JSFiddle - React, Tailwind, and code Playground

HTML

<div class="a">
click
</div>

CSS

.a {
  width:100px;
  height:100px;
  border:1px solid red;
  
}

JavaScript

document.querySelector('div').onclick = function() {
var width = window.getComputedStyle(this,null).getPropertyValue("height");
    alert(width);
}