JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-1.12.4.min.js"></script>
<iframe id="Z_test" src="https://c2ect538.caspio.com/dp.asp?AppKey=b8a94000f5fb0d287ec840b89679">
</iframe>

JavaScript

$(function() {

var AllCountries = ""

$("#Z_test").load(function () {

var eventMethod = window.addEventListener ? "addEventListener" : "attachEvent";
var eventer = window[eventMethod];

var messageEvent = eventMethod == "attachEvent" ? "onmessage" : "message";

// Listen to message from child IFrame window
eventer(messageEvent, function (e) {

       AllCountries = e.data;
       alert('This alert should NOT appear last ' + AllCountries);
}, false);

    alert('iFrame loaded!');
});

// ---------------

alert('This should be the LAST alert appearing with countries list' + AllCountries)
})