index.js

let express = require('express');
let online = 0;
let app = express()

let port = process.env.PORT || 3000
let server = app.listen(port)

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

console.log('RUNING YOUR SERVER ...')
console.log(`hearing requests at localhost:${port}..`)

let socket = require('socket.io')
let io = socket(server)

io.sockets.on('connection', newConnection)

function newConnection(socket) {
    console.log('new connection : ' + socket.id + '  online ' + ++online)
    io.sockets.emit('online', online)

    socket.on('clearall', () => {
        socket.broadcast.emit('clearall')
    })

    socket.on('data', (data) => {
        socket.broadcast.emit('data', data)
    })

    socket.on('mouse', (d) => {
        socket.broadcast.emit('mouse', d)
    })

    socket.on('disconnect', () => {
        console.log('disconnected now online : ' + online)
        online = online - 1
        io.sockets.emit('online', online)
    })

    setInterval(() => {
        socket.emit('online', online)
    }, 500)
}