What is XMLHttpRequest, how to perform a GET request in a complete way, and how to detect errors.

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 '); }                            }                         }                    }                 }

What the

XMLHttpRequest is, how to perform a GET request in its entirety, and how to detect the error.