Affix sidebar vanilla js

scroll-affix.js

  // element height that we want to scroll
  let elheight = document.getElementById("check-sidebar").offsetHeight

  window.addEventListener('scroll', (e) => {
    //last_known_scroll_position = window.scrollY;
		let scrollTop = window.pageYOffset;
		if( scrollTop > elheight ){
			document.getElementById('check-sidebar').style.top="40px";
			document.getElementById('check-sidebar').style.position="fixed";
		}
		else
		{
			document.getElementById('check-sidebar').style.position="relative";
			document.getElementById('check-sidebar').style.top="50px";
		}
  });