This is not what your code returns for this input. This is really weird. I suspect you may have changed something without realizing it. Your code fails on multiple examples, you should focus on solving sample tests correctly first.

Your solution, for some reason, returns undefined, and this confuses tests.
To avoid this error, make sure your solution always returns something.

Do you know what 890625n is?
Do you know what big integers are in JS?

EDIT: OH I am stupid, and the kata is Ruby only.