Dependency

Introduction

Dependency is a relationship where one class (the client) depends on another class (the supplier) because it uses that class in some way. If the supplier changes, the client may need to change too.

Dependency is the weakest form of relationship between classes.

Real-World Analogy: Ordering Food

A Customer depends on a Restaurant:

Customer calls the restaurant to place an order
Customer doesn’t own the restaurant
Customer just needs the restaurant’s service temporarily

If the restaurant changes its menu, the customer may need to change their order.

Types of Dependencies

Type	Description	Example
Method parameter	Passed into a method	`function process(paymentService)`
Local variable	Created inside a method	`const email = new EmailService()`
Return type	Method returns an instance	`getDatabase(): Database`
Static method call	Calling static methods	`Logger.log("message")`

Dependency Injection (DI)

Dependency Injection is a technique where dependencies are “injected” into a class from outside.

Without DI (Tight Coupling)

class OrderService {
  constructor() {
    this.logger = new Logger();
    this.emailService = new EmailService();
    this.paymentService = new PaymentService();
  }
}

With DI (Loose Coupling)

class OrderService {
  constructor(logger, emailService, paymentService) {
    this.logger = logger;
    this.emailService = emailService;
    this.paymentService = paymentService;
  }
}

Best Practices

Depend on abstractions, not concrete implementations (DIP)
Use dependency injection for loose coupling
Minimize dependencies — each class should have few dependencies
Don’t create dependencies inside constructors — inject them

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