Modifying board content breaks the random tests.

Sample tests contain a x, which is not a valid chess piece.

Please use appropriate data types.

This is basically Blind's question, but somewhat more forcefully.

I strongly doubt about the global task and the way you set it up.

• why to pass the values as string while an array would make more sense?
• why to pass the full board when a simple hashmap or even an array would be enough?

You're currently adding useless tasks/"complications" (manner of speaking, there's nothing complicate in handling this) just to somehow justify the kata itself. IMO, that's a bad idea:

• for beginners, you're showing a task that isn't well designed: anyone who would face this task in reality would never use this kind of input (well, for the board, that could be, yes, but in a larger overall task)
• for competent users, you're just adding boring layers over boring layers, especially since those people will see that the inputs aren't given in a meaningful way for this special task.

Overall: I don't think you should keep this kata. ;)

• JS solution setup is java stuff
• please take care of the formatting (indentation) of all code that is visible by the user (that actually applies to the edit panel too, since power users can have to edit/maintain your kata at some point). Currently, tests and solutions setup look rather like... let's say "not properly formatted code", to be nice. ;)