xhrInterceptor.js

const headerStorage = {}; // keeps request headers
function interceptXHR(interceptor) {
  const open = XMLHttpRequest.prototype.open;
  const send = XMLHttpRequest.prototype.send;
  const setRequestHeader = XMLHttpRequest.prototype.setRequestHeader;

  XMLHttpRequest.prototype.setRequestHeader = function (header, value) {
    headerStorage[header] = value;
    setRequestHeader.call(this, header, value);
  };

  XMLHttpRequest.prototype.open = function (
    method,
    url,
    _async,
    user,
    password
  ) {
    this._requestUrl = url;
    open.call(this, method, url, _async, user, password);
  };

  XMLHttpRequest.prototype.send = function (data) {
    this.addEventListener(
      "readystatechange",
      (ev) => {
        interceptor(this, ev);
      },
      false
    );
    send.call(this, data);
  };
}

/**

interceptXHR((request, event) => {
// triggers on readystatechange
});

*/