var xmlhttp;
                     function LoadXmlDoc(url){
                     xmlhttp = null;
                     if(window.XMLHttpRequest){ //code for all new browsers
                         xmlhttp=new XMLHttpRequest();
                     }else if(window.ActiveXObject){//IE5 and IE6
                         xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
                     }
                    if(xmlhttp!=null){
                        xmlhttp.onreadystatechange=function (){
                            if(xmlhttp.readyState==4){ // loaded ...
                                if(xmlhttp.state==200){
                                    //success,...our code here ...
                                }else{
                                    alert(‘Problem retrieving XMl data‘);
                                }
                            }
                         }
                    }
                 }

XMLHttpRequest是什麽、如何完整地運行一次GET請求、如何檢測錯誤。