---
title: Map.prototype.size
slug: Web/JavaScript/Reference/Global_Objects/Map/size
tags:
  - ECMAScript 2015
  - JavaScript
  - Map
  - Property
translation_of: Web/JavaScript/Reference/Global_Objects/Map/size
original_slug: Web/JavaScript/Reference/Global_Objects/Map/size
browser-compat: javascript.builtins.Map.size
---
<div>{{JSRef}}<br>
 </div>

<p><code><strong>size</strong></code> 是可访问属性，用于返回 一个{{jsxref("Map")}} 对象的成员数量。</p>

<p>{{EmbedInteractiveExample("pages/js/map-prototype-size.html")}}</p>

<p>这个示例源码保存在GitHub：<a href="https://github.com/mdn/interactive-examples">https://github.com/mdn/interactive-examples</a>。如果你想贡献代码，修改后在GitHub上发推送请求给我们。</p>

<p> </p>

<h2 id="描述">描述</h2>

<p>size 属性的值是一个整数，表示 Map 对象有多少个键值对。size 是只读属性，用set 方法修改size返回 undefined，即不能改变它的值。</p>

<h2 id="示例">示例</h2>

<pre class="brush:js">var myMap = new Map();
myMap.set("a", "alpha");
myMap.set("b", "beta");
myMap.set("g", "gamma");

myMap.size // 3
</pre>

<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>{{SpecName('ES6', '#sec-get-map.prototype.size', 'Map.prototype.size')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-get-map.prototype.size', 'Map.prototype.size')}}</td>
   <td>{{Spec2('ESDraft')}}</td>
   <td> </td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器兼容">浏览器兼容</h2>

<p>此页的兼容性表格请查阅：<a href="https://github.com/mdn/browser-compat-data">https://github.com/mdn/browser-compat-data</a> <br>
 如果你想更新数据，请在GitHub上给我们发推送请求。</p>

<div class="warning">
<p><strong>警告：</strong><strong>异常提醒</strong></p>
</div>

<ul>
 <li>在JavaScript中直接书写Map.prototype.size可能得到以下异常：<br>
  <code>Uncaught TypeError: Method get Map.prototype.size called on incompatible receiver #&lt;Map&gt;</code><br>
  异常原因查看：<a href="/zh-CN/docs/">https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Called_on_incompatible_type</a></li>
</ul>

<h2 id="相关阅读">相关阅读</h2>

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