scroll to fix navbar

scrollfixnavbar.js

$(document).ready(function() {
  var navpos = $('#mainnav').offset();
  console.log(navpos.top);
    $(window).bind('scroll', function() {
      if ($(window).scrollTop() > navpos.top) {
        $('#mainnav').addClass('fixed');
       }
       else {
         $('#mainnav').removeClass('fixed');
       }
    });
});