Add Data to Response of WP-JSON Root

function filterResponse($response){
  $data = $response->data;
  $data['extra_field'] = 'some data';
  $response->set_data($data);
  return $response;
 }

add_filter('rest_index', 'filterResponse');

Thanks to Jeff

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)