code: function notificationClickListener(event) {
      console.log('Leanplum: [Service Worker] Notification click received.');
  
      event.notification.close();
  
      if (!event.notification || !event.notification.tag) {
        console.log('Leanplum: No notification or tag/id received, skipping open action.');
        return;
      }
  
      var notificationId = event.notification.tag;
      var openActionUrl = openActions[notificationId];
      if (!openActionUrl) {
        console.log('Leanplum: [Service Worker] No action defined, doing nothing.');
        return;
      }
  
      Reflect.deleteProperty(openActions, 'notificationId');
  
      /** @namespace clients.openWindow **/
      event.waitUntil(clients.openWindow(openActionUrl));
    }
docs: Callback that handles clicks on the notification.
  @param {object} event The notification event object.
  @param {object} event.notification The notification object.
  @param {function} event.waitUntil The browser will keep the service worker running until the
  promise you passed in has settled.