対象となる要素がスクロールによって画面から消えたときに、.is-active を付与する

jquery_wsfixnav.js

/*
jquery_wsfixnav.js
--------------------
https://gist.github.com/webbingstudio/afe5e8056cdae8a4f1c62ac08404a3bd
Author: WebbingStudio
Released under the MIT license
http://opensource.org/licenses/MIT
 */
;(function($){
"use strict";

var WsFixNav = window.WsFixNav || {};

WsFixNav = (function() {

    function WsFixNav(element, options) {
        var
            t = this;

        t.initials = {
            et: 0
        };
        t.settings = $.extend({}, t.initials, options);

        t.init(element);
        t.run(element);

        window.onscroll = function() {
            t.run(element);
        };
        return false;

    }

    return WsFixNav;

}());

WsFixNav.prototype.init = function(element) {
    var
        t = this;

    t.et = $(element).position().top;
    return false;
};

WsFixNav.prototype.run = function(element) {
    var
        t = this,
        st;

    st = $(window).scrollTop();
    if (st > t.et) {
        $(element).addClass('is-active');
    } else {
        $(element).removeClass('is-active');
    }

    return false;
};

$.fn.WsFixNav = function() {
    var
        element = this,
        options = arguments[0],
        args = Array.prototype.slice.call(arguments, 1),
        i;
    for (i = 0; i < element.length; i++) {
        if (typeof options == 'object' || typeof options == 'undefined') {
            element[i].WsFixNav = new WsFixNav(element[i], options);
        }
    }
    return element;

};

$(function(){

    $('.js-fixnav').WsFixNav();

});

})(jQuery);

Usage:

HTML
<div class="globalnav js-fixnav">(navigation)</div>

CSS
.globalnav.is-active { position: fixed; top: 0; left: 0; width: 100%; z-index: 999; }