Workshop › Chess › Step 41 of 48

A score for a position

What we are doing

Now the computer will play black. First it needs to judge a position with a number: count material (pawn 100, knight 320, bishop 330, rook 500, queen 900) and add a little for pieces near the middle, where they control more squares. White's pieces count plus, black's minus.

```js
const VALUES = { P: 100, N: 320, B: 330, R: 500, Q: 900, K: 0 }

// Material, plus a little for pieces near the middle, from white's side: positive is good for white.
function evaluate() {
  let score = 0
  for (let r = 0; r < 8; r++) {
    for (let c = 0; c < 8; c++) {
      const piece = board[r][c]
      if (!piece) continue
      const center = piece[1] === 'K' ? 0 : 6 - Math.abs(3.5 - r) - Math.abs(3.5 - c)
      score += (VALUES[piece[1]] + center * 4) * (piece[0] === 'w' ? 1 : -1)
    }
  }
  return score
}
```

What it means

• Math.abs is the distance from zero: Math.abs(-2) is 2. |3.5 - r| + |3.5 - c| is how far the square is from the center (1 for the four middle squares, 7 for a corner), so center goes from 5 down to -1.
• Each piece adds its value plus center * 4, times 1 for white or -1 for black.

Your turn

1. Under DIAGONAL write VALUES.
2. Above const same = ... write the comment and evaluate, with an empty line after it.

Skills: Functions, Game states

← Show the last move Thinking ahead →

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