$.ajax({
               url:      '/url',
               method:   'GET',
               dataType: "json",
               success:  function (r) {
                   console.log(r);
               },
           })

<?php
namespace DrupalcustomController;
class CustomController {

        public function search(){return 'test';}
}

custom_search:
path: /url
defaults:
    _controller: DrupalcustomControllerCustomController::search
    _title: 'Get List'
requirements:
    _permission: 'access content'