server.js

const express = require('express');
const path = require('path');
const bodyParser = require('body-parser');
const cors = require('cors');
const mongoose = require('mongoose');
const socket = require('socket.io');

const config = require('./config/configs');

// Use Node's default promise instead of Mongoose's promise library
mongoose.Promise = global.Promise;

// Connect to the database
mongoose.connect(config.db);

let db = mongoose.connection;

db.on('open', () => {
  console.log('Connected to the database.');
});

db.on('error', (err) => {
  console.log(`Database error: ${err}`);
});

// Instantiate express
const app = express();

// Don't touch this if you don't know it
// We are using this for the express-rate-limit middleware
// See: https://github.com/nfriedly/express-rate-limit
app.enable('trust proxy');

// Set public folder using built-in express.static middleware
app.use(express.static('public'));

// Set body parser middleware
app.use(bodyParser.json());

// Enable cross-origin access through the CORS middleware
// NOTICE: For React development server only!
if (process.env.CORS) {
  app.use(cors());
}

// Initialize routes middleware
app.use('/api/users', require('./routes/users'));

// Use express's default error handling middleware
app.use((err, req, res, next) => {
  if (res.headersSent) return next(err);
  res.status(400).json({ err: err });
});

// Start the server
const port = process.env.PORT || 3000;
const server = app.listen(port, () => {
  console.log(`Listening on port ${port}`);
});

// Set up socket.io
const io = socket(server,{
  cors:{
    origin: config.react_app_url,
  }
});
let online = 0;

io.on('connection', (socket) => {
  online++;
  console.log(`Socket ${socket.id} connected.`);
  console.log(`Online: ${online}`);
  io.emit('visitor enters', online);

  socket.on('add', data => socket.broadcast.emit('add', data));
  socket.on('update', data => socket.broadcast.emit('update', data));
  socket.on('delete', data => socket.broadcast.emit('delete', data));

  socket.on('disconnect', () => {
    online--;
    console.log(`Socket ${socket.id} disconnected.`);
    console.log(`Online: ${online}`);
    io.emit('visitor exits', online);
  });
});