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XPath support in IE

Microsoft saw fit to build XPath support right into the XML DOM object. Each node has two methods

that can be used to retrieve nodes matching an XPath pattern:

selectNodes()

, which returns a collec-

tion of nodes matching a pattern, and

selectSingleNode()

, which returns the first node that matches

a given pattern.

Using the same data as the previous section, you can select all

<name/>

elements that are children of an

element by using the following code:

var lstNodes = oXmlDom.documentElement.selectNodes(“employee/name”);

Because

selectNodes()

is called as a method of

oXmlDom.documentElement

, the document element

is considered the context node for the XPath expression. The method returns a

NodeList

containing all

elements that match the given pattern, meaning that you can iterate through the elements like so:

for (var i=0; i < lstNodes.length; i++) {

alert(lstNodes[i]);

}

Even if there are no matches to a given pattern, a

NodeList

is still returned. If it is empty, its

length

property is equal to

If you want only the first element matching the pattern, then

selectSingleNode()

is the method to use:

var oElement = oXmlDom.documentElement.selectSingleNode(“employee/name”);

The

selectSingleNode()

method returns an

Element

as the function value if found, otherwise it

returns

null

XPath support in Mozilla

As you may have guessed, Mozilla supports the XPath according to the DOM standard. A DOM Level 3

addition called DOM Level 3 XPath defines interfaces to use for evaluating XPath expressions in the

DOM. Unfortunately, this standard is more complicated than Microsoft’s fairly straightforward

approach.

Although a handful of XPath-related objects exist, the two most important ones are

XPathEvaluator

and

XPathResult

. An

XPathEvaluator

is used to evaluate an XPath expression with a method named,

appropriately enough,

evaluate()

The

evaluate()

method takes five arguments: the XPath expression, the context node, a namespace

resolver, the type of result to return, and an

XPathResult

object to fill with the result (usually

null

The result of

selectNodes()

is a living list. So, if you update the document with

another element that matches the XPath expression, that element is automatically

added to the

NodeList

in the appropriate position.

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