Socket.IO 2 Express

app.js

var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

app.get('/', function(req, res){
  res.sendFile(__dirname + '/index.html');
});

io.on('connection', function(socket){
    console.log('a user connected');
    socket.on("increment", (counter) => {
      console.log("increment");
        io.sockets.emit('COUNTER_INCREMENT', counter + 1)
    })
    socket.on("decrement", (counter) => {
      console.log("decrement");
        io.sockets.emit('COUNTER_DECREMENT', counter - 1)
    })
});

http.listen(5000, function(){
  console.log('listening on *:5000');
});

module.exports = app;