JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block">2</div>
<div class="block">3</div>
<div class="block">4</div>
<div class="block">5</div>
<div class="block">6</div>

JavaScript

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


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

function addcls(el, cn){
  if(el == null) return;
  if(el.length){
  	Array.from(el).forEach(i => {
      if(i == null) return;
     	i.classList.add(cn);
	    console.log(i);
    });
  } else {
	  console.log(el);
	  el.classList.add(cn);
  }
}