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.

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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Fixing Type Inference in a Custom React Hook

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