let x = 20;

let speed = 0.001;

let ac = 0.0001;

​

function setup() {

  createCanvas(710, 400);

  background(0); 

  noStroke();

  ellipseMode(RADIUS);

  frameRate(10)

​

}

​

function draw() {

  background(0);

    drawGradient(width/2, height / 2);

​

}

​

function drawGradient(f, y) {

​

  let h = 0;

   if (x >= 20 && x < 200) {

​

    speed = speed

    ac++

​

  } else {

    speed = -speed

    ac = -(ac++)

​

  }

​

  for (let r = x; r > 0; --r) {

    fill(255, 255, 255, h);

    ellipse(f, y, r, r);

    h = h +0.2    

  }

    x = x + speed + ac

}