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: '*' } }); // 
// CORS policy 'Access-Control-Allow-Origin' fix https://stackoverflow.com/questions/24058157/socket-io-node-js-cross-origin-request-blocked


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 client
  socket.emit("FromAPI", response);
};

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