Document: evaluate() method

{{ ApiRef("DOM") }}

The evaluate() method of the {{domxref("Document")}} interface selects elements based on the XPath expression given in parameters.

XPath expressions can be evaluated on both HTML and XML documents.

Syntax

evaluate(xpathExpression, contextNode, namespaceResolver, resultType, result)

Parameters

• xpathExpression

  • : A string representing the xpath to be evaluated.

• contextNode

  • : The context node for the query. It's common to pass document as the context node.

• namespaceResolver

  • : 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. The value null is common for HTML documents or when no namespace prefixes are used.

• resultType

  • : An integer that corresponds to the type of result XPathResult to return. The following values are possible:
    • 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.

        [!NOTE] Results of this type 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.

    • 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.

        [!NOTE] Results of this type 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.

    • 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.

        [!NOTE] Results of this type 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.

    • 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.

        [!NOTE] Results of this type 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.

    • 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.

• result

  • : An existing XPathResult to use for the results. If set to null the method will create and return a new XPathResult.

Return value

An {{domxref("XPathResult")}} linking to the selected nodes. If result was null, it is a new object, if not, it is the same object as the one passed as the result parameter.

Examples

Finding all H2 headings by XPath

const 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. */
let thisHeading = headings.iterateNext();
let 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 give a significant performance improvement, especially on very large documents. This is because the evaluation of the query 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.

Getting element by xml:id

This function is a replacement for {{domxref("Document.getElementById()")}} for when you need to search by xml:id instead.

function getElementByIdWrapper(xmlDoc, id) {
  return xmlDoc.evaluate(
    `//*[@xml:id="${id}"]`,
    xmlDoc,
    () => "http://www.w3.org/XML/1998/namespace",
    XPathResult.FIRST_ORDERED_NODE_TYPE,
    null,
  ).singleNodeValue;
}

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{domxref("Document.createExpression()")}}
• {{domxref("XPathResult")}}