---
title: Array.length
slug: Web/JavaScript/Reference/Global_Objects/Array/length
translation_of: Web/JavaScript/Reference/Global_Objects/Array/length
---
<div>{{JSRef}}</div>

<p><code><strong>length</strong></code> 為<code>Array物件的屬性</code> ，可供設定或回傳該陣列實體中包含的元素個數。其值必為一大於零、32位元、且恆大於該陣列最大索引數的正整數。</p>

<pre class="brush: js notranslate">var items = ['shoes', 'shirts', 'socks', 'sweaters'];
items.length;

// returns 4</pre>

<h2 id="描述">描述</h2>

<p><code>length</code> 屬性的值必為一正整數，其值必介於 0 ～ 2<sup>32</sup> (不包含)之間.</p>

<pre class="brush: js notranslate">var namelistA = new Array(4294967296); //2<sup>32</sup><sup> = </sup>4294967296
var namelistC = new Array(-100) //負數

console.log(namelistA.length); //RangeError: Invalid array length
console.log(namelistC.length); //RangeError: Invalid array length



var namelistB = [];
namelistB.length = Math.pow(2,32)-1; //將長度設定介於 0 ~ 2<sup>32 </sup>-1
console.log(namelistB.length);

//4294967295</pre>

<p>你可以透過改變 <code>length</code> 屬性來改變陣列的長度。當你透過 <code>length</code> 屬性來增加陣列的長度時，陣列中實際的元素也會隨之增加。舉例來說，當你將 array.length 由 2 增加為3，則改動後該陣列即擁有3個元素，該新增的元素則會是一個不可迭代(non-iterable)的空槽(empty slot)。</p>

<pre class="notranslate">const arr = [1, 2];
console.log(arr);
// [ 1, 2 ]

arr.length = 5; // 將arr的length由2改成5
console.log(arr);
// [ 1, 2, &lt;3 empty items&gt; ]

arr.forEach(element =&gt; console.log(element)); // 空元素無法被迭代
// 1
// 2</pre>

<p>如上所見，<code>length</code> 屬性不盡然代表陣列中所有已定義的元素個數。詳見 <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#Relationship_between_length_and_numerical_properties" title="Relationship between length and numerical properties">length 與數值屬性的關係</a>。</p>

<p>{{js_property_attributes(1, 0, 0)}}</p>

<div>
<ul>
 <li><code>Writable</code>: 如果此屬性值為<code>false</code>，則該屬性的內容值無法被改動。</li>
 <li><code>Configurable</code>: 如果此屬性值為<code>false</code>，任何刪除屬性或更改其屬性的操作(<code>Writable</code>, <code>Configurable</code>, or <code>Enumerable</code>)皆會失敗。</li>
 <li><code>Enumerable</code>: 如果此屬性值為<code>true</code>，該內容值可倍 <a href="/en-US/docs/Web/JavaScript/Reference/Statements/for">for</a> 或 <a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...in">for..in</a> 迴圈迭代處理。</li>
</ul>
</div>

<h2 id="範例">範例</h2>

<h3 id="對陣列進行迭代處理">對陣列進行迭代處理</h3>

<p>以下範例中, 陣列 <code>numbers</code> 透過 <code>length</code> 屬性進行迭代操作，並將其內容值加倍。</p>

<pre class="brush: js notranslate">var numbers = [1, 2, 3, 4, 5];
var length = numbers.length;
for (var i = 0; i &lt; length; i++) {
  numbers[i] *= 2;
}
// numbers 內容值變為 [2, 4, 6, 8, 10]
</pre>

<h3 id="縮減陣列">縮減陣列</h3>

<p>以下範例中, 陣列 <code>numbers</code>  的長度若大於 3，則將其長度縮減至 3。</p>

<pre class="brush: js notranslate">var numbers = [1, 2, 3, 4, 5];

if (numbers.length &gt; 3) {
  numbers.length = 3;
}

console.log(numbers); // [1, 2, 3]
console.log(numbers.length); // 3
</pre>

<h2 id="規格">規格</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>{{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="瀏覽器相容性">瀏覽器相容性</h2>

<div>


<p>{{Compat("javascript.builtins.Array.length")}}</p>
</div>

<h2 id="其他">其他</h2>

<ul>
 <li>{{jsxref("Array")}}</li>
</ul>