Symfony: Send variable throught GET to handler from Ajax

I would like to use AJAX in my Symfony3.0.3 project. The communication works, but I can't get variable from JS to the handler. In the direction handler to JS, it works fine.

I'm trying to get the variable from the request with "$request->query->get('id'))" but I only get "null".

In an other way I'm trying to use the variable from the URL but I get this error:

"An exception has been thrown during the rendering of a template ("Some mandatory parameters are missing ("id") to generate a URL for route "admin_ajax".") in CommonBundle:Default:index.html.twig at line 421."

I don't mind using a solution or an other (I'll use the best one depending of your advices), but I still would like the solution for both error.

JS

function selectClient(idClient)//idClient = 1
{
    alert(idClient);
    $.post('{{path('admin_ajax')}}',{idClient: id}, 
            function(response)
            {
                if(response.code == 100 && response.success)
                {
                    alert(response.id);//Show null if using $request->query->get('id')) in handler but should be 1
                }}, "json");    
}

routing:

admin_ajax:
    defaults: { _controller: CommonBundle:Default:getClient }
    path:     /ajax/{id}

handler:

public function getClientAction($id)
{
    $request = $this->container->get('request_stack')->getCurrentRequest();
    $isAjax = $request->isXMLHttpRequest();
    if ($isAjax)
    {
        $response = array("code" => 100, "success" => true, "id" => $request->query->get('id'));
        return new Response(json_encode($response));
    }
    $response = array("code" => 0, "success" => false);
    return new Response(json_encode($response));
}

EDIT: Thank for Rim, and Rvanlaak answer, I used the FOSJsRoutingBundle.

JS

function selectClient(idClient)
{
    $.get(Routing.generate('ajax_getclient', { id:idClient }), 
            function(response)
            {
                if(response.code == 100 && response.success)
                {
                    alert(response.id);
                }
                else
            }, "json");
}

routing:

ajax_getclient:
    defaults: { _controller: CommonBundle:Default:getClient }
    path:     /ajax/{id}
    options:
        expose: true

Note that the option "expose: true" was necessary to works.