This exercise begins with this `Attributes` interface and an empty `MutuallyExclusive<T>`:

```tsx
interface Attributes {
  id: string;
  email: string;
  username: string;
}

type MutuallyExclusive<T> = unknown;
```

## Challenge

Your challenge is to update `MutuallyExclusive` to be a type helper that will turn the `Attributes` object into a discriminated union.

Check your work by hovering over `ExclusiveAttributes` as you go.

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

The solution here is pretty dense and looks a little complicated:

```tsx
type MutuallyExclusive<T> = {
  [K in keyof T]: Record<K, T[K]>;
}[keyof T];
```

Let's work through it by going back to first principles, and checking our work by hovering over `ExclusiveAttributes`:

```tsx
type ExclusiveAttributes = MutuallyExclusive<Attributes>; 
```

When written this way, `MutuallyExclusive` will remap over the attributes of the object that is passed in and return them to us.

```tsx
type MutuallyExclusive<T> = {
  [K in keyof T]: T[K];
};
```

We can check this by hovering over `ExclusiveAttributes`:

```tsx
type ExclusiveAttributes = {
  id: string;
  email: string;
  username: string;
}
```

In order to create a union, we want to create an intermediary representation of that union as an object. From there, we'll remap over that object to get back our proper union values.

### Create an Intermediary `Record` Type

The members of the union that we want need to be stuck into the `T[K]` slot that we wrote above.

For now, we'll replace that slot with a new `Record` type that will take in `K` and an empty object:

```tsx
type MutuallyExclusive<T> = {
  [K in keyof T]: Record<K, {}>;
};
```

What this says now is for each of the properties of the object passed in, create a record that has the key and a new empty object:

```tsx
// Hovering over ExclusiveAttributes at the current step

type ExclusiveAttributes = {
  id: Record<"id", {}>;
  email: Record<"email", {}>;
  username: Record<"username", {}>;
}
```

Since we want to represent the value of the thing inside of the attributes, we can swap the empty object with the `T[K]`. Recall that `T` is the `Attributes` being passed in, and `K` is the index of the key we are currently on:

```tsx
type MutuallyExclusive<T> = {
  [K in keyof T]: Record<K, T[K]>;
};
```

Since each thing inside of `Attributes` is typed as a string, that's what we'll end up with now:

```tsx
// Hovering over ExclusiveAttributes at the current step

type ExclusiveAttributes = {
  id: Record<"id", string>;
  email: Record<"email", string>;
  username: Record<"username", string>;
}
```

### Map over the `Record`s to Create a Union

Now that we have `id`, `email`, and `username` available to us, we need to map over them to grab the members of the union.

We do this by appending `[keyof T]` to the end of `MutuallyExclusive`:

```tsx
type MutuallyExclusive<T> = {
  [K in keyof T]: Record<K, T[K]>;
}[keyof T];
```

Create a Discriminated Union through Intermediary Transformations

0:00 The solution here is pretty dense and complicated. You can see this mutually exclusive type. We're obviously doing a mapping type, and we've quite a lot of annotations here. Let's remove a bit of this and go back to first principles.

0:14 If we just start from this setup, which we've seen before, then what we're going to end up with is it's going to remap over attributes and return them to us.

0:26 Starting from here, we know that in order to create a union, we want to create an intermediary representation of that union in an object, and then remap over that object in order to get back our proper union values. That's the way to do it.

0:44 The members of this union that we want are what we want to stick in this slot there, which is in the value. Let's say Record<T>, and let's just say...In fact, it's going to be K, and let's actually just make an empty object here. I'm going to say const example is ExclusiveAttributes. Let's just see what this is here.

1:07 What this is doing now is it's basically saying, for each of the properties, I'm going to create a record where it has one key, which is id in our username, and then we're going to parse something random into that. Currently, we're just parsing an empty object.

1:25 I can do this with email, id, id, and then username, username, username. Each of these, we don't want to parse this here actually. We want this to represent the value of the thing inside attributes. That means we can say T K.

1:41 T then is going to be this thing we're parsing in, which is attributes, and the K is the index or the key that we're currently on. This means that each of these then is going to end up being a string, because that's how they're typed inside attributes.

1:56 That's pretty good. We're getting there, but now we've got each of these available to us, we just need to map over id, email, and username to grab the members of the union. For that, we say keyof T, and now the example that we get at the end is going to be email, blah, blah, blah. That's going to be enough to parse this.

2:17 The cool thing about this is that I have to parse one of these. This is really nice for like form handlers and things like these, where you have to at least parse one, so we have line of id and that makes it parse.

2:31 This is pretty cool. We end up with this record of id string, or record email string, or record username string.

Transform an Object into a Discriminated Union

0:00 This next challenge takes an object, takes in any old object you can pass in, and turns it into a discriminated union of all of the individual properties of that object. Attributes here, what we should end up with is id or email or username.

0:19 This is a cool challenge. It tests your brain a little bit, but there's nothing new in here. Nothing you haven't seen before. All we want to do is turn MutuallyExclusive into this type helper, which can transform this.

0:32 You can use ExclusiveAttributes here to check how you're doing. Good 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.



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

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Extract Function Parameters Into A Type

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Use Utility Types To Extract a Promise's Result

Extract The Awaited Result of a Promise

With help from one of TypeScript’s most useful utility types, we can create a union type directly by extracting keys off of a runtime construct.

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.

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Typing Promises and Async Requests

Inference Basics

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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

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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

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

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

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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

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

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

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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

Create a Union of Strings with All Possible Permutations of Two Unions

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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

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

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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

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

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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

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

If you find yourself in some complex conditional logic, harness the power of infer.

Infer Elements Inside a Conditional with Infer

Introducing infer for Conditional Logic

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

Use infer with Generics to Extract Types from Arguments

Extract Type Arguments to Another Type Helper

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Pattern Matching on Template Literals with Infer

Extract Parts of a String with a Template Literal

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

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Optionally Infer the Return Type of a Function

Extract the Result of an Async Function

Long ternary statements become difficult to read. Luckily, there’s an alternative way to write longer conditionals with some help from a built-in type.

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

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

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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

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Mapped Types

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Transform a Discriminated Union with Unique Values to an Object

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 an Object into a Discriminated Union

Challenge

Transcript