function to hide a nav on scroll down, then show it on scroll up

hidenav.js

function hideNav(el, howMuch) {
    var prevScrollpos = window.pageYOffset;

    window.addEventListener('scroll', function () {
        var currentScrollPos = window.pageYOffset;
        if (prevScrollpos > currentScrollPos) {

            el.style.transform = 'translateY(0%)'
        } else {
            el.style.transform = 'translateY(' + howMuch + '%)'
        }
        prevScrollpos = currentScrollPos;
    });
}