what si socket.join code example

Example: rooms in socket io

io.on('connection', socket => {
  socket.join('some room');
});
//And then simply use to or in (they are the same) when broadcasting or emitting:

io.to('some room').emit('some event');
//You can emit to several rooms at the same time:

io.to('room1').to('room2').to('room3').emit('some event');
//In that case, an union is performed: every socket that is at least in one of the rooms will get the event once (even if the socket is in two or more rooms).
//You can also broadcast to a room from a given socket:

io.on('connection', function(socket){
  socket.to('some room').emit('some event');
});