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.

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.

## 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 Complete Volume is a set of five full TypeScript workshops for the advanced-beginner to expert professional TypeScript programmer that wants to take their skills to the next level. This bundle includes all of the Total TypeScript self-paced workshops.

TypeScript Pro Essentials

Type Transformations

TypeScript Generics

Advanced TypeScript Patterns

Advanced React with TypeScript

TypeScript Expert Interviews

TypeScript Pro Complete

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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.

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Discriminated Unions for Conditional Props in TypeScript

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Allow Optional Props Using A Discriminated Union Branch With Undefined Types

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The `keyof typeof` Pattern

Syncing Types without Manual Updates

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

Solving Partial Autocompletion

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

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

Ensuring Correct Inference for Prop Types

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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

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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

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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

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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

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

Add a Generic Type Argument to a Props Interface

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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

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

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Adding Generic Type Arguments to Type Helpers

Generic Inference through Multiple Type Helpers

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Refactoring from Generics to a Discriminated Union

Generics vs Discriminated Unions

Using Generics with Components

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Use 'as const' to Infer a Tuple return type

Fixing Type Inference in a Custom React Hook

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

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Handling Complex State Management with TypeScript Unions

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Handling Different State Values with Discriminated Unions

Using Discriminated Unions in useState

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Overloading Functions in TypeScript

Use Function Overloads for Better Type Inference

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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!

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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.

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Exploring the React Namespace

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

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Discriminated Tuples in Custom Hooks

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