Workshop › Klondike Solitaire › Step 38 of 40

Snapshots of the table

What we are doing

Every good card game has undo. The idea is a stack of snapshots: before every change we save a copy of the whole table in history. The next step will take them back.

```js
let history // earlier positions, for undo

  history = []

const save = () => history.push(JSON.stringify(piles))

function flipStock() {
  save()

  if (!allowed) return false
  save()
```

What it means

• JSON.stringify(piles) turns the whole table into one long text: a frozen copy.
• Why not history.push(piles)? That would save the same object, which the next move changes anyway.
• save() is called at the start of flipStock, and in tryMove only once the move is allowed.

Your turn

1. Under let moves write let history ...; in deal, under moves = 0, write history = [].
2. Under canFound, leave an empty line and write save.
3. Call save() first thing in flipStock, and in tryMove under if (!allowed) return false.

Skills: Arrays and objects, Game states

← The best time Undo →

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