Workshop › Klondike Solitaire › Step 19 of 40

Rule 1: building a column

What we are doing

In a tableau column a card goes on a face-up card one rank higher and of the other color: a red 9 on a black 10. Only a king may start an empty column. canStack answers "may this card go on this pile?" and changes nothing.

```js
// Tableau: one lower, the other color; only a king goes into an empty column.
function canStack(card, pile) {
  const under = last(pile)
  if (!under) return card.rank === 13
  return under.up && under.rank === card.rank + 1 && isRed(under) !== isRed(card)
}
```

What it means

• under is the card it would go on. For an empty pile it is undefined, so !under is true: only a king.
• Three conditions joined with && must all be true: under is face up, one rank higher, and of the other color.
• isRed(under) !== isRed(card): two true/false values differ exactly when one card is red and the other is not.

Your turn

Under cardY, leave an empty line and write canStack with its comment.

Skills: Functions

← Tap the stock Rule 2: building a foundation →

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