JSFiddle - React, Tailwind, and code Playground

HTML

<div id="menu"></div>

CSS

#menu{
 font-size: 12px;
 width: 20em;   
}

JavaScript

var div1 = document.createElement('div'), 
css = getMatchedCSSRules(document.getElementById('menu'));

console.log(css); //works but dont' konw how to access width

div1.style.width = window.innerWidth - <!-- menu width -->;