JSFiddle - React, Tailwind, and code Playground

JavaScript

//Cross browser compatibility ///////////////////////////
var myRequest;
if (window.XMLHttpRequest) {
    myRequest = new XMLHttpRequest();
} else if (window.ActiveXObject) {
    myRequest = new ActiveXObject("Microsoft.XMLHTTP");
}

//My variable ///////////////////////////////////////////
var foo = "";

//Response handler //////////////////////////////////////
myRequest.onreadystatechange = function() {
    if (this.readyState === 4) {
        if (this.status === 200) {
            foo = "New value:" + this.responseText;
        } else {
            alert('There was a problem with the request.');
        }
    }
};

//Send request //////////////////////////////////////////
myRequest.open('GET', "/");
myRequest.send();

//Alert updated variable ////////////////////////////////
alert(foo);