JSFiddle - React, Tailwind, and code Playground

HTML

<div class="a" z-index="1">
<div class="a" z-index="3">
<div class="a" z-index="4">
<div class="a" z-index="5">
<div class="a" z-index="6">
<div class="a" z-index="11">

JavaScript

var top = $(".a").get().reduce(function (a, b) {
       
       return Math.max(typeof a === 'object' && $(a).attr('z-index') || a, $(b).attr('z-index'));
    });
console.log(top);