Fantastic! Thanks for pointing that out. My cursory glance over the code in ai.js led me to believe that you were weighting it by score, now I see the full picture.

One more thing - have you looked into storing the game tree? I noticed it is starting the search from the beginning every time. I'd expect you would see a branching factor of around 10, so this would only really make a difference at depths greater than 4.

You started an excellent and inspiring GitHub project - I feel like the AI research into this game has only just begun.