Implementing Google OAuth 2.0 for User Login

// server.js (using Express)
const express = require('express');
const axios = require('axios');
const querystring = require('querystring');
require('dotenv').config(); // Ensure your .env variables are loaded

const app = express();
const PORT = 3000;

const GOOGLE_CLIENT_ID = process.env.GOOGLE_CLIENT_ID;
const GOOGLE_CLIENT_SECRET = process.env.GOOGLE_CLIENT_SECRET;
const GOOGLE_REDIRECT_URI = 'http://localhost:3000/auth/google/callback';

// Step 1: Redirect user to Google for authentication
app.get('/auth/google', (req, res) => {
    const authUrl = 'https://accounts.google.com/o/oauth2/v2/auth?' +
        querystring.stringify({
            client_id: GOOGLE_CLIENT_ID,
            redirect_uri: GOOGLE_REDIRECT_URI,
            response_type: 'code',
            scope: 'profile email', // Requesting user profile and email
            access_type: 'offline', // To get refresh token
            prompt: 'consent' // To ensure refresh token is always returned
        });
    res.redirect(authUrl);
});

// Step 2: Google redirects back to your app, exchange code for tokens
app.get('/auth/google/callback', async (req, res) => {
    const { code } = req.query;

    if (!code) {
        return res.status(400).send('Authorization code missing.');
    }

    try {
        const tokenResponse = await axios.post('https://oauth2.googleapis.com/token',
            querystring.stringify({
                code: code,
                client_id: GOOGLE_CLIENT_ID,
                client_secret: GOOGLE_CLIENT_SECRET,
                redirect_uri: GOOGLE_REDIRECT_URI,
                grant_type: 'authorization_code'
            }),
            {
                headers: {
                    'Content-Type': 'application/x-www-form-urlencoded'
                }
            }
        );

        const { access_token, id_token, refresh_token } = tokenResponse.data;

        // Optionally, fetch user info using the access_token
        const userInfoResponse = await axios.get('https://www.googleapis.com/oauth2/v3/userinfo', {
            headers: { Authorization: `Bearer ${access_token}` }
        });

        console.log('User Info:', userInfoResponse.data);
        console.log('Access Token:', access_token);
        // In a real app, store refresh_token securely and create a session for the user

        res.send(`<h1>Welcome, ${userInfoResponse.data.name}!</h1><p>You are logged in.</p>`);

    } catch (error) {
        console.error('Error during Google OAuth:', error.response ? error.response.data : error.message);
        res.status(500).send('Authentication failed.');
    }
});

app.get('/', (req, res) => {
    res.send('<h1>Home Page</h1><a href="/auth/google">Login with Google</a>');
});

app.listen(PORT, () => {
    console.log(`Server running on http://localhost:${PORT}`);
    console.log('Ensure GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET are set in your .env file.');
});