In this exercise, we have a custom React hook called `useID`.

This hook takes in a default ID, uses it in a `useState`, and then returns the result of the `useState` back from `useID`:

```typescript
export const useID = (defaultID: string) => {
	const [id, setID] = useState(defaultID);

	return [id, setID];
};
```

However, when we use `useID`, we don't have methods like `toUpperCase` on it, even though it's inferred as a string.

## Challenge

Your task is to figure out how you can change this function without altering the runtime behavior to actually return the right thing.

Hint: Hover over `setID` and `ID` to see what's happening.

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

Hovering over our variables inside of `useId` is going to tell us what's going wrong.

Inside of `useId`, when hovering over `id` in the return statement we can see a `const id: string`, and hovering over `setId` shows `React.Dispatch` with a string:

```typescript
// inside of useId

return [id, setId];

// hovering over id
const id: string;

// hovering over setId
const setId: React.Dispatch<React.SetStateAction<string>>;
```

However, when we hover over `useId` itself, we can see that it is creating a union of `id` and `setId` and putting it inside of an array.

```tsx
// hovering over useId
const useId: (
	defaultId: string,
) => (string | React.Dispatch<React.SetStateAction<string>>)[];
```

This means that when we go to use `useId`, both `setId` and `id` have the same type.

```typescript
const [id, setId] = useId("1");

// hovering over id and setId both show:
const id: string | React.Dispatch<React.SetStateAction<string>>;
```

We need to make TypeScript understand that the thing we're returning from the function is not going to change or be appended to.

To fix this, we will tell TypeScript that `useId` is a tuple instead of an array.

There are two ways to do this:

## Solution 1: Strongly Type the Values Inside of a Tuple

The first solution is to set the return type as a tuple with the values typed inside of it:

```tsx
export const useId: (defaultId: string): [
  string,
  React.Dispatch<React.SetStateAction<string>>
  ] => {
    const [id, setId] = useState(defaultId);

    return [id, setId];
};
```

This works because it explicitly tells TypeScript what the function returns.

If we forget to leave one off, TypeScript will yell at us because the target requires two elements. If we add another element, TypeScript will yell at us for having too many.

## Solution 2: Use `as const` on the Returned Array

The second option is to use `as const` on the returned array:

```tsx
export const useId: (defaultId: string) => {
  const [id, setId] = useState(defaultId);

  return [id, setId] as const;
};
```

This tells TypeScript to infer that the returned array is a tuple instead of a big array:

```typescript
const useId: (
  defaultId: string,
) => readonly [string, React.Dispatch<React.SetStateAction<string>>];
```

Using `as const` is my preferred solution because it's just beautiful.

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

[00:00] So, let's take a look and actually hover over UseID, because this is what's going to tell us what's going wrong. You can see that actually, when we return the things from UseID, it gets inferred as a string here, and this kind of React.dispatch.setStackAction thing with a string in there.

[00:16] And that's all good, that's fine, but if we hover over this, you can see that it actually unions those and puts them inside an array here. So this is now an array of those possible things. So that means when we come down to here, both of these have the same type.

[00:33] This SetID is a string or the dispatch, and this one is an ID, string or dispatch. So the key here is to make TypeScript understand that the thing we're returning from this is not going to change, it's not going to be appended to, right? Because if you think about this, we've just basically got an array here, right?

[00:51] So if I just add R here, and technically, I could push things to this. So I could push a new string to this, right? So we need to tell TypeScript that this is a tuple, not an array. The way we can do this, there's two different ways. We can either say, okay, let's strongly type this inside here.

[01:10] And so this one is a React.dispatch, React.setStateFunction or ActionString, and we can stick that in there and that will work. The reason that works is because it explicitly tells TypeScript what this returns, and now if we forget to leave, like if we just leave one off, then it's going to yell at us because the target requires two elements.

[01:29] And if we add another one to this, like this, then it's going to yell at us again. There is a really nice different way to do this though, which is you can use asConst on the returned array. This is super nice because what this does is it basically tells TypeScript to infer that this is a tuple and not a big array.

[01:48] So you can see here that if we remove the asConst, we get that kind of strange union type that we were dealing with before just there. But then if we put this on here, suddenly it takes a read-only thing, which is useful in itself, and we've got a tuple,

[02:04] the first element containing a string and the second element containing React.dispatch. This I think is my preferred solution. It's just beautiful.

Fixing Type Inference in a Custom React Hook

[00:00] In this exercise, we have a hook called useID. And what useID is doing is it takes in a default ID, sticks that in a useState, and then returns the results of the useState back from useID. This all seems fine, but when we actually go to useID, you can see that we don't have things like toUpperCase on it,

[00:18] which is surprising because inside here, it's inferred as a string. Your job here is to try to figure out how you can change this function without changing the runtime behavior to actually return the right thing. And a couple of clues here might be gleaned from hovering over setID and hovering over ID to see what's happening.

[00:38] So that's your job. 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 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

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

Extend JSX.IntrinsicElements with Declaration Merging

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Updating the Global Namespace for an Additional Attribute

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Types Deep Dive

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

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Infer Shared Props for Multiple Components

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The Problem With forwardRef

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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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Fixing Type Inference in a Custom React Hook

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