Added proper random tests, feel free to check them out :)

You might want to add edge cases where the property is inherited through the prototype chain. This will invalidate solutions that use in operator because it searches through the prototype chain as well.

You just have to have a bad day :-) console.log works fine and description is ok (not perfect but quite stright forward)

This is an incredibly STUPID kata! Can't see what the inputs are. Can't see what you're returning with neither return nor console.log. Errors are thrown constantly and because you can't see what you've returned you have no clue what or where to debug. Disgusting. Just disgusting.

No random tests.

Should work with falsy values

The subject of this test is not mentioned anywhere.

There are no checks for the correct format! (completely forgot about it and tests passed anyway)

Needs random tests, and please replace Test.expect with Test.assertEquals.

There was a bit more to breaking the Enigma of course ..

OK. Point taken.

Every letter has to map to some other letter, and the mapping has to be reversible.

But a letter cannot map to itself, even though it is reversible. (This is tested. It's not in the description btw.)

This is a known weakness in cryptanalysis, which is not necessary for the mapping to be reversible.

I do not see a point in requiring this weakness in your kata. No need to teach people bad habits, is there?

I am not sure I understand your point. Can you give an example?

I haven't been active for a while; too busy with projects. What can I help you with?

OK, so it has to be a very simple reciprocal substitution cipher.

But WHY OH WHY can occasional letters not map to themselves, when plenty of others don't?

That one cost the Germans the Enigma for crying out loud.