Dependency injection framework in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Dependency Injection Framework?

A Dependency Injection Framework is a tool that helps automatically provide the parts (dependencies) a piece of code needs to work, so you don't have to create or find them yourself.

beginner

Why use Dependency Injection Frameworks?

They make code easier to manage, test, and change by separating how parts are created from how they are used, like having a helper who gives you exactly what you need when you need it.

beginner

What is 'injection' in Dependency Injection?

Injection means giving an object the things it needs (dependencies) from outside, instead of the object making them itself. It's like getting tools handed to you instead of buying them yourself.

intermediate

Name three common types of Dependency Injection.

1. Constructor Injection: dependencies are given when creating an object.
2. Setter Injection: dependencies are set after the object is created.
3. Interface Injection: dependencies are provided through an interface method.

intermediate

How does a Dependency Injection Framework improve testing?

It allows you to easily swap real parts with fake or test parts, so you can check if your code works correctly without needing the full system, like testing a car engine separately before building the whole car.

What does a Dependency Injection Framework mainly do?

A Runs your program faster

B Writes code for you

C Automatically provides required parts to code

D Creates user interfaces

Dependency Injection Frameworks provide the parts (dependencies) your code needs automatically.

Which is NOT a type of Dependency Injection?

A Setter Injection

B Loop Injection

C Interface Injection

D Constructor Injection

Loop Injection is not a recognized type of Dependency Injection.

How does Dependency Injection help with testing?

A By making code run faster

B By generating test data

C By hiding errors automatically

D By allowing easy replacement of parts with test versions

Dependency Injection allows swapping real parts with test parts to simplify testing.

In Dependency Injection, what is 'injection'?

A Giving an object its needed parts from outside

B Writing code inside a function

C Deleting unused code

D Compiling code

Injection means providing dependencies to an object from outside.

Which benefit is NOT typically associated with Dependency Injection Frameworks?

A Automatic UI design

B Easier code management

C Better testing

D More flexible code changes

Dependency Injection Frameworks do not design user interfaces automatically.

Explain in your own words what a Dependency Injection Framework does and why it is useful.

Think about how having someone hand you the tools you need can make your work easier.

Describe the three common types of Dependency Injection and give a simple example for each.

Consider when and how the needed parts are given to an object.

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