adding a class using classList code example

Example 1: js add class

var element = document.getElementById('element');
element.classList.add('class-1');
element.classList.add('class-2', 'class-3');
element.classList.remove('class-3');

Example 2: add to classlist javascript

Add multiple classes to a 
 element:
document.getElementById("myDIV").classList.add("mystyle", "anotherClass", "thirdClass");

Remove a class from a 
 element:
document.getElementById("myDIV").classList.remove("mystyle");

Remove multiple classes from a 
 element:
document.getElementById("myDIV").classList.remove("mystyle", "anotherClass", "thirdClass");

Toggle between two classes for a 
 element:
document.getElementById("myDIV").classList.toggle("newClassName");

Find out if an element has a "mystyle" class:
document.getElementById("myDIV").classList.contains("mystyle");