Why NestJS exists - See It in Action

Understanding Why NestJS Exists

📖 Scenario: You are a developer learning backend frameworks. You want to understand why NestJS was created and what problems it solves compared to plain Node.js or other frameworks.

🎯 Goal: Build a simple NestJS project that shows the basic setup and a simple controller to demonstrate the framework's structure and purpose.

📋 What You'll Learn

Create a basic NestJS application structure

Add a configuration variable for the port number

Create a simple controller with one route

Complete the main application bootstrap code

💡 Why This Matters

🌍 Real World

NestJS is used to build backend servers for web and mobile apps, APIs, and microservices with a clean and maintainable structure.

💼 Career

Many companies use NestJS for scalable Node.js backend development, so understanding why it exists helps in writing better server-side code and working in teams.

1

Create the basic NestJS application structure

Create a NestJS application by importing NestFactory from @nestjs/core and a root module called AppModule as an empty class.

NestJS

import { NestFactory } from '@nestjs/core';

// Create an empty root module called AppModule
class AppModule {}

// Your code here

Hint

Start by importing NestFactory and define an empty class named AppModule.

2

Add a configuration variable for the port number

Add a constant variable called PORT and set it to 3000 to configure the server port.

NestJS

import { NestFactory } from '@nestjs/core';

class AppModule {}

// Define the server port
const PORT = 3000;
// Your code here

Hint

Use const PORT = 3000; to set the port number.

3

Create a simple controller with one route

Create a controller class called AppController with a method getHello that returns the string 'Hello from NestJS!'. Use the @Controller() decorator and @Get() decorator from @nestjs/common.

NestJS

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

class AppModule {}

const PORT = 3000;

// Create AppController with @Controller and @Get decorators
// Your code here

Hint

Use @Controller() above the class and @Get() above the method that returns the greeting string.

4

Complete the main application bootstrap code

Write an async function called bootstrap that creates the NestJS app with NestFactory.create(AppModule), listens on PORT, and call bootstrap() at the end.

NestJS

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

@Controller()
class AppController {
  @Get()
  getHello() {
    return 'Hello from NestJS!';
  }
}

class AppModule {}

const PORT = 3000;

// Write bootstrap function to start the app
// Your code here

Hint

Use an async function to create and start the NestJS app on the port.

Practice

1. Why was NestJS created as a framework for backend development?

easy

A. To replace frontend frameworks like React and Angular

B. To create mobile applications easily

C. To only support legacy JavaScript without TypeScript

D. To provide a clear and organized way to build scalable server-side apps

Solution

1. Step 1: Understand NestJS purpose

   NestJS is designed to help developers build backend applications that are easy to maintain and scale.

2. Step 2: Compare options with NestJS goals

   Options A, C, and D do not match NestJS's backend focus or modern language support.

3. Final Answer:

   To provide a clear and organized way to build scalable server-side apps -> Option D

4. Quick Check:

   Backend clarity and scalability = B [OK]

Hint: NestJS focuses on backend clarity and scalability [OK]

Common Mistakes:

• Confusing NestJS with frontend frameworks
• Thinking NestJS only supports old JavaScript
• Assuming NestJS is for mobile apps

2. Which of the following is the correct way to create a basic NestJS controller?

easy

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

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

B. import { Controller } from 'nestjs';

@Controller('/cats')
class Cats {
getAll() {
return 'Cats list';
}
}

C. const controller = require('nestjs').Controller;

controller('/cats', () => {
return 'Cats';
});

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

@Controller('cats')
export class CatsController {
findAll() {
return 'All cats';
}
}

Solution

1. Step 1: Check import and decorator usage

   import { Controller, Get } from '@nestjs/common'; @Controller('cats') export class CatsController { @Get() findAll() { return 'This action returns all cats'; } } correctly imports from '@nestjs/common' and uses @Controller and @Get decorators properly.

2. Step 2: Verify method and return value

   import { Controller, Get } from '@nestjs/common'; @Controller('cats') export class CatsController { @Get() findAll() { return 'This action returns all cats'; } }'s method findAll returns a string as expected for a GET route. Other options have wrong imports, missing decorators, or wrong HTTP method.

3. Final Answer:

   Option A code snippet -> Option A

4. Quick Check:

   Correct decorators and imports = A [OK]

Hint: Look for @Controller and @Get from '@nestjs/common' [OK]

Common Mistakes:

• Using wrong import paths
• Missing HTTP method decorators
• Using require instead of import

3. Consider this NestJS service code:

import { Injectable } from '@nestjs/common';

@Injectable()
export class MathService {
  double(value: number): number {
    return value * 2;
  }
}

const service = new MathService();
console.log(service.double(5));

What will be printed in the console?

medium

A. 10

B. 5

C. undefined

D. Error: Injectable decorator missing

Solution

1. Step 1: Understand the double method

   The method multiplies the input value by 2, so double(5) returns 10.

2. Step 2: Check usage of Injectable decorator

   The @Injectable decorator marks the class for NestJS dependency injection but does not affect direct instantiation or method output.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   5 times 2 equals 10 [OK]

Hint: Method doubles input; decorator doesn't block direct use [OK]

Common Mistakes:

• Thinking @Injectable prevents direct instantiation
• Expecting undefined return
• Confusing decorator effects with runtime errors

4. This NestJS controller code has an error:

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

@Controller('dogs')
export class DogsController {
  @Get()
  getDogs() {
    return this.dogs;
  }
}

What is the main problem here?

medium

A. The import statement is wrong

B. The controller is missing a constructor to initialize 'dogs'

C. The @Get decorator is used incorrectly

D. The method getDogs should be async

Solution

1. Step 1: Identify usage of 'this.dogs'

   The method returns 'this.dogs' but the class does not define or initialize 'dogs'.

2. Step 2: Check for initialization

   Without a constructor or property assignment, 'this.dogs' is undefined, causing runtime errors.

3. Final Answer:

   The controller is missing a constructor to initialize 'dogs' -> Option B

4. Quick Check:

   Undefined property needs initialization [OK]

Hint: Check if all used properties are initialized [OK]

Common Mistakes:

• Assuming decorators fix property issues
• Thinking async is required without async code
• Ignoring missing property initialization

5. You want to build a scalable backend app using NestJS that handles many users and keeps code organized. Which feature of NestJS helps most with this goal?

hard

A. Its use of global variables for shared state

B. Its built-in support for frontend UI components

C. Its modular architecture allowing separation of features into modules

D. Its automatic generation of mobile app code

Solution

1. Step 1: Identify scalability needs

   Scalable apps need clear separation of concerns and organized code structure.

2. Step 2: Match NestJS features to needs

   NestJS modules let you group related code, making it easier to maintain and scale. Other options do not support backend scalability.

3. Final Answer:

   Its modular architecture allowing separation of features into modules -> Option C

4. Quick Check:

   Modular design = scalable backend [OK]

Hint: Modular design is key for scalable backend apps [OK]

Common Mistakes:

• Confusing backend with frontend features
• Thinking NestJS generates mobile code
• Using global variables instead of modules