JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="gallery">
    <li id="attachment-32" class="featured"><a href="..." title=""><img src="..." alt="" /></a></li>
    <li id="attachment-34">1</li>
    <li id="attachment-38">2</li>
    <li id="attachment-64">3</li>
    <li id="attachment-75">4</li>
    <li></li>
</ul>

JavaScript

var ids = $("#gallery li").map(function () {
    return this.text();
}).get().join(",");
alert(ids);