initial draft of a scroll spy like in twitter by arian. to be made more like js garden one.

bootstrap-scrollspy.js

var BootstrapScrollSpy = new Class({

    initialize: function(element, wrapper){
        element = this.element = document.id(element);
        this.wrapper = wrapper || window;

        var links = this.links = element.getElements('a');
        var elements = this.elements = [];

        links.each(function(el){
            var href = el.get('href'), target;
            if (href.slice(0, 1) == '#') target = document.id(el.get('href').slice(1));
            elements.push(target);
        });
        
        this.attach();
    },
    
    attach: function(){
        if (!this.boundScroll) this.boundScroll = this.scroll.bind(this);
        this.wrapper.addEvent('scroll', this.boundScroll);
    },
    
    detach: function(){
        if (this.boundScroll) this.wrapper.removeEvent('scroll', this.boundScroll);
    },

    scroll: function(){
        var top = this.wrapper.getScroll().y;
        var last, index;
        this.elements.some(function(el, i){
            if (!el) return false; // skip empty entry
            var y = el.getPosition(this.wrapper == window ? document.body : this.wrapper).y;
            if (y < top) index = i;
            return y >= top;
        }, this);

        if (index != this.active){
            if (this.active != null) this.links[this.active].removeClass('active');
            this.active = index;
            if (index != null) this.links[index].addClass('active');
        }

    }
    
});

// var scroll = new BootstrapScrollSpy('navbar');

https://github.com/BonsaiDen/JavaScript-Garden/blob/master/site/javascript/garden.js