This is composition, not concatenation. There are two contexts where you can talk about concatenating functions but neither really make sense in javascript (reader monoid and endomorphism monoid).

I was confounded by this too. If you are going to specify something for this case, specify throwing an exception.

Indeed, the description of the problem and the test case for it don't match up. Mysteriously, I saw:

Test Failed: Expected: a = 1,b = 2, instead got: a = 1,b = 2,

I just tried it again, and now I can't reproduce it. Might have been wrong.

The description didn't make it obvious that the cache should copy arguments, could it be made explicit that the cache should be aware of mutations?

This kata promises that you will be fed integers, but you are in fact fed strings.

Order shouldn't matter in the results, but the tests given care about this, my first solution failed with things like:
Test Failed: Expected: {"a":25,"b":8}, instead got: {"b":8,"a":25}