How to add a class to a DOM element

TL;DR: Use the add() method on element.classList

When you have a DOM element reference you can add a new class to it by using the add method:

element.classList.add('myclass')

You can remove a class using the remove method:

element.classList.remove('myclass')

Implementation detail: classList is not an array, but rather it is a collection of type DOMTokenList.

You can’t directly edit classList because it’s a read-only property. You can however use its methods to change the element classes.