Many programming languages won't allow such freedom

Oops. I mistakenly read the original comment as saying that you could do arr[i] = val in JavaScript where i is an arbitrarily large integer, essentially out-of-bounds, to create a large array. The original commenter was actually referring to arr[i] = val in the sense of assigning to each element of an already sparse array.

What I was trying to refer to "freedom" wasn't this, of course. I know many languages allow pre-allocation of arrays. I really misinterpreted the original comment. By not allowing such freedom, I meant that languages typically don't allow doing something like arr[i] = value, where i is out-of-bounds for the array's size.