corsProxy.js

const http = require('http');
const https = require('https');
const url = require('url');

/** Mounts a proxy at `basePath` such that requests of the form
 * `${basePath}/http://www.example.com?queryString=value` will be converted to
 * `http://www.example.com?queryString=value`, preserving any request headers.
 * The response is piped directly to the client with no processing.
 */
module.exports = function corsProxy(basePath) {
    const pathOffset = basePath.length;
    return function(req, res, next) {
        const pathIndex = req.url.indexOf(basePath);
        // base path doesn't match, ignore
        if (pathIndex < 0) {
            return next();
        }

        const targetUrl = req.url.substring(pathIndex + pathOffset + 1);
        const config = url.parse(targetUrl);
        config.method = req.method;

        // We need to use a different request class depending on the
        // protocol
        const handler = config.protocol === 'https:' ? https : http;

        const proxyReq = handler.request(config, function(proxyRes) {
            res.writeHead(proxyRes.statusCode, proxyRes.headers);
            proxyRes.pipe(res, {
                end: true
            });
        });

        proxyReq.on('error', err => res.status(500).send(err));

        // Copy over all headers except for 'Host', since that value would
        // point to *this* server, and not the remote server.
        Object.keys(req.headers).forEach(key => {
            if (key.toLowerCase() === 'host') {
                return;
            }
            proxyReq.setHeader(key, req.headers[key]);
        });

        req.pipe(proxyReq, {
            end: true
        });
    };
};