JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="list">
    <li data-id="l1">list 1</li>
    <li data-id="l2">list 2</li>
    <li data-id="l3">list 3</li>
    <li data-id="l4">list 4</li>
</ul>

<p></p>

JavaScript

var arr = new Array();

$('.list li').each(function(){
    var lid = $(this).attr('data-id');
    var lval = $(this).html();
    arr[lid] = lid;
    arr.push(lid);
});

$('p').html('array length = ' + arr.length + ' & index of list 1 is ' + arr.indexOf('l2'));