Isn't the pageIndex function wrong in case you ask for the last item at the end of the array (the item which equals to collection.length)?
It's returning -1 instead of the page index that equals to the last page.
This works because the test used in this kata doesn't check for the very last item of the array.

if you say so :)
but I think that what you said ( "Same" means, here, that the elements in b are the elements in a squared regardless of the order)