Get the scrollbar width.

scrollbarWidth.js

/**
 * Get the scorllbar's width.
 * Note: This script only works on the default zoom level.
 * Ref: https://github.com/twbs/bootstrap/blob/94b4076dd2efba9af71f0b18d4ee4b163aa9e0dd/js/modal.js#L277-L284
 */
function scrollbarWidth() {
  var scrollDiv = document.createElement('div');
  scrollDiv.style.cssText = 'position:absolute;left:-9999px;width:50px;height:50px;overflow:scroll;';
  document.body.appendChild(scrollDiv);
  var width = scrollDiv.offsetWidth - scrollDiv.clientWidth;
  document.body.removeChild(scrollDiv);
  return width;
}