Workshop › Tic-tac-toe › Step 13 of 33

No playing on a taken cell

What we are doing

Right now play can overwrite a mark. It must refuse a cell that is not empty, and answer whether the move happened: true or false.

```js
function play(index) {
  if (board[index] !== '') return false
  board[index] = player
  player = player === 'X' ? 'O' : 'X'
  return true
}
```

What it means

• !== means "is not equal to". If the cell is not empty, return false answers "no move" and stops.
• Otherwise the move is made and return true answers "moved".

Your turn

1. In play, above board[index] = player, add the if line.
2. Under the line that switches player, add return true.

Skills: Game states

← Make a move Listen for clicks →

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