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.

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Mimicking useState Behavior with Function Overloads

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