Document​.evaluate()

Returns an XPathResult based on an XPath expression and other given parameters.

SyntaxSection

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, such as 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

ExampleSection

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.

NotesSection

  • 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 typesSection

(Merge with Template:XPathResultConstants?

These are supported values for the resultType parameter of the evaluate method:

Result TypeValueDescription
ANY_TYPE0Whatever type naturally results from the given expression.
NUMBER_TYPE1A result set containing a single number. Useful, for example, in an XPath expression using the count() function.
STRING_TYPE2A result set containing a single string.
BOOLEAN_TYPE3A result set containing a single boolean value. Useful, for example, an an XPath expression using the not() function.
UNORDERED_NODE_ITERATOR_TYPE4A 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_TYPE5A 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_TYPE6A 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_TYPE7A 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_TYPE8A 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_TYPE9A 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.

Origin
https://developer.mozilla.org/en-US/docs/Web/API/Document/evaluate

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