Vanilla JS – change/add class based on scroll position.

change-class-on-scroll.html

// https://codepen.io/cmykw/pen/gemxJm

// layout
<nav/>

// style
<style>
  body { min-height: 200vh; }

  nav {
    --ease: all 300ms ease;

    position: fixed;
    background: pink;
    height: 72px;
    width: 100%;
    opacity: 0.2;
    transition: var(--ease);
  }

  nav.fade-in { opacity: 1; }
</style>

// logic
<script>
  let scrollpos = window.scrollY
  const header = document.querySelector("nav")
  const header_height = header.offsetHeight

  const add_class_on_scroll = () => header.classList.add(".fade-in")
  const remove_class_on_scroll = () => header.classList.remove(".fade-in")

  window.addEventListener('scroll', function() { 
    scrollpos = window.scrollY;

    if (scrollpos >= header_height) { add_class_on_scroll() }
    else { remove_class_on_scroll() }

    console.log(scrollpos)
  })
</script>