let y;

let d; // direction

let c; // colour

let r; // radius

let b; // background

let h; //height

​

function setup() {

  createCanvas(400, 400);

  x = 30

  y = 200

  d = +10

  c = -255

  r = 10

  b = 200

  h = +3

}

​

function draw() {

  background(0, b ,b);

  noStroke()

  fill(c)

  circle(x, y, r*2);

  if (x > r-1 && x < width-r) {

    c = c;

    d = d

  }

  else {

   c = -c;

   d = -d

  } 

   x = x+d

   y = y+h

}