Let's look at a new pattern that I'll be calling the Type Helper pattern.

We'll start with a `ReturnWhatIPassIn` type helper where anything we pass in will be returned.

For example, if we create a type `Something` and call `ReturnWhatIPassIn`, we should get back `'Something'`:

```tsx
type ReturnWhatIPassIn = unknown;

type Something = ReturnWhatIPassIn<'Something'>
```

More examples of the desired behavior can be seen in our tests:

```tsx
type tests = [
  Expect<Equal<ReturnWhatIPassIn<1>, 1>>,
  Expect<Equal<ReturnWhatIPassIn<"1">, "1">>,
  Expect<Equal<ReturnWhatIPassIn<true>, true>>,
  Expect<Equal<ReturnWhatIPassIn<false>, false>>,
  Expect<Equal<ReturnWhatIPassIn<null>, null>>,
];
```

## Challenge

In our code's current state, the result of calling `ReturnWhatIPassIn` ends up being typed as `any`.

Your challenge is to update `ReturnWhatIPassIn` to behave as expected.

Hint: Currently we get the `any` type returned because the things we pass in aren't generic. This keyword (generic!) should give you a clue about the syntax you want to find <a href="https://www.typescriptlang.org/docs/handbook/2/types-from-types.html" target="_blank" rel="noopener">in the TypeScript Docs</a>.

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

The solution to our `ReturnWhatIPassIn` type helper has some syntax we haven't seen yet:

```tsx
type ReturnWhatIPassIn<T> = T; 
```

The angle brackets next to `ReturnWhatIPassIn` instantiate a type argument called `T`. After the equals sign, we 'return' it by using `T`.

Note that we could name `T` whatever we want, and the type helper would still work as expected:

```tsx
type ReturnWhatIPassIn<Whatever> = Whatever; 
```

For example, if we call our type helper like this:

```tsx
type Something = ReturnWhatIPassIn<'Something'> 
```

The type `Something` will equal whatever we pass in to `ReturnWhatIPassIn`.

## Expanding our Type Helper

We could update `ReturnWhatIPassIn` to return either the same thing that was passed in or `undefined`:

```tsx
type ReturnWhatIPassIn<TWhatever> = TWhatever | undefined; 
```

Now if we update our `Something` example to pass the number 12 into `ReturnWhatIPassIn`:

```typescript
type Something = ReturnWhatIPassIn<12> 
```

Hovering over it in VS Code shows us that our type is either 12 or `undefined`:

```typescript
// Hovering over Something
type Something = 12 | undefined
```

## Recapping the Type Helper Pattern

This type helper pattern allows us to create type functions which can return other types.

With this syntax, `ReturnWhatIPassIn` is now a function, `<T>` is the argument to the function, and `T` is what it returns:

```tsx
type ReturnWhatIPassIn<T> = T; 
```

If we remove the `<T>`, then `ReturnWhatIPassIn` becomes a static value:

```tsx
type ReturnWhatIPassIn = T; 
```

We can also add as many arguments as we want, and use other features like default values and constraints.

## Introducing Generics

This syntax and pattern is our first look at generics on the type level. The type we've created here is a 'generic type'. That term gets thrown around a lot. There's a whole module on generics coming up later. So, for clarity, I'm going to refer to this 'generic within a type' pattern as a 'type helper'.

Type helpers are a critical part of the way TypeScript works.

For example, if we create a `Record` type:

```tsx
type Yeah = Record<any, any> 
```

We can use VS Code to navigate to the source of `Record` and see that it follows this pattern:

```tsx
// `extends` removed for clarity
type Record<K, T> = {
  [P in K]: T;
}
```

We'll be going much deeper into generics throughout Total TypeScript.

For now, the takeaway is that type helpers allow you to pass in arguments and return things from that function.

Create Functions that Return Types

0:01 There's a lot of new syntax here, so let's examine what's going on. We have a type helper which is ReturnWhatIPassIn. This has now a couple of little angled brackets here, and it creates this variable called T. I can call this whatever I want. I can call it TWhatever, and it will still work the same way.

0:23 What happens is, now when I have type Something = ReturnWhatIPassIn<something>, this is going to equal whatever I pass in. We can really, really run with this. Imagine if we have 12 here, for instance. I could do this or undefined, for instance. Now it would be 12 or undefined.

0:47 What this allows us to do is create functions which can return other types. This is a function now. This is the argument to the function, and this is what the function returns. If we were to remove this, then it would no longer be a function. It would be a static value. This is just 12 here.

1:07 Whereas, if I declare it with this, then we can do really, really crazy stuff with this. We can start creating these functions which return other types. I can add as many of these as I want to here -- TWhatever2, TWhatever3. I can keep going and going and going. I can give them default values. I can make them constrained to certain things. That's what we're going to learn about in this section here.

1:35 This whole idea of type functions, this is a lot about what we call generics on the type level. You might have heard of generics before, and I've got an entire module talking about generics. Generics is a pretty widely used term. I'm going to refer to these mostly as type helpers because that's sort of what they are. We have a function that returns another type.

2:00 Generics in functions, generics in classes, they do slightly different things, but use similar syntaxes. I wanted to introduce this first because this is like the basis for what a lot of TypeScript is built on. If we look at, for instance, the record time, so if I say Yeah = Record, and I'll put any, any in here now, and if we command-click to this, then you can see this uses the same sort of pattern.

2:25 If we ignore this extends for now, then what we get is type record equals this, so it's a function with two arguments, K and T, and it returns something here. That's the whole idea about these type helpers. You can pass in arguments and return things from that function.

Introducing Type Helpers

0:00 It's now time to look at a new pattern here, called the type helper pattern. What we're going to do is we want to make this a ReturnWhatIPassIn type helper, where anything that we pass in will be returned.

0:16 What that means is if we have, let's say, type Something = ReturnWhatIPassIn, I should be able to pass Something in there and have Something end up as Something. Currently, it's just any, because this is not particularly valid, because type ReturnWhatIPassIn is not generic. That should give you a clue about what type of syntax we're looking for here. 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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TypeScript Pro Essentials

Type Transformations

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Advanced TypeScript Patterns

Advanced React with TypeScript

TypeScript Expert Interviews

TypeScript Pro Complete

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

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.

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

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

'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

Template literals in TypeScript have some very interesting properties. Here we’ll examine how they interact with unions.

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

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

Adding constraints to a type helper ensures type safety by limiting the types that can be passed as arguments to the type helper.

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

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

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

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

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

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

It can be useful to transform an object’s keys while you’re mapping them. You can do this in a mapped type without changing your original object!

Remapping Object Keys in a Mapped Type

Transforming Object Keys in Mapped Types

The ImportedDataState type in Excalidraw’s codebase is a great example of using a mapped types to support legacy types.

How Excalidraw uses Mapped Types to Save Lines of Code

You can selectively remap an object’s properties by pattern matching against a template literal.

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

Objects can be mapped into unions of tuples. This is useful for when you want to guarantee values and their order.

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.

Create a Discriminated Union through Intermediary Transformations

Transform an Object into a Discriminated Union

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

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

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

Introducing Type Helpers

Transcript