Why does array.slice behave differently for (length, n)

Question

If I have an array a:

1. a[a.length] returns nil. Good.
2. a[a.length, x] returns []. Good.

Answer 1

Look to your friendly Lispy languages for the answer. The philosophy you're looking for began with languages whose specialty was LISt Processing. For instance, here's one way of creating lists in Haskell:

1:[] => [1] 
1:2:3:[] => [1,2,3] 

This is called cons-ing, for 'constructing' a list. If the idea hasn't clicked yet, consider this: an array is created by adding elements to an empty list, not to 'nil'.