let ball = document.body.appendChild(document.createElement('div'));
Object.assign(ball.style, {
  position: 'absolute',
  background: 'red', 
  width: '100px', height: '100px', 
  borderRadius: '50%'
});

let x = -100;
let y = window.innerHeight / 5;
let vx = 3;
let vy = -3;
let grav = .3;
var decay = 0.5;

function loop() {
  let floor = window.innerHeight - 100;
  x += vx;
  y += vy;
  
  vy += grav;
  if (y > floor) {
    vy *= -decay;
    vx *= decay;
    y = floor;
  }
  
  ball.style.transform = `translate(${x}px, ${y}px)`;
  window.requestAnimationFrame(loop);
}
loop();