What is the difference between io.to() and socket.to()?
io.to(roomId).emit() broadcasts to ALL sockets in the room, including the sender. socket.to(roomId).emit() broadcasts to all sockets in the room EXCEPT the sender. Use io.to() when the sender should also see the message, socket.to() for notifications to others.
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More FAQs in Socket.io Rooms and Broadcasting Group Communication in Node.js
Rooms are named channels that sockets can join and leave. When you broadcast to a room, only sockets in that room receive the message. Use socket.join('roomName') to join, socket.leave('roomName') to leave, and io.to('roomName').emit() to broadcast to all sockets in the room.
Create a deterministic room ID by sorting and joining the two user IDs: [userId1, userId2].sort().join('_'). Both users join the same room. Broadcast messages with io.to(roomId).emit(). This ensures only the two users in the conversation receive messages.
Maintain a Map of online users (userId → socketId). On connection, add the user and broadcast 'user:online'. On disconnect, remove the user and broadcast 'user:offline'. Provide an API or Socket.io event to check if a specific user is online.
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