求这段代码解释，详情及注释

<html>
<body>
<script type="text/javascript">
function loadXMLDoc(dname)
{
if (window.XMLHttpRequest)
  {
  xhttp=new XMLHttpRequest();
  }
else
  {
  xhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xhttp.open("GET",dname,false);
xhttp.send("");
return xhttp.responseXML;
}

xml=loadXMLDoc("/b/books.xml");
path="/bookstore/book/title"
// code for IE
if (window.ActiveXObject)
{
var nodes=xml.selectNodes(path);

for (i=0;i<nodes.length;i++)
  {
  document.write(nodes[i].childNodes[0].nodeValue);
  document.write("<br />");
  }
}
// code for Mozilla, Firefox, Opera, etc.
else if (document.implementation && document.implementation.createDocument)
{
var nodes=xml.evaluate(path, xml, null, XPathResult.ANY_TYPE, null);
var result=nodes.iterateNext();

while (result)
  {
  document.write(result.childNodes[0].nodeValue);
  document.write("<br />");
  result=nodes.iterateNext();
  }
}
</script>

</body>
</html>
--------------------编程问答-------------------- 生成节点树  根据浏览器的型号来判断  首先是判断是IE 还是非IE 然后ajax 返回个xml对象根据浏览器型号选择解析xml 方法并构建成节点树

补充：Java ,  Web 开发