title: Null is not an object (evaluating
body: <p>I want to add a class to an element with the id=&quot;key&quot; but it doesn't work why ?? I am a beginners in js : this is the code :</p>
  <pre><code>audio.classList.add('yellow');
  </code></pre>
  <p>This is the error :</p>
  <pre><code>null is not an object (evaluating 'document.getElementById(key).classList')
  </code></pre>