let currentVal=0;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(currentVal);

  bounce(0,255);

  fill(currentVal,50,180);

  circle(width/2,height/2,currentVal);

  bounce(0,200);

}

​

function bounce(minVal,maxVal){

  currentVal += bounceRate;

  // currentVal =currentval+bounceRate;

  if(currentVal>=maxVal|| currentVal<=minVal){

  bounceRate=bounceRate*-1;

  // bounceRate *= -1;

  }

}