Or, solving the scope issue of callback functions without resorting to global variables!

//set the var so we can scope the callback var _this = this;
//callback will be an anonymous function that calls back into our class
//this allows the call back in which we handle the response (_onData())
// to have the correct scope.
this._request.onreadystatechange = function(){_this._onData()};

Encapsulating XmlHttpRequest Calls within JavaScript classes