Untitled

<!DOCTYPE html>
<html>
<head>
    <title>demo</title>
</head>

<body>

<script src="https://www.gstatic.com/firebasejs/3.6.3/firebase.js"></script>

<script>
  // Initialize Firebase
  var config = {
    apiKey: "AIzaSyCW-lP25n8nxq4OuOAaV1PE1rLW_u5GBnw",
    authDomain: "fir-f4a5f.firebaseapp.com",
    databaseURL: "https://fir-f4a5f.firebaseio.com",
    projectId: "fir-f4a5f",
    storageBucket: "",
    messagingSenderId: "356303445704"
  };
  firebase.initializeApp(config);
</script>

<script type="text/javascript">

    var messaging = firebase.messaging();
    messaging.requestPermission().
    then(function () {
        console.log("hello world");
        return messaging.getToken();
    })
    .then(function (token) {
        console.log(token);
    })
    .catch(function (err) {
        console.log("error occured");
    })

</script>