Socket.io2 - no idea what this is

I'm trying to help out a bud and noticed that the person before me had used socket.io and socket.io2. The guy used socket.io (which looks like normal socket) and socket.io2 which I have no idea what it is, but it looks like another library. It looks too big to be a hand-made thing but was wondering if anyone had heard of anything like "socket.io2"? Thanks!