Workshop › Tic-tac-toe › Step 29 of 33

Rule three: block the opponent

What we are doing

If the computer cannot win, it should stop you from winning. That is the same question with X: which free cell would complete a line for X? Take it.

```js
return completes('O') ?? completes('X') ?? (cells[4] === '' ? 4 : random)
```

What it means

• The rules are tried in order: win, else block, else center, else random.

Your turn

In the return line, add completes('X') ?? after completes('O') ?? .

Skills: Functions

← Rule two: win when you can Score a finished game →

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