What is the role of a controller in NestJS?

Controllers handle incoming HTTP requests and return responses. They are decorated with @Controller('route-prefix') and their methods use HTTP-method decorators like @Get(), @Post(), etc. Controllers are thin — they delegate business logic to providers/services.

A controller is responsible for handling incoming requests and returning responses to the client. It should remain thin and delegate actual business logic to services.

Basic controller example

Key responsibilities of a controller:

1
Map routes to handler methods using decorators.
2
Extract data from the request (params, query, body, headers).
3
Delegate logic to services.
4
Return the response (NestJS handles serialization automatically).

Difficulty: 5/10

Topics: routing, request handling, dependency injection

Scenario Questions

0-2 years experience

1
We need a new endpoint /users that returns a list of users; walk me through how you would create the controller and what its responsibilities are.
2
A request hits a route defined in a controller but the response body is always empty; what might be missing in the controller method?
3
How would you use decorators in a controller to extract query parameters and pass them to a service?

2-5 years experience

1
Our controller calls several services and sometimes returns a 500 error; how would you structure error handling and status code mapping inside the controller?
2
During a review you see a controller directly accessing the database; why is that a problem and how would you refactor it?
3
If you need to protect all routes in a controller with authentication, what options do you have and what trade‑offs do they involve?

5-8 years experience

1
Our API gateway forwards requests to multiple NestJS microservices; how would you design controllers to keep latency low and avoid coupling business logic to the controller layer?
2
When scaling to thousands of concurrent requests, what considerations around async handling, interceptors, or pipes become critical in controller design?
3
We need to version our API without duplicating controller code; how would you approach versioning at the controller level while keeping the codebase maintainable?

8+ years experience

1
We are migrating a large legacy Express codebase to NestJS; how would you plan the transition of routing logic and controllers to minimize disruption across teams?
2
Different teams have written controllers with varying patterns; what governance or architectural guidelines would you establish to standardize controller responsibilities and promote reuse?
3
If we want to expose both REST and GraphQL endpoints for the same business operations, how would you structure controllers (or their equivalents) to avoid duplication and keep contracts clear?

Follow-up Questions

❓What would happen if you placed business logic directly inside a controller method?
❓How does NestJS instantiate controllers and what lifecycle hooks are available?
❓Can you explain how you would apply a global interceptor to all controllers?