[00:00] In this exercise, our canvas node is still in place. We've still got X and Y as the defaults, and we've still got our first test that we had before should be able to move our canvas node. But we're now also expecting it to be able to receive an initial position. Our canvas node should be able to receive an optional initial position,

[00:17] which basically sets the X and Y as the starting position here. Your job here is to try to find a way to get this working with the setup that we have already. So you should, you're probably gonna need a constructor, and you might need to have an argument in that constructor. Good luck.

We're continuing to work with the `CanvasNode` class with default `x` and `y` positions and a `move` method:

```typescript
class CanvasNode {
  x = 0;
  y = 0;

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

The test for movability is still in place, and we now have an additional test for receiving an optional initial position:

```typescript
it("Should be able to receive an initial position", () => {
  const canvasNode = new CanvasNode({  // red squiggly line under the entire object
    x: 10,
    y: 20,
  });

  expect(canvasNode.x).toEqual(10);
  expect(canvasNode.y).toEqual(20);
});
```

Currently, there is an error under the `new CanvasNode()` call because the `CanvasNode` class doesn't have a way of accepting an object as an argument when being instantiated.

## Challenge

Your challenge is to modify the `CanvasNode` class so that it can receive an optional initial position. This will require you to add a constructor that can accept an argument.

Classes in TypeScript can have constructors that allow for custom initialization of class instances.

[00:00] Okay, if you want your class to be able to receive an argument you need to specify a constructor So that's what we're going to do We'll say constructor and then we'll say options for instance We'll say you can pass in X which is number and a Y which is a number and then we can open up that scope

[00:16] and we can say this dot X equals ops dot X and let's say or 0 and now it means that it will default to 0 if it passes either a falsy value So 0 or we don't pass anything at all and we'll do the same thing with this dot Y equals ops dot Y

[00:35] Or 0 now we get to do a couple of things. We can simplify a little bit of code instead of just saying equals 0 here we can just Kind of like assign them to number and now because it understands that they're definitely assigning them inside the constructor all is well and now

[00:53] All of our code down here works beautifully and our new canvas node. We can of course move from the initial location You don't need to pass an option and when you hover over canvas node You can see the type of the thing that it expects There's even a really cool solution here too, which is you now no longer actually need the type annotations on the X and Y

[01:11] So you can say X it's just like this X and Y and now X and Y you can see that it Understands that its number again Type script is really clever here Understanding the behavior of what you've put inside your Constructor and understanding that the two types up here are assigned based on that

[01:31] So either of these are really really cool you get again You can see how much work TypeScript has put into making classes a really nice kind of thing to use in JavaScript

In order to make a class capable of receiving arguments, you must specify a constructor.

Using the `constructor()` keyword, you can specify the arguments you want the constructor to accept.

In this case, we'll use `opts` as an optional argument for an options object that includes properties for `x` and `y`, both being numbers:

```typescript
class CanvasNode {
  x = 0;
  y = 0;

  constructor(opts?: { x: number; y: number }) {
    // ...
  }

  ...
```

Then inside of the constructor, we'll assign `this.x` and `this.y` depending on the values passed in. We'll also provide default values of `0` for both `x` and `y` in case they are not provided or are falsy:

```typescript
constructor(opts?: { x: number; y: number }) {
  this.x = opts?.x || 0;
  this.y = opts?.y || 0;
}
```

Now that we've added a constructor, we can update the initial values of `x` and `y` to be typed as `number` instead of assigning them to 0 since the constructor will handle that for us:

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

  ...
```

Now we can create a new instance of `CanvasNode` without passing any options:

```typescript
const canvasNode = new CanvasNode();
```

Or we can create a new instance while passing options:

```typescript
const canvasNode = new CanvasNode({
	x: 10,
	y: 20,
});
```

All of our tests also pass as expected.

## Further Simplification

We can simplify the `x` and `y` types even more by removing the type annotations:

```typescript
class CanvasNode {
  x;
  y;

  ...
```

TypeScript understands the behavior within the constructor, and is able to infer the types of `x` and `y` to be numbers.

These examples help show how TypeScript has enhanced the convenience of using classes when compared to JavaScript.

Specify a constructor in your TypeScript classes to receive arguments and set default values.

Receive Arguments in Class Constructors

Creating Constructors for Classes in TypeScript

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Unexpected Token Error in the Browser

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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

Automating TypeScript Compilation

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Configure the Output Directory for Compiled Files

Compiling TypeScript Files to a Directory

Vite provides a simple and efficient way to build front-end apps for web development.

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

Configure Vite for a Vanilla TypeScript Project

Use Vite for a Frontend App with TypeScript

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When the noEmit option is set to true in the tsconfig.json file, TypeScript acts more like a linter than part of the build process.

TypeScript as a Linter

Comparing Vite with tsc

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TypeScript in a CI/CD System

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Should TypeScript Block Your Dev Server?

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

Quickly Create Scripts with tsx

TypeScript in The Build Process

Learn how to identify and fix common type errors in your functions.

Common sense tells us that the booleans in the `add` function should be replaced with some sort of `number` type.

Fixing Errors in the `add` Function

Understanding Function Errors

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As a general rule, you should always annotate the parameters with their respective types when writing functions.

Annotating Function Parameters

Annotating Empty Parameters

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TypeScript comes with a set of basic types to help define what kind of values can be assigned to variables.

TypeScript Basic Types

Basic Types in TypeScript

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Using the `?` , add optional parameters in TypeScript functions.

Optional Function Parameters in TypeScript

Optional Function Parameters

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Add a Default Parameter with `=`

Default Parameters in JavaScript Functions

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Annotating with {}

Typing Object Literals

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Making Object Property Types Optional

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Creating `type` Aliases

A Single Source of Truth for Type Definitions

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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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Using Rest Parameters in TypeScript

Rest Parameters in TypeScript

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Using Tuple Syntax in TypeScript

Tuples for Precise Array Structures in TypeScript

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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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Fixing Errors with Hovering

Hovering Over a Function Call

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TSDoc Comments

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Quick Renaming in VS Code

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The Organize Imports Hotkey in VS Code

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Combining Literal & Union Types

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Literal Type Assignability

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Creating Constructors for Classes in TypeScript

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