Add a Class to an Element

To add a class to an element, you use the classList property of the element. Suppose you have an element as follows:

<div>Item</div>

To add the box class to the <div> element, you use the following:

const div = document.querySelector('div');
div.classList.add('box');

It’s possible to add multiple classes. The following adds three classes to the <div> element:

const div = document.querySelector('div');
div.classList.add('box','primary','rounded');

Source

https://www.javascripttutorial.net/add-class-to-an-element/