Consider this `GetParametersAndReturnType` type helper that is used to extract the parameters and return types of `T`:

```tsx
type GetParametersAndReturnType<T> = {
  params: Parameters<T>;
  returnValue: ReturnType<T>;
};
```

There are a few test scenarios, all of which are passing:

```tsx
  Expect<
    Equal<
      GetParametersAndReturnType<() => string>,
      { params: []; returnValue: string }
    >
  >,
  Expect<
    Equal<
      GetParametersAndReturnType<(s: string) => void>,
      { params: [string]; returnValue: void }
    >
  >,
  Expect<
    Equal<
      GetParametersAndReturnType<(n: number, b: boolean) => number>,
      { params: [number, boolean]; returnValue: number }
    >
  >,
];
```

The problem comes when hovering over `T` in `GetParametersAndReturnType`:

```
Type 'T' does not satisfy the constraint '(...args: any) => any' 
```

Functions can be used as constraints for Type Helpers, but not all functions have the same shape for their arguments.

The solution is to use the `extends` keyword to constrain `T`. In this case, we need `T` to be typed as a function with a return type of `any`:

```tsx
type GetParametersAndReturnType<T extends () => any> = {} 
```

With this change in place, we no longer have an error in our type helper, but we do with our tests:

```tsx
 Expect<
    Equal<
      GetParametersAndReturnType<(s: string) => void>,
      { params: [string]; returnValue: void }
    >
  >,
```

For example, the above test complains that `(s: string) => void` does not satisfy the constraint `() => any`.

The tests have different parameter shapes, we can't hardcode the type helper to look for strings.

In order to make our type helper work, we will update `GetParametersAndReturnType` to use the rest parameter with `args` and type it as `any`:

```tsx
type GetParametersAndReturnType<T extends (...args: any) => any> = {} 
```

This means that we can accept any number of arguments, including zero, and it will return a function or anything from that function.

By copying this constraint, we've matched up what we expect to pass into this type helper based on what we want to use it for.

This is a nice bit of syntax to remember, as it allows us to use functions as constraints in our type helpers.

Support Function Type Constraints with Variable Arguments

0:00 The solution to this is to make sure that T is constraint to a certain type. The way we do that is with the extends keyword. The way we're going to do that is we're going to type it as a function. This function now, this appears to be passing up here, because we have our Parameters up here and ReturnType. Nothing's erroring up here, but we do get errors down here.

0:24 The proper way to type this is interesting. To do it, I'm going to look at the Parameters type. If I remove this constraints, then this error is giving me a detailed description of what's going on. Which is type T -- which could be unknown, remember -- does not satisfy the constraint '(...args: any) is any.'

0:46 We can come and click over to this. We can see that it has a constraint itself, T extends (...args: any) => any. If we copy over that, we'll see that this starts working now. This is the solution. Why is it the solution there?

1:01 We understand that it can return any here. If we were to specify this can only return strings, then the two things down here, which return void and a number are going to be stop moaning at us, because type 'void' is not assignable to type 'string'.

1:17 We're saying that this function here has to return a string to be valid. We can put in any here. Any says, we can accept any type of function or rather a function that returns anything, because we don't care about what it returns. We want it to be a function.

1:35 What about this section here, this ...args: any? If I remove this, as we saw before, then this says, this is what this function can accept as a Parameter. If I say, a: any here, then it's going to accept this version.

1:52 If I put in a b:any inside here too, then it's going to start accepting this version. We wanted to accept any number of arguments here, which means that we can use this rest parameter and we can say any here. It's also acceptable to do any array here. It doesn't matter.

2:08 What this does is it means that we can accept any number of arguments, including zero arguments too, and it will return a function or return anything from that function there.

2:21 By copying this constraint, we've matched up what we expect to pass into this TypeHelper based on what we want to use it as. This is a nice bit of syntax to remember, except it means, you can always use functions as constraints in your TypeHelpers.

Functions as Constraints for Type Helpers

0:00 For this exercise, we have a TypeHelper here called GetParametersAndReturnType. We have a bit of an issue. All the tests are parsing absolutely fine. We're taking our T here and we're extracting out the Parameters and ReturnType from this T. It's pretty much always going to be a function here.

0:17 You have a function here, which is a function with no arguments that returns a string. This one is a function with one argument which is s, which is a string that returns void. We're getting returnValue: void and params: string there.

0:30 In this one, you have two function arguments that returns a number here and the params get extracted to be a tuple of number and Boolean and returnValue.

0:41 All appears to be working, except here, we have an error saying, "T does not satisfy a certain constraint." Your job here is to figure out what that constraint might be and you probably need to apply it somewhere to make sure that T is typed in a certain way.

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## Inference Basics 

The TypeScript compiler uses type inference to automatically determine the data type of a value depending on the context in which it is used. We can use several utilities and operators to define our own types more precisely.

## Unions & Indexing Unions

Union types play a key role in the type transformation process. In this section we will work at unions and discriminated unions, as well as how to access their properties with indexed access types. Several of TypeScript’s built-in utility types will also be explored.

## Template Literals 

As of TypeScript 4.1, template literals can be used to power amazing inference on strings. In this section we’ll practice using template literals to help create object types, split strings into tuples, and other useful transformations.

## Type Helpers

Type helpers allow you to manipulate and work with types in a more convenient way. They allow you to set constraints that ensure your types are as you expect while retaining flexibility for reuse. The type helper pattern is important to understand, as it forms the basis of many of TypeScript's core features.

## Conditional Types & Infer 

In this section, we will further explore how to leverage the type helpers pattern, paying special attention to conditional types and inference when working with functions, objects, and Promises.

## Mapped Types 

With TypeScript's mapped types, you can transform any type into and out of objects, unions, and tuples. It's an amazingly flexible tool for creating all sorts of different types. You’ll get practice with all of these, as well as using template literals to dynamically customize your types. One learner wrote they'd been using TS for 2 years and finally understood Mapped Types after this section!

## Challenges

To tie up the previous exercises, we've put together several real-world challenges that will put your Type Transformation skills to the test.

Learn to manipulate types for cleaner, maintainable code with over 50 hands-on exercises, from string manipulation to advanced unions and mapped types.

Type Transformations Workshop

Total TypeScript Core Volume is a set of four full TypeScript workshops for the intermediate to expert professional TypeScript programmer that wants to take their skills to the next level. This bundle also includes the Advanced React with TypeScript workshop that will give you expert-level knowledge for typing your React apps.

Type Transformations

TypeScript Generics

Advanced TypeScript Patterns

Advanced React with TypeScript

TypeScript Expert Interviews

Core Volume

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

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

Functions as Constraints for Type Helpers

Transcript