sticky header on scroll

stickyHeader.js

var sections = $('section')
  , nav = $('nav')
  , nav_height = nav.outerHeight();

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