Node.nextSibling & Node.previousSibling

Get the next and previous sibling node of an element, including nodes that aren't elements (such as comments and text fragments).

<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 <!-- The surprise hero -->
let next = hermione.nextSibling;

// returns <li>Radagast</li>
let previous = hermione.previousSibling;