</span> <span class="string token">'\' element:\n left: '</span> <span class="operator token">+</span> nLeft <span class="operator token">+</span> <span class="string token">'px;\n top: '</span> <span class="operator token">+</span> nTop <span class="operator token">+</span> <span class="string token">'px;'</span><span class="punctuation token">)</span><span class="punctuation token">;</span>

<span class="punctuation token">}</span>

<span class="comment token">/* Example call: */</span>

<span class="function token">showOffsetPos</span><span class="punctuation token">(</span><span class="string token">'content'</span><span class="punctuation token">)</span><span class="punctuation token">;</span>

<span class="comment token">// Output:</span>
<span class="comment token">// "Offset position of "content" element:</span>
<span class="comment token">// left: 0px;</span>
<span class="comment token">// top: 153px;"</span></code></code></pre>
<div class="note">
<p><strong>提示：</strong>这是JavaScript中的少数几种情况，其中<strong>分号是强制性的</strong>。实际上，没有分号，循环声明之后的行将被视为语句。</p>
</div>
<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-for-statement" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">for 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-for-statement" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">for 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-12.6.3" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">for 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-12.6.3" hreflang="en" lang="en" rel="noopener">ECMAScript 3rd Edition (ECMA-262)<br/><small lang="zh-CN">for 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-12.6.2" hreflang="en" lang="en" rel="noopener">ECMAScript 1st Edition (ECMA-262)<br/><small lang="zh-CN">for 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>for</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="另请参阅">另请参阅</h2>
<ul>
<li><a href="Reference/Statements/break" title="break 语句中止当前循环，switch语句或label 语句，并把程序控制流转到紧接着被中止语句后面的语句。"><code>break</code></a></li>
<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/empty" title="空语句用来表明没有语句，尽管 JavaScript 语法希望有语句。"><code>empty</code></a></li>
<li><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></li>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/do...while"><code>do...while</code></a></li>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...in"><code>for...in</code></a></li>
<li><a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...of"><code>for...of</code></a></li>
</ul>
</article>