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+---
+title: Array.length
+slug: Web/JavaScript/Reference/Global_Objects/Array/length
+tags:
+  - JavaScript
+  - 参考
+  - 属性
+  - 数组
+translation_of: Web/JavaScript/Reference/Global_Objects/Array/length
+---
+<p>{{JSRef}}</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>{{EmbedInteractiveExample("pages/js/array-length.html")}}</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 class="brush: js">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
+</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></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>{{js_property_attributes(1, 0, 0)}}</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="/zh-CN/docs/Web/JavaScript/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 class="brush: js line-numbers  language-js"><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></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 class="brush: js line-numbers  language-js"><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></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>{{SpecName('ES1')}}</td>
+   <td>{{Spec2('ES1')}}</td>
+   <td>Initial definition.</td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES5.1', '#sec-15.4.5.2', 'Array.length')}}</td>
+   <td>{{Spec2('ES5.1')}}</td>
+   <td> </td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ES6', '#sec-properties-of-array-instances-length', 'Array.length')}}</td>
+   <td>{{Spec2('ES6')}}</td>
+   <td> </td>
+  </tr>
+  <tr>
+   <td>{{SpecName('ESDraft', '#sec-properties-of-array-instances-length', 'Array.length')}}</td>
+   <td>{{Spec2('ESDraft')}}</td>
+   <td> </td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+<div>
+
+
+<p>{{Compat("javascript.builtins.Array.length")}}</p>
+</div>
+
+<h2 id="相关链接">相关链接</h2>
+
+<ul>
+ <li>{{jsxref("Array")}}</li>
+</ul>