A scroll to jQuery plugin

donutScroll.jquery.js

/** A "scroll to" plugin.
    Scrolls to the first element in the jQuery object or to the passed 
    position.
    @author Donovan Mueller (zotobi@gmail.com)
    @url https://gist.github.com/1885908
    ------------------------------------------------------------------------ */
;(function donutScrollPlugin($) {
   var viewport = ($.browser.opera) ? 'html' : 'html, body'
                  // Opera jumps to the top when 'html,body' is used.

   $.fn.donutScroll = function(y, callback)
   {
      var docheight = $(document).height()
        , winheight = $(window).height();
        
      if (typeof y === 'undefined' || y === null) y = this.offset().top;
      if (y < 0) y = 0;
      
      // check that we aren't trying to scroll past what is visible
      // otherwise the animation will be cut short and wont be as pretty
      if ((docheight - y) < winheight) {
         y = docheight - winheight;
      }
   
      // Wheeeeee!
      $(viewport).stop().animate( {scrollTop: y}
                                , { duration: 600
                                  , easing:   'easeOutExpo'
                                  , complete: callback } );
   }; // $.fn.donutScroll()
})(jQuery); // (donutScrollPlugin)()