test if scrollbars exist, and get width
var scrollDiv = document.createElement("div");
scrollDiv.className = "scrollbar-measure";
scrollDiv.style.cssText = 'width: 100px; height: 100px; overflow: scroll; position: absolute; top: -9999px;';
document.body.appendChild(scrollDiv);
// Get the scrollbar width
var scrollbarWidth = scrollDiv.offsetWidth - scrollDiv.clientWidth;
console.log('scrollbar: '+ scrollbarWidth); // Mac: 15
// Delete the DIV
document.body.removeChild(scrollDiv);