Node/TypeScript Backend Patterns

Project layout

• Feature folders over layer folders: src/orders/{orders.router,orders.service,orders.repo,orders.test}.ts beats src/{routes,services,repos}/orders.ts scattered three places.
• One composition root (src/app.ts) wires everything; entry (src/main.ts) only boots. Handlers stay thin — parse/validate → service call → shape response.

Async discipline

• No floating promises: every promise is awaited, returned, or explicitly void-ed with a comment. Unhandled rejection = crash in modern Node.
• Propagate AbortSignal through request-scoped work (fetch, DB timeouts); cancel on client disconnect for expensive handlers.
• No sync I/O (readFileSync, execSync) on request paths — boot time only.
• Concurrency with intent: Promise.all for independent work, for…of await when order matters, a limiter (p-limit) when fan-out is unbounded.

Config

• Typed and validated at boot (zod or equivalent): missing/invalid env kills the process at startup with a clear message — never process.env.X! scattered through the codebase.
• One config.ts exports the parsed object; nothing else reads process.env.

Errors

• Central error taxonomy: AppError subtypes with status + code (NotFound, Validation, Conflict, Upstream). Handlers throw domain errors; ONE error middleware maps them to responses (problem+json shape).
• Never catch (e) {} — swallow nothing; wrap-and-rethrow with context or let it propagate.

DI boundaries

• NestJS: providers with explicit scopes; beware request-scoped bleeding into singletons.
• Express/Fastify: manual factories — services take dependencies as constructor/args, never import singletons directly. Makes the testing shape below possible.

ESM/CJS pitfalls

• Pick ONE module system per package and align "type", tsconfig module, and tooling. Mixed default/named interop errors are config bugs, not code bugs.
• __dirname does not exist in ESM — import.meta.url + fileURLToPath.

Testing shape

• vitest/jest for units (services with faked repos), supertest against the composed app for HTTP behavior (status, body, headers — not internals).
• Test files co-located with the feature. Integration tests own their fixtures.

Logging

• Structured (pino), request-correlated (request-id middleware); never console.log in request paths. Log the error object, not err.message alone (stack + cause matter).