This kata as a whole is not interesting at all because there are only a few possible return values. And the input validation makes no sense either (what is with 'Fermat Friends: []; Mersenne Friends: []'???).

Please write an actually interesting kata next time ;-)

Actually, the description is just plain incorrect. Example solution does not implement the spec either. You don't expect the closest Fermat number, you expect the first bigger, if I read the example solution correctly.

Throwing means raising an Error. You expect a return value.

Testing is lacking. I managed to pass the tests without fully implementing the spec.

Also, no random tests.

So clean!

I agree - so clever!

I think this is a very clear solution. Good job!