在学到XPath遇到的问题

Question

var result = xmldom.evaluate('employee/name',xmldom.documentElement,null,XPathResult.ORDERED_NODE_ITERATOR_TYPE,null);if(result !==null){var element = result.iterateNext(); //iterateNext()的具体作用是什么，书上讲的不太明白。while(element){alert(element.tagName);node= result.iterateNext(): //为什么要重复一次，}}