title: Why can't I see the keys of an Error object?
body: <p>I am mystified by the fact that when I create a new Error object I can see its message or name, but I can't see a list of its keys by using the standard ways.  Why is that?</p>
  
  <pre><code>&gt; err = new Error("an error")
  [Error: an error]
  &gt; err.message
  'an error'
  &gt; err.name
  'Error'
  &gt; Object.keys(err)
  []
  &gt; JSON.stringify(err)
  '{}'
  </code></pre>