socket.js

var sitedata = {
  connectedUsers: 0
}
io.on('connection', function(socket) {
  var userdata = {
    joined: false
  }
  // connected users stuff
  socket.on('join', function() {
    if(userdata.joined) return; // User already joined
    // Set joined to true
    userdata.joined = true;
    // Add a new user
    sitedata.connectedUsers++;
    // Emit to all clients how many users are connected
    io.emit('connectedUsers', {
      num: sitedata.connectedUsers
    });
  });
  // disconnected users stuff
  socket.on('disconnect', function() {
    // so we don't go -1, -2 (just to be sure, should not happen though)
    if(sitedata.connectedUsers > 0) {
      // remove the user from connected
      sitedata.connectedUsers--;
      // emit to all clients new connected user count
      io.emit('connectedUsers', {
        num: sitedata.connectedUsers
      });
    }
  });
});