Not an issue.

This should return null, because there's 10 in array.
I think the array for test case should look like this: Test.assertSimilar(upArray([0, 9, 9, 9]), [1, 0, 0, 0]);

invalid sequence is a string with non numeric character. description updated, please check if it's clearer.
thanks!

It doesn't. That's what your solution returned.

I am a bit confused. How does [9,10,0] represent the number 999? I can see, I guess that the 10 can be converted to a 9(?) but why would it not be 990 then?

999 + 1 = 1000
Alhough the description isn't exactly strict, it looks clear enough for me.

This means you are to find another way, nothing more. It's your problem, not a kata issue.

How [9, 10, 0] should equal [1, 0, 0, 0] ?

All lists are expected to start with element "1", so if the list is (2,3,4) then it is invalid.

Problem definition is not complete. Invalid sequence is not defined. Test cases are not comprehensive enough.I can pass the tests but when I submit my code fails and I do not know why.