and finally you can implement that function:
function slb_save_subscription() {
...
//after saving your data you have to return a json
if( $data_saved ):
$result['status']=1;
$result['message']='Data saved';
endif;
// encode result as json string
$json_result = json_encode( $result );
// return result
die( $json_result );
// stop all other processing
exit;
}