I make some Ajax calls from inside a javascript object.:

myObject.prototye = {
  ajax: function() {
    this.foo = 1;

    var req = new XMLHttpRequest();
    req.open('GET', url, true);
    req.onreadystatechange = function (aEvt) {  
      if (req.readyState == 4) {  
        if(req.status == 200)  {
          alert(this.foo); // reference to this is lost
        }
      }
  }
};

Inside the onreadystatechange function, this does not refer to the main object anymore, so I don't have access to this.foo. Ho can I keep the reference to the main object inside XMLHttpRequest events?