Hacking out a simple "return to top"

scrollup.js

;(function($, window, document, undefined) {
    var totop;
    var show_at;
    var already_shown;
    var didscroll;
    var update_didscroll;
    var maybe_should_display;
    var move;

    /*
     * Skips back to the beginning of the document
     */
    move = function(evt) {
        var change_hash;
        evt.preventDefault();
        change_hash = function() {
            return window.location = '#moved-' + (+new Date).toString();
        };
        return $(document.body).stop().animate({scrollTop: 0}, 500, 'linear', change_hash);
    };
    totop = $('.return-to-top').eq(0);
    totop.bind('click', move);


    /*
     * Decides whether or not the return to top link should be visible
     */
    show_at = totop.outerHeight();
    already_shown = false;
    function should_display() {
        var current_pos;
        var should_fadein;
        var should_fadeout;

        current_pos = $(window).scrollTop();
        should_fadein = current_pos > show_at && already_shown === false;
        if (should_fadein === true) {
            totop.fadeIn(500);
            already_shown = true;
            return true;
        }

        should_fadeout = current_pos < show_at && already_shown === true;
        if (should_fadeout === true) {
            totop.fadeOut(500);
            already_shown = false;
            return false;
        }
    }
    $(document).ready(should_display);


    // see http://ejohn.org/blog/learning-from-twitter/
    didscroll = false;
    update_didscroll = function() {
        didscroll = true;
    };
    $(window).scroll(update_didscroll);

    maybe_should_display = function() {
        if (didscroll === true) {
            didscroll = false;
            return should_display.call(this);
        }
        return false;
    };
    setInterval(maybe_should_display, 500);
})(jQuery, window, document);