JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="stack">
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
  <li>5</li>
</ul>

JavaScript

function setClass(){
    var ul = document.getElementById("stack");
    for(var i = 0; i < ul.children.length; i++){
        ul.children[i].className = 'class' + (i+1);
    }
}

setClass();