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JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>JAVAAAAAAAAAA</title>
</head>
<body>
    <button id="btn">Нажми меня</button>
</body>
</html>

CSS

.btn{
  font-weight: bold;
}

JavaScript

'use strict';

// находим кнопку
const btn = document.querySelector('#btn'); 

//функция, которая переключает класс на кнопке
function btnBold() {
    btn.classList.toggle('btn');
}

//обработчик события на кнопку
btn.addEventListener('click', () => btnBold());

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JSFiddle - React, Tailwind, and code Playground

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