firebase.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    
        <input type="text" value="name" id="txt" ><br>
        <input type="submit" value="click me" id="234" onclick="sub()">
    
    
<script src="https://www.gstatic.com/firebasejs/5.5.8/firebase.js"></script>
<script>
    // Initialize Firebase
    var config = {
      apiKey: "AIzaSyB5AkS01wbDGgvFQ5k2XxMHARL4PC8yAA8",
      authDomain: "webdemo-542bb.firebaseapp.com",
      databaseURL: "https://webdemo-542bb.firebaseio.com",
      projectId: "webdemo-542bb",
      storageBucket: "webdemo-542bb.appspot.com",
      messagingSenderId: "346052208882"
    };
    firebase.initializeApp(config);
  </script>

<script src="index.js"></script>
</body>
</html>