Calculate client-server latency using socket.io

client.js

var socket = io.connect('http://localhost');
var startTime;

setInterval(function() {
  startTime = Date.now();
  socket.emit('ping');
}, 2000);

socket.on('pong', function() {
  latency = Date.now() - startTime;
  console.log(latency);
});

server.js

io.sockets.on('connection', function (socket) {
  socket.on('ping', function() {
    socket.emit('pong');
  });
});

https://stackoverflow.com/a/45216554/1221082