Workshop › Chess › Step 36 of 48

En passant

What we are doing

When an enemy pawn has just stepped two squares past your pawn, your pawn may capture it as if it had moved one: diagonally onto the skipped square. The captured pawn is not on that square but beside the moving pawn, so makeMove removes it there and undo remembers it.

```js
          if (enPassant && enPassant[0] === tr && enPassant[1] === tc) add(r, c, tr, tc, { ep: true })

  const undo = { m, piece, captured: board[tr][tc], castling: { ...castling }, enPassant, taken: '' }
  board[tr][tc] = m.promo ? piece[0] + m.promo : piece
  board[fr][fc] = ''
  if (m.ep) {
    // The pawn taken en passant stands beside the moving pawn, not on the square it moves to.
    undo.taken = board[fr][tc]
    board[fr][tc] = ''
  }

  if (undo.m.ep) board[fr][tc] = undo.taken
```

What it means

• In the pawn's diagonal loop, the enPassant square is a capture even though it is empty; the move gets ep: true.
• The captured pawn stands on the mover's start row fr, in the target column tc.
• undo.taken starts as '' and holds that pawn, so undoMove can put it back.

Your turn

1. In the pawn's diagonal loop, under the capture line, write the enPassant line.
2. In makeMove, add taken: '' to undo, and under board[fr][fc] = '' write the if (m.ep) block.
3. In undoMove, under board[tr][tc] = undo.captured, write the restore line.

Skills: Game states

← Remember the double step Checkmate and stalemate →

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