Add Class in Section on Scroll

custom.js

jQuery( document ).ready(function($) {
var sections = $('.animateiT'),
  nav = $('.jump-left'),
  nav_height = nav.outerHeight();
$(window).on('scroll', function() {
  var cur_pos = $(this).scrollTop();
  sections.each(function() {
    var top = $(this).offset().top - 300;
    var bottom = top + $(this).outerHeight();
    if (cur_pos >= top && cur_pos <= bottom) {
      nav.find('a').removeClass('animated-div');
      sections.removeClass('animated-div');
      $(this).addClass('animated-div');
      nav.find('a[href="#' + $(this).attr('id') + '"]').addClass('animated-div');
    }
  });
});

});