[00:00] In this exercise, we have a class called CanvasNode, which is currently acting like an empty object. We can initiate CanvasNode like this by calling new CanvasNode on it. And currently we're expecting it to have two properties on that class. We're expecting it to have a property of X, which is gonna be zero,

[00:18] and a property of Y, which is gonna be zero. And we're also hoping that it's going to be read-only too. So you've got a couple of different jobs here that you should know how to solve vaguely if you understand or remember the object syntax that we saw before. And we're also gonna need to probably add a constructor to CanvasNode, but there's actually a solution

[00:38] where you don't add a constructor as well. So your job is to investigate the basic properties of classes based on the TypeScript documentation, which I'll link to below, and try to get this working. Good luck.

Here we have a class called `CanvasNode` that currently functions identically to an empty object:

```typescript
class CanvasNode {}
```

Inside of a test case, we instantiate the class by calling `new CanvasNode()`.

However, have some errors since we expect it to house two properties, specifically `x` and `y`, each with a default value of `0`:

```typescript
it("Should store some basic properties", () => {
  const canvasNode = new CanvasNode();

  expect(canvasNode.x).toEqual(0); // red squiggly line under x
  expect(canvasNode.y).toEqual(0); // red squiggly line under y

  // @ts-expect-error Property is readonly
  canvasNode.x = 10;

  // @ts-expect-error Property is readonly
  canvasNode.y = 20;
});
```

As seen from the `@ts-expect-error` directives, we also expect these properties to be readonly.

## Challenge

Your challenge is to apply what you've learned about object syntax and implement a constructor in the `CanvasNode` class to resolve the errors.

Note that there's also a solution that doesn't require the use of a constructor. 

For help, check out the TypeScript documentation for classes.

Take a first look at creating classes in TypeScript.

[00:00] Okay, let's look at classes in TypeScript. So, a class is kind of like a special object where it behaves pretty much exactly like an object, but you get to use some special syntax. And every time you declare it or instantiate a new class, then it basically kind of, you use this new CanvasNode syntax,

[00:18] or we can call this whatever we want. I'm calling this CanvasNode because this is gonna sort of carry on through the next few exercises. And our CanvasNode has to have an X and a Y property. So, just like we did with our object syntax, we can say X is a number and Y is a number. Okay, now we're getting somewhere,

[00:38] but we get an error inside here by saying property X has no initializer and is not definitely assigned in the constructor. Well, what's the constructor? When we call this kind of like function here, new CanvasNode, we're like calling it, passing it zero arguments, we can actually kind of like add that behavior

[00:58] into our CanvasNode by adding a constructor function. And that constructor is gonna be a function like this. And what this does is it says this.X equals zero. And when I add this line of code, then the error up here goes away. So, what was the error? Is not definitely assigned in the constructor.

[01:17] This is the constructor function. And when I assign to this.Y, well, the error goes away. And now our tests are gonna start passing too because we have CanvasNodeX to equal Y, we're expecting to equal zero, we're expecting that to be true. And we're also expecting Y to equal zero too.

[01:37] But we're also expecting these properties to be read only, right? Well, we can fix that. We can say read only X and read only Y. Very, very cool. Now what this means now is that we can no longer like assign to it from the outside. So we can no longer kind of assign to it.

[01:56] Beautiful stuff. And it means we can use this kind of like syntax, this read only syntax inside our class properties. There is though a different style of solution for this where you can actually combine the kind of like type annotation at the top here and the assignment below it.

[02:14] And this looks like this. Well, what you say is class CanvasNode and you say read only X number equals zero. This is really, really cool. It means that you can kind of like combine the two and you don't need to declare a constructor in this case. And so this is a really nice kind of added syntax

[02:34] that we get with classes. You notice again, classes are just like objects, but just a little bit more powerful, a little bit more interesting in various directions. And we're gonna keep on covering them over the next few exercises.

Classes in TypeScript are like a special object that allows for some special syntax. Every time you declare a class, you can instantiate a new instance of it using the `new` keyword.

To create a `CanvasNode` class with with `x` and `y` properties, we can fill in the class body with the following:

```typescript
class CanvasNode {
	x: number; // red squiggly line under x
	y: number; // red squiggly line under y
}
```

With this change, we now have some errors.

The `x` and `y` properties have error messages that say they have no initializer:

```typescript
// Hovering over `x` shows:
Property 'x' has no initializer and is not definitely assigned in the constructor.
```

To fix this error, we need to add a constructor function in the `CanvasNode` class that assigns `0` to both `x` and `y`:

```typescript
class CanvasNode {
	x: number;
	y: number;

	constructor() {
		this.x = 0;
		this.y = 0;
	}
}
```

After adding the constructor, the errors under `x` and `y` go away.

However, there are also errors under the `@ts-expect-error` directives because we expect the `x` and `y` properties to be readonly:

```typescript
// inside of the test case

// @ts-expect-error Property is readonly // red squiggly line under this line
canvasNode.x = 10;

// @ts-expect-error Property is readonly // red squiggly line under this line
canvasNode.y = 20;
```

In order to make these properties readonly, we can add the `readonly` keyword to the property declarations:

```typescript
class CanvasNode {
	readonly x: number;
	readonly y: number;

	constructor() {
		this.x = 0;
		this.y = 0;
	}
}
```

## A More Streamlined Solution

As mentioned in the introduction to this challenge, there is a more streamlined solution that doesn't require the use of a constructor.

When creating the class, we can declare the property as a read-only number and initialize it right there in the declaration:

```typescript
class CanvasNode {
	readonly x: number = 0;
	readonly y: number = 0;
}
```

This technique eliminates the requirement for a constructor to initialize the values, providing an easier and more compact method of working with TypeScript classes.

Classes are similar to objects, but with a bit more power. In the upcoming exercises, we will explore them further.

There are multiple ways to create a class with default properties in TypeScript.

Creating a Class with Default Properties

Classes in TypeScript

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TypeScript in The Build Process

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Representing an Array of Objects

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The `void` Type

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Classes in TypeScript

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