Here we have a `User`, `Post`, and `Comment` that all share an `id` property:

```typescript
interface User {
  id: string;
  firstName: string;
  lastName: string;
}

interface Post {
  id: string;
  title: string;
  body: string;
}

interface Comment {
  id: string;
  comment: string;
}
```

There are tests in place to ensure that all of the `id` values are the same.

## Challenge

Your challenge is to do some refactoring to DRY out this code.

Consult <a href="https://www.typescriptlang.org/docs/handbook/2/objects.html" target="_blank" rel="noopener">the TypeScript docs</a> to determine how you can share the `id` property without writing the same line of code repeatedly.

Interfaces can inherit properties from other interfaces. This allows us to DRY out our code and have a neater way to work with complex data models.

The first thing to do is create a new `Base` interface with the `id` property:

```typescript
interface Base {
  id: string;
}
```

With that in place, we can use the `extends` keyword with each of the other interfaces and remove the `id`:

```typescript
interface User extends Base {
  firstName: string;
  lastName: string;
}

interface Post extends Base {
  title: string;
  body: string;
}

interface Comment extends Base {
  comment: string;
}
```

Adding `extends Base` makes it so the types inherit the properties of the `Base` interface.

Note that if the `User`, `Post`, and `Comment` were defined using the `type` keyword, we would not be able to use `extends`:

```typescript
// This won't work!
type Base {
  id: string;
}

// Syntax errors
type Comment extends Base {
  comment: string;
}
```

This is because `extends` is a property of `interface` that `type` doesn't have.

Interfaces can extend from other interfaces. This allows us to change things in just one place, which is really useful.

It's even possible to extend multiple interfaces.

For example, we can have `Post` extend from `Base` and `User`:

```typescript
interface Post extends Base, User {
  title: string;
  body: string;
}
```

Then when creating a new `post` variable, we would have autocomplete on all of the properties required.

Interfaces can be composed together in a neater way than if we were just using types alone. This is particularly useful for situations where we have a complex data model that you want to express in TypeScript.

Extend Interfaces From Other Interfaces

0:00 Now we have a base interface. This base interface, it contains our ID string, and now each of user, post, and comment will inherit from that base interface using extends.

0:13 This is a property of interface that type doesn't have. If these were types instead, we wouldn't be able to do this in the same way. In fact, we're now littered with syntax errors.

0:25 Interfaces can extend from other interfaces, which is really, really useful. What it means is that if I want to change number here, or change ID to number, then I only need to change it in one place, which is extremely useful.

0:40 If I just changed the tests to work, then that's going to do the same thing. This is really, really nice. We can even have it so that it extends a couple of different things if we want to. Post now extends base and user. If I want to add const post is a post here, then I need to add body, first name, title, or last name.

1:03 This means that you can compose interfaces together in a neater way than if you were just using types. This syntax is pretty useful for some situations, especially if you have this complex data model that you want to express in TypeScript.

Inheriting Interface Properties

0:00 In this exercise, we've got a bunch of duplicated interfaces where they all share this ID property. We've got User, Post, and Comment. We've got a bunch of tests down here to make sure that everything stays as it should be. If I remove title from here, then it's going to yell at me because this Post type is not equal to id:string; title:string; body:string.

0:25 Right from the off, this is like a refactor challenge. Your challenge is to make sure the tests continue to pass while getting rid of some of this duplication here. Ideally, we would find some way that User, Post, and Comment would inherit from an object which contained id:string. That's your challenge.

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To attempt a solution, you’ll need to:

1. Watch and read the `problem` introduction.
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4. Give it a go to see if it solves the problem (there may be more than one solution!)

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### Prerequisites

You’ll need a working understanding of JavaScript.

### Running the Exercises Locally

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Free interactive video tutorial that will help you get started with TypeScript.

Beginner's TypeScript Tutorial Repo

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The Implicit ‘Any’ Type Error

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Denote an Optional Property

Set Properties as Optional

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Mark a Function Parameter as Optional

Optional Parameters

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Make a Variable Conform to an Interface

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Constraining Value Types

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Two Ways to Represent Arrays

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Annotate a Function to Specify its Return Type

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Change Function Returns Based on Type

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Techniques for Coercing an Unknown Type

Typing Errors in a Try-Catch

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Combine Inline or at the Type Level

Combining Types to Create New Types

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Two Techniques for Creating New Types Based on Others

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Specify the Type of a Function

Typing Functions

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Inheriting Interface Properties

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