JSFiddle - React, Tailwind, and code Playground

HTML

<div></div>

CSS

div   {
  width: 150px;
  height: 150px;
  padding: 10px;
  margin: 10px;
  border: 10px solid #999;
}

JavaScript

let div = document.querySelector('div'),
cs = getComputedStyle(div);
console.log(div.offsetWidth + parseInt(cs.marginLeft) + parseInt(cs.marginRight));