title: Access member method from async call
body: <p>I am trying to access a method from my "class" from a method called asynchronously by the code.</p>
  
  <p>Here is my reduced class:</p>
  
  <pre><code>function FileProcessor(file, callback) {    
      this.file = file;
      this.callback = callback;
  }
  
  FileProcessor.prototype.readerOnload = function(e) {
      this.callback(e.target.result);
  };
  
  FileProcessor.prototype.process = function() {
      var reader = new FileReader();
      reader.onload = this.readerOnload;
  
      if (this.file.type == 'text/xml') {
          reader.readAsText(this.file);
      }
  };
  </code></pre>
  
  <p>My cal to this.callback works from the constructor FileProcess, works from process(), but doesn't work from readerOnload. I get:</p>
  
  <p>Uncaught TypeError: undefined is not a function</p>
  
  <p>Any idea what I am doing wrong?</p>
  
  <p>Thanks</p>