Node.js Express Server-Side Proxy for External APIs

const express = require('express');
const axios = require('axios');
const cors = require('cors'); // To handle CORS for our own API

const app = express();
const port = 3001; // Port for our proxy server

// Allow requests from specific origins, or all origins for development
app.use(cors({ origin: 'http://localhost:3000' })); // Example: only allow frontend on port 3000
// app.use(cors()); // Allow all origins (use with caution in production)

// Middleware to parse JSON request bodies
app.use(express.json());

// Our external API key (THIS SHOULD BE IN ENVIRONMENT VARIABLES IN PRODUCTION)
const EXTERNAL_API_KEY = process.env.EXTERNAL_API_KEY || 'YOUR_SUPER_SECRET_EXTERNAL_API_KEY';
const EXTERNAL_API_BASE_URL = 'https://api.example.com'; // Replace with your target API base URL

/**
 * Proxies a GET request to an external API.
 * @param {string} externalPath - The path segment for the external API.
 * @returns {Function} Express middleware.
 */
app.get('/proxy/external/:externalPath', async (req, res) => {
    const { externalPath } = req.params;
    const queryParams = req.query; // Forward query parameters

    const targetUrl = `${EXTERNAL_API_BASE_URL}/${externalPath}`;
    console.log(`Proxying GET request to: ${targetUrl} with query:`, queryParams);

    try {
        const response = await axios.get(targetUrl, {
            params: queryParams,
            headers: {
                'Authorization': `Bearer ${EXTERNAL_API_KEY}`, // Add API key for external service
                // Forward any specific headers if needed, e.g., 'X-Custom-Header': req.headers['x-custom-header']
            },
        });

        // Forward the status and data from the external API to our client
        res.status(response.status).json(response.data);
    } catch (error) {
        console.error(`Error proxying GET to ${targetUrl}:`, error.message);
        // Handle different error types (network, API error responses)
        if (error.response) {
            res.status(error.response.status).json(error.response.data);
        } else {
            res.status(500).json({ message: 'Internal Server Error during proxy request' });
        }
    }
});

/**
 * Proxies a POST request to an external API.
 * This example demonstrates forwarding a request body.
 */
app.post('/proxy/external/:externalPath', async (req, res) => {
    const { externalPath } = req.params;
    const requestBody = req.body; // The client's request body

    const targetUrl = `${EXTERNAL_API_BASE_URL}/${externalPath}`;
    console.log(`Proxying POST request to: ${targetUrl} with body:`, requestBody);

    try {
        const response = await axios.post(targetUrl, requestBody, {
            headers: {
                'Authorization': `Bearer ${EXTERNAL_API_KEY}`,
                'Content-Type': 'application/json', // Assuming JSON body
            },
        });
        res.status(response.status).json(response.data);
    } catch (error) {
        console.error(`Error proxying POST to ${targetUrl}:`, error.message);
        if (error.response) {
            res.status(error.response.status).json(error.response.data);
        } else {
            res.status(500).json({ message: 'Internal Server Error during proxy request' });
        }
    }
});


app.listen(port, () => {
    console.log(`Proxy server listening at http://localhost:${port}`);
    console.log('Example usage from client (assuming client is on http://localhost:3000):');
    console.log(`fetch('http://localhost:${port}/proxy/external/data?param=value')`);
    console.log(`fetch('http://localhost:${port}/proxy/external/submit', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ item: 'new item' }) })`);
});