Workshop › Tic-tac-toe › Step 30 of 33

Score a finished game

What we are doing

The rules have a hole: play a corner, then the opposite corner, and you can set a trap. Instead of more rules, the computer will look ahead at every possible future. First it needs to score a finished game from O's point of view.

```js
// How good `cells` is for O if both sides play perfectly: 1 = O wins, -1 = X wins, 0 = draw.
function score(cells, turn) {
  const end = outcome(cells)
  if (end === 'O') return 1
  if (end === 'X') return -1
  if (end === 'draw') return 0
}
```

What it means

• score(cells, turn) takes a board and whose turn it is (turn is used in the next step).
• For a finished game it returns 1, -1 or 0. For a game still going it returns nothing yet.

Your turn

Above function computerMove, write the comment and score, with an empty line after it. Press Run.

Skills: Functions

← Rule three: block the opponent Look ahead: minimax →

All steps

1. Get the canvas and the pen
2. Paint the board
3. The first grid line
4. Repeat with a loop
5. Finish the grid
6. The board as a list
7. A function that draws
8. Write an X
9. From cell number to position
10. Draw what the board holds
11. A color for each mark
12. Make a move
13. No playing on a taken cell
14. Listen for clicks
15. Which cell was clicked?
16. Clicks on any screen size
17. The eight winning lines
18. Find three in a row
19. Win, draw or go on
20. The round ends
21. Show who won
22. Light up the winning line
23. A function for a fresh round
24. Click to play again
25. A computer that picks a cell
26. The computer answers
27. Rule one: take the center
28. Rule two: win when you can
29. Rule three: block the opponent
30. Score a finished game
31. Look ahead: minimax
32. A computer that never loses
33. Build it yourself: keep score