True, so far this is one of the closest katas to real work tasks I've encountered.

Bad description, solution only achievable upon running the code and fixing it based on the errors that pop up.

I had an issue with the word "Mount" sometimes being expect on the address and sometimes not. Weird. I had to keep pressing the "Submit" button until it actually went OK.

you can do it with string and byte only... one digit needs one byte. Now you can do it like in school

BigInt for Javascript is disabled because this kata was created before BigInt was introduced to Javascript, and it was not enabled afterwards to keep the kata worth its 4 kyu reward. With BigInt enabled, it would be white. The idea of the task itself here is to do it not only difficult way, but also to do it correctly! Anyway, someone had to put some similar mechanism into BigInteger, didn't they? With this kata, you can learn what it is. There are people out there who do things in a "wrong, difficult and arduous way" , just so you could write let num = 1n; in your Javascript program ;)

Well, like I said. I've read the FAQ and that doesn't explain why I can't use a function constructor . Nowhere in the kata instructions does it say we aren't allowed to use a function constructor.

"This kata sucks, because my code works only on my machine" - hmmm, interesting approach.

Please read through this FAQ, you could find some helpful hints there. If you still have some specific problems, tell us about them.

The fact that eval is disaled is somewhat implied by the rank of the kata. As you mentioned, if eval were there, the JS version would be nothing else but easy points (lots of!) and no learning, but still a significant effort in other languages.
The question is whether this kata should have JavaScript version or not. If not, then great, we can delete it. But if you want to have JS version of it, then it whould be either blue with no eval, or a separate white kata with eval. Here, it's a translation of a blue kata. Therefore no eval for you, sorry.

On JavaScript I'm getting "eval is not a function" error.

EDIT

Yep, it's manually disabled. At least specify it in the kata's description FFS.
Also, it's just my personal opinion but disabling native language features is really unpragmatic and goes against the "clever" mentality.
I get that this would be an 8 kyu if eval was enabled but my point still stands.