Imagine that we're working with an external library that has a new global element available to us. We want to strongly type the element so that it can only be used with the correct attributes.

In this case, we want to create a `<something>` element and give it a required `id` attribute, which is a string:

```tsx
<something id="123"></something>
```

However, when we use this element TypeScript tells us that it doesn't exist on the `JSX.IntrinsicElements` interface:

```typescript
// hovering over `something` shows:

Property 'something' does not exist on type 'JSX.IntrinsicElements'.
```

It's not the code that's incorrect, it's the types that are insufficient.

## Challenge

Your goal is to strongly type the `something` element so that it can only be used with the correct attributes.

Hint: You'll need to use declaration merging with something in the global namespace that we've seen before.

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

For the solution to this challenge, we'll name the element `<something-solution>`.

From the error message, we know that the `something-solution` element does not exist on the type `JSX.IntrinsicElements`. 

This tells us that we need to use declaration merging to extend the `JSX.IntrinsicElements` interface.

Like we've seen before, we'll use `declare global` and target the `JSX` namespace. Then we'll extend the `IntrinsicElements` interface with the new `something-solution` element:

```typescript
declare global {
  namespace JSX {
    interface IntrinsicElements {
      "something-solution": {
        id: string;
      };
    }
  }
}
```

After extending the interface the error disappears.

When using the `something-solution` element, we have to pass a string for the `id` prop.

As seen in the tests, not passing the `id` prop or passing a number for the `id` prop will throw an error:

If we pass the wrong type of prop, TypeScript will throw an error, ensuring that we only pass the correct prop.

```tsx

<something-solution />
// Error: Property 'id' is missing in type '{}' but required in type '{ id: string; }'.

<something-solution id={123} />
// Error: Type 'number' is not assignable to type 'string'.
```

## Alternative Solution
Recall that `JSX.IntrinsicElements` is available both in the global `JSX` namespace and in the `React` namespace. 

The first solution uses the global `JSX` namespace, but we could also use the `React.JSX` namespace.

```typescript
declare global {
  namespace React{
    namespace JSX {
      interface IntrinsicElements {
        "something-solution": {
          id: string;
        };
      }
  }}
}
```

While both of these options will work, it's generally recommended to use `React.JSX.IntrinsicElements` when working with React because it's more specific.

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

Extend JSX.IntrinsicElements with Declaration Merging

[00:00] So the thing we know from this error is that JSX.IntrinsicElements is the thing we want to target with our declaration merge. We want to pull this into the global scope here, so we'll declare global. To declare global, we're going to basically say,

[00:16] we need to go onto the namespace JSX and inside there, we're going to go interface intrinsic elements. I have to make sure to spell this right, intrinsic elements, and then say something solution, because it has to be inside a string here, otherwise it's invalid,

[00:33] and we're going to say ID string. Now here, it starts to work and we actually get, you can see here, there we go, JSX.IntrinsicElements something solution ID string. If we command click, we actually go to this definition up here, whereas if we were to command click on a div, let's say, we would actually go all the way

[00:51] into JSX.IntrinsicElements here. You're starting to see how declaration merging can be pretty powerful here, because we can actually describe custom stuff that we can render, which is really, really interesting. So now, if we pass in like the wrong thing, if we don't pass in an ID, it's missing in type empty object, but required in this,

[01:09] and also if we pass it as the wrong thing, this is going to yell at us because it's expecting a string. Very, very cool. And you notice as well, that JSX.IntrinsicElements, it's also available inside the namespace React, JSX.IntrinsicElements. So up here, for instance, this also will work.

[01:29] This is interesting because JSX.IntrinsicElements, it's available on the JSX namespace, but that's global. And if for whatever reason, you have types that are coming in from, let's say, React, but also another framework that uses JSX, like SolidJS or Preact, for instance, you actually want to say,

[01:48] okay, this is only available inside React's namespace of JSX. So it gets a little bit confusing here, but really the main thing is that both of these will work. And probably you want to be more specific, I would say. So you probably want to say React.JSX.IntrinsicElements. But then again, how often are you going to be combining

[02:06] two frameworks in the same project? So I would usually just say, namespace JSX.IntrinsicElements, add your extra kind of elements on there and type the props. Not bad.

Add a New Global Element in TypeScript

[00:00] In this exercise, I want you to imagine that we're working with an external library that has a new global element available to us. We want to strongly type this so that it can only be used with the correct attributes. In this case, we want to create a something element and give it a required ID attribute, which is a string.

[00:17] We're going to need to declaration merge with something in the global namespace that we've seen before. I think you're getting the idea here. In fact, the error that we're getting here is property something does not exist on type JSX.IntrinsicElements. In this case, we're not wrong, it's the types that are wrong.

[00:37] Learning what you've figured out so far, see if you can piece all that together and make this work. Good luck.

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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.





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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

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

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.

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Add a New Global Element in TypeScript

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