pipe http request stream as gateway: sample code

[slow-groovin]

on koa server.

/**
 * This approach worked. Referenced from https://github.com/typestack/routing-controllers/issues/576.
 * 
 * A new Duplex stream must be created to allow the response to be piped into it.
 * The stream is returned within a promise, which is then handled by the framework
 * to write to the HTTP response.
 */
@Get("/pipe")
async pipeSample(@Ctx() ctx: Context) {
    return new Promise((resolve, reject) => {
        const stream = new PassThrough();  // Create a new PassThrough stream to buffer the response data.
        let id = (Math.random() * 50).toFixed(0);  // Generate a random ID between 0 and 50, as a string.
        
        // Make an HTTP request to an external API.
        http.request("http://jsonplaceholder.typicode.com/todos/" + id, (response) => {
            // Set the response headers and status code from the external API response.
            ctx.res.writeHead(response.statusCode ?? 200, response.statusMessage, response.headers);
            
            logger.info(`'status', ${response.statusCode}`);  // Log the status code of the response.
            
            response.pipe(stream);  // Pipe the response data into the PassThrough stream.
            
            // Resolve the promise with the stream once the response ends.
            response.on('end', () => {
                resolve(stream);
            });
            
            // Reject the promise if an error occurs during the response.
            response.on('error', (err) => {
                reject(err);
            });
        }).end();  // End the request, sending it to the server.
    });
}