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

const doc = document;

const block1 = doc.querySelectorAll('#block');
const block2 = doc.querySelector('#block');
const block3 = doc.querySelector('.block');
const block4 = doc.querySelectorAll('.block');
const block5 = doc.getElementById('block');
const block6 = doc.getElementsByClassName('block');

console.log(block1, block2, block3, block4, block5, block6);

function addClass(element, className){
	element.classList.add(className);
  
	console.log(element);
}

addClass(block1, 'init');
addClass(block2, 'init');
addClass(block3, 'init');
addClass(block4, 'init');
addClass(block5, 'init');
addClass(block6, 'init');