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

<article id="wikiArticle">
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<p></p><p></p>
<h2 id="概述">概述</h2>
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<p><code><strong>startsWith()</strong></code>方法用来判断当前字符串是否是以另外一个给定的子字符串“开头”的，根据判断结果返回 <code>true</code> 或 <code>false</code>。</p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre><code class="language-javascript"><code class="brush:js;"><em>str</em>.startsWith(<em>searchString</em> [, <em>position</em>])</code>;</code></pre>
<h3 id="参数">参数</h3>
<dl>
<dt><code>searchString</code></dt>
<dd>要搜索的子字符串。</dd>
<dt><code>position</code></dt>
<dd>在 <code>str</code> 中搜索 <code><var>searchString</var></code> 的开始位置，默认值为 0，也就是真正的字符串开头处。</dd>
</dl>
<h2 id="Examples" name="Examples">示例</h2>
<pre><code class="language-js;">var str = "To be, or not to be, that is the question.";

alert(str.startsWith("To be"));         // true
alert(str.startsWith("not to be"));     // false
alert(str.startsWith("not to be", 10)); // true</code></pre>
<h2 id="Polyfill">Polyfill</h2>
<pre><code  class="language-javascript">/*! http://mths.be/startswith v0.2.0 by @mathias */
if (!String.prototype.startsWith) {
  (function() {
    'use strict'; // needed to support `apply`/`call` with `undefined`/`null`
    var defineProperty = (function() {
      // IE 8 only supports `Object.defineProperty` on DOM elements
      try {
        var object = {};
        var $defineProperty = Object.defineProperty;
        var result = $defineProperty(object, object, object) &amp;&amp; $defineProperty;
      } catch(error) {}
      return result;
    }());
    var toString = {}.toString;
    var startsWith = function(search) {
      if (this == null) {
        throw TypeError();
      }
      var string = String(this);
      if (search &amp;&amp; toString.call(search) == '[object RegExp]') {
        throw TypeError();
      }
      var stringLength = string.length;
      var searchString = String(search);
      var searchLength = searchString.length;
      var position = arguments.length &gt; 1 ? arguments[1] : undefined;
      // `ToInteger`
      var pos = position ? Number(position) : 0;
      if (pos != pos) { // better `isNaN`
        pos = 0;
      }
      var start = Math.min(Math.max(pos, 0), stringLength);
      // Avoid the `indexOf` call if no match is possible
      if (searchLength + start &gt; stringLength) {
        return false;
      }
      var index = -1;
      while (++index &lt; searchLength) {
        if (string.charCodeAt(start + index) != searchString.charCodeAt(index)) {
          return false;
        }
      }
      return true;
    };
    if (defineProperty) {
      defineProperty(String.prototype, 'startsWith', {
        'value': startsWith,
        'configurable': true,
        'writable': true
      });
    } else {
      String.prototype.startsWith = startsWith;
    }
  }());
}</code></pre>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<div><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>
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<div id="compat-desktop">
<table class="compat-table">
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<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Edge</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
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<tr>
<td>Basic support</td>
<td>41</td>
<td><a href="/en-US/Firefox/Releases/17" title="Released on 2012-11-20.">17</a> (17)</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #f00;">未实现</span></td>
<td>28</td>
<td>9</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: #f00;">未实现</span></td>
<td>36</td>
<td>17.0 (17)</td>
<td><span style="color: #f00;">未实现</span></td>
<td><span style="color: #f00;">未实现</span></td>
<td><span style="color: #f00;">未实现</span></td>
</tr>
</tbody>
</table>
<p> </p>
<p>请注意，尽管MSDN文档有清晰地指示这个方法 (https://msdn.microsoft.com/en-us/library/mt146831(v=vs.94).aspx) 不支持Internet Explorer,，但是这个方法似乎会在很久之前的Internet Explorer 8下生效。</p>
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<h2 id="See_also" name="See_also">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/String/endsWith" title="endsWith()方法用来判断当前字符串是否是以另外一个给定的子字符串“结尾”的，根据判断结果返回 true 或 false。"><code>String.prototype.endsWith()</code></a> <span title="这是一个实验性的 API，请尽量不要在生产环境中使用它。"><i class="icon-beaker"> </i></span></li>
<li><a href="Reference/Global_Objects/String/contains" title="此页面仍未被本地化, 期待您的翻译!"><code>String.prototype.contains()</code></a> <span title="这是一个实验性的 API，请尽量不要在生产环境中使用它。"><i class="icon-beaker"> </i></span></li>
<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></li>
</ul>
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