You're right. I corrected this mistake and all JS tests passed. Thank you for your help.

You're using the built-in function in line 2 incorrectly.

Thank you for your answer. I guess there is something wrong with my JS solution then.

Both seem fine to me. I just passed on both with very similar solutions.

All random test failed for solution in JavaScript. Submitted the same solution in Ruby, all tests passed. Is there a problem with JavaScript random tests?

It would probably be better to do a null check before a length check (arr == null || arr.length == 0), if the order is reversed, null check becomes redundant.

I guess that checking for a for-each-loop use in a solution means checking for an iterator method call. So maybe you can extend ArrayList (or other implementation to be used as a fn parameter) and override iterator method. Like that:

class MyList<T> extends ArrayList<T> {
        @Override
        public Iterator<T> iterator() {
            throw new UnsupportedOperationException();
//            return super.iterator();
        }
    }

Maybe do the same for some other methods (get for example).