Here we have a `useData` hook that returns a `Result<T>`.

`Result<T>` is defined as a tuple containing three different statuses: `loading`, `success`, and `error`.

```tsx
export type Result<T> = [
	"loading" | "success" | "error",
	T | Error | undefined
];
```

The `useData` hook fetches data from a URL, processes the response, and sets the state.

```typescript
export const useData = <T>(url: string): Result<T> => {
  const [result, setResult] = useState<Result<T>>(["loading", undefined]);
  useEffect(() => {
    fetch(url)
      .then((response) => response.json())
      .then((data) => setResult(["success", data]))
      .catch((error) => setResult(["error", error]));
  }, [url]);

  return result;
};
```

Although the `result` seems to be typed well, there's an issue when we try to use it.

Inside of `useState`, hovering over the `value` shows us that it's either an `Error`, `title: string`, or `undefined`.

When using the `useData` hook in a component, we get errors because we don't have `value.message` or `value.title` defined.


```tsx
const Component = () => {
	const [status, value] = useData<{ title: string }>(
		"https://jsonplaceholder.typicode.com/todos/1"
	);

	if (status === "loading") {
		return <div>Loading...</div>;
	}

	if (status === "error") {
		return <div>Error: {value.message}</div>; // error!
	}

	return <div>{value.title}</div>; // error!
};
```

## Challenge

Your task is to find out what kind of other type signature we could define for the `Result` type that will resolve these errors.

You shouldn't need to change anything inside the `useData` hook– just make a small change to the type definition, and you should start to see it working.

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

The solution is to update `Result` to be a discriminated tuple.

Basically, we'll take all of  `Result`'s content and put it inside of 3 different tuples.

Here's what `Result` looked like before:

```tsx
// starting point
export type Result<T> = [
	"loading" | "success" | "error",
	T | Error | undefined
];
```

And here's what `Result` looks like as a discriminated tuple:

```tsx
// discriminated tuple solution
export type Result<T> =
	| ["loading", undefined]
	| ["success", T]
	| ["error", Error];
```

Because `loading`'s value is `undefined`, we can mark it as optional and not have to pass it in:

```tsx
export type Result<T> =
	| ["loading", undefined?]
	| ["success", T]
	| ["error", Error];
```

With this change, everything's working.

TypeScript is able to understand that when you create this tuple, it has three different parts for `error`, `loading`, and `success`.

This means when we go to use it, we get nice autocompletion:

```tsx
// inside of useData
const [result, setResult] = useState<Result<T>>([""]);
// autocomplete for error, loading, or success
```

Down in the `Component` when we use `useData`, TypeScript understands the relationship between `status` and `value` when they are destructured. 

It knows that if `status` is a certain thing, then `value` is going to be a certain thing, too:

```tsx
const Component = () => {
	const [status, value] = useData<{ title: string }>(
		"https://jsonplaceholder.typicode.com/todos/1"
	);

// hovering over value shows
const value: Error | {
  title: string;
} | undefined
```

This behavior breaks the traditional understanding of destructuring.

You might think that destructuring a discriminated union of objects wouldn't work, but surprisingly, with tuples it does!

It's surprising that more libraries don't take advantage of this technique to improve code readability and maintainability.

Don't be afraid to explore this powerful approach when working with discriminated unions of objects, as it can lead to cleaner and more understandable code for the people consuming your functions.


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

[00:00] So, you might have figured out the solution here. It's really, really neat, which is we can use a discriminated tuple for this. So, let's say we take all of this and we basically put it inside three different tuples. So, tuple 1, tuple 2 and tuple 3. For the first

[00:18] tuple, we're going to give the first member loading and the second member is going to be undefined here. For the second tuple, we can have success and this is going to be with our T attached to it. And you can see, as I filled in that, then this one worked here.

[00:35] The other one that's not working is error any. So, let's put error and instead of any, let's use an error instead. So, now we've got our results properly typed. We've got loading undefined, success T, error, error. And now suddenly everything's working. Why

[00:51] is it working? So, there's a few things to break down here. First of all, it's really smart that TypeScript understands that when you create this tuple, you get three different parts here, error, loading and success. I can't pass in an error, obviously, into success

[01:05] because this is, like, it's not assignable to T. So, I have to start off with loading undefined. And by the way, you can also specify this as a kind of like an optional member of this tuple. So, I don't even need to pass this here if I don't want to, which is pretty

[01:23] beautiful, actually. Then down here, we basically, this is really, really smart. When you destructure this, TypeScript understands the relationship between these two components. It understands

[01:37] that if status is a certain thing, then value is going to be a certain thing, too. So, this breaks the rule or breaks our understanding of destructuring because you think of, like, destructuring a discriminated union of objects, this wouldn't work, surely. But with tuples,

[01:53] it does. This is so, so smart. And it means, actually, we just get a really clean API up here and a really clean API down here. So, I would recommend actually using this more often, and I'm surprised that more libraries don't. It's just a really, really lovely way

[02:09] to kind of make your code more understandable for the people that are consuming the functions of your library. Lovely stuff.

Discriminated Tuples in Custom Hooks

[00:00] In this exercise, we have a UseDataHook and the UseDataHook returns a ResultTee. ResultTee is defined as a tuple with three different statuses in there. And we've got Loading or Success or Error. And here it's defined as either Tee or Error or Undefined.

[00:19] So this basically, what it does, it just fetches from a URL, then grabs the response, then sets it to SuccessData. So this result is sort of seemingly typed pretty well, except for the fact that when we go to use it, they're not sort of discriminating based on each other.

[00:37] So here, Value is either Error or TitleString or Undefined, and Status is Loading or Error or Success. But what we can see here is that when we have this error here, we don't have an error defined as the value. When we've filtered out Loading and Error,

[00:56] we don't have a kind of value.title here either. So this isn't right. Ideally, what we would like here is for these things to be grouped together. So your job is to work out what kind of other type of type signature we could define for this result in order to make this all work.

[01:15] You shouldn't have to change anything in here, I don't think. It should just be a little change up here, and then you'll start to see it working. 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

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

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.

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Discriminated Tuples in Custom Hooks

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