snippet.js

var numUsers = 0;

// when the client emits 'add user', this listens and executes
socket.on('add user', function (username) {
  if (addedUser) return;

  // we store the username in the socket session for this client
  socket.username = username;
  ++numUsers;
  addedUser = true;
  socket.emit('login', {
    numUsers: numUsers
  });
  // echo globally (all clients) that a person has connected
  socket.broadcast.emit('user joined', {
    username: socket.username,
    numUsers: numUsers
  });
});