In this exercise we're going to examine a little wrinkle in conditional types.

Consider the example below:

```tsx
type Fruit = "apple" | "banana" | "orange"

type AppleOrBanana = Fruit extends "apple" | "banana" ? Fruit : never

// Fails
type tests = [Expect<Equal<AppleOrBanana, "apple" | "banana">>]
```

You would expect `AppleOrBanana` to return a result of `"apple" | "banana"`, but it doesn't.

## Challenge

Read through <a href="https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types" target="_blank" rel="noopener">the Distributive Conditional Types section of the TS docs</a>, then update the example code so tests run correctly.

Hint: there are two possible solutions to this problem.

There’s a gotcha when working with union and conditional types known as the Distributive Conditional Type. In this exercise, we'll find workarounds.

We're going to have to adjust our current mental model of conditional types to understand why we have this issue with distributive conditional types.

For reference, here is the failing code from the problem:

```tsx
type Fruit = "apple" | "banana" | "orange"

type AppleOrBanana = Fruit extends "apple" | "banana" ? Fruit : never
```

As mentioned, there are two ways to solve this challenge.

## Using a Generic

The distributed conditional types problem can be solved by using a generic.

Consider the solution code below:

```tsx
type GetAppleOrBanana<T> = T extends "apple" | "banana" ? T : never

type AppleOrBanana = GetAppleOrBanana<Fruit>
```

We create a new `GetAppleOrBanana` that accepts `T`. If `T` extends apple or banana, return `T`. Otherwise, return `never`.

Then we update `AppleOrBanana` to be `GetAppleOrBanana`, passing in `Fruit`.

This technique will work even when changing the `Fruit` type to include other options.

It works because when you pull a union into a generic like `T`, the members of the union distribute across it.

In other words, `T` will become _each individual member_ of the union, and the conditional type will iterate over each one.

### Check Your Understanding

Read the following code, and think through what is happening with `T`:

```tsx
type Fruit = "apple" | "banana" | "orange";
type GetAppleOrBanana<T> = T extends "apple" | "banana" ? T : never
```

This line of code checks if `T` is either `"apple"` or `"banana"`, and will return it if it is.

Think of it as iterating through the members of the union type to find what matches.

Now compare that solution to the original problem:

```tsx
type Fruit = "apple" | "banana" | "orange";

type AppleOrBanana = Fruit extends "apple" | "banana" ? Fruit : never;
```

In the problem code, we're considering the _entire_ `Fruit` union itself.

Because `"apple" | "banana" | "orange"` is not the same as `"apple" | "banana"`, the code will return `never`.

This issue can really catch you off-guard if you aren't aware of it!

## Solving with `infer`

It's also possible to solve this problem without a generic context:

```tsx
type AppleOrBanana = Fruit extends infer T
  ? T extends "apple" | "banana"
    ? T
    : never
  : never
```

Here we create a conditional type with `Fruit extends infer T`.

This infers what's inside of `Fruit`, and treats it as an iterable because it is now within a generic context!

With `T` acting as the iterable, the individual members of `Fruit` are checked similarly to the solution above.

While this behaviour is useful, you'll usually be able to wrap things in a type helper and have it just work.

Using Generic Context to Avoid Distributive Conditional Types

0:00 The solution here is pretty weird. It means you need to break a little bit of your mental model when it comes to conditional types. There is a difference between a union type that's expressed as a type here and one that you can get via a generic, so one that you pass in via a type argument.

0:21 Let's actually look at the second solution first. Inside here, we're basically saying we've got GetAppleOrBanana T. We say that if T extends apple or banana, then return T. Otherwise, return never. This means that we're able to extract apple or banana from inside Fruit there.

0:40 If we add in another one here, let's say a pear, for instance, then we still end up with apple or banana. If we remove one of these, then we end up with just apple. We're basically extracting the element from there.

0:53 Inside the problem situation, this doesn't work because we seem to be still hitting never here. Apple or banana is never in this situation, which is really weird. The reason this is happening is when you use a generic like this, when you pull it out into T, then what happens is the members of the union distribute across it. T comes to represent each individual member of the union type.

1:26 What we can say is, does T extend apple or banana? Sure, that works for apple. That works for banana. Doesn't work for orange. Doesn't work for pear. It maps over them and filters through them. You can think of this as like iterating through the members of that union type.

1:41 If you don't do this, then this is no longer in a generic context. This is just itself Fruit. It compares the entire thing against the entire thing. It says, "Does this contain apple or banana?" It does contain apple or banana, but it also contains orange. What we end up with is never.

2:02 This is something that will just catch you out a bunch of different times if you don't know about it, which is why I'm teaching it to you here.

2:09 There is a really weird solution if you want to do it without a generic context, which is if you say, "Fruit extends infer T" -- this is a conditional type, and you're just inferring the thing that's in the Fruit -- now it will be treated as though now you can iterate over it because it's in a generic context.

2:29 This T now is basically acting as the iterable. You iterate over. We check against apple, check against banana. If we do, then we return T. A really, really weird property of union types and conditional types that I really wanted to cover.

2:45 You'll know when you need this. You'll know when this situation comes up because you'll just be thinking, "Why on Earth isn't this working?" You'll realize that, "Oh God. OK, it's distributive conditional types. This one I should put in a generic context first. Then I can mess about with it."

Distributivity in Conditional Types

0:00 In this exercise, we're going to examine a horrible little gotcha with conditional types, and it really extends to this. What we're trying to do is we're saying apple or banana here. We should be able to check, if fruit extends apple or banana, then we return the fruit. Otherwise, we return never.

0:23 know that apple or banana is in here, so we should be able to say apple's in there and banana's in there, so we should return apple or banana, and then we're good to go. This is quite a contrived version of this problem.

0:37 The issue here is something called distributive conditional types. Your job is to look for that in the TypeScript handbook and see if you can work out what's happening here. There are two possible solutions to this problem. Both of them are a little bit tricksy, so good luck.

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## Inference Basics 

The TypeScript compiler uses type inference to automatically determine the data type of a value depending on the context in which it is used. We can use several utilities and operators to define our own types more precisely.

## Unions & Indexing Unions

Union types play a key role in the type transformation process. In this section we will work at unions and discriminated unions, as well as how to access their properties with indexed access types. Several of TypeScript’s built-in utility types will also be explored.

## Template Literals 

As of TypeScript 4.1, template literals can be used to power amazing inference on strings. In this section we’ll practice using template literals to help create object types, split strings into tuples, and other useful transformations.

## Type Helpers

Type helpers allow you to manipulate and work with types in a more convenient way. They allow you to set constraints that ensure your types are as you expect while retaining flexibility for reuse. The type helper pattern is important to understand, as it forms the basis of many of TypeScript's core features.

## Conditional Types & Infer 

In this section, we will further explore how to leverage the type helpers pattern, paying special attention to conditional types and inference when working with functions, objects, and Promises.

## Mapped Types 

With TypeScript's mapped types, you can transform any type into and out of objects, unions, and tuples. It's an amazingly flexible tool for creating all sorts of different types. You’ll get practice with all of these, as well as using template literals to dynamically customize your types. One learner wrote they'd been using TS for 2 years and finally understood Mapped Types after this section!

## Challenges

To tie up the previous exercises, we've put together several real-world challenges that will put your Type Transformation skills to the test.

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Typeof Keyword, and Type Level

TypeScript gives us a powerful utility function that can extract the parameters from a function.

Use a Utility Type to Extract Function Parameters

Extract Function Parameters Into A Type

TypeScript can access the return value from within a Promise with a type helper.

It’s quite useful to be able to extract the awaited result of a Promise.

Use Utility Types To Extract a Promise's Result

Extract The Awaited Result of a Promise

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Create Unions from Objects Using Two Operators

Create a Union Type From an Object’s Keys

Creating types from types is one of the most powerful features of TypeScript

Creating Types from Types

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

The 'typeof' Operator

The 'keyof' operator can be used to extract the keys of an object type.

The 'keyof' Operator

Built-in TypeScript utilities that assist with transforming types.

Utility Types

This lesson from the Beginner's Tutorial is a refresher on the Promise type.

Typing Promises and Async Requests

Inference Basics

It’s important to understand the terminology around unions. 

Learn three different types of union in this example: enums, unions, and discriminated unions.

Union Terminology Examples

Understand The Terminology Around Unions

Often we’ll need just a part of a union type. 

TypeScript has a utility type that can directly grab specific members of a discriminated union.

Extract From A Union Using a Utility Type

Extracting Members of a Discriminated Union

Sometimes, you need a union with just a small part omitted.

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

TypeScript gives you a way to a look up a type’s properties individually.

Use Indexed Access Types to Extract Object Properties

Extract Object Properties into Individual Types

At times, you may need to access a single key from each member of a discriminated union. TypeScript makes this much easier than it may seem!

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

Resolve an Object’s Values as Literal Types

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Extract Specific Members From A Union with Indexed Access

Create a Union From an Object's Values

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There is an operator in TypeScript that gives us an elegant solution to this problem.

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

Get All of an Object’s Values

We can also create unions out of arrays. 

There are ways in TypeScript to make a union from the entire array, or just some specified elements.

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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Extracting String Pattern Matches with Template Literals

Extract Union Strings Matching a Pattern

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Passing Unions Into Template Literals

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Using S From ts-toolbelt to Split a String Into a Tuple

Splitting A String into a Tuple

Create an object whose keys are derived from a union of template literals with help from a utility type that just receives the union, and the desired type.

Use a Utility Type to Create An Object From A Union

Create an Object Whose Keys Are Derived From a Union

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Transform String Literals To Uppercase

Examine real-world use cases for template literals by going through Mattermost’s source code.

Template Literals in Mattermost

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Template Literal Types

Template Literals

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Create Functions that Return Types

Introducing Type Helpers

A Maybe type helper can help you safely handle missing values in your code.

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

The Unconstrained Maybe Type Helper

Creating a Maybe Type Helper

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Use Constraints to Limit Type Parameters

Ensure Type Safety in a Type Helper

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Create a Reusable Type Helper

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Set a Default Type Value in a Type Helper

Optional Type Parameters in Type Helpers

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Support Function Type Constraints with Variable Arguments

Functions as Constraints for Type Helpers

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

Exclude null and undefined from the Maybe Type

Constraining Types for Anything but null or undefined

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Enforce a Minimum Array Length in a Type Helper

Constraining Type Helpers to Non-Empty Arrays

Explainer: Type Helpers in Redux

Type Helpers

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Compare and Return Values with Extends and the Ternary Operator

Add Conditional Logic to a Type Helper

We need to handle cases where something undesirable is passed in to a conditional. We’ll do this with a union, nested ternaries, and a special type.

Prevent Unwanted Type Scenarios from Happening

Refine Conditional Logic in a Type Helper

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How TypeScript Added Conditional Types

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Infer Elements Inside a Conditional with Infer

Introducing infer for Conditional Logic

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Template Literal Types Were Nearly Regexes

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Map Over the Keys of an Object

Mapped Types with Objects

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Remapping Object Keys in a Mapped Type

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Selective Remapping with Conditional Types and Template Literals

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Two Techniques for Mapping a Discriminated Union to an Object

Map a Discriminated Union to an Object

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Create a Union of Tuples by Reindexing a Mapped Type

Map an Object to a Union of Tuples

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Map an Object to a Union of Template Literals

Transform an Object into a Union of Template Literals

Use the key remapping technique to transform a discriminated union into a union

Iteratively Map and Remap to Transform Types

Transform a Discriminated Union into a Union

Mapped Types in TypeScript let you dynamically create objects from other types.

Mapped Types

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Distributivity in Conditional Types

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