How can I change a button Id-tag into class-tag and use getElementByClassName right?

I need it this button id for several buttons thats why I want to change it into a class-tag. But if I do so it does not work. Is this not possible to use get ElementByClassName ?

<button class="myBtn">Overlay</button>

So basically I want to change the ID-tag into a Class-tag. I tried to change the getElementById into getElementByClassName. Unfortunatly it does not work.

In the following I have added an example from W3Schools. Which is working with the id-tag.

<!DOCTYPE html>
<html>
<head>
<style>
/* The Overlay (background) */

.overlay {
display: none; /* Hidden by default */
position: fixed; /* Stay in place */
z-index: 1; /* Sit on top */
left: 0;
top: 0;
width: 100%; /* Full width */
height: 100%; /* Full height */
overflow: auto; /* Enable scroll if needed */
background-color: rgb(0,0,0); /* Fallback color */
background-color: rgba(0,0,0,0.4); /* Black w/ opacity */
-webkit-animation-name: fadeIn; /* Fade in the background */
-webkit-animation-duration: 0.4s;
animation-name: fadeIn;
animation-duration: 0.4s
}


/* Overlay Content */

.overlay-content {
position: fixed;
bottom: 0;
background-color: #fefefe;
width: 100%;
-webkit-animation-name: slideIn;
-webkit-animation-duration: 0.4s;
animation-name: slideIn;
animation-duration: 0.4s
}

/* The Close Button */

.close {
color: white;
float: right;
font-size: 28px;
font-weight: bold;
}

.close:hover,
.close:focus {
color: #000;
text-decoration: none;
cursor: pointer;
}

.overlay-header {
padding: 2px 16px;
background-color: #5cb85c;
color: white;
}

.overlay-body {padding: 2px 16px;}

.overlay-footer {
padding: 2px 16px;
background-color: #5cb85c;
color: white;
}

/* Add Animation */

@-webkit-keyframes slideIn {
from {bottom: -300px; opacity: 0} 
to {bottom: 0; opacity: 1}
}

@keyframes slideIn {
from {bottom: -300px; opacity: 0}
to {bottom: 0; opacity: 1}
}

@-webkit-keyframes fadeIn {
 from {opacity: 0} 
to {opacity: 1}
}

@keyframes fadeIn {
from {opacity: 0} 
to {opacity: 1}
}
</style>
</head>
<body>

<h2>Overlay</h2>

 <!-- Trigger/Open The Overlay -->
 <button id="myBtn">Overlay</button>

 <!-- The Overlay -->
 <div id="overlay" class="overlay">

 <!-- Overlay content -->
 <div class="overlay-content">
 <div class="overlay-header">
  <span class="close">&times;</span>
  <h2>Container</h2>
 </div>
 <div class="overlay-body">
  <p>Container Information</p>
  <p>Some other text...</p>
 </div>
 <div class="overlay-footer">
  <h3>Footer</h3>
 </div>
 </div>

 </div>

 <script>
// Get the overlay

var overlay = document.getElementById('overlay');

// Get the button that opens the overlay

var btn = document.getElementById("myBtn");

// Get the <span> element that closes the overlay

var span = document.getElementsByClassName("close")[0];

// When the user clicks the button, open the overlay 

btn.onclick = function() {
overlay.style.display = "block";
}

// When the user clicks on <span> (x), close the overlay

span.onclick = function() {
overlay.style.display = "none";
}

// When the user clicks anywhere outside of the overlay, close it

window.onclick = function(event) {
if (event.target == overlay) {
    overlay.style.display = "none";
}
}
</script>

</body>
</html>

You are looking for getElementsByClassName. getElementByClassName does not exist.