Document.evaluate()

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句法
范例
注意事项
Result types
规范
浏览器兼容性
另请参阅
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返回 XPathResult based on an XPath expression and other given parameters.

句法

var xpathResult = document.evaluate(
  xpathExpression,
  contextNode,
  namespaceResolver,
  resultType,
  result
);

xpathExpression is a string representing the XPath to be evaluated.
contextNode specifies the context node for the query (see the XPath specification ). It's common to pass document as the context node.
namespaceResolver is a function that will be passed any namespace prefixes and should return a string representing the namespace URI associated with that prefix. It will be used to resolve prefixes within the XPath itself, so that they can be matched with the document. null is common for HTML documents or when no namespace prefixes are used.
resultType is an integer that corresponds to the type of result XPathResult to return. Use named constant properties ，譬如 XPathResult.ANY_TYPE , of the XPathResult constructor, which correspond to integers from 0 to 9.
result is an existing XPathResult to use for the results. null is the most common and will create a new XPathResult

范例

var headings = document.evaluate("/html/body//h2", document, null, XPathResult.ANY_TYPE, null);
/* Search the document for all h2 elements.
 * The result will likely be an unordered node iterator. */
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); // Alerts the text of all h2 elements

Note, in the above example, a more verbose XPath is preferred over common shortcuts such as //h2 . Generally, more specific XPath selectors as in the above example usually gives a significant performance improvement, especially on very large documents. This is because the evaluation of the query spends does not waste time visiting unnecessary nodes. Using // is generally slow as it visits every node from the root and all subnodes looking for possible matches.

Further optimization can be achieved by careful use of the context parameter. For example, if you know the content you are looking for is somewhere inside the body tag, you can use this:

document.evaluate(".//h2", document.body, null, XPathResult.ANY_TYPE, null);

Notice in the above document.body has been used as the context instead of document so the XPath starts from the body element. (In this example, the "." is important to indicate that the querying should start from the context node, document.body. If the "." was left out (leaving //h2 ) the query would start from the root node ( html ) which would be more wasteful.)

见 Introduction to using XPath in JavaScript 了解更多信息。

注意事项

XPath expressions can be evaluated on HTML and XML documents.
While using document.evaluate() works in FF2, in FF3 one must use someXMLDoc.evaluate() if evaluating against something other than the current document.

Result types

(Merge with Template:XPathResultConstants ?

These are supported values for the resultType 参数为 evaluate 方法：

结果类型	值	描述
`ANY_TYPE`	0	Whatever type naturally results from the given expression.
`NUMBER_TYPE`	1	A result set containing a single number. Useful, for example, in an XPath expression using the `count()` 函数。
`STRING_TYPE`	2	A result set containing a single string.
`BOOLEAN_TYPE`	3	A result set containing a single boolean value. Useful, for example, an an XPath expression using the `not()` 函数。
`UNORDERED_NODE_ITERATOR_TYPE`	4	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.
`ORDERED_NODE_ITERATOR_TYPE`	5	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.
`UNORDERED_NODE_SNAPSHOT_TYPE`	6	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.
`ORDERED_NODE_SNAPSHOT_TYPE`	7	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.
`ANY_UNORDERED_NODE_TYPE`	8	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.
`FIRST_ORDERED_NODE_TYPE`	9	A result set containing the first node in the document that matches the expression.

Results of NODE_ITERATOR 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.

Results of NODE_SNAPSHOT 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.

规范

规范	状态	注释
DOM (文档对象模型) 3 级 XPath 规范 The definition of 'Document.evaluate' in that specification.	推荐	Initial specification

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`evaluate`	Chrome 1	Edge 12	Firefox 1.5	IE No	Opera 9	Safari 5	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 10.1	Safari iOS 4.2	Samsung Internet Android 1.0

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句法

范例

注意事项

Result types

规范

浏览器兼容性

图例

另请参阅

元数据

相关话题