Listen node content changes

var target = document.querySelector('.some-class');
var observer = new MutationObserver(function(mutations) {
    if (mutations[0].target.classList.contains("some-child-class")) {
        // Do something
    }
}
var config = { subtree: true, characterData: true, childList: true };
observer.observe(target, config);

Тут могла быть ваша реклама.