Is there any documentation about Socket.IO? Its website just has a poor wiki and there is no full documentation explaining connect, emit, require and so on
See
As far as I know, no API documentation exists without inspecting the code on your own.
require is a part of node, not socket.io. See node docs: modules, the same applies for emit
I know this page is quite old but there are more docs that could aid in Socket.io developoment.This may or may not be as helpful since it isn't the direct Socket.io docs but, I found that this helped me more than the actual documentation since it gives a more detailed explanation for each of the different classes & methods.
https://github.com/Automattic/engine.io#engineio-the-realtime-engine
This is the doc for Engine.io, this is how Socket.io works under the hood and will explain how to setup both the Server and Client connections beyond the basic explanations supplied by the Socket.io Docs