JSFiddle - React, Tailwind, and code Playground

HTML

<div class="a" z-index="1">a</div>
<div class="a" z-index="3">b</div>
<div class="a" z-index="4">c</div>
<div class="a" z-index="5">d</div>
<div class="a" z-index="6">e</div>
<div class="a" z-index="11">f</div>

<div class="b" z-index="">g</div>

JavaScript

$(function(){
    var idx = 0;
    $('.a').each(function(){
        if(idx < parseInt($(this).attr('z-index'))){
            idx = parseInt($(this).attr('z-index'));
        }
    });
    $('.b').attr('z-index',idx+1);
});