server.js

var currentClick = 0;
var num = 1;
// list of all connected users
var users = [];
io.on('connection', function(socket){
  var newUser = {
    name: 'user'+(num++),
    numOfClicks: 0
  }
  // add newUser
  users.push(newUser);
  // send info of newUser to the client
  socket.emit('currentUser', newUser);
  // send all users to all socket
  io.sockets.emit('user joined', users);
  // send current click to all socket
  io.sockets.emit('update currentClick', currentClick);

  // when a socket emit click => increase currentClick, increase num click of that socket
  socket.on('click', function(){
    currentClick += 1;
    newUser.numOfClicks +=1;
    // emit event to update counter to all socket
    io.sockets.emit('update user', newUser);
    io.sockets.emit('update currentClick', currentClick)
  });

  // remove user when socket disconnect
  socket.on('disconnect', function(){
    users.splice(users.indexOf(newUser), 1);
  })

  // check if user on socket is not the one has least click.
  socket.on('win', function(){
    // sort users depend on num of click of user;
    var sortedUsers = users.sort(function(a, b){ return a.numOfClicks - b.numOfClicks; });
    // if user has least click => no win
    if(sortedUsers.indexOf(newUser) === 0){
      io.sockets.emit('no win', newUser)
    } else{
      io.sockets.emit('win', newUser);
    }
    // reset all num of click of all socket
    users = users.map(function(user){ return { name: user.name, numOfClicks: 0 }})

    // io.sockets.emit('user joined', users);
    // reset current click
    currentClick = 0;
  })
})