Extend jQuery with `scrollbarWidth` property.

$.scrollbarWidth.js

(function ($) {
    var scrollDiv = document.createElement("div");
    
    scrollDiv.style.width = "100px";
    scrollDiv.style.height = "100px";
    scrollDiv.style.position = "absolute";
    scrollDiv.style.top = "-9999px";
    scrollDiv.style.overflow= "scroll";
    
    document.body.appendChild(scrollDiv);
    
    // number without suffix "px"
    $.scrollbarWidth = scrollDiv.offsetWidth - scrollDiv.clientWidth;
    
    document.body.removeChild(scrollDiv);
})(jQuery);