javascript - using ajax to get data from database -

everyone. trying use ajax data server , not work. got stacked on problem couple days , have no idea how handles. point out how can solve problem? thanks

        function loadxmldoc()          {             user_input = document.getelementbyid('u_name').value;             var url_s = url_seller + user_input;             var url_b = url_bidder + user_input;             var req = new xmlhttprequest();             req.onreadystatechange = function()              {                 if (req.readystate == 4 && req.status == 200)                  {                     var res = json.parse(req.responsetext);                 }             };             req.open("get", url_s, true);             req.send();         }        

i call function using button submit