<h1 id="title">こんにちは CodePen！</h1>
<button id="changeBtn">文字を変える</button>

body {
  font-family: sans-serif;
  text-align: center;
  margin-top: 50px;
}

button {
  padding: 10px 20px;
  font-size: 16px;
}

const button = document.getElementById("changeBtn") as HTMLButtonElement;
const title = document.getElementById("title") as HTMLHeadingElement;

button.addEventListener("click", () => {
  title.textContent = "ボタンがクリックされました！";
});