Use the className Property
If you want to add a class to an HTML element without replacing its existing class, you can do something as shown in the following example:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Add Class to an Element Using JavaScript</title>
<style>
.content{
padding: 20px;
border: 2px solid #ccc;
}
.highlight{
background: yellow;
}
</style>
</head>
<body>
<div class="content" id="myDiv">
<p>This is a paragraph of text.</p>
<button type="button" onclick="addNewClass();">Add Class</button>
</div>
<script>
function addNewClass(){
// Select div element by its id attribute
var elem = document.getElementById("myDiv");
elem.className += " highlight";
}
</script>
</body>
</html>
