Allow to emit message to several rooms at once

[angel1st]

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Current behaviour

Thanks to #2879 it is possible to join several rooms at once.

It would be nice to have the same for sending a message to multiple rooms.
Currently, we can achieve it with following code snippet:

['room1', 'room2', 'room3'].forEach(room => {
  // send message to all clients in the room, but the sender
  socket.broadcast.to(room).emit('message', 'nice game');
});

It would be much better if we can have it like that:

io.in(['room1', 'room2', 'room3']).emit('message', 'cool game');
socket.broadcast.to(['room1', 'room2', 'room3']).emit('message', 'nice game');

[vadibezdushnyi]

And I have a little addition.
io.to(socketId).ifNotIn('room').emit('message', 'cool game');

[Strate]

At least we can handle multiple arguments. I use this kind of patched function:

io.to = (function(to) {
  return function patchedTo(...rooms) {
    return rooms.reduce(
      (thisArg, room) => to.call(thisArg, room),
      this
    )
  }
})(io.to);

[darrachequesne]

Duplicate of #3048

[darrachequesne]

For future readers: this feature was implemented by 085d1de and included in socket.io@4.0.0.

Syntax:

io.to(["room1", "room2", "room3"]).emit(/* ... */);

socket.to(["room1", "room2", "room3"]).emit(/* ... */);

Documentation: https://socket.io/docs/v3/migrating-from-3-x-to-4-0/#Allow-to-pass-an-arrow-to-io-to