The starting point is our solution from the last exercise:

```tsx
type YouSayGoodbyeAndISayHello<T> = T extends "hello" ? "goodbye" : "hello"; 
```

## Challenge

There's one more thing to add to this code. Recall that if we pass something in that is not "hello" that we will be returned "hello."

Your challenge is to update the conditional type to handle cases where `T` extends neither "hello" or "goodbye".

Think of it as saying we _never_ want something that isn't hello or goodbye to be passed in.

Hint: check out the <a href="https://www.typescriptlang.org/docs/handbook/2/functions.html#never" target="_blank" rel="noopener">TypeScript docs on ,`never`</a> and be ready to nest some ternaries.

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.

There's a lot going on to this solution, so let's construct it from scratch.

Here's our starting point:

```tsx
type YouSayGoodbyeAndISayHello<T> = T extends "hello" ? "goodbye" : "hello"; 
```

The first thing we want to do is check if `T` extends "hello" or "goodbye". For now we'll say if does that we'll return "wow", and if it doesn't we'll return `never`:

```tsx
type YouSayGoodbyeAndISayHello<T> = T extends "hello" | "goodbye"
  ? "wow"
  : never;
```

We can test this with an `Example` type to see if we pass this initial check by hovering:

```tsx
type Example = YouSayGoodbyeAndISayHello<"whatever">
// results in `type Example = never`

type Example = YouSayGoodbyeAndISayHello<"goodbye">
// results in `type Example = "wow"`
```

Using a union type like this allows saves us from having to say "if `T` extends hello do something, otherwise if `T` extends goodbye do something."

Now the we know our check works, we can replace `"wow"` with a nested ternary that contains the logic of what to return for "hello" or "goodbye" being passed in:

```tsx
type YouSayGoodbyeAndISayHello<T> = T extends "hello" | "goodbye"
  ? T extends "hello"
    ? "goodbye"
    : "hello"
  : never;
```

Now when our tests pass in "alright pal" or 1, we'll end up in the `never` branch instead of the branch where our actual logic is.

## Conditional Types and `never`

There's no limit to how many conditional types you can stack.

In fact, a lot of the complicated libraries use these huge stacks of conditional types that are ugly to look at but drive a lot of the complex logic.

The pattern of returning `never` allows our conditional types to specify their "else" logic without having to worry too much about what it is. If you try to use something that's been typed as `never` you will get an error.

We'll look at `never` again in later exercises.

Prevent Unwanted Type Scenarios from Happening

0:00 Oh, boy. Here we go. This, YouSayGoodbyeAndISayHello, we're now, first of all, checking this. This looks a little bit crazy. In fact, let me construct this from scratch so you understand. I'm going to, first of all, say -- this is what the problem started with -- we want to first check if T extends hello or goodbye.

0:23 If it does, then let's just say we return wow. Otherwise, we're going to return never. Now, if I say type Example = YouSayGoodbyeAndISayHello, and I pass in whatever, then this doesn't extend this, so we're getting a never here. Whereas, if I say, hello, then we should say wow. If I say goodbye, then it's going to say wow as well.

0:50 We now know what we can make pass this initial check here. It's kind of cool that we get to use a union type here. We don't need to say T extends hello, then do something, and otherwise T extends goodbye, and do something. We can say T extends hello or goodbye, and then we can enter that branch.

1:09 Inside here, we can say T extends hello, then return goodbye, otherwise return hello. We end up with the kind of monstrosity that we have here. That means now that when we pass in something weird, we're going to hit this never branch instead of the branch that contains our actual logic.

1:28 There's no limits here to how many of these you can stack. In fact, a lot of the really, really complicated libraries use these huge stacks of conditional types that are ugly as hell to look at, but drive a lot of the complex logic. This example then is never. This means if we're passing alright pal, we're going to end up with never. If we pass in 1, we're going to end up with never.

1:49 Never is a way of signaling to your types that something should never happen. We're going to break this down in a separate part of the module, and you may indeed have seen it already. This pattern though of returning never allows your conditional types to specify their else logic without having to worry too much about what it is.

2:11 Really, if we're passing something random in here, and we try to use this never somewhere, then this never is going to throw an error pretty much wherever it's used because it should never happen.

Refine Conditional Logic in a Type Helper

0:00 In this exercise, we extend the conditional type problem from last time, which is we have this YouSayGoodbyeAndISayHello, except we now want to handle the cases where you don't pass in either hello or goodbye.

0:15 We're going to handle them in a specific way, where we want to check, first of all, if T extends hello or goodbye. If it does, then allow this check to be performed and return the correct thing. Otherwise, we're going to ask it to return never.

0:32 I'm going to explain more about the never type in the solution. Really, never is a way of saying this should never happen. It's very useful in conditional types, a very common pattern to basically say, "OK, this value isn't allowed," or "This should never happen."

0:46 That's your challenge. You may need to nest some ternaries in here. The syntax is going to be looking a little bit weird by the time you're finished, but I promise, even if it does look strange, if you get all of these solutions passing, you're probably doing something right.

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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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

Use a Utility Type to Extract a Function’s Return Type

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

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

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

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

Refine Conditional Logic in a Type Helper

Challenge

Transcript