let xspeed= 5

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  // x++;

  // x--;

  x+=xspeed;

  fill(x/(width/255));

  if(x> width || x < 0) xspeed *= -1;

​

  ellipse (x, height/2,20,20)

}