let x = -rSize/2; 

function setup() {

  createCanvas(400, 500);

  rectMode(CENTER);

}

​

function draw() {

  background(220);

  rect(x,height/2,rSize, rSize);

  x = x + 3;

  if(x >=width+ rSize/2) {

    x = -rSize/2;

  } 

}