Why controllers handle incoming requests in NestJS

Introduction

Controllers in NestJS act like traffic officers. They listen for incoming requests and decide what to do with them.

When you want to respond to a user visiting a webpage or calling an API.

When you need to organize how your app handles different types of requests.

When you want to separate the logic of receiving requests from the logic of processing data.

When building REST APIs to handle GET, POST, PUT, DELETE requests.

When you want to keep your code clean and easy to manage by grouping related request handlers.

Syntax

NestJS

import { Controller, Get, Post } from '@nestjs/common';

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

  @Post()
  create() {
    return 'This action adds a new cat';
  }
}

@Controller('route') defines the base route for all methods inside the controller.

Methods inside the controller use decorators like @Get() or @Post() to handle specific HTTP requests.

Examples

This controller handles GET requests to '/dogs' and returns a simple message.

NestJS

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

@Controller('dogs')
export class DogsController {
  @Get()
  getDogs() {
    return 'List of dogs';
  }
}

This controller handles POST requests to '/birds' to add new birds.

NestJS

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

@Controller('birds')
export class BirdsController {
  @Post()
  addBird() {
    return 'New bird added';
  }
}

Sample Program

This controller manages a list of fruits. It returns all fruits on GET requests and adds an orange on POST requests.

NestJS

import { Controller, Get, Post } from '@nestjs/common';

@Controller('fruits')
export class FruitsController {
  private fruits = ['apple', 'banana'];

  @Get()
  getAllFruits() {
    return this.fruits;
  }

  @Post()
  addFruit() {
    this.fruits.push('orange');
    return this.fruits;
  }
}

Important Notes

Controllers only handle routing and request reception. Business logic should be in services.

Use decorators to clearly define which HTTP method each controller method handles.

Summary

Controllers listen for and handle incoming requests in NestJS.

They organize routes and connect requests to the right code.

Keep controllers focused on routing; use services for data and logic.

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