Question

Do you know any cross-browser methods?

Answer 1

window.scrollBarWidth = function() { 
  document.body.style.overflow = 'hidden';  
  var width = document.body.clientWidth; 
  document.body.style.overflow = 'scroll';  
  width -= document.body.clientWidth;  
  if(!width) width = document.body.offsetWidth - document.body.clientWidth; 
  document.body.style.overflow = '';  
  return width;  
}