Hello, welcome back! Today, we will decode the fundamentals of Revising Basic Design Patterns - Composition! A vital component of software design patterns, Composition aids us in creating complex classes using simpler ones. Our journey today includes an understanding of the concept of composition, its value in software development, and how to practically implement it in JavaScript.

To kick-start our exploration, let's understand Composition. In object-oriented programming (OOP), composition allows a class to include other classes, paving the way for the creation of complex systems out of simpler components. For instance, when building a car, we bring together independent pieces like the engine, wheels, and seats — a perfect reflection of composition in everyday life. Note that in composition, should the parent object (the car) be destroyed, the child objects (the components) also cease to exist.

Now, let's translate the theory into a JavaScript code application. Transforming the previously mentioned car example, a Car class in JavaScript is created by making objects of the Engine, Wheels, and Seats classes. The Car class owns these child objects; their existence is dependent on the Car.

JavaScript
Copy to clipboard
1class Engine {
2    start() {
3        console.log("Engine starts");  // Engine start message
4    }
5
6    stop() {
7        console.log("Engine stops");   // Engine stop message
8    }
9}
10
11class Wheels {
12    rotate() {
13        console.log("Wheels rotate");  // Wheel rotation message
14    }
15}
16
17class Seats {
18    adjust(position) {
19        console.log(`Seats adjusted to position ${position}`); // Seat adjustment message
20    }
21}
22
23class Car {
24    constructor() {
25        this.engine = new Engine();
26        this.wheels = new Wheels();
27        this.seats = new Seats();
28    }
29
30    start() {
31        this.engine.start();  // Call to start engine
32        this.seats.adjust('upright');  // Adjust seat position
33        this.wheels.rotate();  // Get wheels rolling
34    }
35}
36
37const myCar = new Car();
38myCar.start();  // Begin car functions
39
40/*
41Prints:
42Engine starts
43Seats adjusted to position upright
44Wheels rotate
45*/

In the above code, the Car class encapsulates Engine, Wheels, and Seats objects, which are independent but part of the Car class, forming a Composition pattern.

In OOP, Composition and Inheritance are two significant ways to express relationships between classes. While Inheritance implies an "is-a" relationship, Composition suggests a "has-a" relationship. For instance, a Car IS A Vehicle (Inheritance), but a Car HAS an Engine (Composition).

Superb job! You've now decoded composition and even implemented it in JavaScript! Next, you'll encounter stimulating exercises where you'll gain hands-on experience with composition in JavaScript. Stay curious, and keep practicing to fortify your concepts!