json1-2  Exercise 2: Reading and Sending JSON Over HTTP
=====================================================

Run with Node 24.18.0:  node ex2.mjs   (the servers use a random local port; it does not appear in this output)

--- ex2.mjs ---
// Exercise 2: reading JSON over HTTP safely
import http from 'node:http';
const routes = {
  '/ok':        [200, 'application/json', '{"id":7,"name":"Ada"}'],
  '/notfound':  [404, 'application/json', '{"error":"no such user"}'],
  '/html':      [502, 'text/html', '<html><body>Bad Gateway</body></html>'],
  '/empty':     [204, 'application/json', ''],
  '/badjson':   [200, 'application/json', '{"id":7,'],
  '/wrongtype': [200, 'text/plain', '{"id":8}'],
};
const server = http.createServer((req, res) => { const [s, t, b] = routes[req.url]; res.writeHead(s, { 'content-type': t }); res.end(b); });
await new Promise(r => server.listen(0, '127.0.0.1', r));
const base = 'http://127.0.0.1:' + server.address().port;

async function naive(url) { return (await fetch(url)).json(); }
async function getJson(url) {
  const res = await fetch(url);
  const type = res.headers.get('content-type') ?? '';
  const body = await res.text();                       // read once, as text
  if (!res.ok) throw new Error('HTTP ' + res.status + ': ' + (type.includes('json') ? body : body.slice(0, 40)));
  if (res.status === 204 || body === '') return null;
  try { return JSON.parse(body); }
  catch (e) { throw new Error('bad JSON (' + e.message + '), starts with ' + JSON.stringify(body.slice(0, 12))); }
}
const show = async (fn, path) => { try { return JSON.stringify(await fn(base + path)); } catch (e) { return 'THREW ' + e.message; } };
console.log('path'.padEnd(11) + 'naive: (await fetch(u)).json()'.padEnd(95) + 'getJson(u)');
for (const p of Object.keys(routes)) console.log(p.padEnd(11) + (await show(naive, p)).padEnd(95) + await show(getJson, p));

const r1 = await fetch(base + '/ok'); await r1.json();
try { await r1.json(); } catch (e) { console.log('\nreading the same body twice: ' + e.constructor.name + ': ' + e.message); }

// sending JSON: Content-Type matters
const echo = http.createServer((req, res) => { let b = ''; req.on('data', c => b += c); req.on('end', () => { res.writeHead(200, { 'content-type': 'application/json' }); res.end(JSON.stringify({ type: req.headers['content-type'], body: b })); }); });
await new Promise(r => echo.listen(0, '127.0.0.1', r));
const u = 'http://127.0.0.1:' + echo.address().port;
const send = async init => JSON.stringify(await (await fetch(u, { method: 'POST', ...init })).json());
console.log('\nPOST body JSON.stringify, no header :', await send({ body: JSON.stringify({ a: 1 }) }));
console.log('POST body JSON.stringify, header    :', await send({ headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ a: 1 }) }));
console.log('POST body an object (no stringify)  :', await send({ headers: { 'Content-Type': 'application/json' }, body: { a: 1 } }));
server.close(); echo.close();

Output:
path       naive: (await fetch(u)).json()                                                                 getJson(u)
/ok        {"id":7,"name":"Ada"}                                                                          {"id":7,"name":"Ada"}
/notfound  {"error":"no such user"}                                                                       THREW HTTP 404: {"error":"no such user"}
/html      THREW Unexpected token '<', "<html><bod"... is not valid JSON                                  THREW HTTP 502: <html><body>Bad Gateway</body></html>
/empty     THREW Unexpected end of JSON input                                                             null
/badjson   THREW Expected double-quoted property name in JSON at position 8 (line 1 column 9)             THREW bad JSON (Expected double-quoted property name in JSON at position 8 (line 1 column 9)), starts with "{\"id\":7,"
/wrongtype {"id":8}                                                                                       {"id":8}

reading the same body twice: TypeError: Body is unusable: Body has already been read

POST body JSON.stringify, no header : {"type":"text/plain;charset=UTF-8","body":"{\"a\":1}"}
POST body JSON.stringify, header    : {"type":"application/json","body":"{\"a\":1}"}
POST body an object (no stringify)  : {"type":"application/json","body":"[object Object]"}

WHY THIS WORKS AS AN ANSWER
---------------------------
The server is started inside the script, so every route's behaviour is known exactly and nothing depends on a remote service. Side by side, the shortcut and the helper differ on four of the six routes: the 404 with a JSON
body looks like success to the shortcut (fetch does not reject on a 404); the 502 HTML page gives the shortcut a bare "Unexpected token '<'" while the helper reports the status and the start of the body; the 204 makes the
shortcut throw "Unexpected end of JSON input" where the helper returns null; and the cut-off JSON gives the helper's message the first characters of the body. The text/plain route shows that .json() ignores Content-Type.
The echo route shows the sending side: without the header the server sees text/plain;charset=UTF-8, and passing an object without JSON.stringify sends the text "[object Object]". The helper is one reasonable design, not the only one:
whether a 204 should be null, and how much of an HTML body to quote, are choices for your own code. Not tested: timeouts, redirects, compressed bodies, and servers that send a charset other than UTF-8.