JSFiddle - React, Tailwind, and code Playground

HTML

<div class="test">Test</div>
	<div class="test2">One more</div>

JavaScript

var $ = function(selectors) {
	return document.querySelectorAll(selectors);
}

NodeList.prototype.addClass = function (param) {
	var myNodeList = this;
	if (typeof param === "string"){
		for (var i = 0; i < myNodeList.length; i++) {
			var oldClass = myNodeList[i].className;
			myNodeList[i].className = oldClass + " " + param;
		}
	} else {
		console.log("argument is not a String");
	}
	return this;
}

NodeList.prototype.each = function (callback) {
	var myNodeList = this;
	for (var i = 0; i < myNodeList.length; i++) {
		callback.call(myNodeList[i], param);
	}
	return this;
}
console.log($(".test").each(addClass('newClass')));