app.js

// index.js

// Required dependencies:
// npm install jsonwebtoken axios
// You'll also need to have your private key file (.pem) from your GitHub App settings.

const jwt = require('jsonwebtoken');
const axios = require('axios');
const fs = require('fs');
const path = require('path');

// --- Configuration ---
// Replace these values with your GitHub App's specific details.

// The unique ID of your GitHub App. Found in your app's settings page.
const APP_ID = '1845091'; 

// The installation ID for the repository you want to access.
// You get this when a user installs your app on their repository.
// For testing, you can find it in the URL after installing the app on a repo
// (e.g., https://github.com/settings/installations/INSTALLATION_ID)
const INSTALLATION_ID = '82827805';

// The path to your private key (.pem file) downloaded from your GitHub App settings.
const PRIVATE_KEY_PATH = path.join(__dirname, './private.pem');

// Repository details
const REPO_OWNER = 'harshit-bstack'; // e.g., 'octocat'
const REPO_NAME = 'flask-otel';   // e.g., 'Hello-World'
const BRANCH_NAME = 'test'; // The branch to compare against 'main'

/**
 * Generates a JSON Web Token (JWT) to authenticate as the GitHub App.
 * The JWT is valid for 10 minutes, which is the maximum allowed by GitHub.
 * @returns {string} The generated JWT.
 */
function generateJwt() {
    // Read the private key from the file system.
    const privateKey = fs.readFileSync(PRIVATE_KEY_PATH, 'utf8');

    // Get the current time in seconds.
    const now = Math.floor(Date.now() / 1000);

    // Create the payload for the JWT.
    const payload = {
        iat: now - 60,      // Issued at time (60 seconds in the past to account for clock drift)
        exp: now + (10 * 60), // Expiration time (10 minutes from now)
        iss: APP_ID         // Issuer: your GitHub App's ID
    };

    // Sign the JWT with your private key using the RS256 algorithm.
    console.log('Generating JWT...');
    return jwt.sign(payload, privateKey, { algorithm: 'RS256' });
}

/**
 * Gets an installation access token for a specific repository.
 * This token is used to make API requests on behalf of the installation.
 * @param {string} appJwt - The JWT for the GitHub App.
 * @returns {Promise<string>} A promise that resolves with the installation access token.
 */
async function getInstallationAccessToken(appJwt) {
    console.log('Requesting installation access token...');
    try {
        const response = await axios.post(
            `https://api.github.com/app/installations/${INSTALLATION_ID}/access_tokens`,
            {}, // No request body is needed
            {
                headers: {
                    'Accept': 'application/vnd.github.v3+json',
                    'Authorization': `Bearer ${appJwt}`, // Authenticate with the JWT
                    'X-GitHub-Api-Version': '2022-11-28'
                }
            }
        );
        
        const accessToken = response.data.token;
        console.log('Successfully received installation access token.');
        return accessToken;
    } catch (error) {
        console.error('Error getting installation access token:', error.response ? error.response.data : error.message);
        throw error;
    }
}

/**
 * Fetches the diff between the 'main' branch and a specified branch.
 * @param {string} accessToken - The installation access token.
 * @param {string} owner - The repository owner's username or organization name.
 * @param {string} repo - The repository name.
 * @param {string} branch - The name of the branch to compare.
 * @returns {Promise<string>} A promise that resolves with the diff content.
 */
async function getBranchDiff(accessToken, owner, repo, branch) {
    const base = 'main'; // The base branch for comparison
    const head = branch; // The head branch (the one with new changes)
    const compareUrl = `https://api.github.com/repos/${owner}/${repo}/compare/${base}...${head}`;

    console.log(`Fetching diff from: ${compareUrl}`);

    try {
        const response = await axios.get(compareUrl, {
            headers: {
                // IMPORTANT: The 'Accept' header is crucial to get the diff format.
                'Accept': 'application/vnd.github.v3.diff',
                'Authorization': `token ${accessToken}`, // Authenticate with the installation token
                'X-GitHub-Api-Version': '2022-11-28'
            }
        });
        
        console.log('Successfully fetched diff!');
        return response.data;
    } catch (error) {
        console.error('Error fetching diff:', error.response ? error.response.data : error.message);
        throw error;
    }
}

/**
 * Main function to run the entire process.
 */
async function main() {
    try {
        // Step 1: Generate the JWT to authenticate as the app.
        const appJwt = generateJwt();

        // Step 2: Use the JWT to get an installation-specific access token.
        const accessToken = await getInstallationAccessToken(appJwt);

        // Step 3: Use the installation access token to fetch the diff from the repo.
        const diff = await getBranchDiff(accessToken, REPO_OWNER, REPO_NAME, BRANCH_NAME);

        // Step 4: Print the resulting diff to the console.
        console.log('\n--- GIT DIFF ---');
        console.log(diff);
        console.log('--- END OF DIFF ---\n');

    } catch (error) {
        console.error('The process failed:', error.message);
    }
}

// Execute the main function.
main();