scrollbar.js

$(document).ready(function () {
    var winHeight = $(window).height(),
        docHeight = $(document).height(),
        progressBar = $('.wpuprogress'),
        max, value;

    max = docHeight - winHeight;

    $(document).on('scroll', function(){
        value = $(window).scrollTop();
        progressBar.css('width', value / max * 100 + '%' );
    });

    $(window).on('resize', function() {
        winHeight = $(window).height();
        docHeight = $(document).height();

        max = docHeight - winHeight;
        progressBar.css('width', value / max * 100 + '%');
    });
});