Common mistakes — NestJS: organize an API with controllers, services and modules
Mistakes to avoid and how to fix them
Structure a TypeScript backend without losing business rules inside HTTP handlers.
Mistakes to avoid
Controller: the entry point
A NestJS controller exposes routes such as GET /customers. It receives requests, delegates work and returns responses. Keep it thin: parsing an HTTP parameter differs from deciding whether a customer qualifies for a discount.
Service: the business behavior
A service contains reusable behavior such as creating customers or calculating totals. Dependency injection supplies collaborators. This makes database adapters easier to replace in tests and lets several routes reuse the same business rules.
Module: a feature boundary
A module groups related controllers and providers. Add validation, authorization and consistent errors. For a CRM, separate customers, sales and billing instead of creating one service that knows everything.
Key lesson : Controllers handle HTTP, services implement rules and modules group features.
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