Ajax small package get, post request

Ajax small encapsulation A small encapsulation of ajax get request

function Ajax() {
 var xhr =null;
 if(window.XMLHttpRequest) {
 xhr = new XMLHttpRequest();
 } else {

 xhr = new ActiveXObject("Microsoft.XMLHttp");
 }
 this.get=function(url,success,fail){ //get request
 xhr.open("GET", "1.jsp",true);
 xhr.onreadystatechange=function(){
 if( xhr.readyState==4) {
 alert(xhr.status);
 if(xhr.status==200) {
 var txt = xhr.responseText;
 txt = eval("("+txt+")");
 var ch = txt.charAt(0);
 if(ch=="<") { //xml type
 var xml = xhr.responseXML;
 success(eval("("+xml+")")) ;
 } else if(ch=="["||ch=="{") {//json type
 txt = eval("("+txt+")");
 success(txt);
 } else {//If you don’t know, just return

 success(txt);
 }
 } else {
 if(fail) {
 fail(xhr.status);
 }
 }
 }

 };
 xhr.send(null );

 };


 this.post = function (url,param,success,fail) {//post request
 xhr.open("POST", "1.jsp",true);
 xhr.onreadystatechange =function(){
 if(xhr.readyState==4) {
 alert(xhr.status);
 if(xhr.status==200) {
 var txt = xhr.responseText;
 var ch = txt.charAt (0);
 if(ch=="<") { //xml type
 var xml = xhr.responseXML;
 success(eval("("+xml+")"));
 } else if(ch ==="["||ch=="{") {//json type
 txt = eval("("+txt+")");
 success(txt);
 } else {//Don't know, return directly

 success(txt);
 }

 } else {
 if(fail) {
 fail(xhr.status);
 }
 }
 }

 };
 xhr.setRequestHeader("Content-Type", " application/x-www-form-urlencoded");
 xhr.send(param);
 };

}