In this exercise we'll be using the mapped type again, but this time working with an object instead of a union type.

We’ll starting with an `Attributes` interface and an empty type helper `AttributeGetters`:

```tsx
interface Attributes {
  firstName: string
  lastName: string
  age: number
}

type AttributeGetters
```

## Challenge

Your challenge is to use a mapped type to transform the members of `Attributes` into a set of getter functions.

These functions should have the same key as the property they're getting, and they should return the type of the property as seen in the test:

```tsx
{
  firstName: () => string
  lastName: () => string
  age: () => number
}
```

Hint: This solution follows a similar pattern to the previous exercise!

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

The solution to this challenge contains a similar structure to the `[R in Route]` that we used in the previous challenge:

```tsx
type AttributeGetters = {
  [K in keyof Attributes]: () => Attributes[K]
}
```

The same general procedure is there as well:

The left side declares what our key is, and the right side declares what we'll do with that key.

## Steps to the Solution

For the sake of clarity, let's simplify the right side of the solution to just have `K`:

Functionally, the this is the same as the previous exercise, where `K` is used as both the key and the value:

```tsx
type AttributeGetters = {
  [K in keyof Attributes]: K
}
```

In order to access the `Attribute` key's value, we need to use an indexed access type:

```tsx
type AttributeGetters = {
  [K in keyof Attributes]: Attributes[K]
}
```

Now we have an object with the same keys and values as `Attributes`.

However, the challenge was to return getter functions.

In order to do this, we just need to return `Attributes[K]` from a function.

```tsx
type AttributeGetters = {
  [K in keyof Attributes]: () => Attributes[K]
}
```

That's our solution!

Map Over the Keys of an Object

0:00 Let's examine this syntax that we have here. This is the solution. If you look at this, it's very, very similar actually to the structure that we saw before here, which is R in Route R. If you think about these little pieces here, they're really still being reflected in the setup here, which is on this side, we declare what the key is. Then we declare what we're going to do with the key.

0:27 Imagine if we strip this out and we just have K here. Now what we end up with is we end up with AttributeGetters K in keyof Attributes. This is really powerful, because what we're saying here is we're taking the keyof Attributes and using it to create a new object. Doing this allows you to create a new object with the pieces of the old object. Extremely useful.

0:54 Here, what we're doing is we're saying, basically, "OK, we've got K and let's just put it in there." We end up with keys and the values being the same, which is the same as the previous exercise. What if we actually want to access the value of that? What if we want to access string, string, and number here?

1:12 Well, we have access to the object and we have access to the key there. All we need to do is say, "Attributes," and then use an index access type and say, "K," here. What we get there is now first name is string, last name is string, and age is a number. This isn't keyof. It's the specific key that we're mapping over. It's first name, last name, or age, depending on where we are in the iteration.

1:41 This is like an iterable basically where we're iterating through something. Now in order to make this pass, all we need to do is structure this so that it becomes a Getter. Now this is a function which returns Attributes K, and we end up with the correct type signature.

1:57 There's a lot going on here but the real key is this idea of K in keyof Attributes. Notice, by the way, that if we were to do this, we actually end up with the same type as in Attributes up here, because we're mapping over it and then extracting the values out of it.

2:12 This can be a useful setup by itself if you want to, for instance, make it undefined or if you want to make it readonly. There's lots of cool tricks here, but this is how you pass this solution.

Mapped Types with Objects

0:00 We're going to continue building on this map type. Instead of looking at a union now, we're going to look at an object.

0:07 This object here, we've got this interface Attributes here. What we really want to do is transform this Attributes into a set of getters, which are functions which will return the value here. The age function should return number. The lastName should return string. The firstName should return string.

0:26 You'll need to use a map type for this and also maybe something that we've seen a little bit earlier, too.

0:33 This is a really interesting problem. 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

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

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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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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

Create Unions out of Array Values

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

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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

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

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

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

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

Infer Elements Inside a Conditional with Infer

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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

In this exercise, you’ll use what you know about infer and template literals to get only a person’s last name from their full name.

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

Extract the Result From Several Possible Function Shapes

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

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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Remapping Object Keys in a Mapped Type

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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

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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

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Map an Object to a Union of Template Literals

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Mapped Types

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

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

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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.

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Mapped Types with Objects

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