Setup

const el = document.getElementById("my-el")

Set

el.className = 'abc def open'

View

el.className
// 'abc def open'
el.classList
// [ "abc", "def" ]

Add

You can only add one class at a time like this. No spaces allowed. The class can only be added once and after that will be ignored.

el.classList.add("xyz")

el.className
'abc def xyz'

Remove

el.classList.remove("abc")

Toggle

Add or remove a class, depending on the current state.

el.classList.toggle("abc")