Sending messages to multiple rooms using Socket.io

Question

Is it possible to send messages to multiple rooms using socket.io?

Sending to 1 room:

io.sockets.in(room).emit("id", {})

Sending to N rooms:

io.sockets.in(room1, room2, roomN).emit("id", {})

Answer 1

Updated as of Socket.IO v2.0.3

// sending to all clients in 'game1' and/or in 'game2' room, except sender
socket.to('game1').to('game2').emit('nice game', "let's play a game (too)");

https://socket.io/docs/emit-cheatsheet/

Answer 2

The sockets.in method only accepts one room as an argument, so to broadcast to multiple rooms you would have to reset the room, in between emissions. Something like this should work:

['room1', 'room2', 'room3'].forEach(function(room){
    io.sockets.in(room).emit("id", {});
});

Answer 3

Yes, it's possible to emit to multiple rooms altogether. From the tests:

socket.on('emit', function(room){
  sio.in('woot').in('test').emit('a');
  sio.in('third').emit('b');
});

That's because when you use to or in you're appending the room to the list of rooms to be targeted. From the source code (lib/socket.js):

Socket.prototype.to =
Socket.prototype.in = function(name){
  this._rooms = this._rooms || [];
  if (!~this._rooms.indexOf(name)) this._rooms.push(name);
  return this;
};