JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">Dimentions are:
    <br />200x200 px</div>

CSS

.box {
    width: 200px;
    height: 200px;
    overflow: hidden;
    margin: 10px 0;
    border: 10px solid #999999;
    cursor: pointer;
    background-color: #cccccc;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

JavaScript

var box = document.querySelector('.box');
var gBCR_width = box.getBoundingClientRect().width;
var wGCS = window.getComputedStyle(box).width;
alert('getBoundingClientRect: ' + gBCR_width + '\ngetComputedStyle: ' + wGCS);