Node.js WebSockets Skill

Master real-time bidirectional communication for building chat apps, live notifications, collaborative tools, and real-time dashboards.

Quick Start

WebSocket server in 3 steps:

1. Setup Server - Socket.io or ws library
2. Handle Connections - Manage client lifecycle
3. Emit Events - Send/receive messages

Core Concepts

Socket.io Server Setup

const express = require('express');
const { createServer } = require('http');
const { Server } = require('socket.io');

const app = express();
const httpServer = createServer(app);
const io = new Server(httpServer, {
  cors: {
    origin: 'http://localhost:3000',
    methods: ['GET', 'POST']
  }
});

io.on('connection', (socket) => {
  console.log('User connected:', socket.id);

  // Handle events
  socket.on('chat:message', (data) => {
    // Broadcast to all clients
    io.emit('chat:message', {
      ...data,
      timestamp: Date.now()
    });
  });

  // Join rooms
  socket.on('room:join', (roomId) => {
    socket.join(roomId);
    socket.to(roomId).emit('room:user-joined', socket.id);
  });

  // Handle disconnect
  socket.on('disconnect', () => {
    console.log('User disconnected:', socket.id);
  });
});

httpServer.listen(3000);

Socket.io Client

import { io } from 'socket.io-client';

const socket = io('http://localhost:3000', {
  auth: { token: 'jwt-token-here' }
});

socket.on('connect', () => {
  console.log('Connected:', socket.id);
});

socket.on('chat:message', (message) => {
  console.log('Received:', message);
});

// Send message
socket.emit('chat:message', {
  text: 'Hello!',
  userId: 'user123'
});

Learning Path

Beginner (1-2 weeks)

• ✅ Socket.io server setup
• ✅ Basic event emission
• ✅ Connect/disconnect handling
• ✅ Simple chat application

Intermediate (3-4 weeks)

• ✅ Rooms and namespaces
• ✅ Authentication middleware
• ✅ Broadcasting patterns
• ✅ Error handling

Advanced (5-6 weeks)

• ✅ Horizontal scaling with Redis
• ✅ Binary data transfer
• ✅ Reconnection strategies
• ✅ Performance optimization

Native ws Library

const WebSocket = require('ws');
const { createServer } = require('http');

const server = createServer();
const wss = new WebSocket.Server({ server });

wss.on('connection', (ws, req) => {
  const ip = req.socket.remoteAddress;
  console.log('Client connected from', ip);

  // Handle messages
  ws.on('message', (data) => {
    const message = JSON.parse(data);
    console.log('Received:', message);

    // Echo back
    ws.send(JSON.stringify({
      type: 'echo',
      data: message
    }));
  });

  // Ping/pong heartbeat
  ws.isAlive = true;
  ws.on('pong', () => {
    ws.isAlive = true;
  });

  ws.on('close', () => {
    console.log('Client disconnected');
  });
});

// Heartbeat interval
const interval = setInterval(() => {
  wss.clients.forEach((ws) => {
    if (!ws.isAlive) return ws.terminate();
    ws.isAlive = false;
    ws.ping();
  });
}, 30000);

wss.on('close', () => clearInterval(interval));

server.listen(3000);

Rooms and Namespaces

// Namespaces for feature separation
const chatNamespace = io.of('/chat');
const notificationsNamespace = io.of('/notifications');

chatNamespace.on('connection', (socket) => {
  // Chat-specific logic
  socket.on('message', (msg) => {
    chatNamespace.emit('message', msg);
  });
});

notificationsNamespace.on('connection', (socket) => {
  // Notification-specific logic
  socket.on('subscribe', (topic) => {
    socket.join(topic);
  });
});

// Rooms within namespace
socket.on('join-channel', (channelId) => {
  socket.join(`channel:${channelId}`);

  // Send only to room
  io.to(`channel:${channelId}`).emit('user-joined', {
    userId: socket.userId,
    channelId
  });
});

// Leave room
socket.on('leave-channel', (channelId) => {
  socket.leave(`channel:${channelId}`);
});

Authentication Middleware

const jwt = require('jsonwebtoken');

// Socket.io middleware
io.use((socket, next) => {
  const token = socket.handshake.auth.token;

  if (!token) {
    return next(new Error('Authentication required'));
  }

  try {
    const decoded = jwt.verify(token, process.env.JWT_SECRET);
    socket.userId = decoded.id;
    socket.userRole = decoded.role;
    next();
  } catch (err) {
    next(new Error('Invalid token'));
  }
});

// Namespace-level auth
const adminNamespace = io.of('/admin');
adminNamespace.use((socket, next) => {
  if (socket.userRole !== 'admin') {
    return next(new Error('Admin access required'));
  }
  next();
});

Scaling with Redis Adapter

const { createAdapter } = require('@socket.io/redis-adapter');
const { createClient } = require('redis');

const pubClient = createClient({ url: 'redis://localhost:6379' });
const subClient = pubClient.duplicate();

Promise.all([pubClient.connect(), subClient.connect()]).then(() => {
  io.adapter(createAdapter(pubClient, subClient));
  console.log('Redis adapter connected');
});

// Now events are broadcast across all server instances
io.emit('notification', { message: 'Hello from any server!' });

Real-Time Patterns

Chat Application

// Server
io.on('connection', (socket) => {
  socket.on('chat:join', ({ room, username }) => {
    socket.join(room);
    socket.to(room).emit('chat:user-joined', { username });
  });

  socket.on('chat:message', ({ room, message }) => {
    io.to(room).emit('chat:message', {
      user: socket.userId,
      message,
      timestamp: Date.now()
    });
  });

  socket.on('chat:typing', ({ room }) => {
    socket.to(room).emit('chat:typing', { user: socket.userId });
  });
});

Live Notifications

// Server
function sendNotification(userId, notification) {
  io.to(`user:${userId}`).emit('notification', notification);
}

// Join user's personal room
socket.on('authenticate', () => {
  socket.join(`user:${socket.userId}`);
});

// From any service
notificationService.on('new', (userId, data) => {
  sendNotification(userId, data);
});

Live Dashboard

// Server: broadcast metrics periodically
setInterval(async () => {
  const metrics = await getSystemMetrics();
  io.emit('metrics:update', metrics);
}, 1000);

// Or on-demand updates
database.onChange((change) => {
  io.to(`dashboard:${change.collection}`).emit('data:update', change);
});

Error Handling

// Client-side reconnection
const socket = io({
  reconnection: true,
  reconnectionAttempts: 5,
  reconnectionDelay: 1000,
  reconnectionDelayMax: 5000
});

socket.on('connect_error', (err) => {
  console.log('Connection error:', err.message);
});

socket.on('reconnect', (attemptNumber) => {
  console.log('Reconnected after', attemptNumber, 'attempts');
});

socket.on('reconnect_failed', () => {
  console.log('Failed to reconnect');
  // Fallback to polling or show offline UI
});

// Server-side error handling
socket.on('error', (error) => {
  console.error('Socket error:', error);
});

Unit Test Template

const { createServer } = require('http');
const { Server } = require('socket.io');
const Client = require('socket.io-client');

describe('WebSocket Server', () => {
  let io, serverSocket, clientSocket;

  beforeAll((done) => {
    const httpServer = createServer();
    io = new Server(httpServer);
    httpServer.listen(() => {
      const port = httpServer.address().port;
      clientSocket = Client(`http://localhost:${port}`);
      io.on('connection', (socket) => {
        serverSocket = socket;
      });
      clientSocket.on('connect', done);
    });
  });

  afterAll(() => {
    io.close();
    clientSocket.close();
  });

  it('should receive message from client', (done) => {
    serverSocket.on('hello', (arg) => {
      expect(arg).toBe('world');
      done();
    });
    clientSocket.emit('hello', 'world');
  });

  it('should broadcast to clients', (done) => {
    clientSocket.on('broadcast', (arg) => {
      expect(arg).toBe('everyone');
      done();
    });
    io.emit('broadcast', 'everyone');
  });
});

Troubleshooting

Problem	Cause	Solution
Connection drops	No heartbeat	Enable pingInterval/pingTimeout
Messages not received	Wrong room	Verify room membership
Scaling issues	No Redis	Add Redis adapter
Memory leak	Listeners not removed	Clean up on disconnect

When to Use

Use WebSockets when:

• Real-time bidirectional communication
• Low-latency updates needed
• Live notifications/chat
• Collaborative applications
• Gaming/trading platforms

Related Skills

• Express REST API (HTTP fallback)
• Microservices (distributed events)
• Redis (pub/sub scaling)

Resources

• Socket.io Documentation
• ws Library
• Redis Adapter