---
title: Document.evaluate()
slug: Web/API/Document/evaluate
tags:
  - API
  - DOM
  - XPath
  - 参考
  - 方法
translation_of: Web/API/Document/evaluate
---
<div>{{ ApiRef("DOM") }}</div>

<p>根据传入的 <a href="/zh-CN/docs/XPath">XPath</a> 表达式以及其他参数，返回一个 {{domxref("XPathResult")}} 对象。</p>

<h2 id="Syntax" name="Syntax">语法</h2>

<pre class="syntaxbox">var xpathResult = document.evaluate(
  xpathExpression,
  contextNode,
  namespaceResolver,
  resultType,
  result
);</pre>

<ul>
 <li><code>xpathExpression</code> 表示要计算的 Xpath 字符串。</li>
 <li><code>contextNode</code> 表示本次查询的<em>上下文节点</em>（参照XPath规范 <a href="http://www.w3.org/TR/xpath">http://www.w3.org/TR/xpath</a>）。通常会使用 <code>document</code>。</li>
 <li><code>namespaceResolver</code> 是函数。传入名空间前缀，返回跟此前缀相关的名空间URI（字符串）。通常用来解析Xpath内的前缀，以便对文档进行匹配。HTML文档或者不使用名空间前缀的文档，通常传入 <code>null</code>。</li>
 <li><code>resultType</code> 是整数。指定所返回的 <code>XPathResult</code> 的类型，常使用 <a href="#Result_types">named constant properties</a>，如 <code>XPathResult.ANY_TYPE</code>，范围 0 到 9，见下表。</li>
 <li><code>result</code> 为 <code>XPathResult</code> 型，用以存储查询结果。通常传入 <code>null</code>，此时将创建新的 <code>XPathResult</code> 对象。</li>
</ul>

<h2 id="示例">示例</h2>

<pre class="brush: js">var headings = document.evaluate("/html/body//h2", document, null, XPathResult.ANY_TYPE, null);
/* 在 document 中查找所有的 h2 元素。
 * 结果可能是无序节点迭代器。 */
var thisHeading = headings.iterateNext();
var alertText = "Level 2 headings in this document are:\n";
while (thisHeading) {
  alertText += thisHeading.textContent + "\n";
  thisHeading = headings.iterateNext();
}
alert(alertText); // 显示所有 h2 节点的文本
</pre>

<p>注意，在上述例子中，最好写更冗长的XPath，而不是常用的简写，比如 <code>//h2</code>。 通常，像上述例子所示，更具体的XPath选择器会得到显著的性能提升，特别是在非常大的文档中。这是因为查询计算不会将时间浪费在查看不需要的节点上。使用 // 通常很慢，这是因为它要从根节点和所有子节点中查找所有可能匹配的节点。</p>

<p>通过谨慎使用上下文参数能得到进一步的优化。比如，如果你知道你要查找的内容在 <code>body</code> 标签的某处，你可以这样做：</p>

<pre class="brush: js">document.evaluate(".//h2", document.body, null, XPathResult.ANY_TYPE, null);
</pre>

<p>注意上面的 <code>document.body</code> 已经替代了document作为上下文，所以 XPath 从 body 元素开始查找。 （在这个例子中，<code>"."</code> 很重要，因为它指示了查找要从document.body这个上下文节点开始。如果遗漏了 <code>"."</code> (剩下 <code>//h2</code>) ，查找会从根节点（<code>html</code>）处开始，这样会很浪费。）</p>

<p>查阅 <a href="/en-US/docs/Introduction_to_using_XPath_in_JavaScript" title="Introduction to using XPath in JavaScript">Introduction to using XPath in JavaScript</a> 获得更多信息。</p>

<h2 id="注意">注意</h2>

<ul>
 <li>XPath 表达式能在 HTML 和 XML 文档上计算。</li>
 <li>如果要计算别的而不是当前文档，在 FF3 上必须使用<code>someXMLDoc.evaluate()</code> ，虽然在 FF2 上 <code>document.evaluate()</code> 也有效。</li>
</ul>

<h2 id="结果的类型">结果的类型</h2>

<p>(Merge with <a href="/Template:XPathResultConstants">Template:XPathResultConstants</a>?</p>

<p>这些是 <code>evaluate</code> 方法的 <code>resultType</code> 参数支持的值：</p>

<table class="standard-table">
 <tbody>
  <tr>
   <td class="header">Result Type</td>
   <td class="header">Value</td>
   <td class="header">Description</td>
  </tr>
  <tr>
   <td><code>ANY_TYPE</code></td>
   <td>0</td>
   <td>Whatever type naturally results from the given expression.</td>
  </tr>
  <tr>
   <td><code>NUMBER_TYPE</code></td>
   <td>1</td>
   <td>A result set containing a single number. Useful, for example, in an XPath expression using the <code>count()</code> function.</td>
  </tr>
  <tr>
   <td><code>STRING_TYPE</code></td>
   <td>2</td>
   <td>A result set containing a single string.</td>
  </tr>
  <tr>
   <td><code>BOOLEAN_TYPE</code></td>
   <td>3</td>
   <td>A result set containing a single boolean value. Useful, for example, an an XPath expression using the <code>not()</code> function.</td>
  </tr>
  <tr>
   <td><code>UNORDERED_NODE_ITERATOR_TYPE</code></td>
   <td>4</td>
   <td>A result set containing all the nodes matching the expression. The nodes in the result set are not necessarily in the same order they appear in the document.</td>
  </tr>
  <tr>
   <td><code>ORDERED_NODE_ITERATOR_TYPE</code></td>
   <td>5</td>
   <td>A result set containing all the nodes matching the expression. The nodes in the result set are in the same order they appear in the document.</td>
  </tr>
  <tr>
   <td><code>UNORDERED_NODE_SNAPSHOT_TYPE</code></td>
   <td>6</td>
   <td>A result set containing snapshots of all the nodes matching the expression. The nodes in the result set are not necessarily in the same order they appear in the document.</td>
  </tr>
  <tr>
   <td><code>ORDERED_NODE_SNAPSHOT_TYPE</code></td>
   <td>7</td>
   <td>A result set containing snapshots of all the nodes matching the expression. The nodes in the result set are in the same order they appear in the document.</td>
  </tr>
  <tr>
   <td><code>ANY_UNORDERED_NODE_TYPE</code></td>
   <td>8</td>
   <td>A result set containing any single node that matches the expression. The node is not necessarily the first node in the document that matches the expression.</td>
  </tr>
  <tr>
   <td><code>FIRST_ORDERED_NODE_TYPE</code></td>
   <td>9</td>
   <td>A result set containing the first node in the document that matches the expression.</td>
  </tr>
 </tbody>
</table>

<p>Results of <code>NODE_ITERATOR</code> types contain references to nodes in the document. Modifying a node will invalidate the iterator. After modifying a node, attempting to iterate through the results will result in an error.</p>

<p>Results of <code>NODE_SNAPSHOT</code> types are snapshots, which are essentially lists of matched nodes. You can make changes to the document by altering snapshot nodes. Modifying the document doesn't invalidate the snapshot; however, if the document is changed, the snapshot may not correspond to the current state of the document, since nodes may have moved, been changed, added, or removed.</p>

<h2 id="规范">规范</h2>

<table class="standard-table">
 <thead>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
 </thead>
 <tbody>
  <tr>
   <td>{{SpecName("DOM3 XPath", "xpath.html#XPathEvaluator-evaluate", "Document.evaluate")}}</td>
   <td>{{Spec2("DOM3 XPath")}}</td>
   <td>Initial specification</td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器兼容性">浏览器兼容性</h2>



<p>{{Compat("api.Document.evaluate")}}</p>

<h2 id="参见">参见</h2>

<ul>
 <li>{{domxref("Document.createExpression()")}}</li>
 <li>{{domxref("XPathResult")}}</li>
 <li><a href="/zh-CN/docs/Code_snippets/XPath">XPath Code Snippets</a></li>
 <li><a href="http://codepen.io/johan/full/ckFgn">Check for browser support</a></li>
</ul>