title: What is a type mismatch error mean when assigning onreadystatechange?
body: <p>What is a JavaScript type mismatch error?</p>
  
  <p>And what are the likely causes?</p>
  
  <p>I can't include all the code, but here is the line that gets the error.</p>
  
  <pre><code>xmlObject.onreadystatechange = null;
  </code></pre>