Ajax call to controller

[BartekPresta]

I have ajax 

$.ajax(ajaxSettings).fail(onFail).success(onSuccess)

 

But i dont know how to pass from controller data with error to get to onFail function.

 

I tried 

        $data = array(
            'hasError' => true,
            'order_id' => null,
        );
        $this->ajaxDie(json_encode($data));

 

but ajax always ended in success function