code: function handleNotificationPush(event) {
    logger.log('Push notification received');
  
    if ($Log.notificationReceived) {
      event.waitUntil(logNotificationReceived(event));
    }
  
    // Show notification or fallback
    if (event.data && event.data.title) {
      event.waitUntil(showNotification(event.data));
    } else if ($Notifications.fallbackURL) {
      event.waitUntil(
        self.registration.pushManager.getSubscription()
          .then(fetchNotification)
          .then(convertResponseToJson)
          .then(showNotification)
          .catch(showNotification)
      );
    } else {
      logger.warn('No notification.data and no fallbackURL.');
      event.waitUntil(showNotification());
    }
  }
docs: /*         -------- NOTIFICATIONS HANDLERS ---------