var diam;

var vx;

var vy;

​

function setup() {

  createCanvas(600, 600);

  bx = width/2;

  by = height/2;

  diam = 50;

  vx = 5.3;

  vy = 6.6;

}

​

function draw() {

  background('black');

  noStroke();

  strokeWeight(2);

  ellipseMode(CENTER);

  ellipse(bx,by, diam, diam);

  bx = bx + vx;

  by = by + vy;

  var rad = diam/2;

  if(bx + rad > width){

    vx = 0-vx;

  }

  if(bx - rad < 0){

    vx = 0-vx;

  }

  if(by + rad > height){

    vy = 0-vy;

  }

  if(by - rad < 0){

    vy = 0-vy;

  }

}