JSFiddle - React, Tailwind, and code Playground

HTML

<div id='output'>
  Processing... :p
</div>
<div>
  Sooo... How do I get the width and height of whats-my-dims1?
</div>
<div id='some-hidden-div'>
  <div class='whats-my-dims' id='whats-my-dims1'></div>
</div>
<div class='whats-my-dims' id='whats-my-dims2'></div>

CSS

#some-hidden-div{
  display: none;
}
.whats-my-dims{
  width: 69px;
  height: 42px;
  background-color: #f00;
}

JavaScript

var o = document.getElementById('output');
var wmd1 = document.getElementById('whats-my-dims1');
var wmd2 = document.getElementById('whats-my-dims2');
var hiddenDiv = document.getElementById("some-hidden-div");
hiddenDiv.style.display = "block";
o.innerHTML = 'wmd1: "' + wmd1.clientWidth + '", "' + wmd1.clientHeight + '", wmd2: "' + wmd2.clientWidth + '", "' + wmd2.clientHeight + '"';
hiddenDiv.style.display = "";