In this exercise we have an interface `MyComplexInterface` which is acting as a type helper.

The interface takes arguments for `Event`, `Context`, `Name`, and `Point`, each being put into `getEvent`, `getContext`, `getName`, and `getPoint`, respectively.

```tsx
interface MyComplexInterface<Event, Context, Name, Point> {
  getEvent: () => Event;
  getContext: () => Context;
  getName: () => Name;
  getPoint: () => Point;
}
```

There's also an `Example` that shows how `MyComplexInterface` is called:

```tsx
type Example = MyComplexInterface<
  "click",
  "window",
  "my-event",
  { x: 12; y: 14 }
>;
```

## Challenge

Your challenge is to create another type helper called `GetPoint`, where when we pass in to `MyComplexInterface` we return the `{ x: 12; y: 14 }` as seen in our test.

The goal is for you to focus on trying to extract the generic from the argument.

Hint: using `infer` will allow you to extract `Point` without having to change anything within `MyComplexInterface`!

Reference <a href="https://www.typescriptlang.org/docs/handbook/2/generics.html" target="_blank" rel="noopener">the Generics section of the TS Docs</a> for more.

infer can extract out all the members of the type arguments without needing to dive deep into the element itself to understand its structure.

Before we get to the solution, let's take a look at another example.

Here `Example2` is calling `GetPoint` on `MyComplexInterface`, and we're trying to return the `4`:

```tsx
type Example2 = GetPoint<MyComplexInterface<l, 2, 3, 4>>; 
```

For a first pass at a solution for `GetPoint`, we call `MyComplexInterface` and pass in `any` for each of the slots:

If we change `Example2` to call `GetPoint` with a single random argument, we'll end up with `never` because of the branching logic.

```tsx
type GetPoint<T> = T extends MyComplexInterface<any, any, any, any>
  ? any
  : never;

// As of right now, if we hover over the `4` in `Example2`
// we'll see that it is currently typed as `any` as expected.
```

## Use `infer` in a Type Argument

So what we need to do is somehow inside here is use an infer to extract out one of the slots.

### The Not-Ideal Solution

One option to make the test pass would be to update `GetPoint` to use `ReturnType<T["getPoint"]>`:

```tsx
type GetPoint<T> = T extends MyComplexInterface<any, any, any, any>
  ? ReturnType<T["getPoint"]>
  : never;
```

The issue with this solution is that it ties the conditional type to the internal structure of `MyComplexInterface`. This pattern might not be something that you always want to do.

### The Better Solution

Instead of tying into the structure of the interface, we should instead just look at its public declaration because it is less likely to change.

```tsx
MyComplexInterface<any, any, any, any>
```

We can add `infer` to any of the slots in the declaration (where "slot" is anything between angle brackets). In this case, all of the slots above are `any`.

Because `Point` was the fourth argument, we can replace the corresponding slot with `TPoint` then return it for the matching conditional branch:

```tsx
type GetPoint<T> = T extends MyComplexInterface<any, any, any, infer TPoint>
  ? TPoint
  : never;
```

This approach gives us all of the benefits of being able to extract out all the members of the type arguments without needing to dive deep into the element itself to understand its structure.

You could add `infer` for the other slots, too, but in this case `GetPoint` is only interested in `TPoint`.

Being able to extract type arguments to another type helper is another really cool feature of `infer`.

Use infer with Generics to Extract Types from Arguments

0:00 Let's take a look at the solution here. I've got an Example2 here, which is going to call GetPoint on MyComplexInterface, and we're trying to return this 4 here. As you'll see, it's currently hovering over any.

0:15 What's going on here is that, inside the GetPoint function itself, what we're doing is saying T extends MyComplexInterface, and we're passing any into each of the slots. If we don't pass any of these, then it's going to yell at us because it requires four type arguments.

0:29 What we can do is we now know that T extends that thing. If I try to pass in something random like 1-2-blah-blah-blah, then I'm going to get never because it's going to hit that branch there instead of this branch. What we need to do is somehow, inside here, use an infer to extract out one of the slots.

0:51 One thing we could do is say T, let's say getPoint, return type getPoint. That makes our tests pass. We end up with the 4 here that we're passing in. If I change that to 12, then we get 12, but it sort of ties the conditional type to the internal structure of this interface here, which might not be something that we always want to do.

1:16 We want to look at the public declaration of that interface, which is the type arguments, which are pretty unlikely to change, or less likely to change let's say. Inside here, what we can do is let's say any for now. We can add an infer to any one of these slots here. Let's say we infer TPoint here. We can then just return TPoint.

1:42 Now, you get all the benefits of being able to extract out all the members of the type arguments without needing to dive deep into the element itself and understand its structure. We can even do more of this. We can say infer -- what's that one -- if infect TEvent, infer TContext, and then infer T whatever. What you end up with is a bunch of different infers.

2:12 This has no behavioral difference. I'm just showing you that you can infer multiple slots if you want to. We can return the TContext if we want to or the TEvent or something. Having them as any is also fine because all we really care about in this case is extracting the TPoint. This is another really cool use case of infer because you can just extract out the type arguments to another type helper.

Extract Type Arguments to Another Type Helper

0:00 In this exercise, we have an interface here, which is acting as a type helper. On this interface, we have Event, Context, Name, and Point. These are all the arguments. They get put into getEvent, getContext, getName, getPoint.

0:14 Now, we want to create another type helper called GetPoint, where when we pass in a MyComplexInterface, we return the Point that's there. We could do some interesting stuff here. Really, I want you to focus on trying to extract the generic from the generic slots in there, from the generic argument.

0:36 This Point, what we can do is, using infer, we can actually extract that out directly without having to go into MyComplexInterface itself and grab getPoint and grab the return type or something crazy like that.

0:49 That's your job here, is to try and find a way to use infer to extract out something from the type arguments of something that's being passed in. 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.

Learn to manipulate types for cleaner, maintainable code with over 50 hands-on exercises, from string manipulation to advanced unions and mapped types.

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Understanding how to transform types to other types is crucial because many problems in TypeScript come down to how to get one type to become another.

Type Transformations Workshop Welcome

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Get the Return Type of a Function

Gabriel Vergnaud explains the typeof keyword and the type/value level.

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

Discover the power of union types in TypeScript and how they handle different possibilities over time, creating a multiverse in your code.

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

TypeScript gives us an annotation that infers an object / array’s values as literals and makes them read-only.

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

Resolve an Object’s Values as Literal Types

TypeScript gives you a couple of different ways to pass a union to extract multiple members of another union.

Extract Specific Members From A Union with Indexed Access

Create a Union From an Object's Values

You’ll often need to get all of an object’s values. 

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 help describe different possibilities for what a type can be.

Union Types

Discriminated unions are unions which contain a single key (the discriminator) to tell them apart.

Discriminated Unions

'as const' lets you specify a type as immutable and readonly, meaning its values get inferred as their literal type.

'as const' Operator

Indexed access types let you access a member of a type: an object value, array member, or class method.

Indexed Access Types

Unions and Indexing

Template literals allow us to be very specific with the types of strings we use throughout our applications.

Template Literal with Strings

Only Allow Specified String Patterns

Sometimes you’ll want to create a union based on strings that match a pattern. Template literals can combine with other TS features to make this a breeze!

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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TypeScript allows for strings to be split on the type level. This is rather easy to accomplish using a certain method from `ts-toolbelt`.

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

TypeScript has a number of useful type helpers that can manipulate string literals in various ways. 

Manipulate String Literals Using Type Helpers

Transform String Literals To Uppercase

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

Template Literals in Mattermost

Template Literal types let you express dynamic strings in TypeScript,

Template Literal Types

Template Literals

Type Helpers allow us to create functions that can return other types. They are the basis for many of TypeScript’s core features.

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

Type helpers support multiple type arguments, and it’s easier than you might think.

Add Support for Multiple Types in a Type Helper

Create a Reusable Type Helper

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

Set a Default Type Value in a Type Helper

Optional Type Parameters in Type Helpers

Functions can be used as constraints for Type Helpers, but not all functions have the same shape for their arguments.

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

Using TypeScript’s built-in types allows you to add constraints when working with arrays.

Enforce a Minimum Array Length in a Type Helper

Constraining Type Helpers to Non-Empty Arrays

Explainer: Type Helpers in Redux

Type Helpers

Conditional types are powerful, and are where TypeScript becomes less about "annotations you add to your JS" and more like its own meta programming language.

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

Daniel Rosenwasser discusses how new features are added to TypeScript, using the example of conditional types inspired by Knockout.js.

How TypeScript Added Conditional Types

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Infer Elements Inside a Conditional with Infer

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Pattern Matching on Template Literals with Infer

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The benefits of using template literal types for expressing string requirements over regexes, as explained by TypeScript's program manager.

Template Literal Types Were Nearly Regexes

You may want to extract from a library function's return, but don't want to declare a type annotation for it. This is solved with infer and a conditional type.

Optionally Infer the Return Type of a Function

Extract the Result of an Async Function

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Two Methods for Extracting the Result of Multiple Possible Functions

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Using Generic Context to Avoid Distributive Conditional Types

Distributivity in Conditional Types

Conditional Types and Infer

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Map Over a Union to Create an Object

Mapped types aren’t just for unions. They can also be used on objects.

Map Over the Keys of an Object

Mapped Types with Objects

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Remapping Object Keys in a Mapped Type

Transforming Object Keys in Mapped Types

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How Excalidraw uses Mapped Types to Save Lines of Code

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Selective Remapping with Conditional Types and Template Literals

Conditionally Extract Properties from Object

TypeScript allows you to create new objects by mapping over discriminated unions to pull out only the data you need.

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

Mapped types allow you to do any type transformation you want. This time, we’ll transform an object to a union of template literals.

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

Combine techniques from previous exercises to extract dynamic parameters from a path.

Extract from String with Mapped Types, Template Literals, and infer.

Transform Path Parameters from Strings to Objects

Discriminated unions are useful when dealing with form handling. With a few transformation steps, we can create one from an existing object.

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Transform an Object into a Discriminated Union

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Extract Type Arguments to Another Type Helper

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