Node.js WebSockets and Socket.IO

What is WebSockets and Socket.IO in Node.js?

WebSockets keep a connection open in both directions so the server can push data without the client polling. Socket.IO adds rooms, automatic reconnection and fallbacks on top.

WebSockets and Socket.IO example

import { Server } from 'socket.io';

const io = new Server(httpServer, { cors: { origin: process.env.CLIENT_URL } });

io.use((socket, next) => {
  try {
    socket.data.user = jwt.verify(socket.handshake.auth.token, process.env.JWT_SECRET);
    next();
  } catch {
    next(new Error('Unauthorized'));
  }
});

io.on('connection', socket => {
  socket.join(`user:${socket.data.user.sub}`);

  socket.on('message', text => {
    io.to(`room:${roomId}`).emit('message', { text, from: socket.data.user.sub });
  });

  socket.on('disconnect', () => console.log('left'));
});

Key points to remember

• Authenticate during the handshake, not after the connection is established.
• Rooms are how you target a user or a group instead of broadcasting to everyone.
• Across multiple processes you need the Redis adapter, or messages reach only one worker.
• Server-Sent Events are simpler when you only need one-way updates.

Common mistakes with WebSockets and Socket.IO

• Clustering without the Redis adapter, so users connected to different workers cannot see each other.
• Emitting to all clients when a room would do.