Let's move on to exploring `JSX.IntrinsicElements`, which is another global interface that you'll see in TypeScript and React.

```typescript
export type Example = JSX.IntrinsicElements;
```

In VS Code on your local machine, you can `cmd + click` on `IntrinsicElements` to jump to the `index.d.ts` file. If you are using the web-based Stackblitz editor, check out [the DefinitelyTyped repo on GitHub](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/8169bc6f114ce1b7d3c00f2a3666c4e4921aa317/types/react/index.d.ts#L3263).

To better understand this interface, you'll need to answer the following questions:

## Question 1: What is `JSX.IntrinsicElements`?

Explore the `IntrinsicElements` interface definition in `index.d.ts` to get an idea of what it is and what it does.

## Question 2: What is the structure of `JSX.IntrinsicElements`?

`JSX.IntrinsicElements` appears to have HTML attributes as properties. What are all of the keys and values telling us?

## Question 3: What happens when adding a new element to `JSX.IntrinsicElements`?

If you are running VS Code locally, try editing the object itself by adding a new element:

```typescript
// in `index.d.ts`
interface IntrinsicElements {
  // ...
  myNewElement: {
    foo: string;
  };
}
```

After this change, the `myNewElement` below fixes its error:

```typescript
<myNewElement foo="123" />
//^^^^^^^^^^^ Property 'myNewElement' does not exist on type 'JSX.IntrinsicElements'.
```

Why does this happen?

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

To start, let's take a look at [the type definition for `JSX.IntrinsicElements`](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/8169bc6f114ce1b7d3c00f2a3666c4e4921aa317/types/react/index.d.ts#L3263):

```typescript
// in `index.d.ts`
interface IntrinsicElements {
    // HTML
    a: React.DetailedHTMLProps<React.AnchorHTMLAttributes<HTMLAnchorElement>, HTMLAnchorElement>;
    abbr: React.DetailedHTMLProps<React.HTMLAttributes<HTMLElement>, HTMLElement>;
    address: React.DetailedHTMLProps<React.HTMLAttributes<HTMLElement>, HTMLElement>;
  ...
}
```

This shows us all the things that are globally available to use as elements in React. Each element is a key in the `IntrinsicElements` interface.

This lists includes virtually all valid HTML5 elements and their attributes. Things like `source`, `span`, `strong`, and `svg` are all available to use as elements here.

Each value associated with a key is related to detailed HTML props, attributes, and elements.

For example, the `a` element is structured like this:

```typescript
// inside of index.d.ts
a: React.DetailedHTMLProps<
  React.AnchorHTMLAttributes<HTMLAnchorElement>,
  HTMLAnchorElement
>;
```

Where `DetailedHTMLProps` is essentially a type helper that extends `HTMLAttributes` for the elements that we pass in, which [in this case is `AnchorHTMLAttributes`](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/8169bc6f114ce1b7d3c00f2a3666c4e4921aa317/types/react/index.d.ts#L2129). This is what allows for the attributes like `href` and `target` to be available on the anchor tag.

```typescript
interface AnchorHTMLAttributes<T> extends HTMLAttributes<T> {
  download?: any;
  href?: string | undefined;
  hrefLang?: string | undefined;
  media?: string | undefined;
  ping?: string | undefined;
  target?: HTMLAttributeAnchorTarget | undefined;
  type?: string | undefined;
  referrerPolicy?: HTMLAttributeReferrerPolicy | undefined;
}
```

## Adding a New Element

At the top of `IntrinsicElements`, add `myNewElement` with a `foo` attribute of type `string`:

```typescript
interface IntrinsicElements {
  myNewElement: {
    foo: string;
  };
  // HTML
  ...
```

Now as far as TypeScript is concerned, `myNewElement` with an attribute of `foo` is a valid element that we can use in our application:

```typescript
<myNewElement foo="123" />
```

However, if we try to pass in a boolean instead of a string, we get an error: "Boolean is not assignable to type string."

```typescript
<myNewElement foo={true} />
//            ^^^ Type 'boolean' is not assignable to type 'string'.
```

The `IntrinsicElements` are core to how React types work.

You might see `IntrinsicElements` tends to pop up when you're trying to use an element that's not supposed to be there or encounter random errors throughout your application.

Don't forget to revert your changes in `index.d.ts`, as we don't want to actually edit the file itself!

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

Understand the Structure of React's JSX.IntrinsicElements

[00:00] Okay, let's check out GSX.IntrinsicElements, see what we're dealing with. Come on, click on this, and you'll see that we've got something here, which we'll have a look at later. But you can see that IntrinsicElements looks like this. This is a massive interface, and this massive interface basically describes all of

[00:18] the things that are globally available to use as elements in React. All of this stuff, all of the SVG stuff down here, and all of the DOM stuff, these are things that are globally available for you to use. This means that we can use select, small, or source, span, strong, style, sub,

[00:37] summary, sub, as elements here. And we can actually append to this if we want to. So if we go back here, go to IntrinsicElements and add a new one up here, let's say my new elements, and let's just leave it, I don't know, what should we make this type here? What should the type actually be?

[00:55] Let's just leave it like this for now. The second question, or the third question, I can't remember, was what is the structure of this? Well, it's very obvious from the keys what's happening here. But what are the values of those keys? Well, each of these are detailed HTML props, HTML attributes, HTML elements, HTML elements. So that's an abbreviation.

[01:14] You can see the anchor element has slightly different ones. Anchor HTML attributes, anchor HTML element, anchor HTML element. We can derive from this that if we take a look at HTML props, it's just literally a kind of like type helper here where we pass in the attributes and then we have class attributes and the E.

[01:33] Let's go back there. And this HTML props, we're actually passing in some anchor HTML attributes. And these ones like download, href, sorry, href, I always say href for some reason, media, ping, target, type. These are things that are available on that element here.

[01:53] So we can see that the keys are the elements that are available and here are the props that are available on those elements. So let's say that my new element takes in an ID, which is a string, and we'll go back here and we can see then that my new element, it's currently got foo on here,

[02:10] but foo string is not assignable to type ID string. Let's actually copy the thing that I meant to copy before, whoopsie daisy. Yeah, there we go. So foo string, now it's going to work. And if I pass in a Boolean here, for instance, so just foo, Boolean is not assignable to type string. So this is really cool.

[02:30] This gives us a sense for what Intrinsic Elements is doing because Intrinsic Elements tends to pop up in various guises when you're kind of like trying to maybe use an element that's not supposed to be there or it kind of, it comes up in all sorts of random errors throughout your application. I'm going to delete this before I forget because I will inevitably forget.

[02:48] But jsx.intrinsic-elements, really, really key type that sits at the center of how you should think about React types.

Exploring JSX.IntrinsicElements

[00:00] In this exercise, we're going to look at JSX.IntrinsicElements, which is a really, really interesting interface which sits globally. And I would like you to basically just walk through each of these questions. So let me set them up. What is JSX.IntrinsicElements? Make sure you command click on IntrinsicElements below to go to its definition

[00:18] and make sure you're going to the original definition in index.d.ts. What is the structure of IntrinsicElements when you're in there? What actually is going on there? What are the keys? What are the values in this object that you're seeing? And try editing the object itself

[00:36] and you'll notice that the element below this error actually kind of like fixes its error there. You'll see an error is happening. It doesn't exist on type JSX.IntrinsicElements. And, you know, you definitely want to go and change that back because we don't want to actually be editing the index.d.ts file itself.

[00:55] So go through that. See if you can figure out what is actually happening here and why this error is even occurring in the first place. Good luck.

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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

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

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

Destructuring vs Accessing Discriminated Union Props

Destructuring Discriminated Unions in React Props

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

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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

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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

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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

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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.

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Currying Hooks

Advanced Hooks

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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

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.

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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

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Extend JSX.IntrinsicElements with Declaration Merging

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Updating the Global Namespace for an Additional Attribute

Add Attributes to All Elements with Declaration Merging

Types Deep Dive

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

Strongly Typing Lazy Loaded Components with Generics

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

Typing the Children Prop for Render Props

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Infer Shared Props for Multiple Components

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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.

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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

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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.

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Exploring JSX.IntrinsicElements

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