---
title: String.prototype.codePointAt()
slug: Web/JavaScript/Reference/Global_Objects/String/codePointAt
translation_of: Web/JavaScript/Reference/Global_Objects/String/codePointAt
---
<div>{{JSRef}}</div>

<p><strong><code>codePointAt()</code></strong> 方法返回 一个 Unicode 编码点值的非负整数。</p>

<h2 id="语法">语法</h2>

<pre class="syntaxbox"><code><var>str</var>.codePointAt(<var>pos</var>)</code></pre>

<h3 id="参数">参数</h3>

<dl>
 <dt><code>pos</code></dt>
 <dd>这个字符串中需要转码的元素的位置。</dd>
</dl>

<h3 id="返回值">返回值</h3>

<p>返回值是在字符串中的给定索引的编码单元体现的数字，如果在索引处没找到元素则返回 {{jsxref("undefined")}} 。</p>

<h2 id="描述">描述</h2>

<p>如果在指定的位置没有元素则返回 {{jsxref("undefined")}} 。如果在索引处开始没有UTF-16 代理对，将直接返回在那个索引处的编码单元。</p>

<p>Surrogate Pair是UTF-16中用于扩展字符而使用的编码方式，是一种采用四个字节(两个UTF-16编码)来表示一个字符，称作代理对。</p>

<h2 id="例子">例子</h2>

<h3 id="使用_codePointAt()">使用 <code>codePointAt()</code></h3>

<pre class="brush: js">'ABC'.codePointAt(1);          // 66
'\uD800\uDC00'.codePointAt(0); // 65536

'XYZ'.codePointAt(42); // undefined
</pre>

<h2 id="替补支持(Polyfill)">替补支持(Polyfill)</h2>

<p>给原生不支持 ECMAScript 6 的浏览器使用<code>codePointAt()</code>方法的的一个字符串扩展方法。</p>

<pre class="brush: js">/*! http://mths.be/codepointat v0.1.0 by @mathias */
if (!String.prototype.codePointAt) {
  (function() {
    'use strict'; // 严格模式，needed to support `apply`/`call` with `undefined`/`null`
    var codePointAt = function(position) {
      if (this == null) {
        throw TypeError();
      }
      var string = String(this);
      var size = string.length;
      // 变成整数
      var index = position ? Number(position) : 0;
      if (index != index) { // better `isNaN`
        index = 0;
      }
      // 边界
      if (index &lt; 0 || index &gt;= size) {
        return undefined;
      }
      // 第一个编码单元
      var first = string.charCodeAt(index);
      var second;
      if ( // 检查是否开始 surrogate pair
        first &gt;= 0xD800 &amp;&amp; first &lt;= 0xDBFF &amp;&amp; // high surrogate
        size &gt; index + 1 // 下一个编码单元
      ) {
        second = string.charCodeAt(index + 1);
        if (second &gt;= 0xDC00 &amp;&amp; second &lt;= 0xDFFF) { // low surrogate
          // http://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
          return (first - 0xD800) * 0x400 + second - 0xDC00 + 0x10000;
        }
      }
      return first;
    };
    if (Object.defineProperty) {
      Object.defineProperty(String.prototype, 'codePointAt', {
        'value': codePointAt,
        'configurable': true,
        'writable': true
      });
    } else {
      String.prototype.codePointAt = codePointAt;
    }
  }());
}
</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-string.prototype.codepointat', 'String.prototype.codePointAt')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ESDraft', '#sec-string.prototype.codepointat', 'String.prototype.codePointAt')}}</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>{{CompatibilityTable}}</div>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>特性</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>基本支持</td>
   <td>{{CompatChrome("41")}}</td>
   <td>{{CompatGeckoDesktop("29")}}</td>
   <td>11</td>
   <td>{{CompatOpera("28")}}</td>
   <td>{{CompatNo}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>特性</th>
   <th>Android</th>
   <th>Chrome for Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>基本支持</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatGeckoMobile("29")}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
  </tr>
 </tbody>
</table>
</div>

<h2 id="相关链接">相关链接</h2>

<ul>
 <li>{{jsxref("String.fromCodePoint()")}}</li>
 <li>{{jsxref("String.fromCharCode()")}}</li>
 <li>{{jsxref("String.prototype.charCodeAt()")}}</li>
 <li>{{jsxref("String.prototype.charAt()")}}</li>
</ul>