JSFiddle - React, Tailwind, and code Playground

HTML

<div id="foo" class="bar">#foo.bar</div>

CSS

.red {
    color: #C00;
}

JavaScript

var $div = $("#foo");
var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutation) {
        if (mutation.attributeName === "class") {
        	var attributeValue = $(mutation.target).prop(mutation.attributeName);
            console.log("Class attribute changed to:", attributeValue);
        }
    });
});

observer.observe($div[0],  {
    attributes: true
});
$div.addClass('red');