JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://dl.dropboxusercontent.com/u/7909102/log.js"></script>

JavaScript

(function() {
    // create XMLHttpRequest proxy object
    var oldXMLHttpRequest = XMLHttpRequest;
    
    // define constructor for my proxy object
    XMLHttpRequest = function() {
        var actual = new oldXMLHttpRequest();
        var self = this;
        
        function modResponse(actualResponse) {
            // modify the response here and return it
            log("actual response: " + actualResponse);
            return '{"msg": "Hello"}';
        }
        
        // generic function for modifying the response that can be used by
        // multiple event handlers
        function handleResponse(prop) {
            return function() {
                if (this.responseText && self[prop]) {
                    self.responseText = modResponse(this.responseText);
                }
                // call callback that was assigned on our object
                if (self[prop]) {
                    return self[prop].apply(self, arguments);
                }
            }
        }
        
        function handleLoadEvent(fn, capture) {
            return actual.addEventListener("load", function(e) {
               if (this.responseText) {
                    self.responseText = modResponse(this.responseText);
                }
                return fn.apply(self, arguments);
            }, capture);
        }
        
        // properties we don't proxy because we override their behavior
        this.onreadystatechange = null;
        this.responseText = null;
        this.onload = null;
        if (actual.addEventListener) {
            this.addEventListener = function(event, fn, capture) {
                if (event === "load") {
                    return handleLoadEvent(fn, capture);
                } else {
                    return actual.addEventListener.apply(actual, arguments);
                }
            }
        }
        
        // this is the actual handler on the real XMLHttpRequest object
        actual.onreadystatechange =...