.classList

// use the classList API to remove and add classes
div.classList.remove("foo");
div.classList.add("anotherclass");

var myElement = document.getElementById("myElementID");
myElement.classList.add("style1 style2");
myElement.classList.remove("style1 style2");

// .classList method returns an array of the classes attached to the element it is called with.
document.getElementById("myDIV").classList
// It returns
// [ 'className1', 'className2', ... ]

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