Why has my sticky navbar stopped working since I changed it to class from id?

I need change my navbar from an id to class, but when I did so the sticky part stopped working. I just want the navbar to be as it was when it was an id, just as here; https://www.w3schools.com/howto/tryit.asp?filename=tryhow_js_navbar_sticky . That it stopped working could have something to do with the script at the end of my html document, but I don't know much about javascript yet.. Could anyone help me? Here's my html;

<!doctype html>
<html>
 <head>
      <meta charset="UTF-8">
      <link rel="stylesheet" type="text/css" href="style.css">
  </head>
  <body>    
<div class="ad">
   Bepaalde boeken tot -50% + gratis levering bij bestellingen boven €50!
  </div>

   <div>    <ul class="navbar">
    <li class="links"><a href="index.html">BookShop</a></li>
      <li class="links"><a></a></li>
    <li class="links"><a href="Talen.html">Talen</a></li>
    <li class="links"><a href="Genres.html">Genres</a></li>
    </ul>    </div>
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<script>
window.onscroll = function() {myFunction()};

var navbar = document.getElementsByClassName("navbar");
var sticky = navbar.offsetTop;

function myFunction() {
  if (window.pageYOffset >= sticky) {
    navbar.classList.add("sticky")
  } else {
    navbar.classList.remove("sticky");
  }
}
</script>    

</body>
</html>

my css;

.ad {
    background-color: #fbf4e9;
    text-align: center;  
    font-size:0.9rem;
    padding:5px;
}
.sticky {
    position: fixed;
    top: 0;
}
ul.navbar {
    overflow:hidden;
    list-style-type:none;
    background-color:#f9eedd;
    width:100%;
    height:auto;
    margin:0;
    padding:0;
    z-index:10;
    } 
ul.navbar li a{ 
    display:block;
    color:#8e8275;
    text-decoration:none;
    text-align: center;
    padding: 13px 10px 13px 10px;
    margin: 10px 7px 10px 7px
    }
ul.navbar li.links{ float:left;}

Solution

The function getElementsByClassName returns an array of elements. If you know that you only have one and you need to select it you need to do the following:

var navbar = document.getElementsByClassName("navbar")[0]; //select first element
var sticky = navbar.offsetTop;