str has assigned as an empty string in the first line of code, and it won't go in to the for loop when 1 <= 0. So it'll directly return ""

Directly from the description: beast and dish may contain hyphens and spaces, but these will not appear at the beginning or end of the string.

The string only contains letters from a to z?

A string also has a length property. Don't need to convert the string to an array with split().

Sorry, wil do!

Use spoiler flag next time please, your post was visible in the homepage discourse.

When no value is passed to reduce(), like ([]), where the initial value is 0. The reduce returns it's initial value.

What he is saying is that the function is mutating the original signature array.
You should make a copy of the signature array. You can do that with const aryintSignature_Copy = [...signature];

Wow, such a clever sollution!
I was breaking my head on excluding those 'edge cases' (n =< signature.length) from the loop.

What would be a good practice to eliminate overcomplification?
Is it to write all possible inputs and outputs in 1 overview to spot the most efficient pattern?

But thnx, this was a great lesson.

Oke, but what is returned when count_sheep(0) is executed? Not an empty ""...

But where is the logic for handling the '-' and ' ' at the first index and last index of each argument?