Injectable JS to capture XHR responses

xhr-capture.js

// http://stackoverflow.com/a/10796951
(function(XHR) {
  "use strict";

  var element = document.createElement('div');
  element.id = "interceptedResponse";
  element.appendChild(document.createTextNode(""));
  document.body.appendChild(element);

  var open = XHR.prototype.open;
  var send = XHR.prototype.send;

  XHR.prototype.open = function(method, url, async, user, pass) {
    this._url = url; // want to track the url requested
    open.call(this, method, url, async, user, pass);
  };

  XHR.prototype.send = function(data) {
    var self = this;
    var oldOnReadyStateChange;
    var url = this._url;

    function onReadyStateChange() {
      if(self.status === 200 && self.readyState == 4 /* complete */) {
        document.getElementById("interceptedResponse").innerHTML +=
          '{"url":"' + url + '", "data":' + self.responseText + '}*****';
      }
      if(oldOnReadyStateChange) {
        oldOnReadyStateChange();
      }
    }

    if(this.addEventListener) {
      this.addEventListener("readystatechange", onReadyStateChange,
        false);
    } else {
      oldOnReadyStateChange = this.onreadystatechange;
      this.onreadystatechange = onReadyStateChange;
    }
    send.call(this, data);
  }
})(XMLHttpRequest);

Nice work