title: why document.getElementsByClassName("className") returns object
body: <p>I have a some dom objects that are selected with :  </p>
  
  <pre><code> var elems = document.getElementsByClassName("royal") ;
  </code></pre>
  
  <p>and also another objects </p>
  
  <pre><code>var collapsedElems =  document.getElementsByClassName("collapsed");
  </code></pre>
  
  <p>my problem occured when i tried to concat elems and collapsedElems  with array concat() method</p>
  
  <pre><code> elems.concat(collapsedElems) 
  </code></pre>
  
  <p>but the return type of <code>getElementsByClassName()</code> is not array actually it is
  object.  I checked it at chrome console with typeof operator. that seems weird to me how can i combine this two group of object. ?  </p>