Workshop › Conway's Game of Life › Step 25 of 32

Paint or erase?

What we are doing

Next we want to drag and paint a whole line. First we remember what the press decided: painting holds the value being painted, 1 when you pressed on a dead cell and 0 (erase) when you pressed on a live one.

```js
let painting = null // while the pointer is down: the value being painted, 1 or 0

// Pressing on a dead cell paints live cells as you drag; pressing on a live one erases.
canvas.addEventListener('pointerdown', (event) => {
  const cell = cellAt(toCanvas(event))
  if (!cell) return
  painting = grid[cell.r][cell.c] ? 0 : 1
  grid[cell.r][cell.c] = painting
})
```

What it means

painting starts as null: nothing is being painted.
On a press it becomes the opposite of the pressed cell, and that value is written into the cell.
A click still flips a cell, as before; the difference is that the choice is now remembered.

Your turn

Under let frames write the painting line.
Above the pointerdown listener write the comment; inside it, replace the last line with the two painting lines.

Skills: Game states

← Click a cell Drag to paint →

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