button go-top always stay o top of footer

got-top.js

    // Button go Top
    $(window).on('scroll' ,function (event) {
        var el = $(".region-footer");
        var elTop = el.offset().top;

        var windowTop = $(window).scrollTop();
        var windowHeight = $(window).height();
        var windowBottom = windowTop + windowHeight;

        var elVisible = windowBottom - elTop;

        if (windowBottom >= elTop) {
            $(".go-top").css({
              'position': 'absolute',
              'bottom': el.height()+ 10 +'px'
            });
        } else {
            $(".go-top").css({
              'bottom': '20px',
              'position': 'fixed',
            });
        }
      });