Finding all class names used in HTML/DOM

Answer 1

Simple ES10 new Set() approach

const allClasses = Array.from(document.querySelectorAll('[class]')).flatMap(e => e.className.toString().split(/\s+/))
const classes = new Set()

allClasses.forEach(c => classes.add(c))

console.log(classes)

Get all the class names and split on space so for example, "child first" would become 'child' and 'first'. If you don't like this behaviour you can remove the toString and split functions.

Then add every class name to the set. There is no need to check for duplicates since the set does that for us. If you do want to store duplicates you can use a Map instead.