Building a Server

Inspect the request — method, URL, headers, body. Build the response — status, headers, body.

4 min read Level 1/5 #nodejs#http#server

What you'll learn

Read req method, URL, headers
Set status, headers, body on res
Send JSON responses

Every request triggers your handler with two objects: req (IncomingMessage) and res (ServerResponse). Both are streams.

Reading the Request

import { createServer } from "node:http";

createServer((req, res) => {
  console.log(req.method);     // 'GET', 'POST', ...
  console.log(req.url);        // '/users/42?expand=true'
  console.log(req.headers);    // { host: ..., 'user-agent': ... }
  res.end("ok");
}).listen(3000);

req.url is the raw URL string. To split path/query:

const url = new URL(req.url, `http://${req.headers.host}`);
console.log(url.pathname);   // '/users/42'
console.log(url.searchParams.get("expand"));   // 'true'

Writing the Response

res.statusCode = 200;
res.setHeader("Content-Type", "text/plain");
res.end("Hello!");

Or all-in-one with writeHead:

res.writeHead(200, { "Content-Type": "text/plain" });
res.end("Hello!");

res.end() finishes the response. Forget to call it and the request hangs.

Sending JSON

No built-in helper — do it explicitly:

function json(res, status, data) {
  res.writeHead(status, { "Content-Type": "application/json" });
  res.end(JSON.stringify(data));
}

createServer((req, res) => {
  if (req.url === "/api/user") {
    return json(res, 200, { id: 1, name: "Ada" });
  }
  json(res, 404, { error: "not found" });
}).listen(3000);

This is the kind of helper Express bakes in (res.json(obj)).

Reading the Request Body

POST/PUT bodies arrive as a stream:

async function readBody(req) {
  const chunks = [];
  for await (const chunk of req) chunks.push(chunk);
  return Buffer.concat(chunks).toString("utf8");
}

createServer(async (req, res) => {
  if (req.method === "POST") {
    const text = await readBody(req);
    const data = JSON.parse(text);
    // ...
    res.end("ok");
  } else {
    res.end("only POST");
  }
}).listen(3000);

Again — Express/Fastify do this for you.

Routing →