As akar-0 points out it is a valid JavaScript variable name (in fact there's a couple popular libaries built around it), but the convention originated in some other language (or perhaps even a linter?) to use _ as a shorthand for "this argument isn't used".

Yes. Its value is not actually used so there's no need to declare it with a handy name (though we could use _ as a variable name in JS).

Is the underscore used as a filler variable in order to access the second argument of map?

Added

What's the problem with that?

This is not an issue with the Kata but an opinion on it so I'm going to close it. Feel free to make an issue if you encounter any problems.

That doesn't sound like a real issue.

This kata seems to have no point other than to snark on kata titles.

Description should make it clearer that an object with numeric keys is the expected return value, preferably with an example.

It's kind to include at least one test case.

Assigning an existing function over creating a new function is what I prefer (i.e. aliasing), and this is exactly that kind of scenario.

Good suggestion! The description has been changed.

This is shooting for minimalist, not most performant.