Socket.io

index.html

<h1>Socket.io</h1>

<div id="msgs"></div>
<input type="text">

<script src="/socket.io/socket.io.js"></script>
<script src="/socket.io-client.js"></script>

socket.io-client.js

/* jshint esversion: 6 */
const socket = io();

const msgs = document.querySelector('#msgs');
const input = document.querySelector('input');

socket.on('connect', () => {
  msgs.innerHTML += `Connected with id: ${socket.id} <br>`;
  socket.emit('msg', `I'm connected ${socket.id}`);
});

socket.on('msg', msg => {
  msgs.innerHTML += `Received message: ${msg}<br>`;
});

input.onkeydown = ({ keyCode }) => {
  if (keyCode === 13) {
    msgs.innerHTML += `You: ${input.value}<br>`;
    socket.emit('msg', input.value);
    input.value = '';
  }
};

socket.io-server.js

/* jshint esversion: 6 */

const http     = require('http');
const express  = require('express');
const socketio = require('socket.io');

const app      = express();
const server   = http.createServer(app);
const io       = socketio(server);

app.get('/', (req, res) => res.sendFile(`${__dirname}/index.html`));

app.get('/client.js', (req, res) => res.sendFile(__dirname + '/client.js'));

io.on('connection', socket => {
  console.log('New connection', socket.id);
  socket.broadcast.emit('msg', `${socket.id} connected`);

  socket.on('msg', msg => {
    console.log(`${socket.id} ${msg}`);
    socket.broadcast.emit('msg', msg);
    socket.join('contador');
  });
});

let counter = 0;

setInterval(() => {
  io.to('contador').emit('msg', counter++);
}, 10000);

server.listen(3000, () => {
  console.log(`Server listening on port ${server.address().port}`);
});