JSFiddle - React, Tailwind, and code Playground

HTML

<div id="exemplo1"></div>
<div id="exemplo2"></div>

CSS

#exemplo1 {
    z-index:4;
}
#exemplo2 {
    z-index:4;
    position: relative;
}

JavaScript

var e1 = document.getElementById('exemplo1');

var pos = e1.style.position;
e1.style.position = "relative";
console.log(window.getComputedStyle(e1).getPropertyValue("z-index"));
e1.style.position = pos;