Add CSS class on scroll

scroll-class.js

$(function(){
  var shrinkHeader = 10;
  $(window).scroll(function() {
    var scroll = getCurrentScroll();
    if ( scroll >= shrinkHeader ) {
      $('body').addClass('scrolled');
    }
    else {
      $('body').removeClass('scrolled');
    }
  });
  function getCurrentScroll() {
    return window.pageYOffset || document.documentElement.scrollTop;
  }
});