var by;

var vx = 1.1;

var vy = 1.3;

var diam;

function setup() {

  createCanvas(600, 600);

  by = height / 2;

  bx = width / 2;

  diam = width / 20;

}

​

function draw() {

  background(0);

  strokeWeight(2);

  fill('white');

  noStroke();

  ellipse(bx, by, diam, diam);

  bx = bx + vx;

  by = by + vy;

  var off = diam  /2;

  if(bx > width - off || bx < off){

    vx *= -1;

  }

  if(by > height - off || by < off){

    vy *= -1;

  }

}