A demo of adding class by using JavaScript className attribute

The Code

Output

<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
<script type="text/javascript">
    function changeClass()
    {
        document.getElementById("class_demo").className = "btn btn-danger btn-lg";
    }
</script>

</head>

<body>
<h3>Click on the button to add classes by JavaScript</h3>
<button id="class_demo" onclick="changeClass()">Add Class by JavaScript</button>

</body>
</html>