Workshop › Tic-tac-toe › Step 32 of 33

A computer that never loses

What we are doing

computerMove no longer needs rules. For every free cell it places O, asks score how that ends with perfect play, undoes it, and keeps the best cell.

```js
function computerMove(cells) {
  let best = -1
  let bestScore = -Infinity
  for (let index = 0; index < 9; index++) {
    if (cells[index] !== '') continue
    cells[index] = 'O'
    const value = score(cells, 'X')
    cells[index] = ''
    if (value > bestScore) {
      bestScore = value
      best = index
    }
  }
  return best
}
```

What it means

• best is the best cell so far, bestScore its score. -Infinity is smaller than any number, so the first cell tried always becomes the best so far.
• For each free cell: try O, score it with X to move, undo.
• value > bestScore keeps the cell only if it is better than every one before.

Your turn

Delete the whole old computerMove (from its first line to its closing }) and write the new one in its place. The click listener stays the same.

Skills: Functions

← Look ahead: minimax Build it yourself: keep score →

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