Why providers encapsulate business logic in NestJS - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to inject a provider into a controller.

NestJS

constructor(private readonly [1]: UsersService) {}

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

AUsersService

BuserService

CusersService

DUserService

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name with uppercase as the property name

Misspelling the service name

The provider instance is injected using a private readonly property with camelCase naming.

2fill in blank

medium

Complete the code to call a method from the provider inside a controller method.

NestJS

return this.usersService.[1](id);

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

AfindUserById

BcreateUser

CdeleteUser

DupdateUser

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a method that creates or deletes instead of fetching

Using a method name that does not exist in the provider

The controller calls the provider method that fetches a user by ID.

3fill in blank

hard

Fix the error in the provider method signature to properly return a Promise.

NestJS

async getUser(id: string): [1] {

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

AObservable<User>

BUser

Cvoid

DPromise<User>

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the type directly without Promise

Using void or Observable instead of Promise

Async methods in NestJS providers should return a Promise of the expected type.

4fill in blank

hard

Fill both blanks to create a provider method that filters users by age.

NestJS

getUsersByAge([1]: number): User[] {
  return this.users.filter(user => user.[2] === age);
}

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

Aage

BageLimit

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for parameter and property causing errors

Using undefined variable names

The method parameter and the user property to compare must both be 'age' to filter correctly.

5fill in blank

hard

Fill all three blanks to create a provider method that returns a dictionary of user names keyed by their IDs for users older than 18.

NestJS

getAdultUserNames(): Record<string, string> {
  return this.users.reduce((acc, user) => {
    if (user.[1] > [2]) {
      acc[user.[3]] = user.name;
    }
    return acc;
  }, {});
}

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

Aage

B18

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong property for the key in the dictionary

Comparing with incorrect age value

The method checks if user age is greater than 18 and uses user id as the key in the result object.

Practice

1. Why do NestJS providers encapsulate business logic instead of placing it directly in controllers?

easy

A. To make the app run faster

B. Because controllers cannot contain any code

C. To keep the code organized and separate concerns

D. Because providers handle user interface rendering

Solution

1. Step 1: Understand roles of controllers and providers

   Controllers handle requests and responses, while providers hold the main business rules and logic.

2. Step 2: Recognize separation of concerns

   Separating logic into providers keeps code organized, easier to test, and reusable.

3. Final Answer:

   To keep the code organized and separate concerns -> Option C

4. Quick Check:

   Separation of concerns = A [OK]

Hint: Remember: providers = logic, controllers = routes [OK]

Common Mistakes:

• Thinking controllers should hold business logic
• Believing providers speed up app performance
• Confusing providers with UI rendering

2. Which of the following is the correct way to define a provider class in NestJS that encapsulates business logic?

easy

A. @Injectable() export class MyService { doTask() { return 'done'; } }

B. export class MyService { constructor() {} }

C. function MyService() { return 'done'; }

D. const MyService = () => 'done';

Solution

1. Step 1: Identify NestJS provider syntax

   Providers must be decorated with @Injectable() to be recognized by NestJS.

2. Step 2: Check class structure

   @Injectable() export class MyService { doTask() { return 'done'; } } shows a class with @Injectable() and a method, which is correct for a provider.

3. Final Answer:

   @Injectable() export class MyService { doTask() { return 'done'; } } -> Option A

4. Quick Check:

   @Injectable() + class = B [OK]

Hint: Look for @Injectable() decorator on classes [OK]

Common Mistakes:

• Omitting @Injectable() decorator
• Using plain functions instead of classes
• Not exporting the provider class

3. Given this provider code, what will be the output when appService.getGreeting() is called?

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

@Injectable()
export class AppService {
  getGreeting() {
    return 'Hello from provider!';
  }
}

medium

A. 'Hello from provider!'

B. undefined

C. Error: Method not found

D. 'Hello from controller!'

Solution

1. Step 1: Analyze the getGreeting method

   The method returns the string 'Hello from provider!'.

2. Step 2: Understand method call result

   Calling appService.getGreeting() returns the exact string defined.

3. Final Answer:

   'Hello from provider!' -> Option A

4. Quick Check:

   Method returns string = A [OK]

Hint: Method returns string literal, expect exact output [OK]

Common Mistakes:

• Confusing provider output with controller output
• Expecting undefined if method is not async
• Assuming error without checking method existence

4. Identify the error in this provider code snippet:

@Injectable()
export class UserService {
  getUser() {
    return this.user;
  }
}

medium

A. getUser method should be async

B. Missing declaration of 'user' property

C. Missing @Controller decorator

D. UserService should not be injectable

Solution

1. Step 1: Check class properties

   The method getUser returns this.user, but 'user' is not declared anywhere in the class.

2. Step 2: Understand consequences

   Accessing an undeclared property will cause runtime errors or undefined returns.

3. Final Answer:

   Missing declaration of 'user' property -> Option B

4. Quick Check:

   Undeclared property access = C [OK]

Hint: Check if all used properties are declared in the class [OK]

Common Mistakes:

• Thinking @Controller is needed on providers
• Assuming async needed without async code
• Believing providers can't be injectable

5. You want to reuse business logic for calculating discounts in multiple controllers. What is the best way to organize this in NestJS?

hard

A. Put the discount calculation code inside each controller separately

B. Use a global variable to store discount values accessible by all controllers

C. Write the discount logic directly in the main.ts bootstrap file

D. Create a provider service with discount logic and inject it where needed

Solution

1. Step 1: Identify code reuse best practice

   Duplicating logic in controllers causes maintenance issues and breaks separation of concerns.

2. Step 2: Use providers for shared logic

   Creating a provider service encapsulates the discount logic and allows injection into any controller.

3. Final Answer:

   Create a provider service with discount logic and inject it where needed -> Option D

4. Quick Check:

   Shared logic in providers = D [OK]

Hint: Use providers to share logic across controllers [OK]

Common Mistakes:

• Duplicating logic in controllers
• Using global variables instead of providers
• Placing logic in bootstrap file