Broadcasting and Messaging Patterns

// SERVER io.on('connection', (socket) => { // 1. Send to THIS socket only socket.emit('message', 'Only you receive this'); // 2. Send to ALL sockets (including sender) io.emit('message', 'Everyone receives this'); // 3. Send to ALL sockets EXCEPT sender socket.broadcast.emit('message', 'Everyone except sender'); // 4. Send to specific room io.to('roomName').emit('message', 'Only room members'); // 5. Send to room EXCEPT sender socket.to('roomName').emit('message', 'Room members except sender'); // 6. Send to multiple rooms io.to('room1').to('room2').emit('message', 'Multiple rooms'); // 7. Send to socket by ID io.to(socketId).emit('message', 'Specific socket by ID'); });

// SERVER io.on('connection', (socket) => { // Broadcast server announcement to everyone io.emit('announcement', { type: 'info', message: 'Server maintenance in 5 minutes', timestamp: new Date() }); // Update user count for everyone const updateUserCount = () => { io.emit('userCount', io.engine.clientsCount); }; updateUserCount(); // Send when someone connects socket.on('disconnect', () => { updateUserCount(); // Send when someone disconnects }); }); // CLIENT socket.on('announcement', (data) => { console.log(`[${data.type}] ${data.message}`); alert(data.message); }); socket.on('userCount', (count) => { document.getElementById('user-count').textContent = count; });

// SERVER io.on('connection', (socket) => { // Notify others when user joins socket.broadcast.emit('userJoined', { userId: socket.id, timestamp: new Date() }); // Typing indicator socket.on('typing', (isTyping) => { socket.broadcast.emit('userTyping', { userId: socket.id, isTyping: isTyping }); }); // User action notification socket.on('userAction', (action) => { socket.broadcast.emit('notification', { userId: socket.id, action: action, timestamp: new Date() }); }); }); // CLIENT socket.on('userJoined', (data) => { console.log(`User ${data.userId} joined`); }); socket.on('userTyping', (data) => { if (data.isTyping) { showTypingIndicator(data.userId); } else { hideTypingIndicator(data.userId); } });

// SERVER io.on('connection', (socket) => { // Join a room socket.on('joinRoom', (roomName) => { socket.join(roomName); // Send to THIS user socket.emit('joinedRoom', roomName); // Send to ALL in room (including sender) io.to(roomName).emit('roomUpdate', { message: `User joined ${roomName}`, userCount: io.sockets.adapter.rooms.get(roomName)?.size || 0 }); // Send to ALL in room EXCEPT sender socket.to(roomName).emit('notification', { message: 'New user joined the room' }); }); // Send message to specific room socket.on('roomMessage', (roomName, message) => { // Broadcast to everyone in the room (including sender) io.to(roomName).emit('roomMessage', { userId: socket.id, room: roomName, message: message, timestamp: new Date() }); }); // Send to multiple rooms socket.on('multiRoomMessage', (rooms, message) => { let emitter = io; rooms.forEach(room => { emitter = emitter.to(room); }); emitter.emit('message', message); }); });

// SERVER const users = new Map(); // Track socket ID to username mapping io.on('connection', (socket) => { // Register user socket.on('register', (username) => { users.set(socket.id, username); socket.username = username; }); // Private message by socket ID socket.on('privateMessage', (targetSocketId, message) => { // Send to recipient io.to(targetSocketId).emit('privateMessage', { from: socket.id, fromUsername: socket.username, message: message, timestamp: new Date() }); // Send confirmation to sender socket.emit('messageSent', { to: targetSocketId, message: message, timestamp: new Date() }); }); // Private message by username socket.on('sendToUser', (username, message) => { // Find socket ID by username let targetSocketId = null; for (const [socketId, user] of users.entries()) { if (user === username) { targetSocketId = socketId; break; } } if (targetSocketId) { io.to(targetSocketId).emit('privateMessage', { from: socket.id, fromUsername: socket.username, message: message }); } else { socket.emit('error', `User ${username} not found`); } }); socket.on('disconnect', () => { users.delete(socket.id); }); }); // CLIENT socket.emit('register', 'John'); // Send private message socket.emit('privateMessage', 'target-socket-id', 'Hello privately!'); // Receive private message socket.on('privateMessage', (data) => { console.log(`Private message from ${data.fromUsername}: ${data.message}`); });

// === BASIC PATTERNS === socket.emit('event', data) // → to sender only io.emit('event', data) // → to all clients socket.broadcast.emit('event', data) // → to all except sender // === ROOM PATTERNS === io.to('room').emit('event', data) // → to all in room socket.to('room').emit('event', data) // → to room except sender io.in('room').emit('event', data) // → to all in room (same as .to()) // === MULTIPLE ROOMS === io.to('room1').to('room2').emit('event', data) // → to room1 AND room2 // === PRIVATE MESSAGE === io.to(socketId).emit('event', data) // → to specific socket // === NAMESPACE === io.of('/namespace').emit('event', data) // → to all in namespace // === VOLATILE (may be lost) === socket.volatile.emit('event', data) // → may be dropped if not ready // === WITH ACKNOWLEDGEMENT === socket.emit('event', data, (response) => {}) // → with callback

// SERVER io.on('connection', (socket) => { // Pattern 1: Client requests, server acknowledges socket.on('sendMessage', (message, callback) => { // Validate message if (!message || message.length === 0) { callback({ success: false, error: 'Empty message' }); return; } // Broadcast message io.emit('message', { from: socket.id, message: message, timestamp: new Date() }); // Acknowledge success callback({ success: true, messageId: Date.now() }); }); // Pattern 2: Server requests, client acknowledges socket.emit('serverQuestion', 'Are you still there?', (response) => { console.log('Client responded:', response); }); // Pattern 3: Timeout for acknowledgement socket.timeout(5000).emit('requestData', (err, response) => { if (err) { console.log('Client did not respond in time'); } else { console.log('Client responded:', response); } }); }); // CLIENT // Respond to pattern 1 socket.emit('sendMessage', 'Hello!', (response) => { if (response.success) { console.log('Message sent, ID:', response.messageId); } else { console.error('Failed:', response.error); } }); // Respond to pattern 2 socket.on('serverQuestion', (question, callback) => { console.log('Server asked:', question); callback('Yes, I am here!'); });

// SERVER io.on('connection', (socket) => { // Volatile: May be lost if client not ready (good for high-frequency updates) socket.volatile.emit('cursorPosition', { x: 100, y: 200 }); // Broadcast volatile socket.volatile.broadcast.emit('gameState', gameData); // Binary: Optimize for binary data socket.binary(true).emit('image', buffer); // Compress: Enable per-message compression (Socket.io 4.0+) socket.compress(true).emit('largeData', bigObject); // Local: Emit only on this server (not to other servers in cluster) socket.local.emit('localEvent', data); // Combine flags socket.volatile.compress(true).to('room').emit('optimizedData', data); });

// SERVER - Complete chat application const io = require('socket.io')(3000); const users = new Map(); const rooms = new Map(); io.on('connection', (socket) => { console.log('User connected:', socket.id); // Register user socket.on('register', (username) => { users.set(socket.id, username); socket.username = username; // Notify everyone io.emit('userJoined', { userId: socket.id, username: username, totalUsers: users.size }); }); // Join room socket.on('joinRoom', (roomName) => { socket.join(roomName); // Track room membership if (!rooms.has(roomName)) { rooms.set(roomName, new Set()); } rooms.get(roomName).add(socket.id); // Confirm to user socket.emit('roomJoined', { room: roomName, userCount: rooms.get(roomName).size }); // Notify room members (except sender) socket.to(roomName).emit('userJoinedRoom', { username: socket.username, room: roomName }); }); // Public room message socket.on('roomMessage', (roomName, message) => { io.to(roomName).emit('roomMessage', { from: socket.id, username: socket.username, room: roomName, message: message, timestamp: new Date() }); }); // Private message socket.on('privateMessage', (targetUsername, message) => { // Find target socket let targetSocketId = null; for (const [socketId, username] of users.entries()) { if (username === targetUsername) { targetSocketId = socketId; break; } } if (targetSocketId) { // Send to recipient io.to(targetSocketId).emit('privateMessage', { from: socket.id, fromUsername: socket.username, message: message, timestamp: new Date() }); // Confirm to sender socket.emit('privateMessageSent', { to: targetUsername, message: message }); } else { socket.emit('error', `User ${targetUsername} not found`); } }); // Typing indicator (volatile, room-specific) socket.on('typing', (roomName, isTyping) => { socket.volatile.to(roomName).emit('userTyping', { userId: socket.id, username: socket.username, isTyping: isTyping }); }); // Global announcement (admin only) socket.on('announcement', (message, adminKey) => { if (adminKey === 'secret-admin-key') { io.emit('announcement', { message: message, timestamp: new Date() }); } }); // Disconnect socket.on('disconnect', () => { const username = users.get(socket.id); users.delete(socket.id); // Remove from room tracking rooms.forEach((members, roomName) => { if (members.has(socket.id)) { members.delete(socket.id); io.to(roomName).emit('userLeftRoom', { username: username, room: roomName }); } }); // Notify everyone io.emit('userLeft', { userId: socket.id, username: username, totalUsers: users.size }); }); }); console.log('Chat server running on port 3000');

// ✅ GOOD - Use appropriate patterns socket.emit('welcome', 'Hello!'); // Send to this user socket.broadcast.emit('newUser', userId); // Notify others // ✅ GOOD - Use volatile for high-frequency updates socket.volatile.emit('cursor', { x, y }); // OK to lose some updates // ✅ GOOD - Use rooms for targeted broadcasting io.to('game123').emit('gameUpdate', data); // Only relevant users // ❌ BAD - Don't send to everyone when you should use rooms io.emit('gameUpdate', data); // Everyone gets it! // ❌ BAD - Don't send large data frequently io.emit('heavyData', largeObject); // Slows down server // ✅ GOOD - Throttle or compress large data socket.compress(true).emit('data', large); // Compress first