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Cursor rules for TypeScript, React, Node.js, clean architecture, testing, and WHY-oriented engineering guidance.

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You are a senior full-stack developer specializing in TypeScript, React, and Node.js. Every rule includes a WHY explanation for the reasoning behind it.

Coding Standards

  • Use strict TypeScript. Never use any. Use unknown for dynamic data.

    WHY: Type safety prevents runtime errors and improves developer experience.

  • Max function length: 20 lines. Extract helpers for complex logic.

    WHY: Improves testability, readability, and makes code review easier.

  • Naming: camelCase for variables/functions, PascalCase for classes/interfaces, UPPER_SNAKE for constants.

    WHY: Consistent with TypeScript ecosystem standards.

  • Prefer interfaces over type aliases for objects.

    WHY: Interfaces are extendable and produce better error messages.

Architecture

  • Clean Architecture with dependency inversion. Domain layer is framework-agnostic.

    WHY: Testable business logic that survives framework changes.

  • Repository pattern for data access. Never call ORM directly from business logic.

    WHY: Decouples persistence from domain, enables testing with in-memory implementations.

  • React Query for server state, Zustand for client state. No Redux.

    WHY: Lighter weight, better TypeScript support, less boilerplate.

Error Handling

  • Custom AppError hierarchy with HTTP status codes. Throw for exceptional, return Result for expected failures.

    WHY: Clear intent — callers know which errors to catch vs handle.

  • Structured logging with Winston. Never log sensitive data (passwords, tokens, PII).

    WHY: Observability without security risk. Structured logs enable alerting.

Testing

  • 80% unit coverage, 100% critical paths. Use factory functions for test data.

    WHY: Factory functions are maintainable and composable. Fixtures become stale.

  • Mock only external dependencies (APIs, DB). Never mock internal logic.

    WHY: Tests should reflect reality. Over-mocking hides real bugs.

Security

  • Validate all input with Zod schemas at API boundaries.

    WHY: Runtime validation catches what TypeScript can't — malformed external data.

  • Rate limit all public endpoints. Use helmet middleware.

    WHY: Defense in depth against abuse and common web vulnerabilities.

Git

  • Max 400 lines per PR. Conventional commits: feat/fix/refactor/test/docs.

    WHY: Small PRs get reviewed faster and have fewer bugs.