How do I add a class to a given element?

Question

I have an element that already has a class:

Answer 1

Sample with pure JS. In first example we get our element's id and add e.g. 2 classes.

document.addEventListener('DOMContentLoaded', function() {
    document.getElementsById('tabGroup').className = "anyClass1 anyClass2";
})

In second example we get element's class name and add 1 more.

document.addEventListener('DOMContentLoaded', function() {
    document.getElementsByClassName('tabGroup')[0].className = "tabGroup ready";
})