nav bar follows page scroll

gistfile1.js

window.onload = function()
{
    var headers = document.querySelectorAll('#docs h2, #guide h1');
    var menu = document.getElementById('menu');
    var init = menu.offsetTop;
    var docked;
    window.onscroll = function ()
    {
        if (!docked && (menu.offsetTop - scrollTop() < 0))
        {
            menu.style.top = 0;
            menu.style.position = 'fixed';
            menu.className = 'docked';
            docked = true;
        }
        else if (docked && scrollTop() <= init)
        {
            menu.style.position = 'absolute';
            menu.style.top = init + 'px';
            menu.className = menu.className.replace('docked', '');
            docked = false;
        }
    };
}