server.js

const port = 8033;


const express = require('express');
const app = express();

const httpServer = require('http').createServer(app);

const {Server} = require('socket.io');
const io = new Server(httpServer, {
    cors: {
        origin: true,
    },
});

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

console.log('starting socket server on :' + port);

httpServer.listen(port);

let clientIdIncrement = 0;

// Memorize the date so we can re-broadcast it when a client joins
let timerEndDate = null;

io.on('connection', function (socket) {
    const clientName = '[socket.io#' + (++clientIdIncrement) + '@' + socket.request.socket.remoteAddress + '] ';

    console.log(clientName + 'connected');

    socket.emit('countDownTo', timerEndDate);

    socket.on('countDownTo', async date => {
        console.log(clientName + 'count down to ' + date);

        timerEndDate = date;

        io.emit('countDownTo', timerEndDate);
    });

    socket.on('ping', async data => {
        console.log(clientName + 'ping');

        io.emit('ping', data);
    });

    socket.on('disconnect', function () {
        console.log(clientName + 'disconnected');
    });
});