Workshop › Chess › Step 31 of 48

Castling moves

What we are doing

Castling moves the king two squares towards a rook. It needs history the board cannot show (has the king or that rook ever moved?), so castling keeps four flags: wK, wQ, bK, bQ (king side and queen side for each color). A king on its first square gets a castling move when the flag is on, the squares between are empty and the rook is in its corner.

```js
let castling // which castlings are still allowed

  castling = { wK: true, wQ: true, bK: true, bQ: true }

      // Castling: the king still on its first square, the squares between free and the rook in its corner.
      if (kind === 'K' && c === 4 && r === (color === 'w' ? 7 : 0)) {
        const rook = color + 'R'
        if (castling[color + 'K'] && !board[r][5] && !board[r][6] && board[r][7] === rook) add(r, c, r, 6, { castle: 'K' })
        if (castling[color + 'Q'] && !board[r][1] && !board[r][2] && !board[r][3] && board[r][0] === rook) add(r, c, r, 2, { castle: 'Q' })
      }
```

What it means

• castling[color + 'K'] reads a flag by a name built from text, like castling['wK'].
• King side: columns 5 and 6 empty and the rook on column 7; the king goes to column 6.
• Queen side: columns 1, 2, 3 empty and the rook on column 0; the king goes to column 2.
• The move carries castle: 'K' or castle: 'Q' through add's extra fields.

Your turn

1. Under let turn ... write let castling ...; in reset, under turn = 'w' write the flags.
2. In pseudoMoves, under the pawn block (after its closing }), write the castling block.

Skills: Game states

← Only legal moves The rook jumps over →

All steps

1. A dark table
2. One row of squares
3. All 64 squares
4. The starting position as text
5. Color and kind
6. Draw the pieces
7. An outline for white
8. Draw again and again
9. The knight's jumps as data
10. A list of moves
11. The king steps too
12. Which square was clicked?
13. Light up the selected square
14. Whose turn is it?
15. Where can it go?
16. Show the targets
17. Play a move
18. Click a dot to move
19. Say whose turn it is
20. The rook slides
21. Bishops and queens
22. Pawns step forward
23. Pawns capture diagonally
24. A pawn becomes a queen
25. Is a square attacked?
26. Attacks from far away
27. Is the king in check?
28. A move that remembers
29. Take a move back
30. Only legal moves
31. Castling moves
32. The rook jumps over
33. Losing the right to castle
34. No castling through check
35. Remember the double step
36. En passant
37. Checkmate and stalemate
38. Check! and the result
39. Click to play again
40. Show the last move
41. A score for a position
42. Thinking ahead
43. Pick the best move
44. Alpha-beta: skip what cannot matter
45. Try the best-looking moves first
46. The computer plays black
47. You are white
48. Build it yourself: the fifty-move rule