This exercise is a more difficult version of one we saw earlier in the workshop.

We start with the following:

```tsx
type Route =
  | {
      route: "/";
      search: {
        page: string;
        perPage: string;
      };
    }
  | { route: "/about" }
  | { route: "/admin" }
  | { route: "/admin/users" };

type RoutesObject = unknown;
```

## Challenge

Your challenge is to update `RoutesObject` to match how it is seen in the test:

```tsx
{
  "/": {
    page: string;
    perPage: string;
  };
  "/about": never;
  "/admin": never;
  "/admin/users": never;
}
```

Hint: When mapping, check that if the `search` key is present. If it is, return it. Otherwise, return `never`. You'll want to use `infer` as well.

Following an earlier challenge, here we revisit transforming a discriminated union with unique values into an object.

Here's what my solution looks like:

```tsx
type RoutesObject = {
  [R in Route as R["route"]]: R extends { search: infer S } ? S : never;
};
```

As usual, we'll break it down bit by bit.

### Turn the Discriminated Union into an Object

The first thing to do is turn the discriminated union into an object.

We can simplify this like so:

```tsx
[R in Route as R["route"]]: R 
```

With the above, each of the members of the discriminated union is mapped into its own key:

```tsx
// Hovering over RoutesObject
type RoutesObject = {
    "/": {
        route: "/";
        search: {
            page: string;
            perPage: string;
        };
    };
    "/about": {
        route: "/about";
    };
    ...
```

### Use a Conditional to Find `search`

Now that we have the keys we want, we need to find which ones include `search`.

We can't use an indexed access for this, because we know that they don't all have it:

```tsx
// This won't work!
[R in Route as R["route"]]: R['search']
```

Instead, we'll use `extends` to do a conditional check. If `search` is there, return it. Otherwise, return `never`.

There are a couple ways to do this.

One option is to add an object with a `search` key and an `any` type:

```tsx
[R in Route as R["route"]]: R extends { search: any } ? R["search"] : never; 
```

This works, and gets us what we need to make the test pass.

However, I prefer to use `infer` because it allows us to skip the indexed access:

```tsx
[R in Route as R["route"]]: R extends { search: infer S } ? S : never;
```

It looks cleaner, and prevents other people who read the code from having to determine what the `any` is for.

Following this pattern to do checks is great in situations where you want to extract things that you aren't sure are present in all members of the union.

Create an Object using Mapped Types, Conditional Types, and TypeScript Keywords

0:01 We know first of all that to take a discriminated union and turn it into object, it's pretty nice. We get to use this R in Route as R route, which we've seen before. If we just take this R now, and if we take R route's objects, we now have each of the members of this discriminated union mapped over into its own key. Very cool.

0:22 That's good, because actually, what we want is to turn it into an object with those keys. This section is done for us. We don't need to think about that anymore. The tricky bit is this next section, because all we want from this routes object is actually to turn this, we want to basically grab the search out of it.

0:42 We have our R here, and R can contain search or it might not contain search. We can't just do this, because R might not contain search at all times. R is going to be basically like one of these. Some of these contain search, some of them don't.

1:01 You can't do a partial on this, or I can't do an index access on something that might not be there. We need to go in using a conditional type and grab it.

1:12 What we're going to do is we could say R extends search any and then say R search, otherwise return never. That actually works and it works pretty well. You can see that it grabs all the pieces here, but I like doing it like this, which is to say infer S.

1:30 Infer S in this position basically does the same thing except it does it without the index access check. It just says if this matches the search then grab it and then return it, which I think is just cleaner than looking at that and having to work out what that any is for.

1:45 If that any is typed to something different, although unknown does work, undefined or something, then it's not going to work. I prefer using the infer S, because it just looks a little bit cleaner, and you get to understand exactly what this check is doing.

2:02 Now this works, and this little check is so useful for being able to extract things that you're not sure are they in all members of the union.

Transform a Discriminated Union with Unique Values to an Object

0:00 This exercise is like a more difficult version of one that we've seen before. In the one that we saw before, we had a Route here, which was a discriminated union, and each route had a search object on it like this, which was an empty object, basically. This makes things easier because, when you're mapping over this, you know that search is always going to be there.

0:27 What you'll see is that, when you try to create a RoutesObject, which is basically going to be a keyed object of this, then you're going to see that it's harder to extract the search from the place that you think it's going to be. You're going to need a bit of infer here.

0:42 We're going to need definitely some map types. I can't remember whether it uses as or not, but I think that's enough of a clue to get you started. Your goal here is basically to say, "OK. When we have this key, because it has a search property on it, then we want to return that search property. Otherwise, we're going to return never." Best of luck.

The Type Transformations workshop is your guide to TypeScript features and patterns that allow you to manipulate types to produce exactly what you want. As you begin to see how the pieces fit together, you’ll find yourself writing less code that is easier to maintain.

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

Typeof Keyword, and Type Level

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Use a Utility Type to Extract Function Parameters

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Use Utility Types To Extract a Promise's Result

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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

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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

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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

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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

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

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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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Discriminated Unions

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Indexed Access Types

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Template Literal with Strings

Only Allow Specified String Patterns

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Extracting String Pattern Matches with Template Literals

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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

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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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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

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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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Add Support for Multiple Types in a Type Helper

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

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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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Use infer with Generics to Extract Types from Arguments

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Pattern Matching on Template Literals with Infer

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Template Literal Types Were Nearly Regexes

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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

TypeScript provides a powerful type useful for turning union types into objects.

Use Mapped Types to Create an Object from a Union

Map Over a Union to Create an Object

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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

Transforming Object Keys in Mapped Types

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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

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Mapped Types

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

Transform Path Parameters from Strings to Objects

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Partials allow you to construct types where some higher-level properties are optional. DeepPartial goes further by allowing all properties to be optional.

Transcript

Transform a Discriminated Union with Unique Values to an Object

Transcript