let rHeight = 0;

let x = rWidth;

let y = rHeight;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  rect(x,y,50,50);

  x = x+2;

  y = y+2;

  if(x >= width) {

    x = rWidth;

  }

  if(y >= height) {

    y = rHeight;

  }

}