Step 4: Validate input and handle errors

const http = require("http");

const todos = [];
let nextId = 1;

function send(res, status, data) {
  res.writeHead(status, { "Content-Type": "application/json" });
  res.end(JSON.stringify(data));
}
function readBody(req) {
  return new Promise((resolve, reject) => {
    let raw = "";
    req.on("data", (chunk) => (raw += chunk));
    req.on("end", () => {
      try {
        resolve(raw ? JSON.parse(raw) : {});
      } catch (err) {
        reject(new Error("Invalid JSON"));
      }
    });
  });
}

function validate(body) {
  if (typeof body.title !== "string" || body.title.trim() === "") return "title is required";
  if (body.title.length > 100) return "title must be 100 characters or fewer";
  return null;
}

async function handle(req, res) {
  try {
    if (req.method === "GET" && req.url === "/todos") return send(res, 200, todos);

    if (req.method === "POST" && req.url === "/todos") {
      const body = await readBody(req);
      const problem = validate(body);
      if (problem) return send(res, 400, { error: problem });
      const todo = { id: nextId++, title: body.title.trim(), done: false };
      todos.push(todo);
      return send(res, 201, todo);
    }
    send(res, 404, { error: "Not found" });
  } catch (err) {
    send(res, 400, { error: err.message });
  }
}

// ---- demo: start on a free port, make a few requests, stop ----
const server = http.createServer(handle);
server.listen(0, async () => {
  const base = `http://localhost:${server.address().port}`;
  const call = async (method, path, body) => {
    const res = await fetch(base + path, { method, body: body ? JSON.stringify(body) : undefined });
    const text = await res.text();
    console.log(method, path, "->", res.status, text || "(empty)");
  };
  await call("POST", "/todos", { title: "   " });
  await call("POST", "/todos", {});
  await call("POST", "/todos", { title: "x".repeat(101) });
  const res = await fetch(base + "/todos", { method: "POST", body: "{not json" });
  console.log("POST /todos (bad JSON) ->", res.status, await res.text());
  await call("POST", "/todos", { title: "  Valid title  " });
  server.close();
});

POST /todos -> 400 {"error":"title is required"}
POST /todos -> 400 {"error":"title is required"}
POST /todos -> 400 {"error":"title must be 100 characters or fewer"}
POST /todos (bad JSON) -> 400 {"error":"Invalid JSON"}
POST /todos -> 201 {"id":1,"title":"Valid title","done":false}