title: I can't access HTMLCollection
body: <p>I can't access HTMLCollection Type When I use getElementsByClassName.</p>
  
  <p>I wanna get the length but I can't get that</p>
  
  <pre><code>var documentHeader = parent.document.all['header'];
  var motionClass = documentHeader.getElementsByClassName('motion');
  </code></pre>
  
  <p>this is a result of motionClass</p>
  
  <pre><code>HTMLCollection []
   0: div.motion
   length: 1
   __proto__ : HTMLColletion
  </code></pre>
  
  <p>If I access length result is 0</p>
  
  <p>How can I result This Issue?</p>