test.tsx

const notifications [
  { id: 1 }
  ]; // store.get("notifications")
  
  const watchers = [
    { notificationId: 1, userId: 1 }
    { notificationId: 1, userId: 2 }
    { notificationId: 2, userId: 2 }
  ];   // store.get("watchers")
  
  
  notifications() {
    const notifications = store.get("notifications");
    notifications
      .map(notification => {
        ...notification,
        watching: watchers.some(({ notificationId, userId }) => notification.id === notificationId && userId === 1);
      });
    return notifications;
  }