using XPath to find node under a context node does not work (firefox/firebug/javascript)

I want to extract information from a web page.

The page has m nodes, which can be found by .evaluate("//div[@class='news']", document, ....).

For each of the above nodes, there are 3 nodes inside them. Each of them has different @class selector. And I want to extract these m 3-tuple records.

I tried to use .evaluate() function as instructed in

https://developer.mozilla.org/en/Introduction_to_using_XPath_in_JavaScript

by using this code

parentNodes = document.evaluate("//div[@class='news']", document, ....).
while (true){
   var node = parentNod开发者_运维问答es.iterateNext();
   var child = document.evaluate("//div[@class='title']", node, ....). 
   ...
}

However, "child" is always assigned to the first node in the document, instead of the first node within "node".

I ran this in firebug console.

Does any one know what's wrong?

You are calling evaluate on the document. Hence, the XPath expression is being evaluated from the root of the XML tree. Also, if you want XPath to select a node from within the current context, e.g. among the children of the current node, you should use the .// context selector.

If you start an XPath expression with "/" then you are starting down from the root node/document node of the context node. So instead of "//div[@class = 'title']" use "descendant::div[@class = 'title']", that way you are selecting the descendant div elements of the context node.

You can use also the location path selector descendant-or-self. Like that: descendant-or-self::div... for reference starting the node passed as context.

I hope I have helped.

Use .// instead of //.

Explanation from the XML Path Language specification (XPath), Version 1.0:

• //para selects all the para descendants of the document root and thus selects all para elements in the same document as the context node

• . selects the context node

• .//para selects the para element descendants of the context node

The current top answer is wrong in that there is no way to call .evaluate on any other object, but document. document.evaluate is correct.