addClassOnScroll.js

var didScroll = false;
var html = document.body.parentNode;
var scrollOffset = 500;
var scrollingClass = 'scrolling';


window.onscroll = addClassToHtml;

function addClassToHtml() {
    didScroll = true;
}

setInterval(function() {

    if(didScroll) {

        didScroll = false;

        var scroll = window.pageYOffset;

        if (scroll >= scrollOffset) {
            html.classList.add(scrollingClass);
        }
        else {
            html.classList.remove(scrollingClass);
        }
    }
}, 100);