Workshop › Connect Four › Step 29 of 38

Try a move and take it back

What we are doing

Now a computer opponent. Its first trick: try a move and take it back. Put a disc on the board, ask wins4, remove it again. The computer imagines the move with the same code the real game uses.

```js
// Would dropping in this column win for `who`? Try it, look, and take it back.
function winsWith(col, who) {
  const row = dropRow(col)
  if (row === -1) return false
  board[row][col] = who
  const result = wins4(row, col) !== null
  board[row][col] = 0
  return result
}
```

What it means

• A full column cannot win.
• The disc is put in, wins4 is asked, and the cell is emptied again before returning the answer.

Your turn

Above function colAt( write the comment and winsWith, followed by an empty line.

Skills: Game states, Functions

← Play again Win, or block →

All steps

1. A dark background
2. A drawing function
3. The blue board
4. One hole
5. A whole row
6. All 42 holes
7. The board in memory
8. Discs in their colors
9. Draw every frame
10. Where does the disc land?
11. Play a column
12. Taking turns
13. Click to play
14. The next disc
15. Whose turn?
16. Playing with the keyboard
17. Keys 1 to 7
18. A disc on its way
19. Gravity
20. Watch it fall
21. Inside the board?
22. Walking along a line
23. Four in a row?
24. We have a winner
25. Circle the four
26. Who won?
27. A draw
28. Play again
29. Try a move and take it back
30. Win, or block
31. Don't help the player
32. Prefer the middle
33. The computer's turn
34. Only on your turn
35. You and the computer
36. Counting wins
37. Show the score
38. Build it yourself: the loser starts