HTTP and JSON: requests, responses, headers and data format

POST /api/orders HTTP/1.1
Host: 127.0.0.1:8080
Content-Type: application/json
Content-Length: 27

{"item": "latte", "qty": 1}

import { createServer } from "node:http";

const orders = new Map([[1, { id: 1, item: "cappuccino", qty: 2 }]]);
let nextId = 2;

function sendJson(res, status, data, headers = {}) {
  const body = JSON.stringify(data);
  res.writeHead(status, {
    "Content-Type": "application/json; charset=utf-8",
    "Content-Length": Buffer.byteLength(body),
    ...headers,
  });
  res.end(body);
}

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

const server = createServer(async (req, res) => {
  const url = new URL(req.url, "http://localhost");
  console.log(req.method, url.pathname, req.headers["content-type"] ?? "-");

  const match = url.pathname.match(/^\/api\/orders\/(\d+)$/);
  if (req.method === "GET" && match) {
    const order = orders.get(Number(match[1]));
    if (!order) return sendJson(res, 404, { error: "Order not found" });
    return sendJson(res, 200, order, { "Cache-Control": "no-store" });
  }

  if (req.method === "POST" && url.pathname === "/api/orders") {
    if (!req.headers["content-type"]?.startsWith("application/json")) {
      return sendJson(res, 415, { error: "Expected Content-Type: application/json" });
    }
    let input;
    try {
      input = JSON.parse(await readBody(req));
    } catch {
      return sendJson(res, 400, { error: "Request body is not valid JSON" });
    }
    if (typeof input?.item !== "string" || !Number.isInteger(input.qty) || input.qty < 1) {
      return sendJson(res, 422, { error: "Fields required: item (string), qty (integer >= 1)" });
    }
    const order = { id: nextId++, item: input.item, qty: input.qty };
    orders.set(order.id, order);
    return sendJson(res, 201, order, { Location: `/api/orders/${order.id}` });
  }

  sendJson(res, 404, { error: "Not found" });
});

server.listen(8080, "127.0.0.1", () => {
  console.log("API: http://127.0.0.1:8080");
});

$ curl -si http://127.0.0.1:8080/api/orders/1
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 36
Cache-Control: no-store
Date: Sun, 11 Oct 2026 21:56:00 GMT
Connection: keep-alive
Keep-Alive: timeout=5

{"id":1,"item":"cappuccino","qty":2}

$ curl -si -X POST http://127.0.0.1:8080/api/orders -H 'Content-Type: application/json' -d '{"item": "latte", "qty": 1}'
HTTP/1.1 201 Created
Content-Type: application/json; charset=utf-8
Content-Length: 31
Location: /api/orders/2
Date: Sun, 11 Oct 2026 21:56:00 GMT
Connection: keep-alive
Keep-Alive: timeout=5

{"id":2,"item":"latte","qty":1}

$ curl -si -X POST http://127.0.0.1:8080/api/orders -H 'Content-Type: application/json' -d "{'item': 'latte'}"
HTTP/1.1 400 Bad Request
Content-Type: application/json; charset=utf-8
Content-Length: 42
Date: Sun, 11 Oct 2026 21:56:00 GMT
Connection: keep-alive
Keep-Alive: timeout=5

{"error":"Request body is not valid JSON"}

$ curl -si -X POST http://127.0.0.1:8080/api/orders -d '{"item": "latte", "qty": 1}'
HTTP/1.1 415 Unsupported Media Type
Content-Type: application/json; charset=utf-8
Content-Length: 51
Date: Sun, 11 Oct 2026 21:56:01 GMT
Connection: keep-alive
Keep-Alive: timeout=5

{"error":"Expected Content-Type: application/json"}

const API = "http://127.0.0.1:8080/api";

async function request(method, path, body) {
  const res = await fetch(API + path, {
    method,
    headers: body === undefined ? {} : { "Content-Type": "application/json" },
    body: body === undefined ? undefined : JSON.stringify(body),
  });
  const data = await res.json();
  console.log(method, path, "→", res.status, res.ok ? "ok" : "error", data);
  return data;
}

await request("GET", "/orders/1");
const created = await request("POST", "/orders", { item: "latte", qty: 1 });
await request("GET", `/orders/${created.id}`);
await request("POST", "/orders", { item: "latte", qty: 0 });
await request("GET", "/orders/999");

$ node client.mjs
GET /orders/1 → 200 ok { id: 1, item: 'cappuccino', qty: 2 }
POST /orders → 201 ok { id: 2, item: 'latte', qty: 1 }
GET /orders/2 → 200 ok { id: 2, item: 'latte', qty: 1 }
POST /orders → 422 error { error: 'Fields required: item (string), qty (integer >= 1)' }
GET /orders/999 → 404 error { error: 'Order not found' }

const order = {
  id: 7,
  createdAt: new Date("2026-10-11T09:30:00Z"),
  comment: undefined,
  discount: NaN,
  format() {},
};
console.log(JSON.stringify(order));
console.log(JSON.stringify({ items: ["latte", undefined] }));
console.log(JSON.stringify({ id: 1, tags: ["new"] }, null, 2));

const broken = [
  "{'item': 'latte'}",
  '{"item": "latte",}',
  '{"qty": 012}',
  "",
];
for (const text of broken) {
  try {
    JSON.parse(text);
  } catch (error) {
    console.log(error.name + ":", error.message);
  }
}

console.log(JSON.parse('{"id": 9007199254740993}').id);

try {
  JSON.stringify({ total: 10n });
} catch (error) {
  console.log(error.name + ":", error.message);
}

const a = { name: "a" };
a.self = a;
try {
  JSON.stringify(a);
} catch (error) {
  console.log(error.name + ":", error.message.split("\n")[0]);
}

$ node json-pitfalls.mjs
{"id":7,"createdAt":"2026-10-11T09:30:00.000Z","discount":null}
{"items":["latte",null]}
{
  "id": 1,
  "tags": [
    "new"
  ]
}
SyntaxError: Expected property name or '}' in JSON at position 1 (line 1 column 2)
SyntaxError: Expected double-quoted property name in JSON at position 17 (line 1 column 18)
SyntaxError: Unexpected number in JSON at position 9 (line 1 column 10)
SyntaxError: Unexpected end of JSON input
9007199254740992
TypeError: Do not know how to serialize a BigInt
TypeError: Converting circular structure to JSON