Dependency injection framework in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a dependency injection container.

LLD

container = [1]()

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

AServiceLocator

BDIContainer

CInjector

DProvider

Attempts:

3 left

💡 Hint

Common Mistakes

Using ServiceLocator instead of DIContainer

Confusing Injector with container

The DIContainer is the standard class to create a dependency injection container.

2fill in blank

medium

Complete the code to register a service class in the container.

LLD

container.[1](MyService)

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

Aadd

Bprovide

Cbind

Dsingleton

Attempts:

3 left

💡 Hint

Common Mistakes

Using bind which may create new instances

Using add which is not a standard method

The singleton method registers a service as a single shared instance.

3fill in blank

hard

Fix the error in the code to retrieve a service instance from the container.

LLD

service = container.[1](MyService)

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

Aresolve

BgetInstance

Cfetch

Dcreate

Attempts:

3 left

💡 Hint

Common Mistakes

Using getInstance which is not standard here

Using create which may instantiate new objects incorrectly

The resolve method is used to get the instance of a registered service.

4fill in blank

hard

Fill both blanks to define a service with constructor injection.

LLD

class MyController:
    def __init__(self, [1]):
        self.service = [2]

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

Aservice

Bself.service

CMyService()

Dservice()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling service as a function

Assigning MyService() directly inside constructor

The constructor takes service as parameter and assigns it to self.service for use in the class.

5fill in blank

hard

Fill all three blanks to register and resolve a service with dependencies.

LLD

container.[1](MyRepository)
container.[2](MyService, lambda: MyService([3]))

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

Aregister

Bsingleton

Ccontainer.resolve(MyRepository)

Dprovide

Attempts:

3 left

💡 Hint

Common Mistakes

Using provide instead of register

Not resolving dependency inside lambda

First, register the repository. Then, register singleton service with a factory lambda that calls container.resolve(MyRepository) to inject the dependency.

Practice

1. What is the main purpose of a dependency injection framework?

easy

A. To store data permanently on disk

B. To automatically provide parts (dependencies) to your code

C. To make your code run faster by compiling it

D. To write all code manually without any helpers

Solution

1. Step 1: Understand what dependency injection means

   Dependency injection means giving the parts your code needs automatically instead of creating them inside the code.

2. Step 2: Identify the role of the framework

   A dependency injection framework helps by managing and providing these parts for you, making your code easier to change and test.

3. Final Answer:

   To automatically provide parts (dependencies) to your code -> Option B

4. Quick Check:

   Dependency injection = automatic parts supply [OK]

Hint: Think: Who gives parts to your code? The injector does! [OK]

Common Mistakes:

• Confusing dependency injection with data storage
• Thinking it speeds up code execution directly
• Believing it replaces manual coding completely

2. Which of the following is the correct way to register a service in a dependency injection framework?

easy

A. injector.register(ServiceClass)

B. ServiceClass.inject()

C. register.injector(ServiceClass)

D. ServiceClass.register()

Solution

1. Step 1: Recall the registration syntax

   In most dependency injection frameworks, you register a service by calling a method on the injector object and passing the service class.

2. Step 2: Match the correct syntax

   The correct syntax is injector.register(ServiceClass), which tells the injector to manage that service.

3. Final Answer:

   injector.register(ServiceClass) -> Option A

4. Quick Check:

   Register service = injector.register() [OK]

Hint: Register services by calling register on the injector [OK]

Common Mistakes:

• Calling register on the service class instead of injector
• Mixing method order or names
• Using non-existent methods like inject() on service

3. Given the code below, what will serviceA.getName() output?

class ServiceA {
  getName() { return 'Service A'; }
}

injector.register(ServiceA);
const serviceA = injector.get(ServiceA);
console.log(serviceA.getName());

medium

A. null

B. undefined

C. Error: Service not found

D. Service A

Solution

1. Step 1: Understand registration and retrieval

   The code registers ServiceA with the injector, then asks the injector to give an instance of ServiceA.

2. Step 2: Check the method call on the instance

   The instance has a method getName() that returns the string 'Service A'. So calling serviceA.getName() returns 'Service A'.

3. Final Answer:

   Service A -> Option D

4. Quick Check:

   Registered service returns its name [OK]

Hint: Registered services return their methods normally [OK]

Common Mistakes:

• Assuming injector.get returns undefined or null
• Forgetting to register before getting
• Expecting an error without registration

4. Identify the error in the following code snippet using a dependency injection framework:

class ServiceB {}

const serviceB = injector.get(ServiceB);
injector.register(ServiceB);

medium

A. ServiceB is registered after trying to get it

B. ServiceB class is missing a constructor

C. injector.get should be injector.fetch

D. injector.register should be called twice

Solution

1. Step 1: Check the order of registration and retrieval

   The code tries to get ServiceB from the injector before registering it, which causes an error because the injector doesn't know about ServiceB yet.

2. Step 2: Confirm correct usage order

   Services must be registered before they can be retrieved from the injector.

3. Final Answer:

   ServiceB is registered after trying to get it -> Option A

4. Quick Check:

   Register before get = correct order [OK]

Hint: Always register before getting a service [OK]

Common Mistakes:

• Trying to get service before registration
• Confusing method names like get vs fetch
• Thinking constructor is required for registration

5. You want to inject a Logger service into a UserService using a dependency injection framework. Which approach correctly applies dependency injection to make UserService easier to test?

class Logger {
  log(msg) { console.log(msg); }
}

class UserService {
  constructor(logger) {
    this.logger = logger;
  }
  createUser(name) {
    this.logger.log(`User ${name} created`);
  }
}

injector.register(Logger);
injector.register(UserService);

How should you get UserService with Logger injected?

hard

A. const userService = injector.get(Logger);

B. const userService = new UserService(new Logger());

C. const userService = injector.get(UserService);

D. const userService = UserService();

Solution

1. Step 1: Understand constructor injection

   UserService expects a Logger instance in its constructor. The injector knows how to create Logger and UserService because both are registered.

2. Step 2: Use injector to get UserService with dependencies

   Calling injector.get(UserService) lets the injector create UserService and automatically provide Logger to it.

3. Final Answer:

   const userService = injector.get(UserService); -> Option C

4. Quick Check:

   Injector provides dependencies via constructor [OK]

Hint: Get the service from injector to auto-inject dependencies [OK]

Common Mistakes:

• Manually creating dependencies instead of using injector
• Getting Logger instead of UserService
• Calling UserService without new keyword