Why controllers handle incoming requests in NestJS - Quick Recap

Recall & Review

beginner

What is the main role of a controller in NestJS?

A controller in NestJS handles incoming HTTP requests and returns responses. It acts like a receptionist who listens to requests and directs them to the right place.

beginner

Why do controllers handle incoming requests instead of services?

Controllers focus on receiving and managing requests, while services handle business logic. This separation keeps code organized and easier to maintain.

intermediate

How does a controller know which requests to handle?

Controllers use decorators like @Get(), @Post() to match specific HTTP methods and routes. This is like setting up signs to guide requests to the right handler.

beginner

What happens inside a controller when a request arrives?

The controller receives the request, extracts needed data, calls the service for processing, and sends back the response to the client.

beginner

Why is it important for controllers to handle requests in a web app?

Controllers act as the first contact point for users. They ensure requests are understood and routed correctly, making the app responsive and organized.

In NestJS, what decorator is used to define a controller method that handles GET requests?

A @Injectable()

B @Get()

C @Controller()

D @Post()

The @Get() decorator marks a method to handle HTTP GET requests.

Which part of a NestJS app is responsible for business logic?

A Service

B Module

C Controller

D Middleware

Services contain business logic, while controllers handle requests.

What does a controller do after receiving a request?

A Call a service to process data

B Directly access the database

C Send the request to the client

D Ignore the request

Controllers call services to process data and then send a response.

Why is separating controllers and services useful?

A It makes the app slower

B It prevents any requests

C It removes the need for modules

D It helps organize code and responsibilities

Separation keeps code clean and easier to manage.

Which decorator defines a class as a controller in NestJS?

A @Module()

B @Injectable()

C @Controller()

D @Service()

The @Controller() decorator marks a class as a controller.

Explain why controllers handle incoming requests in NestJS and how they interact with services.

Think of controllers as the app's receptionist.

Describe the benefits of separating request handling in controllers from business logic in services.

Consider how teamwork works best when roles are clear.

Practice

1. In NestJS, why do controllers handle incoming requests?

easy

A. Because they manage server hardware resources

B. Because they store data and manage databases

C. Because they handle user interface rendering

D. Because they organize routes and connect requests to the right code

Solution

1. Step 1: Understand the role of controllers in NestJS

   Controllers are designed to listen for incoming HTTP requests and decide which code should handle them.

2. Step 2: Differentiate responsibilities

   Controllers focus on routing and connecting requests to services, not on data storage or UI rendering.

3. Final Answer:

   Because they organize routes and connect requests to the right code -> Option D

4. Quick Check:

   Controllers handle routing = A [OK]

Hint: Controllers route requests to code, not store data [OK]

Common Mistakes:

• Thinking controllers manage data storage
• Confusing controllers with UI components
• Assuming controllers handle hardware

2. Which of the following is the correct way to define a controller method to handle GET requests in NestJS?

easy

A. @Controller() get() { return 'Hello'; }

B. function get() { return 'Hello'; }

C. @Get() async handleRequest() { return 'Hello'; }

D. router.get('/hello', () => 'Hello');

Solution

1. Step 1: Recognize NestJS controller method syntax

   In NestJS, @Get() decorator marks a method to handle GET requests inside a controller class.

2. Step 2: Compare options

   @Get() async handleRequest() { return 'Hello'; } uses @Get() decorator correctly; others use incorrect syntax or patterns from other frameworks.

3. Final Answer:

   @Get() async handleRequest() { return 'Hello'; } -> Option C

4. Quick Check:

   @Get() decorator = C [OK]

Hint: Use @Get() decorator for GET routes in controllers [OK]

Common Mistakes:

• Using plain functions outside controller classes
• Confusing @Controller with method decorators
• Using Express router syntax inside NestJS controllers

3. Given this NestJS controller code, what will be the output when a GET request is made to '/greet'?

@Controller('greet')
export class GreetController {
  @Get()
  sayHello() {
    return 'Hello, NestJS!';
  }
}

medium

A. 404 Not Found error

B. 'Hello, NestJS!'

C. An empty response

D. Server crash due to syntax error

Solution

1. Step 1: Analyze the route path

   The controller is set to 'greet' and the method handles GET requests at the root of this path, so the full route is '/greet'.

2. Step 2: Check the method return value

   The sayHello() method returns the string 'Hello, NestJS!'. This string will be sent as the response body.

3. Final Answer:

   'Hello, NestJS!' -> Option B

4. Quick Check:

   GET /greet returns method string = A [OK]

Hint: Controller path + method path = full route [OK]

Common Mistakes:

• Expecting 404 due to missing route
• Thinking method returns nothing
• Assuming syntax errors without checking code

4. Identify the error in this NestJS controller code snippet:

@Controller('users')
export class UserController {
  @Post('/create')
  createUser() {
    // missing request body parameter
    return 'User created';
  }
}

medium

A. The method lacks a parameter to receive the request body

B. The @Post decorator is used incorrectly

C. The controller path should not be 'users'

D. The method should return a Promise, not a string

Solution

1. Step 1: Review method parameters for POST requests

   POST requests usually send data in the request body, which the method should accept as a parameter decorated with @Body().

2. Step 2: Check the method signature

   The createUser() method has no parameters, so it cannot access the incoming data, which is a common mistake.

3. Final Answer:

   The method lacks a parameter to receive the request body -> Option A

4. Quick Check:

   POST methods need @Body() parameter = B [OK]

Hint: POST handlers need @Body() to get data [OK]

Common Mistakes:

• Ignoring request body in POST methods
• Misusing decorators on methods
• Thinking controller path causes error

5. You want to keep your NestJS controller focused only on routing and delegate business logic to a service. Which approach best follows this principle?

hard

A. Inject a service into the controller and call its methods inside controller handlers

B. Handle HTTP requests and business logic in the same controller method

C. Use controllers to directly access the database

D. Write all data processing inside the controller methods

Solution

1. Step 1: Understand separation of concerns in NestJS

   Controllers should only handle routing and delegate business logic to services to keep code clean and maintainable.

2. Step 2: Identify the correct pattern

   Injecting a service into the controller and calling its methods inside controller handlers follows this best practice.

3. Final Answer:

   Inject a service into the controller and call its methods inside controller handlers -> Option A

4. Quick Check:

   Controllers route, services handle logic = D [OK]

Hint: Use services for logic, controllers for routing [OK]

Common Mistakes:

• Putting business logic inside controllers
• Accessing database directly in controllers
• Mixing routing and logic in one place