Selection.selectAllChildren()

sel.selectAllChildren(parentNode)

<main>
  <button>Select Footer</button>
  <p>Welcome to my website.</p>
  <p>I hope you enjoy your visit.</p>
</main>
<footer>
  <address>[email protected]</address>
  <p>© 2019</p>
</footer>

const button = document.querySelector('button');
const footer = document.querySelector('footer');

button.addEventListener('click', (e) => {
  window.getSelection().selectAllChildren(footer);
});