Calculate client-server latency using socket.io

client.js

var socket = require('socket.io-client')('http://spacebro.space:8080');
var startTime;

setInterval(function() {
  socket.emit('latency', Date.now(), function(startTime) {
    var latency = Date.now() - startTime;
    console.log(latency);
  })
}, 2000);

server.js

var io = require('socket.io')(80);

io.on('connection', function (socket) {
  
  socket.on('latency', function (startTime, cb) {
    cb(startTime)
  })

  socket.on('disconnect', function () {
    io.emit('user disconnected')
  })
})