let y = 0;

let GRID_SIZE = 20;

​

function setup() {

  createCanvas(400, 400);

  background(255);

  // strokeWeight(0);

}

​

function draw() {

  // if (random(1) > 0.5) {

  //    strokeWeight(4); 

  // } else {

  //   strokeWeight(1);

  // }

  if (random(1) > 0.5) {

    // fill(255);

    line(x, y, x+GRID_SIZE,y+GRID_SIZE);

  } else {

    // fill(0); 

    line(x, y+GRID_SIZE, x+GRID_SIZE,y);

  }

  // rect(x, y, GRID_SIZE, GRID_SIZE);

  x+=GRID_SIZE;

  if (x > width) {

    x = 0;

    y+=20;

  }

  if (y > height) {

    background(255);

    x = 0;

    y = 0;

  }

}