JS - Javascript HTTP Request Object

requestObject.js

var xhr = new XMLHttpRequest;

    xhr.open("GET", url);
    xhr.send();

// ********************************
// Return Json object from server
// ********************************
var id = 1;
var req = new XMLHttpRequest();
req.overrideMimeType("application/json");
req.open('GET', url?ServerSideHandler.aspx?Event='Test&Id='+id, true);
req.onload  = function() {
   var jsonResponse = JSON.parse(req.responseText);
   // do something with jsonResponse
};
req.send(null);

// ******************************************************
// Retrieve XMLHttpRequest response from JS back to the server
// ******************************************************
// ServerSideHandler.aspx
Event = "Test'
int id = Request.QueryString("id);
                             
var obj = GetData(id);
var json = new JavaScriptSerializer().Serialize(obj);
Response.Clear();
Response.ContentType = "application/json; charset=utf-8";
Response.Write(json);
Response.End(); // data now goes back to the calling JS XMLHttpRequest object