path: root/files/zh-cn/web/javascript/reference/global_objects/string/charat/index.html

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

<h2 id="Summary" name="Summary">概述</h2>

<p><strong>charAt()</strong> 方法从一个字符串中返回指定的字符。</p>

<h2 id="Syntax" name="Syntax">语法</h2>

<pre class="syntaxbox"><em>str</em>.charAt(<em>index</em>)</pre>

<h3 id="Parameters" name="Parameters">参数</h3>

<dl>
 <dt><code>index</code></dt>
 <dd>一个介于0 和字符串长度减1之间的整数。 (0~length-1)</dd>
 <dd>如果没有提供索引，charAt() 将使用0。</dd>
 <dt> </dt>
</dl>

<h2 id="Description" name="Description">描述</h2>

<p>字符串中的字符从左向右索引，第一个字符的索引值为 0，最后一个字符（假设该字符位于字符串 stringName 中）的索引值为 <code>stringName.length - 1</code>。 如果指定的 index 值超出了该范围，则返回一个空字符串。</p>

<h2 id="Examples" name="Examples">示例</h2>

<h3 id="Example:_Displaying_characters_at_different_locations_in_a_string" name="Example:_Displaying_characters_at_different_locations_in_a_string">例子：输出字符串中不同位置的字符</h3>

<p>下例输出字符串 "<code>Brave new world</code>" 不同位置处的字符：</p>

<pre class="brush:js">var anyString = "Brave new world";

console.log("The character at index 0   is '" + anyString.charAt(0)   + "'");
console.log("The character at index 1   is '" + anyString.charAt(1)   + "'");
console.log("The character at index 2   is '" + anyString.charAt(2)   + "'");
console.log("The character at index 3   is '" + anyString.charAt(3)   + "'");
console.log("The character at index 4   is '" + anyString.charAt(4)   + "'");
console.log("The character at index 999 is '" + anyString.charAt(999) + "'");
</pre>

<p>上面代码的输出为：</p>

<pre>The character at index 0 is 'B'
The character at index 1 is 'r'
The character at index 2 is 'a'
The character at index 3 is 'v'
The character at index 4 is 'e'
The character at index 999 is ''
</pre>

<h3 id="例子：获取所有字符">例子：获取所有字符</h3>

<p>以下提供了一种确保通过字符串循环总是提供整个字符的方法，即使该字符串包含不在<a href="https://zh.wikipedia.org/wiki/Unicode%E5%AD%97%E7%AC%A6%E5%B9%B3%E9%9D%A2%E6%98%A0%E5%B0%84#.E5.9F.BA.E6.9C.AC.E5.A4.9A.E6.96.87.E7.A7.8D.E5.B9.B3.E9.9D.A2">基本多文种平面</a>（BMP）中的字符。</p>

<pre class="brush:js">var str = 'A \uD87E\uDC04 Z'; // We could also use a non-BMP character directly
for (var i=0, chr; i &lt; str.length; i++) {
  if ((chr = getWholeChar(str, i)) === false) {
    continue;
  } // Adapt this line at the top of each loop, passing in the whole string and
    // the current iteration and returning a variable to represent the
    // individual character

  alert(chr);
}

function getWholeChar (str, i) {
  var code = str.charCodeAt(i);

  if (isNaN(code)) {
    return ''; // Position not found
  }
  if (code &lt; 0xD800 || code &gt; 0xDFFF) {
    return str.charAt(i);
  }

  // High surrogate (could change last hex to 0xDB7F to treat high private
  // surrogates as single characters)
  if (0xD800 &lt;= code &amp;&amp; code &lt;= 0xDBFF) {
    if (str.length &lt;= (i+1))  {
      throw 'High surrogate without following low surrogate';
    }
    var next = str.charCodeAt(i+1);
      if (0xDC00 &gt; next || next &gt; 0xDFFF) {
        throw 'High surrogate without following low surrogate';
      }
      return str.charAt(i)+str.charAt(i+1);
  }
  // Low surrogate (0xDC00 &lt;= code &amp;&amp; code &lt;= 0xDFFF)
  if (i === 0) {
    throw 'Low surrogate without preceding high surrogate';
  }
  var prev = str.charCodeAt(i-1);

  // (could change last hex to 0xDB7F to treat high private
  // surrogates as single characters)
  if (0xD800 &gt; prev || prev &gt; 0xDBFF) {
    throw 'Low surrogate without preceding high surrogate';
  }
  // We can pass over low surrogates now as the second component
  // in a pair which we have already processed
  return false;
}

</pre>

<p>在允许解构分配的独占JavaScript 1.7+环境（如Firefox）中，以下是一个更简洁和更灵活的替代方法，它会自动递增一个递增变量（如果字符保证它是一个替代对）。</p>

<pre class="brush: js">var str = 'A\uD87E\uDC04Z'; // We could also use a non-BMP character directly
for (var i=0, chr; i &lt; str.length; i++) {
  [chr, i] = getWholeCharAndI(str, i);
  // Adapt this line at the top of each loop, passing in the whole string and
  // the current iteration and returning an array with the individual character
  // and 'i' value (only changed if a surrogate pair)

  alert(chr);
}

function getWholeCharAndI (str, i) {
  var code = str.charCodeAt(i);

  if (isNaN(code)) {
    return ''; // Position not found
  }
  if (code &lt; 0xD800 || code &gt; 0xDFFF) {
    return [str.charAt(i), i]; // Normal character, keeping 'i' the same
  }

  // High surrogate (could change last hex to 0xDB7F to treat high private
  // surrogates as single characters)
  if (0xD800 &lt;= code &amp;&amp; code &lt;= 0xDBFF) {
    if (str.length &lt;= (i+1))  {
      throw 'High surrogate without following low surrogate';
    }
    var next = str.charCodeAt(i+1);
      if (0xDC00 &gt; next || next &gt; 0xDFFF) {
        throw 'High surrogate without following low surrogate';
      }
      return [str.charAt(i)+str.charAt(i+1), i+1];
  }
  // Low surrogate (0xDC00 &lt;= code &amp;&amp; code &lt;= 0xDFFF)
  if (i === 0) {
    throw 'Low surrogate without preceding high surrogate';
  }
  var prev = str.charCodeAt(i-1);

  // (could change last hex to 0xDB7F to treat high private surrogates
  // as single characters)
  if (0xD800 &gt; prev || prev &gt; 0xDBFF) {
    throw 'Low surrogate without preceding high surrogate';
  }
  // Return the next character instead (and increment)
  return [str.charAt(i+1), i+1];
}</pre>

<h3 id="示例：修复charAt以支持非基本多文种平面（BMP）字符">示例：修复charAt以支持非基本多文种平面（BMP）字符</h3>

<p>虽然上面的例子对于那些希望支持非BMP字符的用户可能更有用（因为它不要求调用者知道任何非BMP字符可能出现在哪里），在人们希望的情况下，在选择字符 通过索引，将字符串中的替代对作为它们表示的单个字符，可以使用以下：</p>

<pre class="brush: js">function fixedCharAt (str, idx) {
  var ret = '';
  str += '';
  var end = str.length;

  var surrogatePairs = /[\uD800-\uDBFF][\uDC00-\uDFFF]/g;
  while ((surrogatePairs.exec(str)) != null) {
    var li = surrogatePairs.lastIndex;
    if (li - 2 &lt; idx) {
      idx++;
    } else {
      break;
    }
  }

  if (idx &gt;= end || idx &lt; 0) {
    return '';
  }

  ret += str.charAt(idx);

  if (/[\uD800-\uDBFF]/.test(ret) &amp;&amp; /[\uDC00-\uDFFF]/.test(str.charAt(idx+1))) {
    // Go one further, since one of the "characters" is part of a surrogate pair
    ret += str.charAt(idx+1);
  }
  return ret;
}</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>ECMAScript 1st Edition.</td>
   <td>Standard</td>
   <td>Initial definition.</td>
  </tr>
  <tr>
   <td>{{SpecName('ES5.1', '#sec-15.5.4.4', 'String.prototype.charAt')}}</td>
   <td>{{Spec2('ES5.1')}}</td>
   <td> </td>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-string.prototype.charat', 'String.prototype.charAt')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td> </td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器兼容性">浏览器兼容性</h2>

<p>{{ CompatibilityTable() }}</p>

<div>
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
  </tr>
 </tbody>
</table>
</div>

<div>
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</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>Basic support</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
   <td>{{ CompatVersionUnknown() }}</td>
  </tr>
 </tbody>
</table>
</div>

<h2 id="See_also" name="See_also">相关链接</h2>

<ul>
 <li>{{jsxref("String.prototype.indexOf()")}}, {{jsxref("String.prototype.lastIndexOf()")}}</li>
 <li>{{jsxref("String.prototype.split()")}}</li>
 <li>{{jsxref("String.prototype.charCodeAt()")}}</li>
 <li>{{jsxref("String.fromCodePoint()")}}</li>
 <li>{{jsxref("String.prototype.codePointAt()")}}</li>
</ul>