Fixed nav fade out on scroll-down, fade in on scroll up, vanilla js

scroll.js

// based on the jquery version of https://medium.com/@mariusc23/hide-header-on-scroll-down-show-on-scroll-up-67bbaae9a78c

var didScroll;
var lastScrollTop = 0;
var delta = 8; // to prevent 'accidental' trigger
var navbarHeight = 85; // probably in css
var navbarElem = document.querySelector('.navbar');

window.addEventListener('scroll', function (ev) {
  didScroll = true;
});

setInterval(function() {
  if (didScroll) {
    hasScrolled();
    didScroll = false;
  }
}, 300); // debounce

function hasScrolled() {
  var st = window.scrollY;

  if (Math.abs(lastScrollTop  -  st) <= delta)
    return;

  if (st > lastScrollTop && st > navbarHeight) {
    navbarElem.classList.add('is-up');
  } else {
    if(st - window.innerHeight < window.outerHeight) {
      navbarElem.classList.remove('is-up');
    }
  }
  lastScrollTop = st;
}