We introduced the `useStateAsObject` function in the Generics section of the workshop:

```typescript
export function useStateAsObject<T>(initial: T) {
  const [value, set] = useState(initial);
  return {
    value,
    set,
  };
}
```

Now, we want to make `useStateAsObject` behave exactly like `useState`.

## Challenge

Your challenge is to mimic the function overloads on `useState` while also replicating the generic signature.

Don't worry about the behavior where you can pass a function to `useState`. Instead, focus on the basic initial type so the tests pass.

With the knowledge and tools you've gained from our previous exploration of `useState`, you should be well-equipped to tackle this challenge!

By mimicking the function overloads on `useState`, you can make your custom hooks behave in the same way.

Let's start by trying a naive approach.

## The Naive Approach

First we'll update `useStateAsObject` to make the `initial` parameter optional.

```typescript
export function useStateAsObject<T>(initial?: T) {
	// ...
}
```

Now when we hover over `initial`, TypeScript interprets it as either `T` or `undefined`.

```typescript
// hovering over `initial` shows
(parameter) initial: T | undefined
```

However, this causes an issue.

When we pass a value to `useStateAsObject`, it will hit the first overload of `useState`.

```typescript
useState<T | undefined>(initialState: T | (() => T | undefined) | undefined): [T | undefined, React.Dispatch<React.SetStateAction<T | undefined>>]

(+1 overload)
```

The problem is that TypeScript isn't clever enough to hit the second overload when we pass `undefined`. Instead, it will give us an error that `undefined` is not assignable to `T`.

To fix this issue, we need to copy `useState`'s overloads in any function that wraps it.

## Copying `useState`'s Overloads

Let's first get the outer behavior working by adding an overload.

For the new overload, we'll remove the optional `?` from the `initial` parameter and set its return type to have a `value` of `T` and a `set` of `React.Dispatch<React.SetStateAction<T>>`.

Here's what it looks like so far:

```typescript
export function useStateAsObject<T>(initial: T) {
	value: T;
	set: React.Dispatch<React.SetStateAction<T>>;
}
export function useStateAsObject<T>(initial?: T) {
  // ...
```

Now we need to fix the error where not passing a value should be undefined.

To do this, we'll create another overload. This time, we'll remove the `initial` parameter and set the return type to have a `value` of `T` or `undefined` and a `set` of `React.Dispatch<React.SetStateAction<T | undefined>>`.

```typescript
export function useStateAsObject<T>(initial: T) {
	value: T;
	set: React.Dispatch<React.SetStateAction<T>>;
}
export function useStateAsObject<T>(): {
  value: T | undefined;
  set: React.Dispatch<React.SetStateAction<T | undefined>>;
}
export function useStateAsObject<T>(initial?: T) {
  // ...
```

With this extra overload the code behaves as it did before, but it's more accurate since we know that `value` is not always a number.

## Making the Code More Readable

Our code is working, but we could make it a bit more readable by wrapping the return types in a new `useStateReturnValue` type:

```typescript
type UseStateReturnValue<T> = {
  value: T;
	set: React.Dispatch<React.SetStateAction<T>>;
};

export function useStateAsObject<T>(): UseStateReturnValue<T | undefined>;
export function useStateAsObject<T>(initial: T): UseStateReturnValue<T>;
export function useStateAsObject<T>(initial?: T) {
  const [value, set] = useState(initial);
  ...
```

## Wrapping Up

We've learned that when you're wrapping `useState` and similar logic in TypeScript, you need to mimic the overloads of the hook that you're using in order to get all of its API out.

The process is kind of annoying, but it's a really cool trick to know if you're ever doing a library that directly wraps `useState` or `useReducer` or one of the other hooks that have multiple overloads.

Just remember, you should be careful with function overloads, as it's easy to end up in an impossible position and TypeScript won't warn you!

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

[00:00] So let's take a look here. Let's try a naive approach first, where we just basically make this an optional param here. So what happens if we make it an optional param? Well, useState here ends up actually inferring it as T or undefined,

[00:14] because technically initial here, it could be T or it could be undefined, which hits the first overload of useState here. So what that means then is that what happens if we just pass in this T inside there,

[00:28] essentially removing the idea that this could be undefined? Well, type undefined is not assignable to this. So what we're seeing here is we're basically hitting this first overload again. You might think, okay, TypeScript is clever enough that when we hit the undefined overload, we're going to hit the second overload.

[00:47] But in fact, no, whenever we pass an argument to this, it's going to actually just hit the first overload, meaning that undefined is never assignable to it. So what this means then is we need to copy useState's overloads in any function that wraps it.

[01:03] So let's say then that instead of... let's actually just get the outer behavior working first, because this seems to be working. So what we're going to do is we're going to say export function useState as object. We're going to still keep the T, actually, and we're going to go initial T.

[01:21] And now we end up needing actually a type here to explain what's being returned. So let's say value is T and set is going to be a React.dispatch React.setStateAction T. And this, if I remember correctly, yes, is the thing that we've got here.

[01:39] So now inside our function, it's still working and outside our function, it's still working, except for just this bit here where we don't pass one. So let's do that as well. Let's actually copy this over. And now we just remove the initial T here. There we go. Looking good.

[01:57] No, we're not, because this needs to be now value T or undefined. This one here too, T or undefined. If we didn't have those, you see that it's basically just behaving as it was before, where value is always a number. Whereas, in fact, we know it's not a number. You notice here, too, how easy it is for us to lie with these function overloads.

[02:17] So here, we're basically never passing in undefined here. Well, rather, yeah, we're never going to pass an initial value here, but we're lying and saying that actually this value is T only, never undefined. So you have to be a bit careful with these function overloads because it's very easy to actually end up in an impossible position and TypeScript won't warn you about it.

[02:38] But this one now is working beautifully. So what have we learned here, really? Well, actually, there's one more thing to do here, which is to take these object types and actually put them in a, like, this is very ugly, really. Like, look at this. So here's one I baked earlier, which is you actually can wrap these in a useState return value.

[02:56] This is just a type helper that kind of sits next to these and make sure that these function definitions look a little bit prettier. And we can see, too, that we've also removed some duplication in the undefined bit, too. So we've got our T here and useState return value, T or undefined. You notice here we've got generics on or type arguments on each line.

[03:15] You can actually have, like, different type arguments for different generics, too, or different, like, numbers of type argument per overload, which is extremely cool.

[03:25] But yeah, like this song and dance where you have to mimic the overloads of the hook that you're using in order to get all of its API out is pretty annoying. But it's a really cool trick to need to know if you're ever doing a library that directly wraps useState or useReducer or one of the other hooks that have multiple overloads. There you go.

Mimicking useState Behavior with Function Overloads

[00:00] We first saw our useStateAsObject function back in the generic section. But here, what we want to do is we want to make it behave exactly like useState. Now, we want to actually mimic all of the overloads on useState, but also kind of mimic the generic signature here, too.

[00:16] I don't think we want to worry about the behavior where you can pass a function to useState and sort of do it that way. We just want to do it with the basic initial type here. So we just want to make these types pass, essentially. So this one here, ideally, we should be able to pass nothing to useStateAsObject. And it should be typed as value number or undefined.

[00:38] And we should be able to set a state on it, number or undefined. So really, we're just like increasing the power of our abstraction, making it behave more like useState. Now, what we're going to do is we're going to need some function overloads here. I think from our explanation of useState or our exploration of it before, you should have all the equipment you need to be able to solve this yourself. 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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Type-Checking React Props With Discriminated Unions

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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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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

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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

The Partial Autocompletion Quirk

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

Extracting Keys and Values from a Type

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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

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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

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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

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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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Generic Inference through Multiple Type Helpers

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

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Learn how to properly type complex React state that represents different possibilities using TypeScript.

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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

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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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Types Deep Dive

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Mimicking useState Behavior with Function Overloads

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