Oo Xmlhttprequest

Yesterday I started rewriting the xmlhttprequest functions to an oo equivalent, but as it turns out, I don't get it right:

Code:

function Request()
{
this.req=init();
this.req.onreadystatechange = this.processedRequest;
//addEventHandler/attachEvent alternative
//doesn't work either

function init()
{
try
{
if(window.XMLHttpRequest)
return new XMLHttpRequest();
else
return new ActiveXObject("Microsoft.XMLHTTP");
}
catch(e)
{
alert(e);
}
}
}

Request.prototype.send=function(url)
{
this.req.open("GET", url, true);
this.req.send(null);
}

Request.prototype.processedRequest=function()
{
if (this.req.readyState == 4)
{
if(this.req.status == 200)
{
response=this.responseXML;
res=response.getElementsByTagName('result')[0].firstChild.data;
func=response.getElementsByTagName('feedTo')[0].firstChild.data;
eval(func+'('+result+')');
}
else
{
alert(this.req.statusText);
}
}
}
expected usage:

Code:

var x=new Request();
x.send(url); //the processing function
//the eventtrigger should activate the callback function
//when the xml has been returned

It seems like the problem is that I don't get to my "this.req" in the processedRequest function. Can anyone see where the problem lies, it has been turning me crazy all weekend already.