let xspeed = 1;

​

function setup() {

  createCanvas(400, 400);

  x = width / 2;

}

​

function draw() {

  background(250);

​

  x += xspeed;

​

  if (x <= 0 || x >= width) {

    xspeed *= -1;

  }

fill(0)

  ellipse(x, height / 2, 100, 100);

}

​