title: Add multiple ids in getElementById
body: <p>How to use this script with multiple ids? Does not work on the second id that I try to add</p>
  
  <pre><code>&lt;script type="text/javascript"&gt;
  window.onload = (function () {
  var elem = document.getElementById("id1","id2").onclick = function()
  {
  fbq('track', 'InitiateCheckout');
  }
  });
  &lt;/script&gt;
  </code></pre>