Но ведь js поддерживает индексацию с конца.
arr = [3, 2, 3]
arr[-1] === arr[0] // 3 = 3
sometimes it return error or not?

Abusing JavaScript's silent treatment of sloppy data handling is not something I would call "Best Practices". If intentional though it certainly deserves the "Clever" award!

It does, but javascript will return "undefined" for out-of-bounds errors instead of crashing, so it works anyway.