let rows;

let resolution = 20;

​

function setup() {

  createCanvas(400, 400);

  cols = width / resolution;

  rows = height / resolution;

}

​

function draw() {

  background(220);

  fill(255, 0, 0);

  for (i = 0; i < rows; i++) {

    for (j = 0; j < cols; j++) {

  rect(j*resolution, i*resolution, resolution, resolution);

    }

  }

}