OAuth 2.0 Client Credentials for Server API Calls

/**
 * Obtains an OAuth 2.0 access token using the Client Credentials grant type.
 * @param {string} tokenUrl - The OAuth token endpoint URL.
 * @param {string} clientId - Your application's client ID.
 * @param {string} clientSecret - Your application's client secret.
 * @param {string} scope - The requested scopes (space-separated string).
 * @returns {Promise<string>} The access token.
 * @throws {Error} If token acquisition fails.
 */
async function getOAuthAccessToken(tokenUrl, clientId, clientSecret, scope = '') {
    const authString = Buffer.from(`${clientId}:${clientSecret}`).toString('base64');
    const params = new URLSearchParams();
    params.append('grant_type', 'client_credentials');
    if (scope) {
        params.append('scope', scope);
    }

    const response = await fetch(tokenUrl, {
        method: 'POST',
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded',
            'Authorization': `Basic ${authString}`
        },
        body: params.toString()
    });

    if (!response.ok) {
        const errorData = await response.json().catch(() => ({ message: 'Unknown error' }));
        throw new Error(`Failed to get access token: ${response.status} - ${errorData.error_description || errorData.message}`);
    }

    const data = await response.json();
    return data.access_token;
}

/**
 * Makes an authenticated API call using a provided access token.
 * @param {string} apiUrl - The target API endpoint.
 * @param {string} accessToken - The OAuth access token.
 * @param {object} options - Fetch options.
 * @returns {Promise<Response>} The API response.
 */
async function makeAuthenticatedApiCall(apiUrl, accessToken, options = {}) {
    return fetch(apiUrl, {
        ...options,
        headers: {
            ...options.headers,
            'Authorization': `Bearer ${accessToken}`
        }
    });
}

// Example usage (Node.js environment, as `Buffer` is used):
// (async () => {
//     const TOKEN_URL = 'https://oauth.example.com/token';
//     const CLIENT_ID = process.env.OAUTH_CLIENT_ID;
//     const CLIENT_SECRET = process.env.OAUTH_CLIENT_SECRET;
//     const API_URL = 'https://api.example.com/resource';

//     try {
//         const accessToken = await getOAuthAccessToken(TOKEN_URL, CLIENT_ID, CLIENT_SECRET, 'read write');
//         console.log('Access Token acquired:', accessToken);

//         const apiResponse = await makeAuthenticatedApiCall(API_URL, accessToken, {
//             method: 'GET',
//             headers: { 'Accept': 'application/json' }
//         });

//         if (!apiResponse.ok) {
//             throw new Error(`API call failed: ${apiResponse.status}`);
//         }
//         const data = await apiResponse.json();
//         console.log('API Response:', data);

//     } catch (error) {
//         console.error('Authentication or API call error:', error.message);
//     }
// })();