WebSockets & Socket.io - Real-Time Bidirectional Full-Duplex Communication

Traditional HTTP requests require the browser to constantly poll the server for updates. For real-time applications—such as live multiplayer gaming, financial stock tickers, collaborative document editing, and chat platforms—WebSockets provide a persistent, two-way connection where the server can push updates instantly.

Building a Real-Time Chat Server with Socket.io

npm install express socket.io

// socketServer.js
const express = require('express');
const http = require('http');
const { Server } = require('socket.io');

const app = express();
const server = http.createServer(app);
const io = new Server(server, {
  cors: { origin: '*' } // Allow cross-origin client connections
});

io.on('connection', (socket) => {
  console.log(`[USER CONNECTED] Socket ID: ${socket.id}`);

  // 1. Join a specific chat room
  socket.on('joinRoom', (roomName) => {
    socket.join(roomName);
    console.log(`Socket ${socket.id} joined room: ${roomName}`);

    // Notify room members
    socket.to(roomName).emit('notification', `New student entered ${roomName}!`);
  });

  // 2. Handle incoming room message
  socket.on('chatMessage', ({ room, user, message }) => {
    console.log(`[${room}] ${user}: ${message}`);

    // Broadcast message to everyone in room including sender
    io.to(room).emit('newChatMessage', {
      user,
      message,
      time: new Date().toLocaleTimeString()
    });
  });

  // 3. Handle disconnect
  socket.on('disconnect', () => {
    console.log(`[USER DISCONNECTED] Socket ID: ${socket.id}`);
  });
});

server.listen(3000, () => console.log('Socket.io server running on port 3000'));