Authenticating JavaScript `fetch` Requests with JWT

/**
 * Fetches data from a protected API endpoint using a JWT for authentication.
 * @param {string} url The API endpoint URL.
 * @param {string} jwtToken The JSON Web Token to use for authorization.
 * @param {RequestInit} options Optional fetch options (method, body, etc.).
 * @returns {Promise<any>} A promise that resolves with the API response data.
 */
async function fetchWithJwt(url, jwtToken, options = {}) {
  if (!jwtToken) {
    throw new Error('JWT token is required for authenticated requests.');
  }

  const headers = {
    'Content-Type': 'application/json', // Default content type, can be overridden
    'Authorization': `Bearer ${jwtToken}`,
    ...options.headers, // Allow overriding or adding more headers
  };

  const fetchOptions = {
    ...options,
    headers,
  };

  try {
    const response = await fetch(url, fetchOptions);

    if (!response.ok) {
      const errorBody = await response.text().catch(() => 'Unknown error');
      throw new Error(`API Error: ${response.status} ${response.statusText} - ${errorBody}`);
    }

    return await response.json();
  } catch (error) {
    console.error('Error during authenticated fetch:', error);
    throw error;
  }
}

// Example Usage:
// const myJwt = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...'; // Replace with your actual JWT
// const protectedApiUrl = 'https://api.example.com/user/profile';

// fetchWithJwt(protectedApiUrl, myJwt)
//   .then(data => console.log('User profile:', data))
//   .catch(error => {
//     console.error('Failed to fetch profile:', error.message);
//     if (error.message.includes('401')) {
//       // Handle unauthorized, e.g., redirect to login or refresh token
//       console.log('Token expired or invalid, please log in again.');
//     }
//   });

// Example with POST data:
// const newPost = { title: 'My New Post', content: 'Lorem ipsum...' };
// fetchWithJwt('https://api.example.com/posts', myJwt, {
//   method: 'POST',
//   body: JSON.stringify(newPost)
// }).then(data => console.log('Post created:', data))
//   .catch(error => console.error('Error creating post:', error));