Hello, I need to understand how to work with events in SilverStripe. Example:
For simplicity, I have 2 dropdown A and B. The need to respond to the OnChange And so I choose and obtain the data in b. Using jQuery I can catch OnChange event see code.

<script>
    (function($) {
	$(document).ready(function(){
            $("#MyForm_mujForm_ulice").change(function () {
                $("#MyForm_mujForm_ulice option:selected").each(function () {
                    alert($(this).text());
                    //Now what?
                });
            });
        })
        ;
    })
(jQuery);
 </script>

Now I do not know how to call a function, who have in my control.

function onchange($request) {
            $pol = array(1=>1,2=>2);
        
        FormResponse::update_dom_id('MyForm_test', 'test');
        FormResponse::status_message('Done', 'good');

        return FormResponse::respond();
    }

I modified the controller:

public function index(){
           if(Director::is_ajax() == false){
              return array();
           }
           $data = array( 'Test' => 'value');
           return json_encode($data);

        }

Ajax calls a function:

$("#lab").load('My_Controller', function(vystup){
                        $("#MyForm_mujForm_test").val(vystup.test);
                        alert("echo: " + vystup.test);
                    });

Unfortunately I can not get the value of the test. pops up in my error alert:
echo: undefined

A solution is parsed!!!!!!!
var obj = jQuery.parseJSON(vystup);

P.S. Sorry for my English.