The function determineMembership is lifted (and should not be in function closure btw), making the code more reusable and modular. Nicer for the next dev or myself after 6 weeks to modify this code.

Agreed

+1 for composabel functions.
Similar approach: http://www.codewars.com/kata/reviews/55733d477fbf43cd5500000f/groups/557927c3e7f0fcbf55000064

Running tests gives me 'Test didn't pass: Unknown error'

:D

Array should be provided, not required to copy/ paste. Tests time out.

Failure messages are not desriptive.

http://jsperf.com/what-is-the-best-range/3

Added the repetition.
http://jsperf.com/what-is-the-best-range/3

Eval seems very slow:
http://jsperf.com/codewars-custom-kata-5296455e4fe0cdf2e000059f

Good idea to call .reduce instead of .slice.

Would be more interesting and educating if it can be mixe uppercase and lowercase. Now it is easy to just use .sort() without learing that it deas not sort string containing upper- and lowercase alphabetically, but according to some JS internal representation.

On philospophical side, real martial art katas are something that get repeated thousands of times, if you are any good.
I would love to see that kind of katas. E.g. for refactoring applying tranformation a,b,c & d.