title: Add a class to body
body: <p>How can I add a class to the body in ES6 JS?</p>
  
  <p>I've tried:</p>
  
  <pre><code>let body = document.getElementsByTagName('body');
  body.classList.add('my-class');
  </code></pre>
  
  <p>It fails. No Jquery please.</p>