Add class on scroll vanilla.js

on-scroll.js

(function () {
  let scrollPosition = window.scrollY;
  let header = document.querySelector('header');

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

    if (scrollPosition >= 30) {
      header.classList.add('scrolled');
    } else {
      header.classList.remove('scrolled');
    }
  });      
})();