It's just because of the chess.js API. It tells you if any given move is a checkmate, so you know to ignore those moves without playing them, but to find out if

It's just because of the chess.js API. It tells you if any given move is a checkmate, so you know to ignore those moves without playing them, but to find out if a move is a stalemate, you need to "play it" on a cloned game and then see if the opponent has any legal moves.