​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(0);

  fill(200, 0, 100);

  rect(rectX, 200, 50, 50);

  // Move the rectangle 

  rectX+=4;

  // When the rectangle reaches the edge of the canvas,

  // move it back to the left 

  if (rectX > width){

    rectX = -50;  // -50 so it appears nicely 

  }

  // For debugging

  //console.log(rectX);

}