WebSocket Setup

You are a Node.js backend engineer. The user wants to implement a WebSocket server with room-based communication using Socket.IO.

What to check first

• Verify Node.js is installed: node --version (v14+)
• Check if Socket.IO is in package.json: npm list socket.io
• Confirm an HTTP server package exists: npm list express or npm list http

Steps

1. Install Socket.IO: npm install socket.io (also need npm install express for the HTTP server)
2. Create an Express server and attach Socket.IO to it using http.createServer(app)
3. Initialize Socket.IO with the server instance: const io = require('socket.io')(httpServer)
4. Listen for the connection event on the io instance to handle new socket connections
5. Inside the connection handler, use socket.on('join', callback) to handle room join requests with the room name parameter
6. Use socket.join(roomName) to add the socket to a room
7. Broadcast messages to a specific room using io.to(roomName).emit(eventName, data)
8. Handle disconnection with socket.on('disconnect') and optionally use socket.leave(roomName) before cleanup

Code

const express = require('express');
const http = require('http');
const socketIO = require('socket.io');

const app = express();
const server = http.createServer(app);
const io = socketIO(server, {
  cors: {
    origin: '*',
    methods: ['GET', 'POST']
  }
});

const PORT = process.env.PORT || 3000;

// Track room members
const rooms = {};

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

  // Handle joining a room
  socket.on('join', (roomName, userName) => {
    socket.join(roomName);
    socket.currentRoom = roomName;
    socket.userName = userName;

    if (!rooms[roomName]) {
      rooms[roomName] = [];
    }
    rooms[roomName].push({ id: socket.id, name: userName });

    // Notify others in the room
    io.to(roomName).emit('user-joined', {
      userName: userName,
      userId: socket.id,
      totalUsers: rooms[roomName].length
    });

    // Send room state to the joining user
    socket.emit('room-state', {
      members: rooms[roomName],
      roomName: roomName
    });
  });

  // Handle messages in a room
  socket.on('message', (msg) => {
    if (socket.currentRoom) {
      io.to(socket.currentRoom).emit('new-message', {
        userId: socket.id,
        userName: socket.userName,
        text: msg,
        timestamp: new Date

Note: this example was truncated in the source. See the GitHub repo for the latest full version.

Common Pitfalls

• Treating this skill as a one-shot solution — most workflows need iteration and verification
• Skipping the verification steps — you don't know it worked until you measure
• Applying this skill without understanding the underlying problem — read the related docs first

When NOT to Use This Skill

• When a simpler manual approach would take less than 10 minutes
• On critical production systems without testing in staging first
• When you don't have permission or authorization to make these changes

How to Verify It Worked

• Run the verification steps documented above
• Compare the output against your expected baseline
• Check logs for any warnings or errors — silent failures are the worst kind

Production Considerations

• Test in staging before deploying to production
• Have a rollback plan — every change should be reversible
• Monitor the affected systems for at least 24 hours after the change