We're dealing with a modal which has two variants: one with a title and one without a title. The current `ModalProps` type is defined as follows:

```typescript
type ModalProps =
  | {
      variant: "no-title"
    }
  | {
      variant: "title"
      title: string
    }
```

Now, we want to add a `buttonColor` prop to the `ModalProps` type that is always required across different variants. In other words, regardless of the modal's variant, we should always have to pass in a button color.

Here is the current `Modal` component:

```typescript
export const Modal = (props: ModalProps) => {
  if (props.variant === "no-title") {
    return (
      <div>
        <span>No title</span>
        <button
          style={{
            backgroundColor: props.buttonColor,
          }}
        >
          Click me!
        </button>
      </div>
    )
  } else {
    return (
      <div>
        <span>Title: {props.title}</span>
        <button
          style={{
            backgroundColor: props.buttonColor,
          }}
        >
          Click me!
        </button>
      </div>
    )
  }
}
```

At the moment, we have errors because the `buttonColor` property is not on the `ModalProps` type at all.

## Challenge

Your task is to figure out a way to type `ModalProps` so that `buttonColor` is included on either variant. (Obviously you could manually add this to each variant, but this is not efficient, especially if you've got lots and lots of variants or lots of props to add!)

Your challenge is to find a more efficient way to type this so that you can attach it to each variant.

Good luck!

Learn how to add a required React property to different variants of a discriminated union in TypeScript

The best way to approach this is by using an intersection. Initially, the code looks like this:

```typescript
type ModalProps =
  | {
      variant: "no-title"
    }
  | ({
      variant: "title"
      title: string
    } & {
      buttonColor: string
    })
```

However, saving the file will cause Prettier to rearrange things a bit. You'll notice that it places the brackets in such a way that the `buttonColor` is only attached to one branch.

```
// Error: Property 'buttonColor' does not exist on type '{ variant: "no-title"; };'.
```

If we're on the title variant and inside `no-title`, it actually throws an error because it doesn't exist on type variant `no-title`. Thus, we need a way to tell TypeScript that we want to attach this to both variants.

Even without the parentheses, if you look at the operational order, TypeScript or Prettier is correct. This is because 'and' actually goes before 'or' in the operation order. This means that this gets resolved as one type and it's behaving exactly the same as you can see below, even without the brackets:

```
type ModalProps =
  | {
      variant: "no-title"
    }
  | {
      variant: "title"
      title: string
    } & {
      buttonColor: string
    }
```

This gets resolved as a separate member of the union.

## Bracketing Up the Entire Union

The only proper solution is by bracketing up the entire union. Wrapping these in parentheses is valid syntax, having it just sort of floating before the union.

```typescript
type ModalProps = (
  | {
      variant: "no-title"
    }
  | {
      variant: "title"
      title: string
    }
) & {
  buttonColor: string
}
```

Upon saving this, it will be properly formatted. Now `buttonColor` gets applied to each member of the union.

Even though this looks a bit ugly, this is the smoothest way to do it. It's actually clear which union is being intersected with which.

## Nicer Looking Variant

Now, let's create a `variantModalProps` to try to make this a bit prettier:

```typescript
type VariantModalProps =
  | {
      variant: "no-title"
    }
  | {
      variant: "title"
      title: string
    }

type ModalProps = VariantModalProps & {
  buttonColor: string
}
```

Now the `modalProps` reads a little bit easier.

This might be the best way to go if you're aiming for code readability, but both work equally well.

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

[00:00] The best way to do this then is by using an intersection. And the intersection kind of looks like this, where we use this ampersand symbol and add in button color string here. Now, if we save this file, then Prettier is actually gonna kind of screw us over a little bit. Because if you notice,

[00:17] the way that it places these brackets here, these parentheses, is actually like only attaching button color into one branch here. And we can see this if we go into our component itself. So we're on the title variant. And now if we're inside no title, it's actually erring at us

[00:36] because it doesn't exist on type variant no title. So we need a way to basically say to TypeScript, actually, I want to attach this to both variants. So even without the parentheses, actually, if you look at the operational order, TypeScript is correct, or Prettier is correct here,

[00:54] because and actually goes before or in the operation order. This means that this gets resolved as one type and it's behaving exactly the same as you can see below, even without the brackets. And this gets resolved as a separate member of the union. So the only way to properly solve this is by bracketing up the entire union.

[01:13] So wrapping these in parentheses, and yes, this is valid syntax, having kind of this just sort of floating before the union. And if we save this, then it's gonna properly format it. And you can see that this is a type and then this gets intersected with it. So now button color gets applied to each member of the union.

[01:31] And it means that we have button color on here. And it means that all of these tests are passing below too. So this is, even though this looks a little bit ugly, this is the smoothest way to do it. And it's actually the way it formats it, it's nice and clear kind of which union is being intersected with which. You could even, if you wanted to take this out

[01:51] and say my little props is, let's just leave it like that for now. And let's say, variant modal props. You could add that as kind of like a second declaration there and then add that in variant modal props. And now the modal props reads a little bit easier too. So you don't need to use this parentheses pattern,

[02:11] but that's kind of what's happening under the hood, right? We're just sort of attaching, like using this alias to attach them in parentheses and then intersecting it with the button color and we're good to go. So this might be the best way to go if you're aiming for code readability, but both work equally well.

Adding a Prop Required Across Discriminated Union Variants

[00:00] In this exercise, we're going back to our modal. And this time, in either variant, we want to be able to pass in a button color prop. And that's going to be put on this button here as the background color. So in the no title variant, there's one, and in the title variant, there's one. And this is going to be a required prop.

[00:19] You're always going to have to pass this in. So now it's kind of currently erroring because it's not on the props type at all. So your job is to try to figure out a way to type these modal props so that button color is included on either variant. Of course, you could do this, right? But this is way too much like hard work, especially if you've got lots and lots of variants

[00:38] or lots and lots of props to add here. So your job is to find a way to type this so that you can attach it to each variant.

[00:47] Good luck.

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Using Discriminated Unions to Create Flexible Component Props in React

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Manage TypeScript's control flow limitations in discriminated unions with destructuring: balance prop access and discriminator narrowing.

Destructuring vs Accessing Discriminated Union Props

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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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Allow Optional Props Using A Discriminated Union Branch With Undefined Types

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The `keyof typeof` Pattern

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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

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

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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

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Refactor complex type logic into a type helper.

An "All or Nothing" type helper simplifies complex logic while allowing for flexible object shapes.

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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.

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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.

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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.

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In order to get proper inference, the type arguments need to be passed appropriately to the type helpers.

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Generics enable the useful pattern of inferring type arguments through multiple different type helpers.

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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.

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Explored here are two approaches to the `as` prop in React, with one much more efficient than the other.

Transcript

Adding a Prop Required Across Discriminated Union Variants

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