MutationObserver

var element = document.getElementById('myElement');
setTimeout(function () {
    element.classList.add('hello');
}, 2000);

var observer = new MutationObserver(function(ev) {
    console.log('Classlist has changed: '+ ev[0].target.className);   
})

observer.observe(element, {
    attributes: true,
    attributeFilter: ['class'],
    childList: false,
    characterData: false
})