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Get ready to advance, because object types are where TypeScript truly shines. This lesson shows how to represent complex object shapes in TypeScript.

Advanced Object Types

In TypeScript, it's not always enough to be able to compose types together. Sometimes it's convenient to copy all the type members from one type into another type. We can accomplish this with the `extends` keyword, like in this example:

```ts
interface Shape {
  color: string;
}

interface Square extends Shape {
  sideLength: number;
}

const mySquare: Square = { sideLength: 10, color: 'blue' };
```

In this example, the `Square` `interface` uses the `extends` keyword to copy all the type members from `Shape` into `Square`. Therefore, when we declare a variable like `mySquare`, both a `.sideLength` property with type `number` and a `.color` property of type `string` is required.

Using `extends` can help us organize our code by abstracting out common type members into their own interface, then copying them into more specific types.


Extending Interfaces

As our data gets nested deeper, we'll start to have typed objects that become unwieldy to write and read. Take the following type:

```ts
interface About {
  general: {
    id: number;
    name: string;
    version: {
      versionNumber: number;
    }
  }
}
```
This type has two levels of nesting. This might work for a short program, but as our code expands and we need more features we'll likely encounter two problems:

1. As we add more data, this `interface` may become so nested it's hard for ourselves and other developers to read.
2. It's likely that we may want just a nested part of this type somewhere. For instance, we may want just the `version` object type in our program, and it would be nice if we could use it without all the other type members in `About`.

To solve this, TypeScript allows us to compose types. We can define multiple types and reference them inside other types. Here's the type above, rewritten with individual types composed together:

```ts
interface About {
  general: General;
}

interface General {
  id: number;
  name: string;
  version: Version;
}

interface Version {
  versionNumber: number;
}
```
The resulting code is a little longer, but we addressed the problems with a big interface: we can now read our code easier with named types and we can reuse the smaller `interfaces` in other places in our code.

Composing types together is an essential way to keep our code organized and flexible.


Composed Types

As our programs grow and become more complex, we'll need to add more methods and properties to our objects to accommodate more features. In fact, we may need to add _nested_ methods and properties. Take a look at the following `class`:

```ts
class OneSeries implements Robot {
  about;

  constructor(props: { general: { id: number; name: string; } }) {
    this.about = props;
  }

  getRobotId() {
    return `ID: ${this.about.general.id}`;
  }
}
```

In this `class`, `OneSeries` expects to have an `about` property that is an object with a nested object inside it. Inside `getRobotId()`, `OneSeries` returns `this.about.general.id`. To type an object nested inside another object, we could write an `interface` like this:

```ts
interface Robot {
  about: {
    general: {
      id: number;
      name: string;
    };
  };
  getRobotId: () => string;
}
```

Notice that within the `Robot` `interface`, the `general` typed object is nested inside the `about` typed object. TypeScript allows us to infinitely nest objects so that we can describe data correctly.


Deep Types

In TypeScript, we can define types in a variety of ways to fit our code. We've used the `type` keyword to define types, however, there's another way to define types with the `interface` keyword.

Here's a `type` that you've seen before: 

```ts
type Mail = {
  postagePrice: number;
  address: string;
}

const catalog: Mail = ...
```

And here is an identical type that utilizes `interface`:

```ts
interface Mail {
  postagePrice: number;
  address: string;
}

const catalog: Mail = ...
```

In this example, we used both a `type` and an `interface` to create a typed object named `Mail`. The syntaxes for `type` and `interface` are slightly different, since `interface` does not require an equals sign (`=`) before the typed object. Functionally, the two `Mail` types in the example are identical: both will enforce the typed object at compile time when typing a variable.

The biggest difference between `interface` and `type` is that `interface` can only be used to type objects, while `type` can be used to type objects, primitives, and more. As it turns out, `type` is more versatile and functional than `interface`. So why would we use `interface`?

Sometimes, we don't want a type that can do everything. We'd like our types to be constrained so we're more likely to write consistent code. Since `interface` may only type objects, it's a perfect match for writing object-oriented programs because these programs need many typed objects. So, let's get started writing types with `interface`.


Interfaces and Types

When typing objects in TypeScript, sometimes it's not possible to know the property names for an object, like when we get back information from an outside data source/API. While we may not know the exact property names at compile-time, we may know what the data will look like in general. In that case, it's useful to write an object type that allows us to include a variable name for the property name. This feature is called _index signatures_. 

Imagine we query a map API to get a list of latitudes where a solar eclipse can be viewed. The data might look like:

```ts
{
  '40.712776': true;
  '41.203323': true;
  '40.417286': false;
}
```
We know that all the property names will be strings, and all their values will be booleans, but we don't know what the property names will be. To type this object, we can utilize an index signature to type this object. We could write this object's type like this:

```ts
interface SolarEclipse {
  [latitude: string]: boolean;
} 
```

In the `SolarEclipse` type, there's an index signature used for defining a variable property name of each type member. The `[latitude: string]` syntax defines every property name within `SolarEclipse` as a `string` type with a value of type `boolean`. In the `[latitude: string]` syntax, the `latitude` name is purely for us, the developer, as a human-readable name that will show up in potential error messages later.


Index Signatures

A common scenario in programming is creating a function or class that can take in many arguments, some of which are required, and some that are optional. Every `interface` within this lesson so far assumes that every type member is required, however, TypeScript allows us to make some type members optional. Take a look at this code:

```ts
interface OptionsType {
  name: string;
  size?: string;
}

function listFile(options: OptionsType) {
  let fileName = options.name;

  if (options.size) {
    fileName = `${fileName}: ${options.size}`;
  }

  return fileName;
}
```
In the example, `OptionsType` has an optional type member named `size`. We can denote any type member as optional using the `?` operator after the property name and before the colon (`:`), like this: `size?: string;`. Since `.size` is optional, we added a conditional to check if it exists before trying to use the `.size` property.

The optional parameter allows us to call `listFile()` with a parameter that does not include a `size` property at all, like this:

```ts
listFile({ name: 'readme.txt' })
```

Optional type members in TypeScript allow us to create programs that don't require us to pass in every possible key-value pair, making our programs more concise and easier to reason about.


Optional Type Members

One of the challenges of writing TypeScript is knowing how to apply types in every situation we'll encounter within our code. Take a look at this example:

```ts
class Robot {
  identify(id: number) {
    console.log(`beep! I'm ${id}`);
  }
}
```

Here, we've created a `class` named `Robot`. How could we apply the type `Robot`? Also, some robots may have more functionality than others, or have variable property names. How might we apply types in these situations?

This lesson is about how we can deal with a variety of situations to make sure that our code is typed, no matter what our program does or how it's structured. Types should always help make our code safer, without imposing restrictions on how we write and organize our code.

In this lesson, we'll dive deep into how to use types with object-oriented programming patterns, how to use types together to create combined types, and so much more!


Using a robot that has an internal circuitry and a motherboard as a baseline for what an interface can be. This baseline robot is compared to a "similarly shaped" human which has a heart and thus does not fit the interface. However, another type of robot, Codey, while a different overall shape does have internal circuitry and fits the interface.

Introduction

The `interface` keyword in TypeScript is especially good for adding types to a `class`. Since `interface` is constrained to typed objects and using `class` is a way to program with objects, `interface` and `class` are a great match.

TypeScript gives us the ability to apply a type to an object/class with the `implements` keyword, like this:

```ts
interface Robot {
  identify: (id: number) => void;
}

class OneSeries implements Robot {
  identify(id: number) {
    console.log(`beep! I'm ${id.toFixed(2)}.`);
  }

  answerQuestion() {
    console.log('42!');
  }
}
````

In the example, there's an `interface` named `Robot` and a `class` named `OneSeries`. The `implements` keyword is then used to apply the type `Robot` to `OneSeries`. 

Notice that `Robot` has an `.identify()` method. Since `Robot` is applied to `OneSeries`, `OneSeries` must include a method named `.identify()` that matches the `Robot` `interface`. Additionally, `OneSeries` can have methods and properties of its own, like the `.answerQuestion()` method.

`implements` and `interface` allow us to create types that match a variety of `class` patterns, which makes `interface` a good tool for use on object-oriented programs.


Interfaces and Classes

🙌 Way to go! You have _advanced_ through this entire lesson. No one will _object_ to your superior TypeScript knowledge. Here's an overview of what we learned:

- We can use both `interface` and `type` keywords to declare types.
- `interface` is great for typing objects, especially within object-oriented programs.
- We can apply an `interface` on a `class` using the `implements` keyword.
- Object types can be nested infinitely.
- We can define multiple types and compose them together to organize our code and make it more flexible.
- We can copy the type members of one `interface` into another using the `extends` keyword.
- We can define variable property names within an object type with an _index signature_. An index signature uses syntax like: `[propertyName: string]: string`.
- It's possible to make some type members optional, using the `?`  operator. The syntax looks like `name?: string`.

Use the code editor to polish your understanding of typing objects in TypeScript.
