[franciscop]

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.