Get Scrollbar Size

GetScrollbarWidth.js

var getScrollBarSize = function () {
    var body = document.body,
        size,
        defaultOverflow = document.body.style.overflow,
        dummy = document.createElement("div"), 
        dummyInner = document.createElement("div");

    dummy.style.border = dummy.style.margin = dummy.style.padding =
      dummyInner.style.border = dummyInner.style.margin = dummyInner.style.padding = "0";

    dummy.style.width = "100px";
    dummy.style.height = "100px";
    dummy.style.position = "absolute";
    dummy.style.top = "-1000px";
    dummy.style.overflow = "scroll";

    dummyInner.style.width = "200px";
    dummyInner.style.height = "200px";

    dummy.appendChild(dummyInner);

    body.style.overflow = 'hidden';
    body.appendChild(dummy);

    size = 100 - dummy.clientWidth;

    body.removeChild(dummy);
    body.style.overflow = defaultOverflow;
    
    return size;
}