JS add class after scroll

js-add-class-after-scroll.js

let scrollpos = window.scrollY
const header = document.querySelector("nav")
const header_height = header.offsetHeight

const add_class = () => header.classList.add("fade-in")
const remove_class = () => header.classList.remove("fade-in")

window.addEventListener('scroll', function() { 
  scrollpos = window.scrollY;
  
  if (scrollpos >= header_height) { 
    add_class()
  } else { 
    remove_class() 
  }
    console.log(scrollpos) // optional
  })