Wait for element to be added into DOM then execute JavaScript

dom_observer.js

var observer = new MutationObserver(function (mutations) {
                mutations.forEach(function (mutation) {
                    if (!mutation.addedNodes) return

                    for (var i = 0; i < mutation.addedNodes.length; i++) {
                        var item = Array.prototype.slice.call(mutation.addedNodes).find(function (i) { return i.id == "selector" });
                        if (item) {
                            // Do your stuff here
                            observer.disconnect();
                        }
                    }
                })
            })

            observer.observe(document.querySelector("#parent_tag"), {
                childList: true,
                subtree: true,
                attributes: false,
                characterData: false
            })