JSFiddle - React, Tailwind, and code Playground

HTML

<div class='sidebar'>
  sidebar
</div>
<div class='footer'>
  footer
</div>

CSS

.sidebar {
  position: fixed;
  top: 0;
  background-color: green;
  padding: 10px;
  color: white;
  width: 300px;
  height: 400px;
}
.footer {
  margin-top: 1200px;
  width: 100%;
  height: 620px;
  background-color: gray;
}

JavaScript

var altura = $(document).height() - $(".footer").height();
$(window).scroll(function(){
  if ($(this).scrollTop() > altura){
    $(".sidebar").fadeOut(500);
  }else{
    $(".sidebar").fadeIn(500);
  }
});