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 ;-)

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.