I have some experience building these kinds of games, so last year I put together a beginner's guide on them. It's been getting more attention recently with people wanting to make/experiment with these games during quarantine, so I figured I'd share to get feedback:

- Part 1, Client: https://victorzhou.com/blog/build-an-io-game-part-1/

- Part 2, Server: https://victorzhou.com/blog/build-an-io-game-part-2/

- [demo] An example .io game: https://example-io-game.victorzhou.com/