TypeError in prevous developers code.

Hi,

I’m currently helping to rebuild a theme/website after a hack.
The original developer is nowere to be found & my knowledge of JQuery is limited.

I there is a back to top button, this one works. Next to it there is a button with the text ‘disable automatic scrolling’.

When you press that button only the text changes to ‘enable automatic scrolling’ and in my firebug I see a ” TypeError : jQuery(…).offset(…) is undefined “

which origins from:

var jump=function(e)
{
    //alert('here');
   if (e){
       //e.preventDefault();
       var target = jQuery(this).attr("href").replace('/', '');
   }else{
       var target = location.hash;
   }

   jQuery('html,body').stop(true, true).delay(1000).animate(
   {
       scrollTop: (jQuery(target).offset().top) - 0
   },1100,function()
   {
       //location.hash = target;
   });

}

First of all what is this script trying to do, and how do I make it work again?