Implementing OAuth 2.0 Client Credentials Grant in Node.js

const axios = require('axios');

const OAUTH_CONFIG = {
    TOKEN_URL: 'https://oauth.example.com/token',
    CLIENT_ID: process.env.OAUTH_CLIENT_ID,
    CLIENT_SECRET: process.env.OAUTH_CLIENT_SECRET,
    SCOPE: 'read write' // Optional scope
};

let accessToken = null;
let tokenExpiry = 0;

async function getAccessToken() {
    if (accessToken && Date.now() < tokenExpiry) {
        console.log('Using cached access token.');
        return accessToken;
    }

    console.log('Fetching new access token...');
    try {
        const response = await axios.post(
            OAUTH_CONFIG.TOKEN_URL,
            new URLSearchParams({
                'grant_type': 'client_credentials',
                'client_id': OAUTH_CONFIG.CLIENT_ID,
                'client_secret': OAUTH_CONFIG.CLIENT_SECRET,
                'scope': OAUTH_CONFIG.SCOPE
            }),
            {
                headers: {
                    'Content-Type': 'application/x-www-form-urlencoded'
                }
            }
        );

        accessToken = response.data.access_token;
        // Set expiry a bit before the actual expiry to avoid using an expired token
        tokenExpiry = Date.now() + (response.data.expires_in - 60) * 1000; 
        console.log('New access token obtained.');
        return accessToken;
    } catch (error) {
        console.error('Error fetching OAuth token:', error.response ? error.response.data : error.message);
        throw new Error('Failed to obtain OAuth access token.');
    }
}

async function callProtectedApi(apiUrl) {
    try {
        const token = await getAccessToken();
        const response = await axios.get(apiUrl, {
            headers: {
                'Authorization': `Bearer ${token}`
            }
        });
        return response.data;
    } catch (error) {
        console.error('Error calling protected API:', error.response ? error.response.data : error.message);
        throw error;
    }
}

// Example usage (ensure OAUTH_CLIENT_ID and OAUTH_CLIENT_SECRET are set as environment variables):
// (async () => {
//     try {
//         const apiData = await callProtectedApi('https://api.example.com/protected-resource');
//         console.log('Protected API Data:', apiData);
//     } catch (e) {
//         console.error('Application failed:', e.message);
//     }
// })();