Please reread what I wrote and re-check your tests. As many other users have pointed out, that test fails for them, as well. Case insensitization would mean that the case wouod be ignored, so MoOse and moose would be treated the same way. That isn't happening as it should be; it's not working as intended. Check your work.

There are absolutely ways to ignore casing here. Unfortunately, one of the tests written requires case sensitivity, so your function might be perfectly correct and still fail to clear this kata.

The instructions state that the function should be case INSENSITIVE, but one of the tests requires case sensitivity (i.e.: "MoOse === moose" // false;).

Commenting your code would be helpful

That's understandable. However, BigInt has been widely supported for quite a while now, so the instructions for the Kata should probably be updated to explicitly state that BigInt is to be avoided in any solutions attempted.