https://stackoverflow.com/questions/51814018/issue-with-socket-io-when-hosting-on-heroku?noredirect=1#comment90594875_51814018

server.js

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

app.use(cors());

const server = require('http').Server(app);
const io = require('socket.io')(server, { path: '/octagon/socket.io' });

server.listen(process.env.PORT || 3000);

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

// io.origins('*:*'); // for latest version

io.sockets.on('connection', socket => {
  socket.emit('news', { hello: 'world' });
  console.log('connected socket!', socket.id);
  socket.on('test', message => {
    console.log(message);
  });
});

io.on('error', console.error);