Workshop › Chess › Step 34 of 48

No castling through check

What we are doing

The king may not castle out of check, through an attacked square or into one. The usual try-and-check test already covers the landing square; we add the start square and the square the king passes over.

```js
  return pseudoMoves(color).filter((m) => {
    if (m.castle) {
      const row = m.from[0]
      // No castling out of check or across an attacked square.
      if (attacked(row, 4, other(color)) || attacked(row, m.castle === 'K' ? 5 : 3, other(color))) return false
    }
```

What it means

• Only castling moves are checked here; return false drops the move from the list.
• Column 4 is the king's start square; column 5 or 3 is the square it passes over.

Your turn

In legalMoves, right under the filter((m) => { line, write the castling check.

Skills: Game states

← Losing the right to castle Remember the double step →

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