Why can't I add a class attribute using Javascript [PLEASE HELP]?😩😩😩😩

Question

Am trying to add class attributes to elements I have created using JS but it is not working.
It only works when I add an id but I need to use multiple classes for an element so that I can style the elements using CSS according to their groups.

Answer 1

document.className = "name"

Or

document.classList.add("name")