Fixed header on scroll

gistfile1.js

(function () {
    var el = document.querySelector('#header'),
        className = 'fixed',
        offset = (el.offsetTop + el.offsetHeight),
        scrollTop = function () {
            return document.body.scrollTop || document.documentElement.scrollTop;
        };

    if (scrollTop() > offset) {
        el.className = className;
    }

    window.onscroll = function() {
        if (scrollTop() > offset) {
            if (!el.className.length) {
                el.className = className;
            }
        } else  {
            if (el.className.length) {
                el.className = '';
            }
        }
    };
}());