JSFiddle - React, Tailwind, and code Playground

HTML

<body onload="changeClass()">

<section class="example">
<p>Hello</p>
</section>

<section class="example">
<p>New section</p>
</section>

JavaScript

function changeClass() {
	var x = document.getElementsByClassName('example');
	var width = (window.innerHeight > 0) ? window.innerHeight : screen.Height;
	console.log(width);
	if(width <= 640) {
		while(x.length > 0) {
			x[1].className ='newClass'; 
		}
	} else {
		while(x.length > 0) {
			x[0].className = "example";
		}
	}
}