How to add and remove class on scroll

  <script>
    $(window).scroll(function() {   
    var scroll = $(window).scrollTop();   
    if (scroll >= 5) {
        $(".navbar").addClass("smallheader");
    }else {
        $(".navbar").removeClass("smallheader");
    }
});
  </script>

How to set div size according to background image

<script>
var img = new Image();
img.src = 'images/jquery.jpg';
$(window).on("resize", function () {
    $('.classname').height($('.classname').width() * img.height / img.width);
}).resize();
</script>