↑
Main Page

XPathEvaluator

Element.prototype.selectSingleNode = function (sXPath) {

var oEvaluator = new XPathEvaluator();

var oResult = oEvaluator.evaluate(sXPath, this, null,

XPathResult.FIRST_ORDERED_NODE_TYPE, null);

if (oResult != null) {

return oResult.singleNodeValue;

} else {

return null;

}

This method can then be used the same as the one in IE:

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

alert(oNode);

The last section of

XPathResult

types are the Boolean type, number type, and string type. Each of these

result types returns a single value using the

booleanValue

numberValue

, and

stringValue

proper-

ties, respectively. For the Boolean type, the evaluation typically returns

true

if at least one node matches

the XPath expression and returns

false

otherwise:

var oEvaluator = new XPathEvaluator();

var oResult = oEvaluator.evaluate(“employee/name”, oXmlDom.documentElement, null,

XPathResult.BOOLEAN_TYPE, null);

alert(oResult.booleanValue);

In this example, if any nodes match

“employee/name”

, the

booleanValue

property is equal to

true

For the number type, the XPath expression must use an XPath function that returns a number, such as

count()

, which counts all the nodes that match a given pattern:

var oEvaluator = new XPathEvaluator();

var oResult = oEvaluator.evaluate(“count(employee/name)”, oXmlDom.documentElement,

null, XPathResult.BOOLEAN_TYPE, null);

alert(oResult.numberValue);

This code outputs the number of nodes that match

“employee/name”

(which is 2). If you try using this

method without one of the special XPath functions,

numberValue

is equal to

NaN

For the string type, the

evaluate()

method finds the first node matching the XPath expression, then

returns the value of the first child node, assuming the first child node is a text node. If not, the result is

an empty string. Here’s an example:

var oEvaluator = new XPathEvaluator();

var oResult = oEvaluator.evaluate(“employee/name”, oXmlDom.documentElement, null,

XPathResult.STRING_TYPE, null);

alert(oResult.stringValue);

The previous code outputs

“Nicholas C. Zakas”

, because that is the first text node in the first

<name/>

element under an

element.

470

Chapter 15

18_579088 ch15.qxd 3/28/05 11:42 AM Page 470