Not the blog article author, I just saw an interesting article and posted it here. But I can answer the first question, there's 3 major ways I can think of:

- Read the specification in-depth and understand it well.

- Just testing, if you know JavaScript you can test it in other runtimes, create tests around the methods and then you know how it works well. This is usually more for edge cases though, like what happens if you mutate the 3rd argument of Array.map?

- Look at other implementation's internals, as you said.

I did this for ~10 years, and absolutely no regrets, it was a lot of fun and the side projects gave me energy.

Nowadays it's hard though, learning a new language, with a gf and a full-time demanding job, I don't have a lot of time to be tinkering. I do feel a bit sad about this but just assumed it's just life, and cannot imagine with kids how impossible this'd be.

I did look at doing some basic housekeeping with LLMs (updating deps, standardize testing across projects, etc) and realized I have literally 200+ side projects, most of them websites/JS libraries/React libraries. I was a bit baffled, of course 80% of it is trash, but I was kind of amazed at how many things I've actually done.