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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Expert-level knowledge for typing your React apps with TypeScript.

React TypeScript

Total TypeScript Core Volume is a set of four full TypeScript workshops for the intermediate to expert professional TypeScript programmer that wants to take their skills to the next level. This bundle also includes the Advanced React with TypeScript workshop that will give you expert-level knowledge for typing your React apps.

Type Transformations

TypeScript Generics

Advanced TypeScript Patterns

Advanced React with TypeScript

TypeScript Expert Interviews

Core Volume

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

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

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

Fixing Type Inference in a Custom React Hook

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

ElementType and ComponentType are two React type helpers which help describe the shapes of components.

Understanding React's ElementType and ComponentType

Learn how to add an interface to React's global namespace.

Adding interfaces into global namespaces can be useful in a variety of situations.

Merge Namespaces in TypeScript

Appending to React's Global Namespace

Declaration merging can be used to modify existing interfaces in the global namespace.

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

Modify Existing Interfaces in the Global React Namespace

By utilizing declaration merging, you can inform TypeScript of the types of custom elements from external libraries.

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

Extend JSX.IntrinsicElements with Declaration Merging

Add a New Global Element in TypeScript

The types that React exposes for each element are extensive. Take a deep into these types to understand how they are defined and work.

Navigating HTMLAttribute types

Exploring HTML Attribute and Element Types

Having an optional test attributes available can be useful for testing elements.

Have a clear understanding of the namespace structure and hierarchy of interfaces in React, you'll be better equipped to handle any errors

Updating the Global Namespace for an Additional Attribute

Add Attributes to All Elements with Declaration Merging

Types Deep Dive

Lazy loading allows us to dynamically import and render components, but it can be tricky to type props correctly.

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

Strongly Typing Lazy Loaded Components with Generics

Strongly Typed Lazy Loading

While not as common as it used to be, the Render Props pattern is sometimes the best solution to a problem.

The Render Props pattern allows for flexibility that can't be achieved with hooks alone.

Typing the Children Prop for Render Props

Render Props

Using a Record and the satisfies operator saves you from manually specifying types when multiple components share the same props.

The pattern of using a record with satisfies is useful when you have multiple components that share the same props.

Infer Shared Props for Multiple Components

Records of Components with the Same Props

React's implementation of forwardRef has some properties that make it annoying when working with generic components, but it can be worked around.

The Problem With forwardRef

When overriding global behavior, it's best to do it locally to prevent conflicts in library development scenarios.

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.

Transcript

Adding a Prop Required Across Discriminated Union Variants

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