The "has-a" vs. "is-a"

Part of the Object Oriented Programming section of Coddy's JavaScript journey — lesson 47 of 56.

In object-oriented programming, there are two main types of relationships between objects: "has-a" (composition) and "is-a" (inheritance). While inheritance creates parent-child hierarchies, composition builds objects by combining smaller, focused components.

Let's first understand the "is-a" relationship through inheritance:

class Animal {
  constructor(name) {
    this.name = name;
  }
  
  eat() {
    console.log(`${this.name} is eating.`);
  }
}

// Dog "is-a" Animal - inheritance relationship
class Dog extends Animal {
  bark() {
    console.log(`${this.name} says woof!`);
  }
}

Now let's look at the "has-a" relationship through composition:

class Engine {
  start() {
    console.log("Engine started");
  }
}

// Car "has-a" Engine - composition relationship
class Car {
  constructor() {
    this.engine = new Engine(); // Composition
  }
  
  startCar() {
    this.engine.start();
    console.log("Car is running");
  }
}

The key difference:

"is-a" (inheritance): A Dog IS AN Animal
"has-a" (composition): A Car HAS AN Engine

Challenge

You're given the component classes (Processor, Memory) and most of the Computer class. Your task is to complete the constructor in the computer.js file:

Create a processor property using the Processor class
Create a memory property using the Memory class
Pass processorSpeed to the Processor constructor and memorySize to the Memory constructor

Cheat sheet

In object-oriented programming, there are two main types of relationships between objects:

"is-a" relationship (inheritance): Creates parent-child hierarchies where a subclass extends a parent class
"has-a" relationship (composition): Builds objects by combining smaller, focused components

Inheritance example ("is-a"):

class Animal {
  constructor(name) {
    this.name = name;
  }
  
  eat() {
    console.log(`${this.name} is eating.`);
  }
}

// Dog "is-a" Animal
class Dog extends Animal {
  bark() {
    console.log(`${this.name} says woof!`);
  }
}

Composition example ("has-a"):

class Engine {
  start() {
    console.log("Engine started");
  }
}

// Car "has-a" Engine
class Car {
  constructor() {
    this.engine = new Engine(); // Composition
  }
  
  startCar() {
    this.engine.start();
    console.log("Car is running");
  }
}

Try it yourself

import { Computer } from './computer.js';

const myComputer = new Computer(3.5, 16);
myComputer.start();

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Object Oriented Programming

1Objects & The this Keyword

Quick Review: Objects Adding Methods to Objects Understanding the this Keyword Constructor Functions The new Keyword Recap Challenge

4Methods & Managing State

Methods that modify state Method chaining pattern

7 Inheritance & The extends Key

Inheritance The "is-a" Relationship The extends Keyword The super() Method Inheriting Properties&Methods Recap Challenge

10Polymorphism

What is Polymorphism?Overriding Inherited Methods Using super Recap Challenge

13Composition vs. Inheritance

The "has-a" vs. "is-a"The Limits of Deep Inheritance Building objects

2Organizing Code

What are Modules?Exporting with export Importing with import Default vs. Named Exports

5Encapsulation & Access

Encapsulation Public & Private Class Fields Private Methods Recap Challenge

8Organizing OOP Code

Organize Classes into Modules

11Project: A Shape Renderer

Setup: Shape Class & Export Circle Class Inheritance

Overriding & Area Method

Static Shape Counter

14Library Management System

Designing the Library Book Class with Composition

Book Author Validation

3Classes & Objects

What is a Class?Creating Instances Properties and Methods Recap Challenge

6Project: A Simple Bank Account

BankAccount Class Deposits and Withdrawals

Balance as Private Field

9Static Methods & Properties

Class-Level vs. Instance-Level Static Properties Static Utility Methods Recap challenge

12Getters & Setters

The get and set Keywords Computed Properties Validation and Side Effects Recap Challenge

15 Final Challenges

Game Character Builder E-commerce Product System University Management System Smart Home Device System