That's quite broken (you'd want a different variable inside the body, vs clobbering the iteration counter, else this would process the last item in your list, then exit).

I hope you haven't done this anywhere.

Makes my brain hurt, but I think this will only run through the loop one time looking at the last element of the array.

Uh no don't reassign the loop variable in the inner scope. Use:

    const j = (length - 1) - i;

in that case. Much safer.

You should save the old value of i somewhere and restore it back at the very end of the loop. Or simply define a new j like another comment says.