Clever, and far from obscure.
Makes it very easy to understand what's going on here, and it's very cool o/

Yeah, plus creating a copy of the array costs memory and cycles.

Nice functional style. I like it. It is very easy to see what is going on.

This solution is quite elegant. However, I don't think its performance is good, since adding/removing items from array is much more time consuming than replacing its values.

1-based is not obvious enough

Your description has improved a bit. However, the solution setup and example test cases no longer reflect what's being tested. You switched your test cases from testing the combineArrays function to testing the appendArrays function while leaving the solution setup and example test cases as combineArrays. You need to make sure the test fixture, example test fixture and solution setup are consistent before I can give you further feedback.

jacobb, is this now an acceptable kata after i implemented the changes?

The appendArrays/combineArrays issue became a thing.

You have a function preloaded with user tests predefined, yet your submission tests are a diferent function.

for some reason solution requires to keep both functions, however it's not described