Why controllers handle incoming requests in NestJS - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a controller that handles HTTP GET requests.

NestJS

import { Controller, [1] } from '@nestjs/common';

@Controller('cats')
export class CatsController {
  @[1]()
  findAll() {
    return 'This action returns all cats';
  }
}

Drag options to blanks, or click blank then click option'

APost

BPut

CGet

DDelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Post() instead of @Get() for GET requests

Forgetting to import the decorator

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

2fill in blank

medium

Complete the code to inject a service into the controller constructor.

NestJS

import { Controller } from '@nestjs/common';
import { CatsService } from './cats.service';

@Controller('cats')
export class CatsController {
  constructor(private readonly [1]: CatsService) {}
}

Drag options to blanks, or click blank then click option'

AcatsService

BCatsService

CserviceCats

Dcats

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name with uppercase as parameter name

Using unrelated variable names

The injected service is usually named in camelCase, matching the class name but starting lowercase.

3fill in blank

hard

Fix the error in the controller method to correctly handle a POST request with a body.

NestJS

import { Controller, Post, [1] } from '@nestjs/common';

@Controller('cats')
export class CatsController {
  @Post()
  create(@[1]() createCatDto: any) {
    return 'This action adds a new cat';
  }
}

Drag options to blanks, or click blank then click option'

ABody

BParam

CQuery

DHeaders

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Param() to get body data

Forgetting to import the decorator

The @Body() decorator extracts the body of the POST request.

4fill in blank

hard

Fill both blanks to create a controller method that handles GET requests with a route parameter.

NestJS

import { Controller, Get, [1] } from '@nestjs/common';

@Controller('cats')
export class CatsController {
  @Get(':id')
  findOne(@[2]('id') id: string) {
    return `This action returns a cat with id ${id}`;
  }
}

Drag options to blanks, or click blank then click option'

AParam

BBody

CQuery

DHeaders

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Body() or @Query() for route parameters

Not importing Param decorator

The @Param() decorator is used both in import and method parameter to access route parameters.

5fill in blank

hard

Fill all three blanks to create a controller method that handles PATCH requests with a route parameter and body.

NestJS

import { Controller, Patch, [1], [2] } from '@nestjs/common';

@Controller('cats')
export class CatsController {
  @Patch(':id')
  update(@[3]('id') id: string, @[2]() updateCatDto: any) {
    return `This action updates a cat with id ${id}`;
  }
}

Drag options to blanks, or click blank then click option'

AParam

BBody

CQuery

DHeaders

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up @Param() and @Body() decorators

Not importing both decorators

The @Param() decorator is used to get the route parameter, and @Body() to get the request body.

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