Workshop › Klondike Solitaire › Step 39 of 40

Undo

What we are doing

undo takes the last snapshot off the stack and turns it back into piles. A stack is exactly right: the last thing done is the first thing undone. The Z key calls it.

```js
function undo() {
  if (!history.length || won) return
  piles = JSON.parse(history.pop())
  moves += 1
}

  if (event.key === 'z' || event.key === 'Z') undo()
  else if (event.key === ' ') flipStock()

  ctx.fillText('Best ' + (best ? best + 's' : '-') + '  Z undo', canvas.width - LEFT, 24)
```

What it means

• Nothing to undo, or a won game: do nothing.
• history.pop() takes the last snapshot off; JSON.parse turns the text back into brand new arrays and cards. Even a card that was turned face up comes back face down, with no special code.
• Undoing counts as a move.

Your turn

1. Under save, leave an empty line and write undo.
2. In the keydown listener, write the Z line first and turn the Space line's if into else if.
3. Add + ' Z undo' to the Best text.

Skills: Arrays and objects, Game states

← Snapshots of the table Build it yourself: finish by itself →

All steps

1. The green table
2. Draw one card
3. The face of a card
4. Red and black
5. A deck of 52
6. Shuffle
7. Piles in one object
8. Deal the seven columns
9. A place for every pile
10. Put the piles on the table
11. Fan out the columns
12. Outlines for empty places
13. Turn over a card
14. Space turns a card
15. Go through the stock again
16. What is under the pointer?
17. Hitting a card in a column
18. Tap the stock
19. Rule 1: building a column
20. Rule 2: building a foundation
21. Move cards by the rules
22. Turn over the uncovered card
23. Find where a card goes
24. Pick up a card
25. Tap to move
26. A move counter
27. Count the moves
28. Where did you grab it?
29. Follow the pointer
30. Draw the held cards at the pointer
31. Drop it where you want
32. Drop a king into an empty column
33. A clock
34. Winning
35. You won!
36. A new game
37. The best time
38. Snapshots of the table
39. Undo
40. Build it yourself: finish by itself