What are the req and res objects in Node.js HTTP?

Direct answer

req (IncomingMessage) is a readable stream of the request — method, url, headers, body; res (ServerResponse) is writable — set status/headers, write chunks, end response; both inherit stream backpressure behavior.

Treat req as input and res as output for one HTTP exchange. Large POST bodies should be consumed as streams, not buffered unbounded in memory.

req	res
method	statusCode (default 200)
url	setHeader / writeHead
headers	write / end
Readable stream body	Writable stream body
socket	headersSent flag

Read body — collect chunks: req.on('data'); or use framework parser for JSON.
headersSent — after writeHead/send, cannot change status — guard double responses.
Framework mapping — Fastify request/reply wrap same concepts with validation and serializers.

req-res-body.mjs

function readJsonBody(req) {
  return new Promise((resolve, reject) => {
    const chunks = [];
    req.on("data", (chunk) => chunks.push(chunk));
    req.on("end", () => {
      try {
        resolve(JSON.parse(Buffer.concat(chunks).toString("utf8")));
      } catch (err) {
        reject(err);
      }
    });
    req.on("error", reject);
  });
}

// In handler:
// const body = await readJsonBody(req);
// res.writeHead(201, { "Content-Type": "application/json" });
// res.end(JSON.stringify({ id: 1 }));