ontime/server/app.js

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

const port = process.env.PORT || 4001;
const index = require('./routes/index');

const app = express();
app.use(index);

const server = http.createServer(app);

const io = socketIo(server, {
  cors: {
    origin: 'http://localhost:3000',
    methods: ['GET', 'POST'],
  },
});

let interval;

io.on('connection', (socket) => {
  console.log('New client connected');
  if (interval) {
    clearInterval(interval);
  }
  interval = setInterval(() => getApiAndEmit(socket), 1000);
  socket.on('disconnect', () => {
    console.log('Client disconnected');
    clearInterval(interval);
  });
});

const getApiAndEmit = (socket) => {
  const response = new Date();
  // Emitting a new message. Will be consumed by the client
  socket.emit('FromAPI', response);
};

server.listen(port, () => console.log(`Listening on port ${port}`));