JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="id1" class="mylist">
    <li><a href="">a</a></li>
    <li><a href="">b</a></li>
    <li><a href="">c</a></li>
    <li><a href="">d</a></li>
    <li><a href="">e</a></li>
</ul>
<ul id="id2" class="mylist">
    <li><a href="">f</a></li>
    <li><a href="">g</a></li>
    <li><a href="">h</a></li>
    <li><a href="">i</a></li>
    <li><a href="">j</a></li>
</ul>

JavaScript

var thisEleArr = document.getElementsByTagName('ul');

for (var i=0;i<thisEleArr.length;i++) {
    if (thisEleArr[i].getAttribute('id')==='id1') {
		console.log(i); //this will console write out the index of the matching ID
    }
}