var y = 200;

​

function setup() {

  createCanvas(400, 400);

  frameRate(5);

}

​

function draw() {

  noStroke();

  background(0, 0, 0, 10);

  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;

  }

}