Are websocket rooms persistent? How to destroy a room?

Using Websockets with Socket.io on Nodejs.

I'd like to create dynamic rooms (random names) for users to collaborate, but I am afraid this could creep up in the long run. So it would be good if rooms existed only for the duration of the socket connections.

So my question is: do rooms persist beyond the socket connections?

If they do, how to make sure the rooms are destroyed after the last socket ID leaves them?

If rooms are not persistent and won't break the server at some point, I am fine.