[00:00] We have a similar setup here to our previous exercise with parameters, but this time we're interested in the return type. We have a createUser function, which let's say we don't control at all, comes from a third-party library or a different source that we don't control. And we have a type called user here, which we're expecting to be exactly the same shape

[00:18] as the return type of createUser. Your job is to try to figure out how we can type this user without needing to manually type it again. It's gonna be very similar to the parameters exercise, but this time using something called a return type, type helper.


Here we have a `createUser` function:

```typescript
import { Equal, Expect } from "@total-typescript/helpers";

const createUser = (id: string) => {
  return {
    id,
    name: "John Doe",
    email: "example@email.com",
  };
};
```

For the purposes of this exercise, assume that the `createUser` function is obtained from a third-party library or some other source outside our control.

Currently, `User` is typed as `unknown`, but as seen in our test case, we expect it to have the same shape as the return type of `createUser`:

```tsx
type User = unknown;

type test = Expect<
  Equal< // red squiggly line under Equal<>
    User,
    {
      id: string;
      name: string;
      email: string;
    }
  >
>;
```

## Challenge

Your task is to determine how to attribute a type to `User` without the need to manually type it out all over again. 

This will work similarly to what we did in the parameters exercise. However, this time, you'll need to utilize the `ReturnType` type helper.

The ReturnType type helper can be used to extract the return type of a function.

[00:00] Okay, going by the pattern that we've had before, this should be pretty simple. We can use return type available globally, passing in type of create user. Whoops, create user. There we go. And now we're gonna end up with ID, name, and email all in here. Beautiful stuff. Return type again, it accepts a function,

[00:19] so you can't pass in any object into here. And it just means that this complicated type helper up here just returns the return type of the function, no matter what it is. So now if I add something up here, like phone is whatever, then we get an extra attribute inside phone there too. So this is another way that you can derive types from

[00:39] values, even if those values are relatively complex. Very cool.


Following the pattern we used in the previous exercise, we can use the `ReturnType` type helper to extract the return type of a function.

Update the `User` type to use `ReturnType` with `typeof createUser` passed in:

```typescript
type User = ReturnType<typeof createUser>;
```

Now, `User` will have the same shape as the return type of `createUser`:

```typescript
// hovering over User shows:
type User = {
  id: string;
  name: string;
  email: string;
}
```


Use the ReturnType type helper to extract the return type of a function.

Using the ReturnType Type Helper

Extracting Return Types from Functions

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Extracting Return Types from Functions

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