Edit in JSFiddle

window.addEventListener('scroll', function() {
	var scroll = window.scrollY;
  var sidebar = document.querySelector('#sidebar');
  sidebar.style.transform = 'translateY(' + scroll + 'px)';
});

<div id="wrapper">
  <div id="sidebar"></div>
  <div id="content"></div>
</div>

div {
  padding: 5px;
}
#wrapper {
  display: flex;
}
#sidebar {
  width: 200px;
  height: 70px;
  background-color: #9b59b6;
}
#content {
  flex: 1;
  height: 10000px;
  background-color: #ecf0f1;
}