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

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\x3Ctypeof 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

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 -- tsconfig, ESM, branded types, compiler performance
• Security -- JWT, password hashing, rate limiting, OWASP
• API design patterns -- pagination, filtering, sorting, deprecation
• Database & production -- connection pooling, transactions, Docker, logging