let x = (-rSize)/2;

let y = (-rSize)/2;

function setup() {

  createCanvas(400, 500);

    rectMode(CENTER);

    background(220);

​

}

​

function draw() {

    background(220);

    rect(x,y,rSize,rSize);

    x = x + 3;

    if (x >= width + rSize/2){

        x = -rSize/2;

    }

    y = y + 3;

    if (y >=width + rSize/2){

        y = -rSize/2;

    }

}