index.html

<!DOCTYPE html>
<!--
 Copyright 2016 Google Inc.

 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
-->
<html lang="en">
<head>
    <meta charset="utf-8" />
    <meta name="google-signin-scope" content="profile">
    <meta name="google-signin-client_id" content="YOUR_CLIENT_ID_HERE.apps.googleusercontent.com" />
    <title>appengine-oauth2 sample</title>

    <script src="https://apis.google.com/js/platform.js" async defer></script>
    <script>
    function onSignIn(googleUser) {
        // Useful data for your client-side scripts:
        var access_token = googleUser.getAuthResponse().access_token;


        var xhr = new XMLHttpRequest();
        xhr.open('POST', '/hello');
        xhr.setRequestHeader("Authorization", "Bearer " + access_token);
        xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        xhr.onload = function() {
            document.getElementById('response').innerHTML = xhr.responseText;
            document.getElementById('me').style = "display:inline;";
        };
        xhr.onError = function() {
            document.getElementById('response').innerHTML = "Error";
            document.getElementById('me').style = "display:inline;";
        };
        xhr.send();
    }
    </script>
</head>
<body>
<div class="g-signin2" data-onsuccess="onSignIn" data-theme="dark"></div>
<div id="me" style="display: none;">
    <h1><span id="response"></span></h1>
</div>
</body>
</html>