This `AddRoutePrefix` type helper that has already been scaffolded for us.

It takes in `TRoute` then puts it into a string literal that adds a `/` to the beginning:

```typescript
type AddRoutePrefix<TRoute> = `/${TRoute}`;
```

This is passing most of our tests, but not all.

Using `@ts-expect-error` tells the tests we expect an error when passing a boolean or number to the `AddRoutePrefix` type helper.

## Challenge

Your challenge is to work out what is causing the error, and solve it by putting some constraints on what can be passed in.

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

The solution is to update `AddRoutePrefix` so we can only pass in a string.

To do this, we'll add `extends string` to the type helper to tell it that `TRoute` must be a string:

```tsx
type AddRoutePrefix<TRoute extends string> = `/${TRoute}`; 
```

## More on Constraints

As seen in the introduction to the exercise, for most things we don't need to add `extends` because TypeScript is able to infer the type of what we pass in. However, if we know that later we're going to need to do string-specific operations, it's important to add constraints.

If you don't pass in a type constraint, TypeScript will assume that `TRoute` is unknown. This means that it doesn't have any properties associated with it which puts us a little bit closer to danger.

For the sake of experimenting, we can update `AddRoutePrefix` to extend every type from the tests and they will all pass:

```tsx
type AddRoutePrefix<
  TRoute extends
    | string
    | number
    | bigint
    | boolean
    | null
    | undefined
> = `/${TRoute}`;
```

However, adding an object to the extends will cause an error because `{wow: true}` is not assignable to that type.

Many type helpers add their own constraints, including built-in ones like `Record`.

Let's create an example `Record` then use CMD + click in VS Code to see its source code:

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

In the source for `Record` we can see that it extends `keyof any`:

```tsx
type Record<K extends keyof any, T> 
```

We'll get to what this specifically means later, but for now you can see how `extends` allows us to set different constraints on our type helpers.

Use Constraints to Limit Type Parameters

0:00 The way you fix this error then is we somehow need to make TRoute here, we need to make sure that the only thing you can pass it is a string, and the way you do that is you say TRoute extends string. 0:14 OK. What's the syntax doing here? It's basically saying that TRoute must be a string, and if we try to pass in a Boolean here, then it's going to say type Boolean does not satisfy the constraint string.

0:28 If you mention that this was just a normal function, we say const addRoutePrefix as a function, and we've got our route inside here, then basically, this is the same thing as we're doing here. If we say root: string, we're doing exactly the same thing as addRoutePrefix is doing.

0:48 This is a way of giving a parameter or giving a type to the type parameter that is being passed in. The cool thing about TypeScript is you don't need to do this. Because this is all happening on the Type-Level, it has perfect information so it will infer it based on what you pass in.

1:06 But if you want to do things with it that are required for a string to be able to do, like being a part of another template string, then you need to constrain it properly. We could constrain this to be a number if we want, and this wouldn't complain because you can pass numbers to a template literal, which is pretty cool.

1:25 Yeah, that's the basic idea of this. We can do lots of things with this. We can actually mimic the type that it's complaining about here if we wanted to. If we copy this over, we go string or number or bigints, and we can pass that directly into there and that won't moan.

1:42 For instance, if we add another member to this like wow: 1, then now, it's going to moan because wow: 1 is not assignable to that type, but you get the idea. We can basically now guarantee that TRoutes extends a certain type.

1:54 If you don't do that, if you don't pass in a type into here, a type constraint, then it's going to assume the TRoute is unknown. That's the type that it defaults to. We know that in TypeScript, unknown, so routes., it doesn't really have any properties associated. We really don't know what it is.

2:14 It's like one step above any, in times of danger, but at least we know what the danger is. That's a good way of thinking about it. If you don't specify this, then anything you do with that TRoute is going to be unknown.

2:27 If you do specify constraints, then that means that you can get a sense of what's happening to this, and often, other type helpers will specify their own constraints too. If we say type Yeah = Record<string, string>, then if we look inside Record, command click here, we can see that it extends keyof any.

2:48 We'll get to what that means later, but you get the sense that, OK, I can add these different constraints onto my type helpers to make sure that my users can only pass certain things in.

Ensure Type Safety in a Type Helper

0:00 In this exercise, we have a type helper that's already scaffolded for us, which is an AddRoutePrefix. It takes in a TRoute. TRoutes, we're not using T for some reason. We can use T, but it doesn't matter. We're putting it inside a string literal. Now, this string literal then...What we're expecting from the test is that it's going to add this little character to the start of it. 

0:25 That appears to be working for most of these, except we're getting a few errors here. It looks like, in the tests, we're expecting it to error when we pass this type-helper a Boolean. We're also expecting it to error when we pass it a number. That tells me that this TRoute here, we need to constrain it to be of a certain type.

0:44 If we hover over this, then we're going to see type TRoute is not assignable to type string, number, bigint, Boolean, null, or undefined. Your job is to try and work out what's causing that error, and if we can solve this by somehow constraining the type that we get passed in, in order to make sure this error goes away.

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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

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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

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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

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

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Union Types

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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

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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

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Manipulate String Literals Using Type Helpers

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Template Literals in Mattermost

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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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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

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

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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

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Two Methods for Extracting the Result of Multiple Possible Functions

Extract the Result From Several Possible Function Shapes

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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

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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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Iteratively Map and Remap to Transform Types

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Mapped Types

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Ensure Type Safety in a Type Helper

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