Workshop › Conway's Game of Life › Step 9 of 32

Four rules

What we are doing

One generation: every cell counts its neighbours and follows the rules. The new values go into a new grid, next, because all cells must change at the same moment.

```js
// Every cell changes at the same moment, so the next generation is built in a new grid.
function step() {
  const next = emptyGrid()
  for (let r = 0; r < ROWS; r++) {
    for (let c = 0; c < COLS; c++) {
      const n = countNeighbors(r, c)
      // A live cell survives with 2 or 3 neighbours; a dead cell comes alive with exactly 3.
      next[r][c] = n === 3 || (n === 2 && grid[r][c] === 1) ? 1 : 0
    }
  }
  grid = next
}
```

What it means

next starts empty; the loops fill it cell by cell, reading only from grid.
The four rules fit in one line: alive next time with 3 neighbours, or with 2 if alive now.
|| means "or", && "and". At the end grid = next makes the new generation the world.

Your turn

Write step under countNeighbors, after an empty line.

Skills: Game states, Loops

← Count the neighbours Draw again and again →

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