There's no copyright to the rules of the game itself, but its implementation as a work of Javascript is certainly copyrightable, in the same way that the manual

There's no copyright to the rules of the game itself, but its implementation as a work of Javascript is certainly copyrightable, in the same way that the manual of a board game is copyrightable. There may also be a copyright interest in the "target" and "guessable" word lists as collections, but I'm not certain of the applicable subtleties.