Node.nextElementSibling & Node.previousElementSibling
Get the next and previous sibling element of an element.
<ul>
<!-- The gray wizard -->
<li>Gandalf</li>
<li>Radagast</li>
<li id="hermione">Hermione</li>
<!-- The surprise hero -->
<li>Neville</li>
</ul>
let hermione = document.querySelector('#hermione');
// returns <li>Neville</li>
let next = hermione.nextElementSibling;
// returns <li>Radagast</li>
let previous = hermione.previousElementSibling;