nodejs

Use a layered architecture with routes defining endpoints, controllers handling HTTP requests, services containing business logic, and repositories managing data access. Dependencies flow downward from routes to repositories, keeping each layer focused on a single responsibility.

Wrap async route handlers with an asyncHandler function that catches promise rejections and forwards them to Express error middleware. Define custom error classes like NotFoundError and ValidationError extending a base AppError with status codes for consistent error responses.

Promise.all rejects immediately when any promise fails, making it suitable for operations that must all succeed. Promise.allSettled waits for all promises and returns each result's status, allowing you to handle partial failures when fetching multiple independent resources.

Use zod to define a schema for process.env with types, defaults, and constraints like minimum string lengths for secrets. Parse process.env at startup so the application fails fast with clear errors when required configuration is missing or invalid.

Inject the repository as a constructor dependency and replace it with a mock object using vi.fn() in vitest. Mock repository methods to return test data or reject with errors, then assert the service calls them with correct arguments and handles failures properly.

Controllers should only handle HTTP concerns like parsing requests and formatting responses. Business rules such as email validation, password hashing, and duplicate checks belong in the service layer, while database queries belong in repositories.