This kata is a subject to deduplication process here: https://github.com/codewars/content-issues/issues/190.
Please join the discussion to help us identify duplicate kata and retire them.

a function returns None by default so you can reduce your code

random tests missing in coffeescript

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Python: assert_equals should be used

Python: Needs random tests

Ruby: Needs random tests

Ruby: assert_equals should be used

For C# a note should be added to the description that the Kata is expecting an exception to be thrown for negative values, not returning null.

Crystal and Haskell translations kumited

[The latter not really by me, I just fixed the conflicts]

@galafik, tnx for the nice factorial kata!