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 ;)

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.