This exercise begins with a similar setup to the last.

We've got `CreateDataShape` which takes in `TData` and `TError` as arguments:

```tsx
type CreateDataShape<TData, TError> = {
  data: TData;
  error: TError;
};
```

In the first test we're passing a string for `data`, and `undefined` for `error`.

The test fails because `CreateDataShape` requires two type arguments to be passed in.

## Challenge

Your challenge is to update `CreateDataShape` so that `TError` has a default value. This would allow us to treat it as optional, since the default value would be applied if it wasn't passed a value.

Type Helpers support default values, allowing for arguments to be optional.

The solution to this challenge is simple, but gives us good insight into some new syntax.

Updating `CreateDataShape` to have `TError = undefined` tells TypeScript that the second argument doesn't need to be passed in:

```tsx
type CreateDataShape<TData, TError = undefined> = {
  data: TData;
  error: TError;
};
```

If we test our work by creating an `Example` type, we can confirm that `CreateDataShape` works as expected whether we supply one or both arguments:

```tsx
type Example = CreateDataShape<number> 
```

## Combining a default value with extends

The default value can be combined with `extends`. For example, we can add an `Error` class:

```tsx
type CreateDataShape<TData, TError extends Error = undefined> = { 
```

But then we'd get a type `undefined` doesn't satisfy the constraint `Error` so we can't provide a default that doesn't match the constraint.

A workaround for this is to say that `TError` extends `Error` or undefined equals undefined like so:

```tsx
type CreateDataShape<TData, TError extends Error | undefined = undefined> = { 
```

This does start to get a bit messy, so extracting this out to a `MaybeError` is a good way to clean it up:

```tsx
type MaybeError = Error | undefined

type CreateDataShape<TData, TError extends MaybeError = undefined> = {
```

This syntax is useful, but note that similarly to functions, a required type parameter cannot follow an optional one.

Set a Default Type Value in a Type Helper

0:00 The solution here is super simple and gives us a nice insight into some new syntax here. We can say TError = undefined. Now, what this does is it basically says you don't need to pass that second element then.

0:15 If we add this CreateDataShape, let's say we add this to an example here, then what we're going to get is we're going to get data: string; and error: undefined;. If we change this to a number, then of course we get data: number; and error: undefined;.

0:31 Finally, we can pass anything to this error, actually. If we just parse an error inside here, then we get data: number; and error: Error. We can also combine this with the extends thing if we want to. We can say TError extends an error clause.

0:46 In fact, there, what it's going to do it's going to say Type 'undefined' does not satisfy the constraint 'Error'. We can't provide a default that doesn't match our constraint. We could say error or undefined if we want to. There we start getting a little bit messy, right?

1:00 We can see that we've got TError extends Error or undefined = undefined. You'll find this in some type definitions, is that it just gets quite clumped together and quite grouped up. There's not a huge perfect way to avoid this, really.

1:14 We could go Type this = type MaybeError = Error undefined. Then TError extends MaybeError. That's a way to maybe clean it up nicely. That gives you a sense of what's possible here. Everything in this can have a default. We can't specify just the same with functions if we say TThird here, for instance.

1:32 Then this is impossible because required type parameters may not follow optional type parameters just like in normal functions. This is a really, really useful bit of syntax. It lets us just provide sensible defaults if we don't want to be parsing these, and lets us be more expressive with our type helpers.

Optional Type Parameters in Type Helpers

0:01 We've got a similar setup to what we had last time, which is our CreateDataShape, we've got our TData, and a TError. Except here, we've now only got two tests. The two tests are, we've got a CreateDataShape here, and we're passing in a string for data. We want error to be undefined here, except the error it's giving us is that generic-type CreateDataShape requires two type arguments.

0:26 We've got this here, where we're passing it two type arguments, and they're being put into the DataShape. We've got a data here and an error, but we want to somehow find a default for this TError to default it to undefined. Currently, it demands that we have to pass it.

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There are over 50 exercises in this workshop, each with a challenge designed to help you learn to wield the power of transforming strings into objects, objects into unions, and everything in between.



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Typeof Keyword, and Type Level

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Creating Types from Types

The 'typeof' operator transforms a runtime construct into a type.

The 'typeof' Operator

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The 'keyof' Operator

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Extract From A Union Using a Utility Type

Extracting Members of a Discriminated Union

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TypeScript has a useful utility type that can extract all but a specified part of a union!

Use a Utility Type to Remove a Single Member of a Union

Excluding Parts of a Discriminated Union

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The Power of Union Types in TypeScript

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Use Indexed Access Types to Extract Object Properties

Extract Object Properties into Individual Types

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Simple Syntax Used to Access Parts of a Discriminated Union

Extract the Discriminator from a Discriminated Union

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The Annotation Used to Infer an Object's Values as Literal Types

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Use Two Operators With Indexed Access to Get All of an Object's Values

Get All of an Object’s Values

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Methods Used to Create Unions out of Array Values

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Union Types

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Discriminated Unions

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'as const' Operator

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Indexed Access Types

Unions and Indexing

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Template Literal with Strings

Only Allow Specified String Patterns

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Passing Unions Into Template Literals

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Template Literal Types

Template Literals

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Let's create a Maybe type that can handle null and undefined

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Use Constraints to Limit Type Parameters

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Leverage an essential TypeScript concept to constrain types to anything but null or undefined.

Exclude null and undefined from the Maybe Type

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Enforce a Minimum Array Length in a Type Helper

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Map Over the Keys of an Object

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Two Techniques for Mapping a Discriminated Union to an Object

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Optional Type Parameters in Type Helpers

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