JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="list">
    <li>test</li>
    <li>test</li>
    <li>test</li>

JavaScript

var lis = document.getElementsByTagName('li');

for(var i = 0; i < lis.length; i++) {
    console.log(Array.prototype.indexOf.call(document.getElementById('list').getElementsByTagName('li'), lis[i]));
}