Workshop › Conway's Game of Life › Step 24 of 32

Click a cell

What we are doing

Now a click (or a tap) flips the cell under the pointer: dead becomes alive, alive becomes dead.

```js
canvas.addEventListener('pointerdown', (event) => {
  const cell = cellAt(toCanvas(event))
  if (!cell) return
  grid[cell.r][cell.c] = grid[cell.r][cell.c] ? 0 : 1
})
```

What it means

• pointerdown fires when a mouse button or a finger goes down on the canvas.
• cellAt(toCanvas(event)) turns the event into a cell; off the grid we return and do nothing.
• The last line writes the opposite of the cell: 1 becomes 0, 0 becomes 1.

Your turn

Write the listener under cellAt, above function update() {, with an empty line after it.

Skills: Keyboard and mouse input

← Which cell? Paint or erase? →

All steps

1. The board
2. Measure the grid
3. One living cell
4. A function that draws
5. The world in memory
6. Draw every living cell
7. A random soup
8. Count the neighbours
9. Four rules
10. Draw again and again
11. Next generation with N
12. Count the generations
13. Show the generation
14. A world without edges
15. Count the living
16. A generation every few frames
17. Let it live
18. Running or not
19. One place for the buttons
20. Space to play, N to step
21. Clear and shuffle
22. Where is the pointer?
23. Which cell?
24. Click a cell
25. Paint or erase?
26. Drag to paint
27. Patterns from text
28. The glider gun
29. Fire the gun
30. Buttons on the screen
31. Tap the buttons
32. Build it yourself: a world at rest