Dependency Injection (DI)

Definition

A design pattern in which a class receives its dependencies from external sources rather than creating them itself.

Explain Like I'm New

Imagine a `Car` class. A bad Car builds its own engine inside its constructor (`this.engine = new V8Engine()`). Now that Car is permanently stuck with a V8 engine. Dependency Injection says: 'Don't build the engine. Ask for the engine in the constructor arguments.' (`constructor(engine: IEngine)`). Now you can easily pass in an ElectricEngine or a V8Engine.

Real World Example

Angular and NestJS are built entirely around DI. When you ask for a `DatabaseService` in a controller's constructor, the framework automatically finds it and injects it for you (Inversion of Control).

Common Use Cases

•Highly testable architectures
•Large scale frameworks (Angular/NestJS)

Interactive Example

// The interface contract
interface Logger {
  log(msg: string): void;
}

// Concrete dependencies
class ConsoleLogger implements Logger {
  log(msg: string) { console.log("[CONSOLE]: " + msg); }
}
class FileLogger implements Logger {
  log(msg: string) { /* writes to file */ }
}

// --- BAD WAY (Tightly Coupled) ---
class BadService {
  private logger = new ConsoleLogger(); // Hardcoded!
  doWork() { this.logger.log("Work done"); }
}

// --- GOOD WAY (Dependency Injection) ---
class GoodService {
  // It asks for ANY logger that matches the interface
  constructor(private logger: Logger) {}
  
  doWork() { this.logger.log("Work done"); }
}

// Now we control the dependencies from the outside!
const testService = new GoodService(new ConsoleLogger());
const prodService = new GoodService(new FileLogger());

Interview Questions

basic

What is a 'Dependency' in programming?

intermediate

How does DI make unit testing easier?

advanced

What is an IoC (Inversion of Control) Container?

Flash Cards

Question

How does DI make unit testing easier?

Click to reveal answer

Answer

If a `PaymentProcessor` creates its own `StripeAPI` inside it, you cannot test it without actually charging a credit card. If it uses DI (`constructor(api: IPaymentApi)`), you can pass a `MockPaymentApi` during tests that always returns 'Success' without hitting the internet.

Question

What is an IoC Container?

Click to reveal answer

Answer

It is a framework feature. Instead of you manually writing `new Engine()`, passing it to `new Car()`, and passing that to `new App()`, the IoC Container acts as a smart registry. You just say 'I need an App', and the container automatically builds the entire dependency tree and wires it together.