Element sticky fixed

index.html

<p style="margin-bottom:100px;">Scroll this page.</p>
<div class="sticky"><h3>Super amazing header</h3></div>

script.js

var sticky = document.querySelector('.sticky');
var origOffsetY = sticky.offsetTop;
document.addEventListener('scroll', function(e) {
  window.scrollY >= origOffsetY ? sticky.classList.add('fixed') :
                                  sticky.classList.remove('fixed')
});

style.css

.fixed { position: fixed; top: 0; width: 100%; }