let xspeed = 10;

​

function setup() {

  createCanvas(600, 600);

}

​

function draw() {

  background(20);

  // draw an ellipse in the middle of the left edge of the canvas

  // make the ellipse move to the right side of the canvas

  // when the ellipse reaches the right side of the canvas,

  // it turns around and move back to the left side of the canvas

  // when it reaches the left edge of the canvas,

  // it turns around the move toward the right side of the canvas

  // forever

  ellipse(x, height / 2, 20);

  x += xspeed;

  if (x > width || x < 0) {

    xspeed = xspeed * -1;

  }

​

}