uTools-Manuals/docs/javascript/Reference/Global_Objects/Array/length.html

<article id="wikiArticle">
<p></p><p></p>
<p><code><strong>length</strong></code> 是<code>Array</code>的实例属性。返回或设置一个数组中的元素个数。该值是一个无符号 32-bit 整数，并且总是大于数组最高项的下标。</p>
<div><iframe class="interactive interactive-js" frameborder="0" height="250" src="https://interactive-examples.mdn.mozilla.net/pages/js/array-length.html" width="100%"></iframe></div>
<h2 id="Description" name="Description">描述</h2>
<p><code>length</code> 属性的值是一个 0 到 2<sup>32</sup>-1 的整数。</p>
<pre><code  class="language-javascript">var namelistA = new Array(4294967296); // 2的32次方 = 4294967296 
var namelistC = new Array(-100) // 负号

console.log(namelistA.length); // RangeError: 无效数组长度 
console.log(namelistC.length); // RangeError: 无效数组长度 



var namelistB = []; 
namelistB.length = Math.pow(2,32)-1; //set array length less than 2 to the 32nd power 
console.log(namelistB.length); 

// 4294967295
</code></pre>
<p>你可以设置 <code>length</code> 属性的值来截断任何数组。当通过改变<code>length</code>属性值来扩展数组时，实际元素的数目将会增加。例如：将一个拥有 2 个元素的数组的 <code>length</code> 属性值设为 3 时，那么这个数组将会包含3个元素，并且，第三个元素的值将会是 <code>undefined</code> 。</p>
<pre><code>var arr = [1, 2, 3];
printEntries(arr);

arr.length = 5; // set array length to 5 while currently 3.
printEntries(arr);

function printEntries(arr) {
  var goNext = true;
  var entries = arr.entries();
  while (goNext) {
    var result = entries.next();
    if (result.done !== true) {
      console.log(result.value[1]);
      goNext = true;
    } else
      goNext = false;
  }
  console.log('=== printed ===');
}

// 1
// 2
// 3
// === printed ===
// 1
// 2
// 3
// undefined
// undefined
// === printed ===</code></code></pre>
<p>但是， <code>length</code> 属性不一定表示数组中定义值的个数。了解更多：<a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#Relationship_between_length_and_numerical_properties">长度与数值下标属性之间的关系</a>。</p>
<p></p><table class="standard-table">
<thead>
<tr>
<th class="header" colspan="2"><code>Array.length</code> 属性的属性特性：</th>
</tr>
</thead>
<tbody>
<tr>
<td>writable</td>
<td>true</td>
</tr>
<tr>
<td>enumerable</td>
<td>false</td>
</tr>
<tr>
<td>configurable</td>
<td>false</td>
</tr>
</tbody>
</table><p></p>
<ul>
<li><code>Writable</code> ：如果设置为<code>false</code>，该属性值将不能被修改。</li>
<li><code>Configurable</code> ：如果设置为<code>false</code>，删除或更改任何属性都将会失败。</li>
<li><code>Enumerable</code> ：如果设置为 <code>true</code> ，属性可以通过迭代器<a href="zh-CN/docs/Web/JavaScript/Reference/Statements/for">for</a>或<a href="Reference/Statements/for...in">for...in</a>进行迭代。</li>
</ul>
<h2 id="Examples" name="Examples">示例 </h2>
<h3 id="Example:_Iterating_over_an_array" name="Example:_Iterating_over_an_array">遍历数组</h3>
<p>下面的例子中，通过数组下标遍历数组元素，并把每个元素的值修改为原值的2倍。</p>
<pre><code  class="language-javascript"><code class="language-js"><span class="keyword token">var</span> numbers <span class="operator token">=</span> <span class="punctuation token">[</span><span class="number token">1</span><span class="punctuation token">,</span> <span class="number token">2</span><span class="punctuation token">,</span> <span class="number token">3</span><span class="punctuation token">,</span> <span class="number token">4</span><span class="punctuation token">,</span> <span class="number token">5</span><span class="punctuation token">]</span><span class="punctuation token">;</span>
<span class="keyword token">var</span> length <span class="operator token">=</span> numbers<span class="punctuation token">.</span>length<span class="punctuation token">;</span>
<span class="keyword token">for</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> i <span class="operator token">&lt;</span> length<span class="punctuation token">;</span> i<span class="operator token">++</span><span class="punctuation token">)</span> <span class="punctuation token">{</span>
  numbers<span class="punctuation token">[</span>i<span class="punctuation token">]</span> <span class="operator token">*</span><span class="operator token">=</span> <span class="number token">2</span><span class="punctuation token">;</span>
<span class="punctuation token">}</span>
<span class="comment token">// 遍历后的结果 [2, 4, 6, 8, 10]</span></code></code></pre>
<h3 id="Example:_Shortening_an_array" name="Example:_Shortening_an_array">截断数组</h3>
<p>下面的例子中，如果数组长度大于 3，则把该数组的长度截断为 3 。</p>
<pre><code  class="language-javascript"><code class="language-js"><span class="keyword token">var</span> numbers <span class="operator token">=</span> <span class="punctuation token">[</span><span class="number token">1</span><span class="punctuation token">,</span> <span class="number token">2</span><span class="punctuation token">,</span> <span class="number token">3</span><span class="punctuation token">,</span> <span class="number token">4</span><span class="punctuation token">,</span> <span class="number token">5</span><span class="punctuation token">]</span><span class="punctuation token">;</span>

<span class="keyword token">if</span> <span class="punctuation token">(</span>numbers<span class="punctuation token">.</span>length <span class="operator token">&gt;</span> <span class="number token">3</span><span class="punctuation token">)</span> <span class="punctuation token">{</span>
  numbers<span class="punctuation token">.</span>length <span class="operator token">=</span> <span class="number token">3</span><span class="punctuation token">;</span>
<span class="punctuation token">}</span>

console<span class="punctuation token">.</span><span class="function token">log</span><span class="punctuation token">(</span>numbers<span class="punctuation token">)</span><span class="punctuation token">;</span> <span class="comment token">// [1, 2, 3]</span>
console<span class="punctuation token">.</span><span class="function token">log</span><span class="punctuation token">(</span>numbers<span class="punctuation token">.</span>length<span class="punctuation token">)</span><span class="punctuation token">;</span> <span class="comment token">// 3</span></code></code></pre>
<h2 id="规范">规范</h2>
<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.</td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.4.5.2" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Array.length</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-properties-of-array-instances-length" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Array.length</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-properties-of-array-instances-length" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Array.length</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<div>
<div class="hidden">The compatibility table in 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>length</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>
              12</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>
</div>
<h2 id="相关链接">相关链接</h2>
<ul>
<li><a href="Reference/Array" title="REDIRECT Array"><code>Array</code></a></li>
</ul>
</article>