Array reduce

Implement a standalone reduce(arr, fn, initialValue) that behaves like Array.prototype.reduce. Don't call the built-in reduce or reduceRight.

It walks the array from left to right and keeps an accumulator, which is a running result. For each item it sets acc = fn(acc, value, index, arr). At the end it returns the final acc.

Starting value:

• If initialValue is passed, acc starts as that value and the walk starts at index 0. Passing undefined explicitly still counts as passing a value.
• If it's not passed, acc starts as the first item and the walk starts at the next index. A single item is returned without calling fn.
• With no items and no initial value, throw a TypeError.

Edge cases from the spec:

• Holes in a sparse array (indexes that were never assigned, as in [, , 3]) are skipped. They are never passed to fn and never used as the starting value. An array of only holes counts as empty.
• The length is read once, before the walk. Items pushed onto the array during the walk are not visited.
• If fn is not a function, throw a TypeError.