getScrollbarWidth.js

var scrollbarWidth;
function getScrollbarWidth() {
  if (scrollbarWidth) {
    return scrollbarWidth;
  }
  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);
  scrollbarWidth = (w1 - w2);
  return scrollbarWidth ;
}