title: addEventListener is not a function
body: <p>I have HTML like this</p>
  
  <pre><code>&lt;p class="trigger-overlay"&gt;
  </code></pre>
  
  <p>but got error of addEventListener is not a function</p>
  
  <pre><code>function toggleOverlay(){alert('fire');};
  var triggerBttn = document. getElementsByClassName('trigger-overlay');
  triggerBttn.addEventListener( 'click', toggleOverlay );
  </code></pre>
  
  <p>I use id and it's working, but I want to be able to trigger on multiple classes.</p>
  
  <pre><code>function toggleOverlay(){alert('fire');};
  var triggerBttn = document. getElementById('trigger-overlay');
  triggerBttn.addEventListener( 'click', toggleOverlay );
  </code></pre>