JS for conditional fixing of a CSS element in a page

gistfile1.txt

JavaScript

$(document).scroll(function(){
var elem = $('.subnav');
if (!elem.attr('data-top')) {
    if (elem.hasClass('navbar-fixed-top'))
        return;
     var offset = elem.offset()
    elem.attr('data-top', offset.top);
}
if (elem.attr('data-top') <= $(this).scrollTop() )
    elem.addClass('navbar-fixed-top');
else
    elem.removeClass('navbar-fixed-top');
});


CSS

.subnav {
    background:#FFF;
}
.navbar-fixed-top {
    position:fixed;
}