main.js

function make2DArray(cols, rows) {
  let arr = new Array(cols)
  for (let i = 0; i < arr.length; i++) {
    arr[i] = new Array(rows)
    for (let j = 0; j < arr[i].length; j++) {
      arr[i][j] = 0;
    }
  }
  return arr
}

let grid;
let w = 10;
let cols, rows;

function setup() {
  createCanvas(400, 400);
  cols = width/w;
  rows = height/w;
  grid = make2DArray(cols, rows);
  
  for (let i = 0; i < cols; i++) {
    for(let j = 0; j < rows; j++) {
      grid[i][j] = 0;
    }
  }

}

function mouseDragged() {
  let col = floor(mouseX / w);
  let row = floor(mouseY / w);
  grid[col][row] = 1;
}

function draw() {
  background(0);
  for(let i = 0; i < cols; i++) {
    for (let j = 0; j < rows; j++) {
      stroke(255)
      fill(grid[i][j]*255);
      let x = i * w;
      let y = j * w;
      square(x, y, w);
    }
  }
  
  let nextGrid = make2DArray(cols, rows);
  for(let i = 0; i < cols; i++) {
    for (let j = 0; j < rows; j++) {
      let state = grid[i][j];
      if (state === 1) {
        let below = grid[i][j + 1];
        let belowLeft = grid[i - 1][j + 1];
        let belowRight = grid[i + 1][j + 1]
        if (below === 0) {
          nextGrid[i][j + 1] = 1;
        } else if (belowLeft === 0) {
            nextGrid[i - 1][j + 1] = 1;
        } 
        else if (belowRight === 0) {
          nextGrid[i + 1][j + 1] = 1;
        } 
        else {
          nextGrid[i][j] = 1;
        }
      }
    }
  }
  grid = nextGrid;
}