JSFiddle - React, Tailwind, and code Playground

HTML

<div id="content"></div>

JavaScript

var target = document.getElementById('content');

var observer = new MutationObserver(function (mutations) {
    mutations.forEach(function (mutation) {
        for (var i = 0; i < mutation.addedNodes.length; i++) {
            var addedNode = mutation.addedNodes[i];
            if (addedNode.id == "myElement") {
                alert("#myElement added!");
            }
        }
    });
});

var config = {
    attributes: true,
    childList: true,
    characterData: true
};
observer.observe(target, config);

setTimeout(function () {
    $("div").append('<div id="myElement"/>');
}, 2000);