Continuing with `User` and `Post`, we now have a `getDefaultUserAndPosts` function that has an `unknown` return type:

```typescript
export const getDefaultUserAndPosts = (): unknown => {
  return {
    id: "1",
    firstName: "Matt",
    lastName: "Pocock",
    posts: [
      {
        id: "1",
        title: "How I eat so much cheese",
        body: "It's pretty edam difficult",
      },
    ],
  };
};
```

## Challenge

Your challenge is to update the return type for the function so that it is both `User` and `{ posts: Post[] }`.

TypeScript allows for composing types together to create new types. This is useful when returning data that is a combination of multiple different types.

This syntax might look confusing, but it is exactly what we want:

```typescript
export const getDefaultUserAndPosts = (): User & { posts: Post[] } => {
```

It says that we want to return a `User` as well as an array of `Post`s.

The `&` tells TypeScript that we want to _intersect_ the two.

This is similar to what we saw with `extends` previously, but this time instead of inheriting, we are combining.

With `extends` you inherit, and with `&` you combine.

So we are making a single return type by combining `User` with an array of `Post`s.

It's possible to intersect multiple types, including those we create inline.

For example, we can also add an `age`:

```typescript
export const getDefaultUserAndPosts = (): User & { posts: Post[] } & { age: number} => {
```

If the return type starts to get too messy, you can do the intersection at the type level:

```typescript
type DefaultUserAndPosts =  User & { posts: Post[] } & { age: number}

export const getDefaultUserAndPosts = (): DefaultUserAndPosts => {
```

Now when working with the `DefaultUserAndPosts` type, we would get autocompletion on all of the required properties.

Intersection is usually used for composing types from other types, but can be useful in other situations as well.

Combine Inline or at the Type Level

0:00 The syntax down here might look a little bit confusing, but this is exactly what we want here. We want to say we want to return a user, and we want to return an array of posts. What this allows us to do is this little intersection symbol there means that we intersect the two.

0:19 This is very similar to what we saw last time with extends. Except, with extends, you inherit. With intersection, you combine. You make type that is a combination of user and these posts down here. If I remove one of these, then this is going to yell at me. It's saying, "Object blah-blah blah-blah," because ID doesn't exist on just posts.

0:39 We need to be able to say user and this, and we can do multiple of these if we want. We can say user and age is number, for instance, and age 123. We can even do this at the type level outside if this starts getting a little bit too ugly.

0:58 We can say that defaultUserAndPosts is a type, and type defaultUserAndPosts is this, which is really nice. This symbol there means you combine two types together to create a new type. What TypeScript does when you hover over it is it shows you all of the component parts that make up that intersection. This is really useful for lots of different situations, but usually for composing types from other types.

Combining Types to Create New Types

0:00 In this exercise I want you to find the return type for getDefaultUserAndPosts. We're typing it as unknown here, but what we really want it to be is user and an array of posts in a posts object here.

0:17 So we could do what we did last time and do the extends clause, but I want you to find a different solution here. Your only changes should be within this little area there. You should be able to find a way to get this to work and have this non-error down here.

Get hands-on interactive practice with this free TypeScript tutorial and learn the foundational knowledge and skills you need to become a TypeScript Wizard.

In this TypeScript tutorial you will:

* Learn to type variables, functions, and errors
* Become familiar with the TypeScript documentation
* Gain experience with modern TypeScript tooling

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## Instructions

This Beginner’s TypeScript tutorial is split into several exercises. Each features a `problem` that encourages you to take an active, exploratory learning approach to finding a `solution`.

To attempt a solution, you’ll need to:

1. Watch and read the `problem` introduction.
2. Check out [the TypeScript docs](https://www.typescriptlang.org/docs/).
3. Search for relevant keywords.
4. Give it a go to see if it solves the problem (there may be more than one solution!)

Whether you succeed or get stuck, watch & read the `solution` and check out the included code.

### Prerequisites

You’ll need a working understanding of JavaScript.

### Running the Exercises Locally

Clone [the Beginner’s TypeScript Tutorial repo](https://github.com/total-typescript/beginners-typescript-tutorial), and use npm or yarn to install dependencies.

If you [follow this link to use gitpod](https://gitpod.io/#https://github.com/total-typescript/beginners-typescript-tutorial), you can work in a full VS Code coding environment running in your browser.

Follow the [instructions in the README](https://github.com/total-typescript/beginners-typescript-tutorial) for more.

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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Working with Object Params

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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

Arrays are one of the most common data structures in development. In this lesson, find out how they are represented in TypeScript.

Two Ways to Represent Arrays

Working with Arrays

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Annotate a Function to Specify its Return Type

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Assigning Dynamic Keys to an Object

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Change Function Returns Based on Type

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When using a try-catch in TypeScript, it’s important that you type the error appropriately. There are a few different techniques to accomplish this.

Techniques for Coercing an Unknown Type

Typing Errors in a Try-Catch

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.

Extend Interfaces From Other Interfaces

Inheriting Interface Properties

TypeScript provides us with tools for being more selective when creating new types based on others.

Two Techniques for Creating New Types Based on Others

Selectively Construct Types from Other Types

TypeScript supports typing functions that are passed in as arguments, including the arguments that they take.

Specify the Type of a Function

Typing Functions

TypeScript supports typing the returns from asynchronous functions– great for real world processes like fetching from databases or other third-party APIs.

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Combining Types to Create New Types

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