Let's go deeper into function types.

Here we have a `createThenGetUser` function:

```typescript
const createThenGetUser = async (
  createUser: unknown,
  getUser: unknown,
): Promise<User> => {
  const userId: string = await createUser();

  const user = await getUser(userId);

  return user;
};
```

This function takes in an asynchronous `createUser` function. It returns a promise that includes a `userId` string.

The `userId` string is then used in a call to `getUser` which would be a database call or something similar.

## Challenge

Both `createUser` and `getUser` are currently typed as `unknown`.

Your challenge is to type the functions so that the errors go away.

Reference the function typing and Promise syntax that we've looked at previously for help.

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

Here's the correct syntax for typing these functions:

```typescript
const createThenGetUser = async (
  createUser: () => Promise<string>,
  getUser: (id: string) => Promise<User>,
): Promise<User> => {
  const userId: string = await createUser();

  const user = await getUser(userId);

  return user;
};
```

Same as before, these can be extracted out into their own types as well:

```typescript
type GetUser = (id: string) => Promise<User>;
```

What this is saying is that we have an `id` which is a string, and the function will return a Promise containing a `User`.

If we had `GetUser` only returning the `User` without the Promise, we won't be able to call it as an async function in the test due to errors.

Wrapping function return types with Promises is really useful when working with real world processes like fetching from databases.

Specify the Type for an Async Function

0:00 The correct way to type these functions is using this syntax here. Same as we had before, we can extract these out into a type if we want, type getUser equals this. Then we say getUser inside here. What this is saying is we have our ID, which is a string here. Then, we're saying we're returning a promise containing a user.

0:23 If we don't do this, then it's going to be a little bit funky here because type Promise is missing the following properties. We can't do an async function here. Whereas, if we did this instead, then it would be happy. If we were using real fetches to a database using something like Prisma or something similar, then we would need to wrap them in promises. Having these in promises is really, really useful.

0:50 What we can do is say this promise string here, if we wanted to, we could make this promise number. Then ID would error here because it would be inferred as number. You get getUser number, type user is not assignable to type string. You get the idea. You can manipulate these pretty well by using this promise syntax to indicate the return type of async functions.

Typing Async Functions

0:00 Let's go deeper into function types here. We've got a function which is createThenGetUser. It takes in a createUser function which is asynchronous. It returns a promise, and it returns a promise including a string. Then, we use that string, which is the userID we've just created from createUser, to get the user. This might be a different database call or something similar.

0:26 Both of these are currently types as unknown. Your challenge is to work out how we can type these properly, so that these errors go away, using the function syntax that we've had before and maybe using some of the promise syntax that we've had before too.

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

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Typing Async Functions

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