JSFiddle - React, Tailwind, and code Playground

HTML

<div id="computed" style="width:14em;">This div has width and height set in CSS.</div>

CSS

#computed{
    width: 14em;
    height: 14em;
    background: rgba(00,00,00,.3);
}

JavaScript

document.onload = sayIt();

function sayIt() {
var hop;
hop = document.getElementById("computed").style.width;
    alert(hop);
}