#ajax response handler function
Try this

(function($) {

  Drupal.YOUR_MODULE_NAME = {
    form_id: 'FORM_ID'  
  };

  Drupal.behaviors.YOUR_MODULE_NAME = {
   attach: function(context, settings) {

    for (ajax_el in settings.ajax) {
      if (Drupal.ajax[ajax_el].element.form) {
        if (Drupal.ajax[ajax_el].element.form.id === Drupal.YOUR_MODULE_NAME.form_id) {
          Drupal.ajax[ajax_el].success = Drupal.YOUR_MODULE_NAME.success;
       }
      }
     }
    }   
   };


 Drupal.YOUR_MODULE_NAME.success = function (response, status) {
    // HERE YOU GET THE response
 }

 });