JSFiddle - React, Tailwind, and code Playground

HTML

<div class="posts">
    <div class="post" data-id="5"></div>
    <div class="post" data-id="3"></div>
    <div class="post" data-id="1"></div>
    <div class="post" data-id="4"></div>
</div>

JavaScript

var max = Math.max.apply(Math, $(".posts .post").map(function () {
    return $(this).data("id");
}));
alert(max);