The code was very janky, but it worked

Which itself has a dependency of the function is_number.

dear Lord lmfao

That is one deep rabbit hole.

is this really a feature not built into Java Script?

x % 2 == 0

the is_even package does not provide much worth indeed because it simply negates is_odd and thereby all its benefit.

It's dependency is_odd on the other hand provides at least some additional checks (it also checks if the value is a valid integer below the max int value)

And while I would indeed see uses for such methods (especially with the other checks, no simple oneliners) in some cases, especially in testing: This is stuff you write yourself, throw it in a e.g. NumberUtils class and everything is fine. You do never depend on an external library for that. The benefit (not spending a few seconds to write it) does not outweigh any of the drawbacks that come with external libraries.

Make it faster and jankier with precalculated lookup tables!