JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li  data-details="li1">first</li>
    <li  >second</li>
</ul>

JavaScript

$(document).ready(function () {
    $("ul li:nth-child(2)").data("details", "li1");
    console.log($("ul li").filter(function () { return $(this).data("details") === "li1"; }).length);
});