Nodejs

Node.js backend services

Stand up a plain Node.js HTTP/JSON service — node:http or Express 5 — that a person can read, test, and operate without a DI framework. This skill is about the runtime and the request lifecycle: how to lay out the code, how to keep async correct, how errors become status codes, how config fails fast, and how the process dies cleanly. The contract shape, the type system, and the data layer live elsewhere (see the boundary below).

Boundary

• The app is built around @Module / @Injectable / DI providers, guards, interceptors, pipes, exception filters → nestjs. Nest starts the moment the DI container appears; route away then, do not half-build it here.
• Pure TypeScript type system, generics, tsconfig → typescript. This skill uses TS but does not teach types.
• REST resource modeling, versioning, pagination, status-code semantics (framework-agnostic) → api-design. This skill wires the handlers; api-design decides the contract. Cross-cutting error taxonomy → error-handling.
• Schema, queries, connection pool, migrations → postgresdb / prisma-orm / drizzle-orm. Calling a repository stays here; designing the table does not.
• Containerizing / shipping → docker / deployment. Choosing the logging/metrics/tracing stack → observability. This skill emits structured logs and a shutdown hook; it does not own the pipeline.

Decide first

Pick the runtime shape before writing code — the wrong choice is expensive to undo.

Defaults for a new service: pin Node 24 (Active LTS) in engines.node; Node 22 is Maintenance LTS; Node 26 is Current (released 2026-05-05, enters LTS Oct 2026) — adopt it only if you want Temporal/V8 14.6 and can track Current. Write ESM for new code ("type": "module"), CommonJS only when a hard dependency forces it.

// package.json
{
  "type": "module",
  "engines": { "node": ">=24" },
  "scripts": {
    "dev": "node --watch src/server.ts",
    "test": "node --test"
  }
}

Project layout

Concrete tree. The names are conventions, not magic — but the split is load-bearing.

src/
  app.ts          # build + return the Express app; NEVER calls listen()
  server.ts       # imports app, listens, owns SIGTERM/shutdown
  config/
    env.ts        # validate process.env once, export a typed `config`
  routes/
    users.ts      # router only: path → controller
  controllers/
    users.ts      # parse request, call service, shape response
  services/
    users.ts      # business logic, no req/res objects
  repositories/
    users.ts      # data access; the only layer that touches the DB
  errors/
    app-error.ts  # AppError/HttpError with status + code

Rule — separate app construction from listen(). app.ts builds and returns the app; server.ts binds the port and owns shutdown. Why: tests import app and exercise routes in-process without binding a port, so they run fast and in parallel.

// Bad: index.ts listens at import time — untestable, double-binds in tests
const app = express();
app.get("/health", (_req, res) => res.json({ ok: true }));
app.listen(3000); // side effect on import

// Good: app.ts
export function buildApp() {
  const app = express();
  app.get("/healthz", (_req, res) => res.json({ ok: true }));
  return app; // no listen here
}

Async rules

Every async mistake here is a latent production incident, not a style nit.

• Await or return every promise. A floating promise is a latent process crash — since Node 15 an unhandled rejection terminates the process by default. The error happens later, detached from its handler.
• Never mix callback + promise styles. Promisify once at the boundary (util.promisify or fs/promises) and stay in promises after that. Half-converted code swallows errors.
• Promise.all for independent work; sequential await only for true dependencies. Awaiting independent calls one by one wastes wall-clock time.
• Use AbortSignal.timeout(ms) for per-operation deadlines and thread the signal into fetch/DB/long ops. Compose with AbortController so shutdown can cancel in-flight work (see graceful shutdown).
• Never rely on a global unhandledRejection handler as control flow. Log and exit there if anything; do not use it to keep running.

// Bad: forgotten await — the rejection floats and crashes later, error lost
function handler(req, res) {
  saveAudit(req.body);          // returns a promise nobody awaits
  res.json({ ok: true });       // responds before save resolves/rejects
}

// Good: await it (Express 5 forwards a throw to the error middleware)
async function handler(req, res) {
  await saveAudit(req.body);
  res.json({ ok: true });
}

Error handling

This is the core, and the most common bug. In Express 5 an async handler that rejects or throws is auto-forwarded to the error middleware — no try/catch + next(err) wrapper, no asyncHandler. Source: Express 5 migration guide and the framework's own router tests (a value-less Promise.reject() becomes an Error with message Rejected promise).

• Define one AppError (or HttpError) carrying status and code. Throw it from services; controllers don't translate.
• Register exactly one 4-arg (err, req, res, next) error middleware, LAST, after every route. Arity is what makes Express treat it as an error handler — a 3-arg function is a normal middleware no matter what you name it. Order still matters in v5.
• A 404 fallthrough handler goes just before the error middleware.
• Map known errors → their status; unknown → 500; never leak the stack or raw message in production.

// errors/app-error.ts
export class AppError extends Error {
  constructor(public status: number, public code: string, message: string) {
    super(message);
  }
}

// app.ts — registration ORDER (routes → 404 → error handler)
app.use("/users", usersRouter);

app.use((_req, res) => res.status(404).json({ code: "not_found" }));

// LAST, exactly 4 args — this is the error handler
app.use((err, _req, res, _next) => {
  const status = err instanceof AppError ? err.status : 500;
  const code = err instanceof AppError ? err.code : "internal_error";
  if (status >= 500) logger.error({ err }, "unhandled");
  res.status(status).json({
    code,
    message: status < 500 ? err.message : "Internal Server Error",
    ...(process.env.NODE_ENV !== "production" && { stack: err.stack }),
  });
});

// Bad: the symptom "async route throws but client gets 200 empty body"
app.use((err, _req, res, _next) => { /* error handler */ });  // registered FIRST
app.get("/users/:id", async (req, res) => {
  const u = await findUser(req.params.id);   // throws NotFound
  res.json(u);                                // never reached; no handler after → hangs/empties
});

The error handler placed before the route never sees the throw, so the response is whatever was half-written. Put it last. See references/express5-migration.md for the full v4→5 breaking-change checklist, the middleware-order diagram, and the legacy v4 asyncHandler wrapper.