initFalling.js

const initFalling = ball => {
  /* Constants */
  const ballHeight = 100
  const acceleration = 9.8 / 60;
  const { innerHeight } = window;

  /* Variable to denote the speed of the ball */
  let fallingSpeed = 0;
  const animateFall = () => {
    const top = parseInt(ball.style.top);
    const newTop = `${top + fallingSpeed}px`;

    /* To break the fall, when the ball is near the surface */
    if (parseInt(newTop) >= innerHeight - ballHeight) {
      ball.style.top = this.innerHeight - ballHeight + "px";
      ball.style.background = "red";
      return null;
    }
    
    /* Else set the top to the new value */
    ball.style.top = newTop;
    fallingSpeed = fallingSpeed + acceleration;
    requestAnimationFrame(animateFall);
  };

  /* Fire the first animation */
  requestAnimationFrame(animateFall);
};