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 maxValue = Math.max.apply( Math, $('.a').map(function() {
    return +$.attr(this, 'z-index');
}));

alert( maxValue );