var d = 100; // diameter

const r = d/2;  // radius

let x = d / 2; // position on x axis

​

function setup() {

  createCanvas(600, 400);

}

​

function draw() {

  background(0);

  stroke(255);

  strokeWeight(4);

  noFill();

  ellipse(x, 200, d, d);

​

  if (x > width - r || x < 0 + r) {

    speed *= -1;

  }

  x += speed;

}