how to make bottom bar that disappears js

  /* When the user scrolls down, hide the navbar. When the user scrolls up, show 
  the navbar */
var prevScrollpos = window.pageYOffset;
window.onscroll = 
  function() {
  var currentScrollPos = window.pageYOffset;
  if 
  (prevScrollpos > currentScrollPos) {
    
  document.getElementById("navbar").style.top = "0";
  } else {
    
  document.getElementById("navbar").style.top = "-50px";
  }
  
  prevScrollpos = currentScrollPos;
}