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

let maxValue = Math.max(...$('.a').get().map(el => +$.attr(el, 'z-index')));

alert(maxValue);