javascript classlist add

 document.getElementById("myDIV").classList.add("mystyle"); 

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

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

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

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

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

if (document.body.classList.contains('thatClass')) {
    // do some stuff
}

//get array of class names in Javascript
var classesArray = document.getElementById('myElementID').className.split(/\s+/);