undici 8.10.0: POST JSON and read the parsed reply with undici.request

{"case":{"caseId":"case:sha256:5eca53004109cf3f84dc692c161ad62b1eeeb43f76509fa0bdb1189e5e70217d","constraints":{"moduleSystem":"esm","runtime":"node"},"contract":["start a local echo server","POST a JSON body with undici.request","assert the server saw application/json with the exact body","assert body.json() returns the parsed reply and statusCode is 200"],"goal":"POST JSON and read the parsed reply with undici.request","kind":"HOW","packages":["pkg:npm/undici@8.10.0"],"schemaVersion":1,"symbols":["undici.request","body.json"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","ecosystem":"npm","executionContext":"node","moduleSystem":"esm","os":"linux","packageManager":"npm","runtime":"node","runtimeVersion":"22","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/undici@8.10.0"],"schemaVersion":1,"symbols":["undici.request","body.json"],"verifierAdapter":"node-typescript@1"}

{
  "name": "csx-sample-undici-request-json",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "csx-sample-undici-request-json",
      "version": "1.0.0",
      "license": "MIT-0",
      "dependencies": {
        "undici": "8.10.0"
      }
    },
    "node_modules/undici": {
      "version": "8.10.0",
      "resolved": "https://registry.npmjs.org/undici/-/undici-8.10.0.tgz",
      "integrity": "sha512-HvltHd7avK13QIw/oLe4qoOLyoVSoafqJ2jYOrtMRBkbYT31eiBQ8O0ehRKZiEZCMEyLFQNIADpgCWC5fALvYQ==",
      "license": "MIT",
      "engines": {
        "node": ">=22.19.0"
      }
    }
  }
}

{
  "name": "csx-sample-undici-request-json",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "license": "MIT-0",
  "dependencies": {
    "undici": "8.10.0"
  }
}

import { request } from 'undici';

// undici.request does not serialize or parse for you: the body must be a
// string and the response body is a stream with .json()/.text() helpers.
// Skipping await on body.json() leaks the connection back into the pool
// unread, which stalls later requests.
export async function postJSON(url, payload) {
  const { statusCode, body } = await request(url, {
    method: 'POST',
    headers: { 'content-type': 'application/json' },
    body: JSON.stringify(payload),
  });
  return { statusCode, data: await body.json() };
}

import http from 'node:http';
import { strict as assert } from 'node:assert';
import { postJSON } from '../src/index.mjs';

const payload = { hello: 'codesamplex', n: 7 };
let seen = null;
const server = http.createServer((req, res) => {
  let raw = '';
  req.on('data', (c) => (raw += c));
  req.on('end', () => {
    seen = { type: req.headers['content-type'], raw, method: req.method };
    res.writeHead(200, { 'content-type': 'application/json' });
    res.end(JSON.stringify({ echoed: JSON.parse(raw) }));
  });
});
await new Promise((r) => server.listen(0, '127.0.0.1', r));

const out = await postJSON(`http://127.0.0.1:${server.address().port}/echo`, payload);
server.close();

assert.equal(seen.method, 'POST');
assert.match(seen.type, /^application\/json/);
assert.deepEqual(JSON.parse(seen.raw), payload);
assert.equal(out.statusCode, 200);
assert.deepEqual(out.data.echoed, payload);
console.log('CONTRACT PASS: undici.request posted JSON and body.json() parsed the reply');