socket.io determine if a user is online or offline

Question

We can trace if a connection is established or disconnected by this code

console.log(\'a user connected\');
    socket.on(\'disconnect\', function () {
              


        

Answer 1

If your clients have specific user IDs they need to send them to socket.io server. E.g. on client side you can do

And on server you will put something like

const users = {};
io.on('connection', function(socket){
  console.log('a user connected');

  socket.on('login', function(data){
    console.log('a user ' + data.userId + ' connected');
    // saving userId to object with socket ID
    users[socket.id] = data.userId;
  });

  socket.on('disconnect', function(){
    console.log('user ' + users[socket.id] + ' disconnected');
    // remove saved socket from users object
    delete users[socket.id];
  });
});

Now you can pair socket ID to your user ID and work with it.