Workshop › Tic-tac-toe › Step 25 of 33

A computer that picks a cell

What we are doing

Now the computer will play O. Its first version is simple: make a list of the free cells and pick one at random.

```js
function computerMove(cells) {
  const free = []
  cells.forEach((cell, index) => {
    if (cell === '') free.push(index)
  })
  const random = free[Math.floor(Math.random() * free.length)]
  return random
}
```

What it means

• free starts as an empty array; free.push(index) adds each empty cell's number to its end.
• Math.random() is a random number from 0 up to (not including) 1. Times free.length, rounded down, it is a valid position in free.
• The function returns the chosen cell number.

Your turn

Under outcome, leave an empty line and write computerMove (above function play). Press Run.

Skills: Arrays and objects, Functions

← Click to play again The computer answers →

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