Building a WebSocket Server with Node.jsLesson 3.3

How to implement WebSocket rooms for group messaging

room data structure with Map, joining and leaving rooms, per-room broadcast, client metadata, room membership tracking, cleanup on disconnect, room capacity limits

Rooms Are Just Sets of Clients

The ws package has no built-in room concept. Implement rooms with a Map:

const rooms = new Map(); // roomId -> Set of ws clients

function joinRoom(ws, roomId) {
  if (!rooms.has(roomId)) rooms.set(roomId, new Set());
  rooms.get(roomId).add(ws);
  ws.roomId = roomId; // store on client object
}

function leaveRoom(ws) {
  const room = rooms.get(ws.roomId);
  if (room) {
    room.delete(ws);
    if (room.size === 0) rooms.delete(ws.roomId);
  }
}

function broadcastToRoom(roomId, message, exclude = null) {
  const room = rooms.get(roomId);
  if (!room) return;
  const payload = JSON.stringify(message);
  for (const client of room) {
    if (client !== exclude && client.readyState === WebSocket.OPEN) {
      client.send(payload);
    }
  }
}

wss.on('connection', (ws) => {
  ws.on('close', () => leaveRoom(ws));
});

Always call leaveRoom in the close handler to prevent stale references. Delete empty rooms immediately to avoid unbounded Map growth in servers with many transient rooms.