var rad = 10;

var speed = 5;

​

function setup() {

  createCanvas(400, 400);

  x = width / 2;

  y = height / 2;

  var a = Math.random() * 100;

  vx = cos(a) * speed;

  vy = sin(a) * speed;

}

​

function draw() {

  background(250);

  fill(0)

  circle(x, y, rad);

  x += vx;

  y += vy;

  if(x >= width - rad || x <= rad) {

    x = constrain(x, rad, width - rad);

    vx = -vx;

  }

  if(y >= height - rad || y <= rad) {

    y = constrain(y, rad, width - rad);

    vy = -vy;

  }

}