​

let val = 255;

let r = 0;

​

function setup() {

  createCanvas(400, 400);

  noStroke();

  frameRate(10);

}

​

function draw() {

  translate(width/2,height/2);

  background(51);

  fill(255, val);

  ellipse(0, 0, r);

  fill(51);

  ellipse(0,0,r-r/4);

  val -= 3;            //val decreases the alpha

  r += 4;              //r increases the circle radius

  if (r >= 400) {      //resetting the sketch.

    val = 255;

    r = 0;

  }

}