Uncaught TypeError: Cannot read property 'top' of undefined

Check if the jQuery object contains any element before you try to get its offset:

var nav = $('.content-nav');
if (nav.length) {
  var contentNav = nav.offset().top;
  ...continue to set up the menu
}

Your document does not contain any element with class content-nav, thus the method .offset() returns undefined which indeed has no top property.

You can see for yourself in this fiddle

alert($('.content-nav').offset());

(you will see "undefined")

To avoid crashing the whole code, you can have such code instead:

var top = ($('.content-nav').offset() || { "top": NaN }).top;
if (isNaN(top)) {
    alert("something is wrong, no top");
} else {
    alert(top);
}

Updated fiddle.

The problem you are most likely having is that there is a link somewhere in the page to an anchor that does not exist. For instance, let's say you have the following:

<a href="#examples">Skip to examples</a>

There has to be an element in the page with that id, example:

<div id="examples">Here are the examples</div>

So make sure that each one of the links are matched inside the page with it's corresponding anchor.

I know this is extremely old, but I understand that this error type is a common mistake for beginners to make since most beginners will call their functions upon their header element being loaded. Seeing as this solution is not addressed at all in this thread, I'll add it. It is very likely that this javascript function was placed before the actual html was loaded. Remember, if you immediately call your javascript before the document is ready then elements requiring an element from the document might find an undefined value.

I ran through similar problem and found that I was trying to get the offset of footer but I was loading my script inside a div before the footer. It was something like this:

<div> I have some contents </div>
<script>
  $('footer').offset().top;
</script>
<footer>This is footer</footer>

So, the problem was, I was calling the footer element before the footer was loaded.

I pushed down my script below footer and it worked fine!