JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li id="objectID1">1</li>
    <li id="objectID2">1</li>
    <li id="objectID3">1</li>
    <li id="objectID4">1</li>
</ul>

JavaScript

var ar = Array.prototype.slice.call(document.querySelectorAll('li')),
    index = ar.map(function (el) {
        return el.id;
    }).indexOf('objectID4');

console.log(index);