JSFiddle - React, Tailwind, and code Playground

HTML

<h1 id="width"></h1>

JavaScript

// set the initial width
	var viewportWidth = document.documentElement.clientWidth;
	var el = document.getElementById("width");
	el.innerHTML = viewportWidth + "px";

	// on resize
	window.addEventListener('resize', function(event){

		var viewportWidth = document.documentElement.clientWidth;
		var el = document.getElementById("width");
		el.innerHTML = viewportWidth + "px";

	});