Document.evaluate()
Returns an XPathResult
based on an XPath
expression and other given parameters.
Syntax
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 passdocument
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 resultXPathResult
to return. Use named constant properties, such asXPathResult.ANY_TYPE
, of the XPathResult constructor, which correspond to integers from 0 to 9. -
result
is an existingXPathResult
to use for the results.null
is the most common and will create a newXPathResult
Example
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.)
See Introduction to using XPath in JavaScript for more information.
Notes
- 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
These are supported values for the resultType
parameter of the
evaluate
method:
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
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.
Specifications
Specification |
---|
DOM Standard # dom-xpathevaluatorbase-evaluate |
Browser compatibility
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