In this exercise, we're starting with the same Modal component that has the variants all lined up in a discriminated union:

```typescript
type ModalProps =
  | {
      variant: "no-title"
    }
  | {
      variant: "title"
      title: string
    }
```

However, this time the component has changed from using just props to instead destructure `variant` and `title`:

```typescript
export const Modal = ({ variant, title }: ModalProps) => {
  if (variant === "no-title") {
    return <div>No title</div>
  } else {
    return <div>Title: {title}</div>
  }
}
```

The `variant` seems to be fine, but `title` doesn't seem to exist on type `ModalProps`.

```typescript
// hovering over title shows the error
Property 'title' does not exist on type 'ModalProps'.
```

Your task is to find a way to fix this error while still destructuring the props.

Hint: Refresh your memory by reading [the Narrowing entry in the TypeScript Docs](https://www.typescriptlang.org/docs/handbook/2/narrowing.html). 

Figure out how to fix a common type error that occurs when destructuring props with discriminated unions in React components.

Here's the initial code we're working with:

```typescript
type ModalProps =
  | {
    variant: "no-title"
    }
  | {
    variant: "title"
      title: string
    }

export const Modal = ({ variant, title }: ModalProps) => {
  if (variant === "no-title") {
    return <div>No title</div>
  } else {
    return <div>Title: {title}</div>
  }
}
```

Let's try a couple of different approaches for destructuring the discriminated union.

## Trying the Rest Syntax

Instead of just having `title` here, let's use the rest syntax with `...props`:
 
```typescript
export const Modal = ({ variant, ...props }: ModalProps) => {
  if (variant === "no-title") {
    return <div>No title</div>
  } else {
    return <div>Title: {props.title}</div>
  }
}
```

Unfortunately, this doesn't work. Hovering over the `...props` shows:

```typescript
(parameter) props: {} | {
  title: string;
}
```

This says that `title` is present in one of the branches but not the other.

This means that TypeScript basically will ignore the fact that `title` exists altogether.

To make this work, you would have to add an `else if` statement to see if `title` is in `props`:

```typescript
  if (variant === "no-title") {
    return <div>No title</div>
  } else if ('title' in props) {
    return <div>Title: {props.title}</div>
  }
```

This isn't ideal because TypeScript should be narrowing that for us already.

## Access Props After Narrowing

The better solution is to check `props.variant`. Then TypeScript will understand that the thing we're narrowing is `props`. After the variant has been narrowed, we can destructure the title from props, though generally I would prefer to just access it off of `props`:

```typescript
export const Modal = (props: ModalProps) => {
  if (props.variant === "no-title") {
    return <div>No title</div>
  } else {
    const { title } = props  // destructuring here
    return <div>Title: {title}</div>
  }
}
```

Manage TypeScript's control flow limitations in discriminated unions with destructuring: balance prop access and discriminator narrowing.

Destructuring vs Accessing Discriminated Union Props

[00:00] Let's try a couple of different approaches here. First, what we're going to try and do is take this title and instead of just having title here, let's go dot dot dot props. Let's imagine that we really want to destructure up in here. And now props, let's see if we can do props dot title here. Nothing. Okay, why is that?

[00:19] Props is okay. It's either an empty object or it's this title here. So title is present in one of the branches but not the other. This means that TypeScript basically will ignore the fact that title exists and you

[00:33] would have to do something like else if, let's say title in props. Now it will understand that props is that branch of the other one, whereas the other. But really it should be narrowing that already.

[00:49] Now this is a slight limitation in TypeScript which is that if you separate the discriminator from the rest of the thing that it's discriminating, then it basically won't apply what it understands about this to the rest of the props.

[01:05] The only way to really get this working is to do this and say props dot variant and now it understands that the thing you're narrowing is that props. Whereas if you were to split them out into two different variables, then it wouldn't understand.

[01:19] So if we do like const my props or let's say underscore props up here equals props, then we can't access underscore props here because this one hasn't been narrowed, this one has. And TypeScript's control flow isn't quite smart enough to pick that up.

[01:37] So if you're really keen on doing destructuring, then the best way to do it is just to do it inside here because now it understands that inside this scope that props is the one with the title so you can do const title equals props and this is absolutely fine.

[01:54] But my preferred solution is really just the one that we started out with which is just to have the props kind of at the top level and just access them based on there because it's really not too much pain and it means that if you're using this discriminated union props, it's going to be the same kind of everywhere you use it. Whereas this solution is perfectly fine if you want to do destructuring or if you have,

[02:14] you know, you're doing a big refactor or something and the previous component was destructured. But I would always prefer to just access it off the props.

[02:22] It's nice and simple.

Destructuring Discriminated Unions in React Props

[00:00] In this exercise, we're starting with our modal components again. In exactly the same setup, we have our modal props with the variants all lined up in the discriminated union, except we've got an error down here. And that's because we've changed it from using just props into doing some destructuring here.

[00:17] So we have our variant being destructured and our title being destructured. Variant seems to be fine, like it's no title or title, lovely. But title doesn't seem to exist on type modal props.

[00:30] Your job here is to try to find a way to fix the error and try to work around this little bit of pain here. Let's imagine that we still want to destructure the props, but we might not be able to do it quite here.

[00:44] So your job is to try and fix this. Good luck.

React is the framework for professional web development.

The scale and complexity of web applications is always expanding. This has led to missed deadlines, higher costs, and deep frustration on developers like you and your team.

TypeScript is the language of choice for professional web developers. TypeScript lets teams build faster, more robust applications.

TypeScript solves a lot of problems, but it's not a silver bullet. It’s a complex language with a lot of features, and is challenging to use effectively– particularly when it comes to React. 

TypeScript with React is a high-wire act. 

You have to keep your components flexible, type-safe, and structured so you can take advantage of inference and not sacrifice maintainability down the line.

As soon as you start throwing `any`s around or duplicating code you know should be reusable, you’ve lost the plot.

Advanced React with TypeScript by Matt Pocock is your secret weapon.

You’ve found the magic wand you need to stop fighting TypeScript and make it do YOUR bidding like a true wizard.

In this Advanced React with TypeScript workshop, you’ll learn how to combine the most important concepts and patterns of React with TypeScript. 

This combination is a superpower that will change the way you and your team build web applications.

You’ll write custom hooks that use generics, share some (but not all) props between components, and remove guesswork by diving into the types behind the scenes of React and other popular libraries.

This workshop has been vetted and tested with professional senior+ React developers and teams like yours.

Advanced TypeScript with React includes over 50 exercises spanning six in-depth sections: 

## Advanced Props with TypeScript
**Handle props and custom components**

Learn how to shape props to support multiple variants, share selected props between different components, use union types for conditional rendering, and other best practices for your components.

## TypeScript Generics in React
**Leverage generics for your components and custom hooks**

TypeScript generics are a key tool in creating flexible, reusable components and hooks in React.

Generics can be used to define how components should be rendered based on the data and structures passed in. You'll work with custom hooks that support async operations and proper error handling, as well as combine generics with useState and useReducer for complex state management. The exercises in this section will fundamentally change the way you build components.

## Advanced React Hooks with TypeScript
**Harness types to create safer hooks**

Using TypeScript with React Hooks provides serious benefits. You will practice with advanced concepts like returning read-only tuples from custom hooks and implementing function overloads for built-in hooks like useState. You'll implement loading and error states for an async hook, while provided a cancellation mechanism to avoid memory leaks. By the end of this section, you'll have the tools to create well-typed hooks that elevate your component development workflow.

## Types Deep Dive
**Read behind the scenes to build your understanding**

The TypeScript type system forms the backbone of React and other libraries you use on a regular basis. But the errors it gives you can be confusing in the extreme.

Examine the various members of the React namespace, and how to use them to build better prop types. From understanding JSX.IntrinsicElements to augmenting React’s types themselves, you’ll learn everything you need to remove the guesswork when solving React’s nastiest TypeScript errors.

## Advanced Patterns in React with TypeScript
**Recognize and follow industry-standard patterns**

Combining community-adopted React patterns with TypeScript features can significantly optimize your applications and the way you write code. You will work with classic component patterns like Render Props and Higher Order Components, solve the forwardRef problem, and master the ‘as’ prop– all with the added benefits of type safety.

The patterns you learn here will give you ideas for refactoring your existing React codebase - and build powerful design systems

## Leveraging External Libraries
**Safely integrate external libraries into your React components**

Libraries supercharge your React development productivity. But they can be a double-edged sword. With TypeScript, you’re not just adding runtime code– you’re adding types to your project.

You'll create type-safe form components with `react-hook-form`, and learn how to master third-party generics with `react-select`. You will look behind the type definitions behind React Query, and apply what you learn to defining a custom hook to encapsulate its functionality. The lessons you learn from diving deep into types in node modules will be applicable to any third-party library you choose to use.

## Prerequisites

You'll need a working knowledge of React, as well as foundational TypeScript knowledge. Working through the [React with TypeScript Tutorial](https://www.totaltypescript.com/tutorials/react-with-typescript) is recommended.





Expert-level knowledge for typing your React apps with TypeScript.

React TypeScript

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

Enhance the type safety of React props by leveraging discriminated unions to restrict possible variations.

Using Discriminated Unions to Create Flexible Component Props in React

Type-Checking React Props With Discriminated Unions

Learn how to add a required React property to different variants of a discriminated union in TypeScript

A guide on how to properly use intersection to resolve union types in TypeScript and ensure code readability.

Resolving Discriminated Union Types with an Intersection

Adding a Prop Required Across Discriminated Union Variants

Handle a situation where a Boolean is used as a discriminator in a discriminated union, and learn how to use a default value to avoid passing unnecessary props.

Learn how to use discriminated unions to handle conditional props in TypeScript.

Discriminated Unions for Conditional Props in TypeScript

Differentiating Props With a Boolean Discriminator

In this exercise, you will learn how to use the Record type to represent an empty object by passing in a string and never type.

Using the Record Type to Represent an Empty Object

In this exercise, we'll be using a discriminated union to handle different behaviors based on a prop value due.

Make props optional in components by typing them as undefined in discriminated union branches.

Allow Optional Props Using A Discriminated Union Branch With Undefined Types

Conditionally Require Props With Discriminated Unions

Explore how to find a better type definition for a mapped React component.

Learn what makes these two React types different and how to use them effectively to ensure type safety in your code.

What's The Difference Between React.ReactNode and React.FC?

Finding a Better Type Definition For A Mapped Component

TypeScript infers data structures, allowing a type to serve as the source of truth, with props inferred from runtime behavior.

The `keyof typeof` pattern in TypeScript to derive types from variables and create dynamic maps for runtime behavior.

The `keyof typeof` Pattern

Syncing Types without Manual Updates

There is a quirk in TypeScript where autocompletion for types with predefined values and strings doesn't work quite as you might expect.

Wrap the string in parentheses and intersect with an empty object to resolve the partial autocompletion quirk in TypeScript.

Solving Partial Autocompletion

The Partial Autocompletion Quirk

Using `as const` and indexed access types helps you extract keys and values from a type.

The combination of `as const` and indexed access types allows you to extract keys and values from a type.

Using `as const` and Indexed Access Types to Extract Keys and Values from a Type

Extracting Keys and Values from a Type

TypeScript has a few different ways to ensure correct inference depending on how restrictive or permissive your checking needs to be.

Differentiate between annotation operators, "as const", and "satisfies" in TypeScript for optimal type inference and validation based on specific use cases.

Comparing `as const`, `as`, and `satisfies`

Ensuring Correct Inference for Prop Types

Discover the power of keyof and satisfies to ensure type safety while maintaining a single source of truth for your type.

Understanding and Implementing Dynamic Props Mapping in React

Inference from a Single Source of Truth

Advanced Props

Generics and type helpers combine to create reusable code that's easy to understand and maintain.

A generic type helper makes our code more reusable and easier to understand.

Implement a Generic Type Helper

DRY out Code with Generic Type Helpers

Refactor complex type logic into a type helper.

An "All or Nothing" type helper simplifies complex logic while allowing for flexible object shapes.

Creating an "All or Nothing" Type Helper for React props

Refactoring to a Type Helper

Generic type parameters can be constrained to accept only specific values, improving the reliability and predictability of your type helpers.

Enforcing constraints helps you prevent errors from occurring, and ensures your type helpers are being used in the correct way.

Add Generic Constraints to Type Helpers

Constraining a Type Helper to Accept Specific Values

Type arguments allow us to pass in a type to a hook, which can be used to provide more accurate return values.

By default, type arguments are inferred as `unknown`. When using this type of API in your code, it is crucial to have it well-documented.

Adding Type Arguments to a Function

Adding Type Arguments to a Hook

Learn how TypeScript behaves when wrapping one generic function inside of another generic function.

TypeScript uses type inference to infer the type of generic functions, making them easier to use.

Type Inference with Generic Functions in TypeScript

Wrapping a Generic Function Inside of Another

Generics are useful for components, especially in situations where different props depend on each other.

By adding a generic type argument to a props interface, we can infer prop types.

Add a Generic Type Argument to a Props Interface

Applying Generics to Components

There are still class components in legacy codebases, so it's important to understand how to make them generic.

Props work the same whether we're using functional components or class components. It's just the generic syntax that's different.

Converting a Class Component to be Generic

Generics in Class Components

Discover how to use type enforcement in React components to address data inconsistencies in a user table, preserving data integrity.

In order to get proper inference, the type arguments need to be passed appropriately to the type helpers.

Use the Angle Brackets Syntax to Pass a type to a Component

Passing Type Arguments To Components

Generics enable the useful pattern of inferring type arguments through multiple different type helpers.

Adding Generic Type Arguments to Type Helpers

Generic Inference through Multiple Type Helpers

Discover how to optimize your code by making use of generic hooks and type constraints in useMutation. 

Comparing function arguments rather than the functions themselves should be considered a best practice when dealing with function assignability in TypeScript. 

Refactoring a Generic Hook for Best Inference

Build a useMutation hook

Generics aren't always the best solution for every problem. Sometimes, a different approach is better suited for the task at hand.

Replacing generics with a discriminated union can make your code cleaner and easier to understand while still maintaining the same inference.

Refactoring from Generics to a Discriminated Union

Generics vs Discriminated Unions

Using Generics with Components

Learn how to get better inference from a custom hook without changing its runtime behavior.

Learn how to identify and resolve type inference problems when using useState in React, ensuring accurate type information for returned values.

Use 'as const' to Infer a Tuple return type

Fixing Type Inference in a Custom React Hook

Learn how to create a strongly typed context in React using the createRequiredContext function, and fix type errors that can occur

Avoid the common issue of creating a context and not knowing what to pass into it.

Using Type Arguments to Create A Strongly Typed Context

Strongly Typing React Context

Learn a pattern for managing complex state that helps you track every possible state your application can be in.

Learn how to properly type complex React state that represents different possibilities using TypeScript.

Handling Complex State Management with TypeScript Unions

Using TypeScript to Manage Complex State

Clever usage of TypeScript types provides a great alternative to useReducer for tracking multiple properties in state.

Learn how to use discriminated unions with useState to effectively manage different states and properties in your React applications.

Handling Different State Values with Discriminated Unions

Using Discriminated Unions in useState

Learn how a different type signature can lead to better inference when working with different statuses in React state.

Discover how using discriminated tuples in TypeScript gives better inference as demonstrated through an example involving loading, success, and error states.

Improved Type Safety with Discriminated Tuples in TypeScript

Discriminated Tuples in Custom Hooks

Function overloads resolve type inference issues when a function can return different types of values.

Overloading Functions in TypeScript

Use Function Overloads for Better Type Inference

By mimicking the function overloads on `useState`, you can make your custom hooks behave in the same way.

Be careful when using function overloads, because it's easy to end up in an impossible position and TypeScript won't warn you about it!

Wrapping useState Functionality with Function Overloads

Mimicking useState Behavior with Function Overloads

This pattern of creating an outer hook and returning a hook from inside it is sometimes referred to as "currying hooks".

Generics need to be captured at the point where they're used as well as where they're being inferred from.

Inferring Type Arguments in Curried Hooks

Currying Hooks

Advanced Hooks

React provides a namespace that contains types, functions, and values. Type definitions can help you understand why and how React works the way it does.

The DefinitelyTyped contributors have structured the React types so that they work in both the runtime and type worlds.

Understanding the React Namespace

Exploring the React Namespace

React gives us a few different types to use for our components. Understanding the differences between them can help you avoid errors and write better code.

Learn how JSX.Element, React.ReactElement, and React.ReactNode affect type inference and return values in React components.

Understanding React's JSX Types

JSX.Element, React.ReactElement, and React.ReactNode

A strongly requested feature for React & TypeScript is to strongly type the children. Is this even possible?

Manually call JSX to force the type of children passed to a component to be a specific type.

Strongly Typing Children in React Doesn't Work

Strongly Typing Children in React

JSX.IntrinsicElements is a global interface that contains HTML attributes as properties that you'll when using TypeScript in React.

JSX.IntrinsicElements is a global interface that contains HTML elements and attributes.

Understand the Structure of React's JSX.IntrinsicElements

Exploring JSX.IntrinsicElements

ElementType and ComponentType are two React type helpers which help describe the shapes of components.

Understanding React's ElementType and ComponentType

Learn how to add an interface to React's global namespace.

Adding interfaces into global namespaces can be useful in a variety of situations.

Merge Namespaces in TypeScript

Appending to React's Global Namespace

Declaration merging can be used to modify existing interfaces in the global namespace.

Learn how to merge interfaces in TypeScript to combine and extend their properties in the same global namespace.

Extend the Global React namespace with Declaration Merging in TypeScript

Modify Existing Interfaces in the Global React Namespace

By utilizing declaration merging, you can inform TypeScript of the types of custom elements from external libraries.

Type external JSX elements through declaration merging by extending JSX.IntrinsicElements.

Extend JSX.IntrinsicElements with Declaration Merging

Add a New Global Element in TypeScript

The types that React exposes for each element are extensive. Take a deep into these types to understand how they are defined and work.

Navigating HTMLAttribute types

Exploring HTML Attribute and Element Types

Having an optional test attributes available can be useful for testing elements.

Have a clear understanding of the namespace structure and hierarchy of interfaces in React, you'll be better equipped to handle any errors

Updating the Global Namespace for an Additional Attribute

Add Attributes to All Elements with Declaration Merging

Types Deep Dive

Lazy loading allows us to dynamically import and render components, but it can be tricky to type props correctly.

It's important to understand constraints when typing third-party libraries.

Strongly Typing Lazy Loaded Components with Generics

Strongly Typed Lazy Loading

While not as common as it used to be, the Render Props pattern is sometimes the best solution to a problem.

The Render Props pattern allows for flexibility that can't be achieved with hooks alone.

Typing the Children Prop for Render Props

Render Props

Using a Record and the satisfies operator saves you from manually specifying types when multiple components share the same props.

The pattern of using a record with satisfies is useful when you have multiple components that share the same props.

Infer Shared Props for Multiple Components

Records of Components with the Same Props

React's implementation of forwardRef has some properties that make it annoying when working with generic components, but it can be worked around.

The Problem With forwardRef

When overriding global behavior, it's best to do it locally to prevent conflicts in library development scenarios.

Override forwardRef's Behavior Locally

Fixing forwardRef Locally

Higher Order Components (HOCs) are great for creating reusable functionality in React, but typing a wrapped component has its challenges.

By making an HOC generic and properly handling the props it is passed, we can create a more flexible component.

Implementing a Generic Higher Order Component

Typing Higher Order Components

Our current Higher Order Component implementation doesn't work with generic components. Let's fix that.

By updating type signatures and changing how props are handled, we can make a Higher Order Component that works with generic components.

Add Generic Component Support to a Higher Order Component

Using Higher Order Components with Generic Components

The `as` prop is one of the most complex in TypeScript, and there are lots of different ways to approach it.

Explored here are two approaches to the `as` prop in React, with one much more efficient than the other.

Transcript

Destructuring Discriminated Unions in React Props

Transcript