Workshop › Sokoban › Step 25 of 29

Undo

What we are doing

undo() takes the last photo off history and puts everything back as it was. The Z key calls it.

```js
function undo() {
  const previous = history.pop()
  if (!previous) return
  ;({ player, boxes, moves } = JSON.parse(previous))
}

  if (event.key.toLowerCase() === 'z') undo()
```

What it means

• history.pop() removes and gives back the last photo, or undefined if there is none; then we return.
• JSON.parse turns the text back into an object, and ({ player, boxes, moves } = ...) puts its three values back into the three variables. The ; and the parentheses are needed because a line cannot start with { here.
• toLowerCase() makes Z (Caps Lock) work too.

Your turn

1. Under the move function, leave an empty line and write undo.
2. In the listener, under the arrow if block, write the z line. Press Run.

Skills: Arrays and objects, Game states

← A photo before every move Restart →

All steps

1. A level written as text
2. The canvas and the pen
3. The warehouse floor
4. A function that draws
5. A name for every tile
6. Read the walls
7. Draw the walls
8. Center the level
9. The goals
10. The boxes
11. The player
12. Two more levels
13. One step
14. Walk with the arrows
15. Boxes are in the way
16. Push a box
17. A box on a goal turns green
18. Solved!
19. On to the next level
20. Which level?
21. A line at the bottom
22. Count the moves
23. A photo of the game
24. A photo before every move
25. Undo
26. Restart
27. A record for every level
28. Show the record
29. Build it yourself: count the pushes