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 [http://www.w3.org/TR/xpath 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

Example

From the Mozilla XPath Tutorial:

   var headings = document.evaluate("//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  

var headings = document.evaluate("//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

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