client.js

var client = require('socket.io-client')('http://10.0.2.2:9000/');
client.on('connect', function () {
  console.log('connect');
});
client.on('disconnect', function () {
  console.log('disconnect');
});

server.js

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

var count = 0;
io.on('connection', function (socket) {
  count++;
  console.log('%d clients connected', count);
  socket.once('disconnect', function () {
    count--;
    console.log('%d clients connected', count);
  });
});

io.listen(9000);