Question

First of all I did not write this code. I found it on somebody else’s website and I want to learn from it by trying it out myself. However I can’t make it work. I’ve googled for the code in case it’s a jQuery plugin that’s freely available or anything, but I can’t find it anywhere on the web.

I have my sidebar (with id #sidebar) and have given it the class “sticky”, I’ve included jQuery at the top of the page, and I’ve put this code in place in the head:

<!-- Floating sidebar jQuery --> 
        <script type="text/javascript"> 
            var Sticky = function( $obj, opts ){

               $(window).scroll( 
                  function(e){
                     Sticky.onScroll(e, $obj, opts );
                  });

            }
            Sticky.onScroll = function( e, $o, opts ){

               var iScrollTop = $(window).scrollTop();
               var sClass = "sticky";

               //set original data
               if( !$o.data(sClass) ){
                  $o.data(sClass, {css:{position:$o.css('position'),top:$o.css('top')}, offset:$o.offset()} );
               }
               var oOrig = $o.data(sClass);
               var bIsSticky = $o.hasClass(sClass);

               if( iScrollTop > oOrig.offset.top && !bIsSticky ){
                  $o.css({position:'fixed',top:0}).addClass(sClass);
               }else if(iScrollTop < oOrig.offset.top && bIsSticky){
                  $o.css(oOrig.css).removeClass(sClass);
               }   

            }

            Sticky( $('#sidebar') );

        </script> 

As you can see, the final JS line Sticky( $('#sidebar') ); fires on the #sidebar element. However, when you scroll down, this error is written to Chrome’s log:

Uncaught TypeError: Cannot read property ‘offset’ of undefined

Firebug is a bit more verbose, and says:

oOrig is undefined: if( iScrollTop >
oOrig.offset.top && !bIsSticky ){

I’m trying my best to understand this but can somebody help me see why it’s not working?

Thanks!

Jack

Answer 1

Wow, new answer…Thanks felix

wrap the function call in a ready function.

$(function() {
    Sticky($('#sidebar'));
});

The dom is most likely not ready when you call Sticky($('#sidebar')) so when .data is used to set data on $o it actual does nothing:

$o.data(sClass, {css:{position:$o.css('position'),top:$o.css('top')}, offset:$o.offset()} );].  

So when it gets the data on line:

var oOrig = $o.data(sClass);

it cannot actualy get the data.

This is because the dom elements are not ready to be manipulated because the dom is not ready yet.

OLD ANSWER (NOT RIGHT!)

$.offset is a function.

The problem in the line:

if( iScrollTop > oOrig.offset.top && !bIsSticky ){

is that: oOrig.offset is a function, not a variable. So oOrig.offset.top is not valid. Simply call the function and it will return a variable with the top property which you can access:

if( iScrollTop > oOrig.offset().top && !bIsSticky ){

Explination:

oOrig.offset is a reference to a function (the offset function in jquery).

You must call the function to access the .top property.