It's all good, we understand your feelings.

And isn't the answer to your question in the explanation after you passed the kata?

You have a problem there with the global var, when calling the function several times, it'll keep the previous value. Even then, recursion is not a good approach for this problem, because the input values are very large and it'll overflow the stack.

Go ahead, I'll try to answer it without giving out the solution.

map and reduce are two loop-alike methods, so your solution isn't O(n).