help me !

I write this code For my web file simply.
but this have one error(bug) in my js file.
this is the js file ;


//----------------------------------------------------------
//get Element by id

function getEl(id){
return document.getElementById(id);
}

//create element by tag name

function createEl(tag){
return document.createElement(tag);
}

//change one object's class

function changeCss(obj,css){
obj.className=css;
}

//create ajax object

function createAjax(){
var xmlHttp=null;
try{
// Firefox, Opera 8.0+, Safari
xmlHttp=new XMLHttpRequest();
}
catch (e){
// Internet Explorer
try{
xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e){
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
}
return xmlHttp;
}
//Ajax Object
function Ajax(div,url){
this.ajax = createAjax();//Create ajax object (xmlhtmlrequest object)
this.url = url; //request url
this.div = getEl(div); //display response message div
this.send = function(){
//ajax=this.ajax;
this.ajax.open("GET",this.url,true);
this.ajax.onreadystatechange=this.ajaxOk;
this.ajax.send(null);
};//send request
this.ajaxOk = function(){
alert(typeof(this.ajax)); // error at this line
if(this.ajax.readyState == 4 && this.ajax.status == 200)
this.div.innerHTML=this.ajax.responseText;
};//get message and display it
}
//----------------------------------------------------------------



then i use it with this code

//-------------------------------------------------------------------

function viewMessageList(action){
var url="NoticeList.asp";
var div="bodyMain";
url+="?Action="+action+"&t="+new Date().getTime();
var ajaxObj=new Ajax(div,url);
ajaxObj.send();
}
//----------------------------------------------------------------------


but It not do i want , it show this "undefine". someone can help!
thinks!