Plain Javascript on scroll event

onScroll.js

var getScroll = function() {
    if (window.pageYOffset != undefined) {
        return [pageXOffset, pageYOffset];
    } else {
        var sx, sy, d = document,
            r = d.documentElement,
            b = d.body;
        sx = r.scrollLeft || b.scrollLeft || 0;
        sy = r.scrollTop || b.scrollTop || 0;
        return [sx, sy];
    }
}
window.onscroll = function(e) {
    var iHScroll = getScroll()[1];
    if (iHScroll > 20) {
        $('nav').addClass('active'); //example
    } else {
        $('nav').removeClass('active'); //example
    }
}