AI solution for JavaScript Quiz on BFE.dev
55. sparse array

The code outputs the following:

1
undefined
undefined
2
[2, NaN, 4]
1
undefined
undefined
2
[1, undefined, undefined, 2]

The arr array has two empty items, which results in gaps or "holes" in the array. When looping through the array with forEach or for...of, the empty items are shown as undefined.

When using map, the result is an array with the same number of elements as arr, but with the empty items changed into undefined, causing the NaN value when multiplied by 2.

Using the spread operator [...arr], the empty items are still present and shown as undefined.