Master the use of composition vs inheritance in classes. Discover when to apply has-a vs is-a relationships to enhance your coding skills.

When To Use

[00:00] Alright. You done? Alright. Let's get into it then. We've got our logger class.

[00:04] We're gonna make it logger and come on, AI. You're supposed to be faster than this. What's going on here? Sometimes sometimes you do funny things, AI. Alright.

[00:14] So we've got our logger class. It's gonna log the message. Returns void. I'm not gonna put that in there. We don't need to put that nonsense in there.

[00:21] And then we're console log the message. Alright. It's it's pretty straightforward. Nothing too exciting going on here. Then we're going to make a file logger.

[00:29] So this extends logger, and it's gonna override the log message, and log with the file log. And we're going to create one of those. Console log that. File, log file system initialize. The instance itself has no properties and so that's why we end up with an empty object right there.

[00:48] Okay. So now we're going to create a console logger, using inheritance. So here we go. We're inheriting logger just like we're doing with the file logger. And this one is going to do a console log.

[00:59] So boom. Now we've got two things that are loggers. This is an is a relationship. So now we're going to change things a little bit and instead, we're gonna make a email service and this has a logger. K?

[01:14] So these other two, they don't have any properties. This next one has a property. It's a logger. And it's going to accept a logger and then we'll have a send email and that is going to use the this, instances logger instance to log the, the send email. So then we can, comment these in and we'll see, right here.

[01:41] I'm looking for Where is this? Oh, yeah. There it is. It's under the file log because, we created a file or we're using the file logger for our email service. And so that's gonna have the file log prefix and then sending email to etcetera etcetera.

[01:56] So you could think of this as, in a like a real world scenario, the file logger would actually write the logs to a file where the, console logger would just write them to the console. But we're keeping things simple. This would end up going into a file somewhere. Okay. And with that then, we've got, that it has a relationship versus the is a relationship.

[02:19] The cool thing about this is that the email service could accept any type of logger and as long as they are a logger, then it can have any one of those things and that can be quite useful. You could also default this just like everything else we've learned. So you can make a new logger that's just the the default logger. And so that's one benefit of having this, not be, a, an interface and actually be a class is because it has a default implementation. So okay.

[02:49] Great. Good job on this one.

[00:00] Alright. In this exercise, we're going to explore kind of the composition versus inheritance with, classes and kind of comparing has a relationships with is a relationship. So a thing has something or a thing is something. So jump into this exercise and we'll see you when you're done.

Most modern JavaScript and TypeScript development leans heavily on functions and plain objects. But classes, interfaces, and inheritance aren't going away. You'll encounter them in libraries, frameworks, and codebases across the industry, and if you don't understand how they work, you'll struggle to work with the code that uses them.

The problem is that without a clear mental model, you'll misuse inheritance where composition would be simpler, skip interfaces where they'd save you from brittle coupling, and create class hierarchies that become impossible to follow three levels deep.

In this workshop, you'll learn how to build classes with proper encapsulation, implement interfaces for polymorphism, extend base classes when inheritance makes sense, and reach for composition when it doesn't.

Every section pairs the "how" with the "why," so you don't just learn the syntax, you learn when each pattern earns its place and when simpler alternatives are the better choice.

## Classes

Classes are the fundamental building block of object-oriented TypeScript. They bundle data and behavior together into a single unit-- a user has a name and email, a shopping cart has items and a total. If you've only worked with plain objects and functions, classes introduce a new way to organize your code.

In this section, you'll build classes from scratch while learning how they work under the hood:

- Creating classes with properties, constructors, and methods
- Using `new` to create instances and understanding what `this` refers to
- Initializing properties with default values instead of constructor arguments
- Making fields private with the `#` syntax to control access
- Refactoring constructors to accept an options object instead of positional arguments

## Interfaces and Classes

An interface on its own is just a shape-- a contract that says "anything claiming to be this type must have these methods." The real power shows up when multiple classes implement the same interface, because now you can write functions that accept any of them without knowing which specific class they'll get.

This section teaches you how interfaces enable polymorphism in practice:

- Defining interfaces with method signatures that classes must implement
- Using the `implements` keyword to enforce a contract on a class
- Implementing multiple interfaces on a single class
- Programming to abstractions by accepting interfaces instead of concrete classes
- Writing functions that work with any implementation of a given interface

## Inheritance and Polymorphism

Inheritance lets one class build on another. A circle is a shape. A square is a rectangle. The child class inherits everything from the parent and can override specific behavior-- like calculating area differently for each shape.

Here you'll work through the mechanics of inheritance and understand where it helps and where it hurts:

- Extending a base class with the `extends` keyword
- Forwarding constructor arguments to the parent with `super`
- Overriding methods to provide specialized behavior in child classes
- Using substitutability to pass any child class where the parent is expected
- Understanding why deep inheritance chains become difficult to maintain

## Composition vs Inheritance

Inheritance says "this thing is a that." Composition says "this thing has a that." A car isn't an engine-- it has one. The distinction matters because inheritance creates tight coupling between parent and child, while composition lets you swap out parts without rewriting the class that uses them.

This section shows you when to reach for each approach and why composition is usually the better default:

- Distinguishing "is a" relationships (inheritance) from "has a" relationships (composition)
- Injecting dependencies through constructor arguments instead of hardcoding them
- Swapping implementations at runtime by passing different objects (file logger, console logger, silent logger)
- Using dependency injection to make classes testable with mock objects
- Defaulting to composition and reserving inheritance for clear "is a" cases


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Master object-oriented principles in TypeScript, including classes, inheritance, and polymorphism, to enhance your development skills and project quality.

Intro to Object Oriented TypeScript

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Intro to Classes

Master the fundamentals of object-oriented TypeScript by creating and implementing classes with essential properties and instances in this engaging exercise.

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Master class inheritance and polymorphism by exploring the extends keyword. Gain essential skills to elevate your coding proficiency today!

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Substitutability 

Master breaking inheritance and polymorphism in coding to enhance your programming skills, making your projects more efficient and adaptable.

Break Inheritance and Polymorphism

3. Inheritance and Polymorphism

Master the key differences between composition and inheritance in object-oriented programming to enhance your coding clarity and efficiency.

Intro to Composition vs Inheritance

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4. Composition vs Inheritance

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Object Oriented TypeScript

Object Oriented TypeScript

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When To Use

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