Why use all this code (i've see in archive plugin) when u can simply use…

This little bit code? ??
<a href="#" class="scrollup">Scroll</a>

.scrollup{
    width:40px;
    height:40px;
    opacity:0.3;
    position:fixed;
    bottom:50px;
    right:100px;
    display:none;
    text-indent:-9999px;
    background: url('icon_top.png') no-repeat;
}

—

<script type="text/javascript">
    $(document).ready(function(){

        $(window).scroll(function(){
            if ($(this).scrollTop() > 100) {
                $('.scrollup').fadeIn();
            } else {
                $('.scrollup').fadeOut();
            }
        });

        $('.scrollup').click(function(){
            $("html, body").animate({ scrollTop: 0 }, 600);
            return false;
        });

    });
</script>

Source + demo: https://gazpo.com/2012/02/scrolltop/

https://www.ads-software.com/extend/plugins/jquery-smooth-scroll/