JSFiddle - React, Tailwind, and code Playground

by evg_

HTML

<button class="btn">Старт</button>

CSS

* {
	margin: 0;
	padding: 0;
	box-sizing: border-box;
}

body {
	display: flex;
	height: 100vh;
}

.btn {
	margin: auto;
	width: 200px;
	height: 50px;
	line-height: 50px;
	text-align: center;
	padding: 0 10px;
	cursor: pointer;
	background: lightgreen;
	color: #fff;
	font-family: "Open Sans";
	text-transform: uppercase;
	font-size: 14px;
	display: block;
	border: none;
}

.btn:focus {
	outline: none;
}

.modifier {
	background: tomato;
}

JavaScript

var btn = document.querySelector(".btn")
		isStart = false;

btn.addEventListener("click", function() {
	if (isStart) {
		btn.classList.remove("modifier");
		btn.textContent = "Старт";
		isStart = false;
		stop();
		return;
	};
	
	btn.classList.add("modifier");
	btn.textContent = "Стоп";
	isStart = true;
	start();
});

function start() {
	console.log("start");
}

function stop() {
	console.log("stop");
}