scrollbarWidth.js

function getScrollbarWidth() {
        if(width !== undefined) {
            return width;
        }

        var outer = document.createElement("div");

        outer.style.visibility = "hidden";
        outer.style.width = "100px";
        outer.style.msOverflowStyle = "scrollbar"; // needed for WinJS apps

        document.body.appendChild(outer);

        var widthNoScroll = outer.offsetWidth;
        // force scrollbars
        outer.style.overflow = "scroll";

        // add innerdiv
        var inner = document.createElement("div");
        inner.style.width = "100%";
        outer.appendChild(inner);

        var widthWithScroll = inner.offsetWidth;

        // remove divs
        outer.parentNode.removeChild(outer);

        width = widthNoScroll - widthWithScroll;

        return width;
    }