Porting my Rubido (playdate version) to Bangle JS2 (chinese checkers / solitaire game)

That looks great!

Looking at the first bit of code you have there I see you do:

BoardPart = BoardParts.Items[Y][X];

But then you do:

      if (BoardParts.Items[Y][X]) {
        if (BoardParts.Items[Y][X].AnimPhase < 2) {
          pegsLeft++;
          result += CPeg_CanMoveTo(BoardParts.Items[Y][X], X + 2, Y, false);
          result += CPeg_CanMoveTo(BoardParts.Items[Y][X], X - 2, Y, false);
          result += CPeg_CanMoveTo(BoardParts.Items[Y][X], X, Y - 2, false);
          result += CPeg_CanMoveTo(BoardParts.Items[Y][X], X, Y + 2, false);
          result += CPeg_CanMoveTo(BoardParts.Items[Y][X], X + 2, Y - 2, false);
          result += CPeg_CanMoveTo(BoardParts.Items[Y][X], X + 2, Y + 2, false);
          result += CPeg_CanMoveTo(BoardParts.Items[Y][X], X - 2, Y + 2, false);
          result += CPeg_CanMoveTo(BoardParts.Items[Y][X], X - 2, Y - 2, false);
        }
      }

So you reference BoardParts.Items[Y][X] a lot - each time Espruino is having to do a lot of lookups to find BoardParts, Items, then Y and X. What if you just replaced all occurrences or BoardParts.Items[Y][X] with BoardPart (which should be the same?).

That might make a reasonable difference to the speed as well