let velocityX, velocityY;

function setup() {

  createCanvas(windowWidth, windowHeight);

  locationX = width/2.0;

  locationY = height/2.0;

  velocityX = random(-10, 10);

  velocityY = random(-10, 10);

}

​

function draw() {

  background(0);

  fill(31, 127, 255);

  noStroke();

  circle(locationX, locationY, 40);

  locationX = locationX + velocityX;

  locationY = locationY + velocityY;

  if(locationX < 0 || locationX > width){

    velocityX = velocityX * -1.0;

  }

  if(locationY < 0 || locationY > height){

    velocityY = velocityY * -1.0;

  }

}