I never decalre them unless they're needed.

I completely can't get that part: h[c]. May you please explain how that works?
I mean for me this is putting currentValue as index of previousValue.
It doesn't look correct, but that worked... how ?

Okey, I'm sorry, that should be question. Can you copy this code and run to see is there something really missing that my solution can't be accepted or there is something specific (because solution is correct). As I wrote, the same code is passed in tests for 'null and empty', but after I click ATTEMPT it passes all tests (correct results of encryption and decryption) beside that one.

UPDATE: We were writing at same time... so probably my question is answered... Thank You! However I didn't try it again. I'll see if your hint was helpful...

There's problem with over 8 characters digits given in fact. Mostly it doesn't accept results - where last 1/2 characters of about 18 long characters digit are rounded up/down by random tests - while mine were more precise.

You changed Url to Urs but not in "Test Area", so the same solution won't run for just tests, but will be passed when you submit final solution.

Good idea to teach new users something more at once would be to use Upper Case in case of first letter so they had to apply 'i' flag after end of their RegEx.

There is mistake in word 'argument' ('argumetn') of sentence: 'The first argument represents the last letter of the first word and the second arguemtn represents the first letter of the second word.'