01-hello-world.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Getting Started with JavaScript</title>

  <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
  <style>
    body { padding: 30px; }
  </style>
</head>
<body>

  <div class="jumbotron text-center">
    <h1>This is my site!</h1>

    <button type="button" class="btn btn-primary" onclick="changeText()">
      Change the Text
    </button>
  </div>

  <!-- 🔥🔥🔥🔥 start javascript 🔥🔥🔥🔥 -->
  <script>
    const header = document.querySelector('h1');

    function changeText() {
      header.innerText = 'my new text!';
    }
  </script>
</body>
</html>