Workshop › Chess › Step 35 of 48

Remember the double step

What we are doing

En passant needs to know one thing from the last move: did a pawn just move two squares, and which square did it skip? A double step is marked double: true; makeMove keeps the skipped square in enPassant for exactly one move, and undo saves and restores it.

```js
let enPassant // the square a pawn could capture onto en passant right now, or null

  enPassant = null

          if (r === start && !board[r + 2 * dir][c]) add(r, c, r + 2 * dir, c, { double: true })

  const undo = { m, piece, captured: board[tr][tc], castling: { ...castling }, enPassant }

  if (m.castle === 'Q') [board[fr][3], board[fr][0]] = [board[fr][0], '']
  enPassant = m.double ? [(fr + tr) / 2, fc] : null

  castling = undo.castling
  enPassant = undo.enPassant
```

What it means

• The skipped square is halfway between the start and target rows: (fr + tr) / 2, in the same column.
• Every other move sets enPassant back to null, so the chance lasts one move.
• enPassant inside { ... } is short for enPassant: enPassant.

Your turn

Add the variable and its reset line, { double: true } on the two-square move, enPassant in the undo object, the line in makeMove under the castling rook lines, and the restore line in undoMove.

Skills: Game states

← No castling through check En passant →

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