JSFiddle - React, Tailwind, and code Playground

HTML

<div id="block"></div>
<div class="block"></div>
<div class="block"></div>
<div class="block"></div>
<div class="block"></div>
<div class="block"></div>

JavaScript

addClass(document.querySelectorAll('.block, #block'),'test');
addClass(document.getElementById('block'),'test2');
addClass([document.getElementById('block'),document.querySelector('.block')],'test3');

function addClass(elements, className){
	if(!NodeList.prototype.isPrototypeOf(elements) && !Array.prototype.isPrototypeOf(elements)) elements = [elements];
	elements.forEach(function(el){
		el.classList.add(className);
    console.log(el);
  });
}