How do I add a class to a given element?

Question

I have an element that already has a class:

Answer 1

document.getElementById('some_id').className+='  someclassname'

OR:

document.getElementById('some_id').classList.add('someclassname')

First approach helped in adding the class when second approach didn't work.
Don't forget to keep a space in front of the ' someclassname' in the first approach.

For removal you can use:

document.getElementById('some_id').classList.remove('someclassname')