A real user may connect through multiple devices or browser tabs.

For example, the same user might be connected through:

• A desktop browser.
• A mobile browser.
• A second desktop tab.

Maintaining only one socket ID per user means a new connection can overwrite an existing connection.

A better approach is to use a user-specific room.

When an authenticated user connects, join a room named using their permanent user ID.

For example:

user:12345

Now all connections belonging to that user can receive the same private notification.

io.on('connection', async (socket) => {
  const userId = socket.data.userId;

  if (!userId) return;

  await socket.join(`user:${userId}`);

  console.log(`User ${userId} connected`);
});

function sendPrivateNotification(io, userId, notification) {
  io.to(`user:${userId}`).emit('notification:new', notification);
}