uTools-Manuals/docs/javascript/Reference/Global_Objects/String/charAt.html

<article id="wikiArticle">
<p></p><p></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><code class="language-javascript"><em>str</em>.charAt(<em>index</em>)</code></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><code class="language-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) + "'");
</code></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 ''
</code></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 class="external" 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" rel="noopener">基本多文种平面</a>（BMP）中的字符。</p>
<pre><code class="language-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; 
}

</code></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><code  class="language-javascript">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]; 
}</code></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><code  class="language-javascript">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;
}</code></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><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.5.4.4" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">String.prototype.charAt</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-string.prototype.charat" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">String.prototype.charAt</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p></p><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
            This compatibility table still uses the old format,
            because we haven't yet converted the data it contains.
            <strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
<div class="htab">
<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
<ul>
<li class="selected"><a>Desktop</a></li>
<li><a>Mobile</a></li>
</ul>
</div><p></p>
<div id="compat-desktop">
<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><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<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><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
</tr>
</tbody>
</table>
</div>
<h2 id="See_also" name="See_also">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/String/indexOf" title="indexOf() 方法返回调用  String 对象中第一次出现的指定值的索引，开始在 fromIndex进行搜索。"><code>String.prototype.indexOf()</code></a>, <a href="Reference/Global_Objects/String/lastIndexOf" title="lastIndexOf() 方法返回指定值在调用该方法的字符串中最后出现的位置，如果没找到则返回 -1。从该字符串的后面向前查找，从 fromIndex 处开始。"><code>String.prototype.lastIndexOf()</code></a></li>
<li><a href="Reference/Global_Objects/String/split" title="split() 方法使用指定的分隔符字符串将一个String对象分割成字符串数组，以将字符串分隔为子字符串，以确定每个拆分的位置。"><code>String.prototype.split()</code></a></li>
<li><a href="Reference/Global_Objects/String/charCodeAt" title="charCodeAt() 方法返回0到65535之间的整数，表示给定索引处的UTF-16代码单元 (在 Unicode 编码单元表示一个单一的 UTF-16 编码单元的情况下，UTF-16 编码单元匹配 Unicode 编码单元。但在——例如 Unicode 编码单元 &gt; 0x10000 的这种——不能被一个 UTF-16 编码单元单独表示的情况下，只能匹配 Unicode 代理对的第一个编码单元) 。如果你想要整个代码点的值，使用 codePointAt()。"><code>String.prototype.charCodeAt()</code></a></li>
<li><a href="Reference/Global_Objects/String/fromCodePoint" title="String.fromCodePoint() 静态方法返回使用指定的代码点序列创建的字符串。"><code>String.fromCodePoint()</code></a></li>
<li><a href="Reference/Global_Objects/String/codePointAt" title="codePointAt() 方法返回 一个 Unicode 编码点值的非负整数。"><code>String.prototype.codePointAt()</code></a></li>
</ul>
</article>