var y = 200;

function setup() {

  createCanvas(400, 400);

  frameRate(5);

}

​

function draw() {

  background(0,0,0,5);

  ellipse(x,y,20,20);

  var dir = (int(random(4)));

  if (dir === 0){

    x = x - 20;

  } else if (dir === 1) {

    x = x + 20;

  } else if (dir === 2) {

    y = y - 20;

  } else {

    y = y + 20;

  }

}