Step 8: Test it

const assert = require("assert");
const http = require("http");

// the app under test: a tiny in-memory counter API
const items = [];
const server = http.createServer((req, res) => {
  if (req.method === "POST" && req.url === "/items") {
    let raw = "";
    req.on("data", (c) => (raw += c));
    req.on("end", () => {
      const body = JSON.parse(raw);
      if (!body.name) {
        res.writeHead(400);
        return res.end(JSON.stringify({ error: "name is required" }));
      }
      items.push(body.name);
      res.writeHead(201);
      res.end(JSON.stringify({ count: items.length }));
    });
  } else {
    res.writeHead(404);
    res.end("{}");
  }
});

const tests = [];
const test = (name, fn) => tests.push({ name, fn });

test("creating an item returns 201 and the new count", async (base) => {
  const res = await fetch(base + "/items", { method: "POST", body: JSON.stringify({ name: "a" }) });
  assert.strictEqual(res.status, 201);
  assert.deepStrictEqual(await res.json(), { count: 1 });
});

test("a missing name is rejected with 400", async (base) => {
  const res = await fetch(base + "/items", { method: "POST", body: JSON.stringify({}) });
  assert.strictEqual(res.status, 400);
});

test("unknown routes give 404", async (base) => {
  const res = await fetch(base + "/nope");
  assert.strictEqual(res.status, 404);
});

server.listen(0, async () => {
  const base = `http://localhost:${server.address().port}`;
  let failed = 0;
  for (const t of tests) {
    try {
      await t.fn(base);
      console.log("ok  ", t.name);
    } catch (err) {
      failed++;
      console.log("FAIL", t.name, "-", err.message);
    }
  }
  console.log(failed === 0 ? "all tests passed" : failed + " failed");
  server.close();
});

ok   creating an item returns 201 and the new count
ok   a missing name is rejected with 400
ok   unknown routes give 404
all tests passed