fixNav.js

const nav = document.querySelector('#nav');
const topOfNav = nav.offsetTop

function fixNav() {
  if(window.scrollY >= topOfNav) {
    document.body.classList.add('fixed-nav')
    document.body.style.paddingTop = nav.offsetHeight + 'px';
  } else {
    document.body.classList.remove('fixed-nav')
    document.body.style.paddingTop = 0;

  }
};

window.addEventListener('scroll', fixNav);