Not entirely sure how this should be handled, but JS Arrays support “holes”, where [1,,3] creates an array without anything at index 1. From the outside, that index appears to contain undefined, but array methods like map and forEach skip over those indices while iterating, and flat removes them, changing the indices of everything that comes after the hole. You can tell the difference between a hole and an undefined value with either index in array or array.hasOwnProperty(index).

I think it’s perfectly fine to simply say that holes are not handled properly since they’re quite rare in practice but I wanted to make sure this language quirk wasn’t missed.

Do present test cases handle this quirk adequately or is there anything else you'd like to be covered in the test suite?

I don’t believe there are any tests that involve array holes in the current test suite so if we want to support it new tests would need to be added.