Document.evaluate()

根据传入的 XPath (en-US) 表达式以及其他参数,返回一个 XPathResult (en-US) 对象。

语法

var xpathResult = document.evaluate(
  xpathExpression,
  contextNode,
  namespaceResolver,
  resultType,
  result
);
  • xpathExpression 表示要计算的 Xpath 字符串。
  • contextNode 表示本次查询的上下文节点(参照 XPath 规范 http://www.w3.org/TR/xpath)。通常会使用 document
  • namespaceResolver 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 null
  • resultType 是整数。指定所返回的 XPathResult 的类型,常使用 named constant properties,如 XPathResult.ANY_TYPE,范围 0 到 9,见下表。
  • resultXPathResult 型,用以存储查询结果。通常传入 null,此时将创建新的 XPathResult 对象。

示例

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

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 //h2。 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。

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 body 标签的某处,你可以这样做:

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

注意上面的 document.body 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,"." 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 "." (剩下 //h2) ,查找会从根节点(html)处开始,这样会很浪费。)

查阅 Introduction to using XPath in JavaScript 获得更多信息。

注意

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

结果的类型

(Merge with Template:XPathResultConstants?

这些是 evaluate 方法的 resultType 参数支持的值:

Result Type Value Description
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() function.
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() function.
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

Specification
DOM Standard
# dom-xpathevaluatorbase-evaluate

浏览器兼容性

BCD tables only load in the browser

参见