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=new XMLHttpRequest();
        }elseif(window.ActiveXObject){    //code for IE5 and IE6
            xmlhttp=new ActiveXObject("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>