forEach on a 'new Array' isn't doing what I expect

Question

I\'m just learning how to use JS higher-order functions (map, forEach, reduce, etc), and have stumbled into confusion. I\'m trying to write a simple \'range\' function, but

Answer 1

The forEach method iterates over the indices of the array. Interestingly enough, when you create a new array via new Array(n), it contains no indices at all. Instead, it just sets its .length property.

> var a = new Array(3);
> console.info(a)
[]
> console.info([undefined, undefined, undefined])
[undefined, undefined, undefined]

MDN describes forEach, and specifically states:

forEach executes the provided callback once for each element of the array with an assigned value. It is not invoked for indexes which have been deleted or elided.

Here's a neat technique to get an array with empty, but existing, indices.

var a = Array.apply(null, Array(3));

This works because .apply "expands" the elided elements into proper arguments, and the results ends up being something like Array(undefined, undefined, undefined).