var speed = 2;

​

function setup() {

  createCanvas(400, 400);

  background(200);

}

​

function draw() {

  background(200);

  var redValue = map(position, 0, width, 0, 255);

  fill(redValue, 100, 100);

    ellipse(position, height/2, 50, 50);

  position = position + speed;

  if (position > width) {

    speed = -2;

  } else if (position < 0) {

    speed = 2; 

  }

}

​

​