XMLHttpRequest——这是什么、怎样完整地执行一次GET请求、怎样检测错误。

<script type="text/javascript">
var xmlhttp;
function loadXMLDoc(url){
xmlhttp = null;
if(window.XMLHttpRequest){    //code for all new browsers
xmlhttp=newXMLHttpRequest();
}elseif(window.ActiveXObject){    //code for IE5 and IE6
xmlhttp=newActiveXObject("Microsoft.XMLHTTP");
}
if(xmlhttp!=null){
xmlhttp.onreadystatechange=state_Change;
xmlhttp.open("GET",url,true);
xmlhttp.send(null);
}else{
alert("Your browser does not support XMLHTTP.");
}
}

function state_Change(){
if(xmlhttp.readyState==4){    //4 = "loaded"
if(xmlhttp.status==200){    //200 = OK
//...our code here...
}else{
alert("Problem retrieving XML data");
}
}
}
</script>