after progress on the question how to create socket.io multicast groups, I found making rooms a great way to do what I needed.
However, it would be great to know about all the rooms, without an extra data-structure.
Is it possible to get a list of all rooms on the server from the server socket?
The short answer:
I analysed
io:I got the following output:
after joining room “public-alfred” from a single client
io.sockets.adapter.roomscontained: