[00:00] Okay, we're back with our class shape, which has X and Y, it has a constructor, which sets the initial X and Y, has a position getter, and it has a move function. And we have up here a type called IShape. And this type, IShape, has a position of X and Y and it has

[00:16] a move function, which has delta X and delta Y. Your job here is to try to find a piece of syntax which lets us guarantee that class shape meets the contract described in type IShape, that it kind of implements this contract. And there is a special piece of syntax in

[00:35] TypeScript that lets you do it. And I did just drop the I word just a little few seconds ago, which might give you a clue as to how you can get this done. Good luck.

Continuing with the `Shape` class. It has properties for `#x` and `#y`, a constructor, a `position` getter, and a `move` function.

We now also have an `IShape` type, which has a position property that includes `x` and `y` coordinates, as well as a `move` function that takes in a `deltaX` and `deltaY`.

Here is the updated code:
```typescript
type IShape = {
  position: { x: number; y: number };
  move: (deltaX: number, deltaY: number) => void;
};

class Shape {
  #x: number;
  #y: number;

  constructor(initial?: { x: number; y: number }) {
    this.#x = initial?.x ?? 0;
    this.#y = initial?.y ?? 0;
  }

  get position() {
    return {
      x: this.#x,
      y: this.#y,
    };
  }

  move(x: number, y: number) {
    this.#x = x;
    this.#y = y;
  }
}
```

There aren't any errors, but there's no guarantee that the `Shape` class follows the contract defined in the `IShape` type.

## Challenge

Your challenge is to find a TypeScript syntax that ensures our `Shape` class aligns with the requirements of the `IShape` type.

A hint to help you along the way involves the letter "I".

TypeScript has a special syntax that allows you to ensure that a class adheres to a contract.

[00:00] Okay, the syntax here is class shape implements I shape. Ooh, interesting. Class shape implements I shape. Now I shape, what we can do is if we add a property to I shape up here, and let's say we add a property, ooh, I don't know, like, let's just say something, which is going to be a string, or maybe even ID,

[00:20] let's say a string. Now we're going to get an error on shape saying class shape incorrectly implements interface I shape. That's funny, even though I shape is actually a type, not an interface, but property ID is missing in type shape, but required in type I shape. Okay, so we would have to say ID equals one,

[00:40] for instance, or A. Now this means then that we've kind of like saying to our class, you need to implement this particular type. This particular set of things needs to be on your public API. Note that it doesn't count for private stuff here, so we don't need to mention X and Y on here.

[00:57] It's really just for things that you want your public class or the public interface of your class to be able to be accessed. So this is really nice and a cool little piece of syntax, and you notice that the naming convention here is kind of interesting. We're using some what's called Hungarian notation, which is what we have the class,

[01:16] which is kind of like the object, and then we say I shape or T shape, for instance, which is kind of like saying this is the interface that describes this class, and that's the kind of relationship between them. You don't necessarily need to do this. We can kind of use class as a type, really, which is kind of fun.

[01:35] So we can say if something needs to receive a shape, we don't need to pass in I shape and implement all that. We can just say this function receives a shape. Very nice. If I just show you that, we can say const func equals a little function, and the shape has to be a shape like that. Very cool. So we don't need this kind of implement stuff,

[01:54] but sometimes it's useful, and sometimes it just means that you can declare a type, for instance, and then say every single instance of this class declaration, for instance, so let's say a triangle needs to implement I shape, a square needs to implement I shape, then it means that you can just be really, really strict about the way that you declare your classes.

[02:13] Quite useful.

The syntax keyword for ensuring classes adhere to a specific interface is `implements`:

```typescript
class Shape implements IShape {
	// ...
}
```

Now if we were to add a property to `IShape`, an error would be thrown at the `Shape` class for not correctly implementing the interface `IShape`:

```typescript
type IShape = {
  id: string; // example added now
  position: { x: number; y: number };
  move: (deltaX: number, deltaY: number) => void;
}

class Shape implements IShape { // red squiggly line under Shape
  // rest of class as before
}

// Hovering over Shape shows:
Class 'Shape' incorrectly implements interface 'IShape'.
```

Note that if we were to add a private property to the `Shape` class, it would not be included in the interface check. This is because interfaces only apply to the public API of the class.

## About the Naming Convention

The name `IShape` uses Hungarian notation. This convention is used to indicate that the type is an interface. The `I` prefix is used for interfaces, while the `T` prefix is used for types.

You don't need to use this naming convention, but it can be quite useful for understanding a type's purpose at a glance.

## Using a Class as a Type

It's possible to use a class itself as a type. This would eliminate the need for a separate type or interface. For example, a function can directly receive a `Shape` object:

A class itself can be used as a type, eliminating the need for a separate type or interface. For example, a function can directly receive a `Shape` object:

```typescript
const moveShape = (shape: Shape) => {
	// ...
};

class Shape {
	// ...
}
```

However, specifying a type and requiring classes to implement a specific interface ensures greater consistency and standardization in your codebase.

Using the `implements` keyword in TypeScript to ensure that a class adheres to an interface's contractual requirements.

Implementing Interfaces in TypeScript with Classes

Ensure a Class Adheres to a Contract

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Troubleshoot an unexpected token error in the browser when trying to import a TypeScript file.

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Browsers Can't Understand TypeScript Syntax

Unexpected Token Error in the Browser

Use the TypeScript CLI to convert a TypeScript file into a JavaScript file.

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Compile TypeScript to JavaScript with the TypeScript CLI

Convert a TypeScript File into a JavaScript File

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Use tsc's Watch Mode to Automatically Compile TypeScript Files

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Compiling TypeScript Files to a Directory

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Create a new project with Vite and TypeScript to improve upon our current setup.

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Use Vite for a Frontend App with TypeScript

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TypeScript as a Linter

Comparing Vite with tsc

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TypeScript in a CI/CD System

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The tsx CLI tool allows you to quickly run scripts written in TypeScript.

Quickly Create Scripts with tsx

TypeScript in The Build Process

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Optional Function Parameters in TypeScript

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Add a Default Parameter with `=`

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Annotating with {}

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Specifying Optional Properties

Making Object Property Types Optional

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Learn about the two different syntaxes for defining arrays in TypeScript.

Two Syntaxes for Arrays

Add an Array to an Object

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There are few different ways in which an array of objects can be expressed

Representing an Array of Objects

Arrays of Objects in TypeScript

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Rest Parameters in TypeScript

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Using Tuple Syntax in TypeScript

Tuples for Precise Array Structures in TypeScript

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The Controversial `any` Type

How to make sure a function and its parameters and return types fit certain criteria, and how to annotate functions passed as arguments

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Function Types for Type Aliases in TypeScript

Function Types

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The `void` Type

Typing an Event Listener

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Hovering Over a Function Call

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TSDoc Comments

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Manually Triggering Autocomplete

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Quick Renaming in VS Code

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The Organize Imports Hotkey in VS Code

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Refactoring with Quick Fixes

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Automatic Code Formatting with Prettier

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Restarting the TypeScript Server in VS Code

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Ensure a Class Adheres to a Contract

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