server.js

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);

app.use(express.static('public'));

io.on('connection', socket => {
    console.log('a user connected');

    socket.on('join-room', (roomId, userId) => {
        socket.join(roomId);
        socket.to(roomId).emit('user-connected', userId);

        socket.on('disconnect', () => {
            socket.to(roomId).emit('user-disconnected', userId);
        });
    });
});

const PORT = process.env.PORT || 3000;
server.listen(PORT, () => {
    console.log(`Server is running on port ${PORT}`);
});