Eventure

const nextEvents = Object.entries(attendance)
  .filter(([, status]) => status === "attending")
  .map(([eventId]) => eventId);

const currentEvents = foundAttendee.attendingEvents || [];
const add = nextEvents.filter((id) => !currentEvents.includes(id));
const remove = currentEvents.filter((id) => !nextEvents.includes(id));

const res = await axios.patch(
  `${API_BASE_URL}/attendees/${foundAttendee.id}`,
  { add, remove }
);

// Trust the server's copy, then update the shared list.
onUpdateAttendees((prev) =>
  prev.map((a) => (a.id === foundAttendee.id ? res.data : a))
);

Instead of overwriting the whole record, the client sends only what changed. The server applies the same add and remove sets, so two views can’t silently undo each other’s changes.

if (!firstName || !lastName || !email ||
    !reasonForAttending || !Array.isArray(attendingEvents)) {
  return res.status(400).json({
    message: "firstName, lastName, email, reasonForAttending, and attendingEvents (array) are required",
  });
}

const attendees = await readJSONFile("attendees");

// Prevent duplicates by email
const exists = attendees.some(
  (a) => a.email?.toLowerCase() === email.toLowerCase()
);
if (exists) {
  return res.status(409).json({ message: "Attendee already exists" });
}

The client also trims and lower-cases emails before searching or posting, so the same person can’t register twice with different capitalisation.

export const readJSONFile = async (fileName) => {
  try {
    const data = await readFile(getFilePath(fileName), "utf-8");
    if (!data.trim()) return [];
    return JSON.parse(data);
  } catch (error) {
    if (error.code === "ENOENT") return [];
    throw error;
  }
};