Send ajax data to php controller in symfony2

Hi guys I am new in symfony 2 and i have little confusing with sending data with ajax to php controller in symfony 2. I want to create project with google map so i create MapController:

   <?php

namespace AppBundle\Controller;

use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\JsonResponse;
class MapController extends Controller
{ 
    /**
     * @Route("/", name="homepage")
     */
    public function indexAction(Request $request)
    {
        // replace this example code with whatever you need
        return $this->render('map/map.html.twig', [
            'base_dir' => realpath($this->getParameter('kernel.root_dir').'/..').DIRECTORY_SEPARATOR,
            'contData' => ['lat' => '51.24591334500776', 'lng'=> '22.56967306137085']
        ]);
    }


    public function saveAction(Request $request)
    {
        $data = $request->request->get('params');

         return new JsonResponse(['data' => $data], Response::HTTP_OK);
    }

}

then i create routing:

    app:
    resource: '@AppBundle/Controller/'
    type: annotation
map:
    path:      /map
    defaults:  { _controller: AppBundle:Map:index }
    requirements:
        page: '\d+' 
map_save:
    path:      /map/save
    defaults:  { _controller: AppBundle:Map:save }
    methods:  [POST] 

so when i go to to route:

http://localhost/googlemap/web/app_dev.php/map

i display my template map.html.twig

there i have javascipt function where i tried to send ajax data to some controller:

    marker.addListener("click", function () {


                    //!
                    var http = new XMLHttpRequest();
                    var url = "map/save";
                    var params = marker.position.lat();
                    http.open("POST", url, true);

//Send the proper header information along with the request
                    http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

                    http.onreadystatechange = function () {//Call a function when the state changes.
                        if (http.readyState == 4 && http.status == 200) {
                            alert(http.responseText);
                        }
                    }
                    http.send(params);

But i have NULL in this response: {"data":null}