code: function XMLHttpRequest() {
    EventTarget.call(this, events);
  
    this.readyState;
  
    if (support.xhr.timeout) {
      this.timeout = 0;
    }
  
    if (support.xhr.cors) {
      this.withCredentials = false;
    }
  
    this.upload;
    this.responseType = '';
    this.responseText = '';
    this.responseXML = null;
    this.readyState = states.UNSENT;
  
    if (support.xhr.response) {
      this.response = '';
    }
  
    if (support.xhr.responseURL) {
      this.responseURL = '';
    }
  
    this.status = 0;
    this.statusText = '';
  }
docs: https://xhr.spec.whatwg.org/#constructors