Skill: iliaal/compound-eng-nodejs-backend (version 3.0.5)

--- name: ia-nodejs-backend class: language description: >- Node.js backend patterns: layered architecture, TypeScript, validation, error handling, security, deployment. Use when building REST APIs, REST endpoints, middleware, Express/Fastify/Hono/NestJS/Koa servers, tRPC procedures, Bun servers, or server-side TypeScript. paths: "**/*.ts,**/*.js,**/*.mjs,**/*.cjs" --- # Node.js Backend **Verify before implementing**: For framework-specific APIs (Express 5, Fastify 5, Node.js 22+ built-ins), search current docs via `search_docs` before writing code. Training data may lag current releases. ## Framework Selection | Context | Choose | Why | |---------|--------|-----| | Edge/Serverless | Hono | Zero-dep, fastest cold starts | | Performance API | Fastify | Higher throughput than Express, built-in schema validation | | Enterprise/team | NestJS | DI, decorators, structured conventions | | Legacy/ecosystem | Express | Most middleware, widest adoption | Ask user: deployment target, cold start needs, team experience, existing codebase. ## Architecture ``` src/ ├── routes/ # HTTP: parse request, call service, format response ├── middleware/ # Auth, validation, rate limiting, logging ├── services/ # Business logic (no HTTP types) ├── repositories/ # Data access only (queries, ORM) ├── config/ # Env, DB pool, constants └── types/ # Shared TypeScript interfaces ``` - Routes never contain business logic - Services never import Request/Response - Repositories never throw HTTP errors - Dependencies point inward only (Clean Architecture rule): routes -> services -> repositories. Never the reverse. - For scripts/prototypes: single file is fine -- ask "will this grow?" ## TypeScript Rules - Use `import type { }` for type-only imports -- eliminates runtime overhead - Prefer `interface` for object shapes (2-5x faster type resolution than intersections) - Prefer `unknown` over `any` -- forces explicit narrowing - Use `z.infer<typeof Schema>` as single source of truth -- never duplicate types and schemas - Minimize `as` assertions -- use type guards instead - Add explicit return types to exported functions (faster declaration emit) - Untyped package? `declare module 'pkg' { const v: unknown; export default v; }` in `types/ambient.d.ts` ## Validation **Zod** (TypeScript inference) or **TypeBox** (Fastify native). Validate at boundaries only: request entry, before DB ops, env vars at startup. Use `.extend()`, `.pick()`, `.omit()`, `.partial()`, `.merge()` for DRY schemas. ## Error Handling Custom error hierarchy: `AppError(message, statusCode, isOperational)` → `ValidationError(400)`, `NotFoundError(404)`, `UnauthorizedError(401)`, `ForbiddenError(403)`, `ConflictError(409)` Centralized handler middleware: - `AppError` → return `{ error: message }` with statusCode - Unknown → log full stack, return 500 + generic message in production - Async wrapper: `const asyncHandler = (fn) => (req, res, next) => Promise.resolve(fn(req, res, next)).catch(next);` Codes: 400 bad input | 401 no auth | 403 no permission | 404 missing | 409 conflict | 422 business rule | 429 rate limited | 500 server fault ## API Design **Contract-first**: define route schemas (Zod schemas, Fastify JSON Schema, or OpenAPI spec) before writing handler logic. The schema is the contract -- implementation follows. Generate OpenAPI/Swagger docs from these schemas for interactive API documentation. - **Hyrum's Law awareness**: every observable response field, ordering, or timing becomes a dependency for callers. Use Zod schemas or Fastify response schemas to control exactly what's serialized -- never return raw ORM objects or untyped objects from handlers. - **Addition over modification**: add new optional fields rather than changing or removing existing ones. Removing a field from a response schema breaks callers silently. Deprecate first (mark in OpenAPI spec), remove in a later version. - **Consistent error envelope**: all errors -- validation, auth, not-found, application -- must produce the same `{ error: { code, message, details? } }` structure. Centralize in the error handler middleware. Callers build error handling once; inconsistent errors force per-endpoint special cases. - **Boundary validation**: validate at the middleware/route handler level (Zod `.parse()` on request body/params, Fastify schema validation). Services and repositories trust that input was validated at entry -- no redundant checks scattered through business logic. - **Third-party responses are untrusted data**: validate shape and content of external API responses before using them in logic, rendering, or decision-making. A compromised or misbehaving service can return unexpected types, malicious content, or missing fields. Parse through a Zod schema before use. - **Resources**: plural nouns (`/users`), max 2 nesting levels (`/users/:id/orders`) - **Methods**: GET read | POST create | PUT replace | PATCH partial | DELETE remove - **Versioning**: URL path `/api/v1/` - **Response**: `{ data, pagination?: { page, limit, total, totalPages } }` - **Queries**: `?page=1&limit=20&status=active&sort=createdAt,desc` - Return `Location` header on 201. Use 204 for successful DELETE with no body. ## Async Patterns | Pattern | Use When | |---------|----------| | `async/await` | Sequential operations | | `Promise.all` | Parallel independent ops | | `Promise.allSettled` | Parallel, some may fail | | `Promise.race` | Timeout or first-wins | Never use readFileSync or other sync methods in production -- use `fs.promises` or stream equivalents. Offload CPU work to worker threads (Piscina). Stream large payloads. ## Production Resilience - **Fail-fast env validation**: parse and validate all environment variables at startup with a Zod schema (`const env = envSchema.parse(process.env)`). If invalid, crash before serving traffic. Never discover a missing env var on the first request that needs it. - **Health endpoints**: expose both `/health` (shallow, always 200 if process is alive) and `/ready` (deep, verifies database, cache, and critical dependencies are reachable). Load balancers probe `/ready` for traffic routing; monitoring probes `/health` for process liveness. Don't conflate them. - **Caching**: Redis cache-aside for DB/API responses; in-memory LRU with TTL for hot paths. Always invalidate on writes. - **Load shedding**: `@fastify/under-pressure` (or equivalent) -- monitor event loop delay, heap, RSS; return 503 when thresholds exceeded. - **Response schemas**: In Fastify, always define response schemas -- enables `fast-json-stringify` for 2-3x faster serialization. - **Circuit breaker**: use `opossum` for outbound service calls. States: CLOSED (normal) -> OPEN (failing, return fallback) -> HALF_OPEN (probe). Prevents cascade failures when downstream services are down. ## Discipline - Simplicity first -- every change as simple as possible, impact minimal code - Only touch what's necessary -- avoid introducing unrelated changes - No hacky workarounds -- if a fix feels wrong, step back and implement the clean solution - Before adding a new abstraction, verify it appears in 3+ places. If not, inline it. - If a fix requires bypassing TypeScript (`as any`, non-null assertions on untrusted data, `// @ts-ignore`), treat it as a design smell and find the typed solution - Verify: `tsc --noEmit && npm test` pass with zero warnings before declaring done ## Verify - `tsc --noEmit` passes with zero errors - `npm test` passes with zero failures - No TypeScript bypasses (`as any`, `@ts-ignore`) in new code ## References - [TypeScript config](./references/typescript-config.md) -- tsconfig, ESM, branded types, compiler performance - [Security](./references/security.md) -- JWT, password hashing, rate limiting, OWASP - [API design patterns](./references/api-design.md) -- pagination, filtering, sorting, deprecation - [Database & production](./references/database-production.md) -- connection pooling, transactions, Docker, logging


Security scan verdicts — ClawScan: clean | static analysis: clean | VirusTotal: clean | SkillSpector: clean (LOW)