navigation highlight script difficulty

navigation highlight script difficulty

 - I have been playing around with the below peice of code for a while now 

  2. $(document).on("scroll", onScroll);
  3.     
  4.     //smoothscroll
  5.     $('.navbar-nav a[href^="#"]').on('click', function (e) {
  6.         e.preventDefault();
  7.         $(document).off("scroll");
  8.         
  9.         $('.navbar-nav li').each(function () {
  10.             $(this).removeClass('active');
  11.         })
  12.         $(this).addClass('active');
  13.       
  14.         var target = this.hash,
  15.            menu = target;
  16.         $target = $(target);
  17.         $('html, body').stop().animate({
  18.             'scrollTop': $target.offset().top+2
  19.         }, 500, 'swing', function () {
  20.             window.location.hash = target;
  21.             $(document).on("scroll", onScroll);
  22.         });
  23.     });

  25.     function onScroll(event){
  26.     var scrollPos = $(document).scrollTop();
  27.     $('.navbar-nav a').each(function () {
  28.         var currLink = $(this);
  29.         var refElement = $(currLink.attr("href"));
  30.         if (refElement.position().top <= scrollPos && refElement.position().top + refElement.height() > scrollPos) {
  31.             $('.navbar-nav li').removeClass("active");
  32.             currLink.addClass("active");
  33.         }
  34.         else{
  35.             currLink.removeClass("active");
  36.         }
  37.     });
  38. }  
  39. });

here is the code in a fiddle :  Jsfiddle

now if you have a look at the if condition in the onScroll function : 

       if (refElement.position().top <= scrollPos && refElement.position().top + refElement.height() > scrollPos) 

somehow this consition fails and the message i get in the console is 

    TypeError: refElement.position(...) is undefined

i get the same error in firefox and also in firebug . 

 getting that message would mean ,  the condition failed , but ironically the next line still execute I.E. 

     $('.navbar-nav li').removeClass("active"); 

how is that possible and why am i getting this error message ?