JAVASCRIPT

html add class

//Use element.classList.add to add a class:

let element = document.getElementById("div1");
element.classList.add("otherclass");

add class name in html

<script type="text/javascript">
    function changeClass(){
        // Code examples from above
    }

    window.onload = function(){
        document.getElementById("MyElement").addEventListener( 'click', changeClass);
    }
</script>
...
<button id="MyElement">My Button</button>