JavaScript を使用してブラウザのスクロールバーのサイズを決定するにはどうすればよいですか?

function getScrollBarWidth () {
  var inner = document.createElement('p');
  inner.style.width = "100%";
  inner.style.height = "200px";

  var outer = document.createElement('div');
  outer.style.position = "absolute";
  outer.style.top = "0px";
  outer.style.left = "0px";
  outer.style.visibility = "hidden";
  outer.style.width = "200px";
  outer.style.height = "150px";
  outer.style.overflow = "hidden";
  outer.appendChild (inner);

  document.body.appendChild (outer);
  var w1 = inner.offsetWidth;
  outer.style.overflow = 'scroll';
  var w2 = inner.offsetWidth;
  if (w1 == w2) w2 = outer.clientWidth;

  document.body.removeChild (outer);

  return (w1 - w2);
}