title: responseXML displaying as null even though xhr object shows it as set
body: <p>I have the following code which eventually populates a table. At this point I create an XHR, where on load it will call my employee collections process XML function</p>
  
  <pre><code>EmployeeCollection.prototype.fetchXML = function(path, callback) {
      var xhr = new XMLHttpRequest();
      xhr.open("GET", path, true);
      xhr.send();
  
      xhr.onload = this.processXML(xhr, callback);
  };
  
  EmployeeCollection.prototype.processXML = function(xhr, callback) {
       console.log(xhr); // this shows an XHR object with 
                         // readyState: 4 and responseXML: document
  
       console.log(xhr.responseXML); // null
  }
  </code></pre>
  
  <p>My question is; how come I can see the responseXML as a document if I access the xhr object and expand the node in the console, yet when I try to directly access this property of the xhr object - I can't? </p>