Workshop › Chess › Step 24 of 48

A pawn becomes a queen

What we are doing

A pawn that reaches the far side is promoted to a queen. The move carries an extra field promo: 'Q', so add learns to take extra fields, and play puts a queen on the board instead of the pawn.

```js
  const add = (r, c, tr, tc, extra = {}) => moves.push({ from: [r, c], to: [tr, tc], ...extra })

        const dir = color === 'w' ? -1 : 1
        const last = color === 'w' ? 0 : 7
        const promote = (tr) => (tr === last ? { promo: 'Q' } : {})
        if (!board[r + dir][c]) {
          add(r, c, r + dir, c, promote(r + dir))

          if (board[tr][tc] && board[tr][tc][0] !== color) add(r, c, tr, tc, promote(tr))

  board[tr][tc] = m.promo ? piece[0] + m.promo : piece
```

What it means

• extra = {} is a default value: if add is called without a fifth argument, extra is an empty object. ...extra copies its fields into the move.
• promote(tr) returns { promo: 'Q' } when the pawn lands on the last row, otherwise {}.
• In play, piece[0] + m.promo makes 'wQ' from 'wP'.

Your turn

1. In pseudoMoves, change the add line.
2. In the pawn block, add last and promote under dir, and pass promote(...) to the two add calls shown.
3. In play, change the board[tr][tc] = piece line.

Skills: Functions, Game states

← Pawns capture diagonally Is a square attacked? →

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