Composition vs Inheritance

Loading "Intro to Composition Vs Inheritance"

Intro to Composition Vs Inheritance

Both composition and inheritance are ways to reuse code, but they solve different problems. Knowing when to use each is crucial for good design.

Inheritance (Is-A)

Inheritance creates an "is-a" relationship:

class Animal {
	makeSound(): void {}
}

class Dog extends Animal {
	// Dog IS-A Animal
}

Use inheritance when:

You have a clear hierarchical relationship
Subclasses are specializations of the parent
You want to share implementation

Composition (Has-A)

Composition creates a "has-a" relationship:

class Engine {
	start(): void {}
}

class Car {
	#engine: Engine // Car HAS-A Engine

	constructor(engine: Engine) {
		this.#engine = engine
	}

	start(): void {
		this.#engine.start()
	}
}

Use composition when:

You want flexibility to change behavior at runtime
You need to combine multiple behaviors
The relationship is "has-a" not "is-a"

Three Types of Composition

Inheritance (is-a): Dog extends Animal
Object Composition (has-a): Car has an Engine
Function Composition (transforms): combine functions into pipelines

// Object composition (OOP)
class EmailService {
	#logger: Logger
	constructor(logger: Logger) {
		this.#logger = logger
	}
	send(email: Email) {
		this.#logger.log('Sending...')
		// ...
	}
}

// Function composition (FP)
const sendEmail = (logger: Logger) => (email: Email) => {
	logger.log('Sending...')
	// ...
}

Both achieve the same goal—loose coupling and flexibility—through different mechanisms.

When to Use Each

Inheritance:

Dog is an Animal ✅
Circle is a Shape ✅
Manager is an Employee ✅

Composition (object or function):

Car has an Engine ✅
Computer has a CPU ✅
User has a Profile ✅
Data transformation pipelines ✅

Favor composition over inheritance when possible. Both object composition and function composition are more flexible and easier to change than inheritance. Use inheritance when you truly have an "is-a" relationship.

In this exercise, you'll choose between composition and inheritance.