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>Document</title>
  </head>
  <body>
    <button id="btn">
      bouton
    </button>
  </body>
</html>

JavaScript

const btn = document.getElementById('btn');

let orFn = () => {
  alert('old');
};

btn.addEventListener('click', orFn);

orFn = () => {
  alert('new');
};