In this exercise, we have the same `Event` discriminated union that we’ve used before:

```tsx
export type Event =
  | {
      type: "click"
      event: MouseEvent
    }
  | {
      type: "focus"
      event: FocusEvent
    }
  | {
      type: "keydown"
      event: KeyboardEvent
    }
```

Your job is to grab the type of `click` or `focus` or `keyDown` from the discriminated union so that the following test passes:

```tsx
type tests = [Expect<Equal<EventType, "click" | "focus" | "keydown">>]; 
```

Don’t overthink it!

This exercise is easier than it seems, and we've already covered all of the pieces you'll need to solve it!

At times, you may need to access a single key from each member of a discriminated union. TypeScript makes this much easier than it may seem!

As simple as it may seem, here’s the solution:

```tsx
type EventType = Event["type"]; 
```

This works because when you access a key in a union type, you are actually accessing every possible permutation of the union.

### Experimenting with the Event Type

For example, let’s remove the `type` discriminator from `"keydown"`:

```tsx
export type Event =
  | {
      type: "click"
      event: MouseEvent
    }
  | {
      type: "focus"
      event: FocusEvent
    }
  | {
      event: KeyboardEvent
    }

```

Now TypeScript will show us an error at our `type EventType = Event["type"];` line because not every member of the `Event` union includes a type.

We can put the `type` back and do a similar experiment where we rename an `event`:

```tsx
export type Event =
  | {
      type: "click"
      mouseEvent: MouseEvent  // renamed event
    }
  | {
      type: "focus"
      event: FocusEvent
    }
  | {
      type: "keydown"
      event: KeyboardEvent
    }

// Error
type EventType = Event["event"]

```

Now If you try to access `"event"` from `Event` and one of the permutations doesn't have it, or is mislabeled, you'll get an error.

It's important to understand you can access parts of discriminated unions, as long as that part belongs to every member of the union.

Simple Syntax Used to Access Parts of a Discriminated Union

0:00 Here you go. This is the solution. As simple as it may seem, when you access a key on a union, you're actually accessing all of the possible permutations of that key. It means that EventType ends up being click or focus or keydown.

0:17 If I were to remove the discriminator here, this would actually not work because it doesn't exist on all of the different branches of the union. It works especially well with discriminated unions because you know in a discriminated union, like type, is always going to be there.

0:32 Whereas we could do this with the Event, for instance, so Event "event," and we'd end up with MouseEvent or FocusEvent or KeyboardEvent. If one of these was actually mislabeled as mouseEvent, like this for instance, then this wouldn't work.

0:46 That's a really important thing to understand about these types is that you can actually access pieces of them as long as they belong on each member of the union.

Extract the Discriminator from a Discriminated Union

0:00 In this exercise, we have a discriminated union, the same one that you've seen before. What I'd like you to do is grab out the type here of click or focus or keydown from the discriminated union.

0:13 This exercise is actually easier than it seems, so don't go too deep into anything too crazy. All of the pieces that you need for this have been covered before. Good luck.

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Typeof Keyword, and Type Level

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Use a Utility Type to Extract Function Parameters

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Use Utility Types To Extract a Promise's Result

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Creating Types from Types

The 'typeof' operator transforms a runtime construct into a type.

The 'typeof' Operator

The 'keyof' operator can be used to extract the keys of an object type.

The 'keyof' Operator

Built-in TypeScript utilities that assist with transforming types.

Utility Types

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Typing Promises and Async Requests

Inference Basics

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Learn three different types of union in this example: enums, unions, and discriminated unions.

Union Terminology Examples

Understand The Terminology Around Unions

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TypeScript has a utility type that can directly grab specific members of a discriminated union.

Extract From A Union Using a Utility Type

Extracting Members of a Discriminated Union

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TypeScript has a useful utility type that can extract all but a specified part of a union!

Use a Utility Type to Remove a Single Member of a Union

Excluding Parts of a Discriminated Union

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The Power of Union Types in TypeScript

TypeScript gives you a way to a look up a type’s properties individually.

Use Indexed Access Types to Extract Object Properties

Extract Object Properties into Individual Types

TypeScript gives us an annotation that infers an object / array’s values as literals and makes them read-only.

The Annotation Used to Infer an Object's Values as Literal Types

Resolve an Object’s Values as Literal Types

TypeScript gives you a couple of different ways to pass a union to extract multiple members of another union.

Extract Specific Members From A Union with Indexed Access

Create a Union From an Object's Values

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There is an operator in TypeScript that gives us an elegant solution to this problem.

Use Two Operators With Indexed Access to Get All of an Object's Values

Get All of an Object’s Values

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There are ways in TypeScript to make a union from the entire array, or just some specified elements.

Methods Used to Create Unions out of Array Values

Create Unions out of Array Values

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Union Types

Discriminated unions are unions which contain a single key (the discriminator) to tell them apart.

Discriminated Unions

'as const' lets you specify a type as immutable and readonly, meaning its values get inferred as their literal type.

'as const' Operator

Indexed access types let you access a member of a type: an object value, array member, or class method.

Indexed Access Types

Unions and Indexing

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Template Literal with Strings

Only Allow Specified String Patterns

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Extracting String Pattern Matches with Template Literals

Extract Union Strings Matching a Pattern

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Passing Unions Into Template Literals

Create a Union of Strings with All Possible Permutations of Two Unions

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Using S From ts-toolbelt to Split a String Into a Tuple

Splitting A String into a Tuple

Create an object whose keys are derived from a union of template literals with help from a utility type that just receives the union, and the desired type.

Use a Utility Type to Create An Object From A Union

Create an Object Whose Keys Are Derived From a Union

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Manipulate String Literals Using Type Helpers

Transform String Literals To Uppercase

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Template Literals in Mattermost

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Template Literal Types

Template Literals

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Create Functions that Return Types

Introducing Type Helpers

A Maybe type helper can help you safely handle missing values in your code.

Let's create a Maybe type that can handle null and undefined

The Unconstrained Maybe Type Helper

Creating a Maybe Type Helper

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Use Constraints to Limit Type Parameters

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Add Support for Multiple Types in a Type Helper

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Set a Default Type Value in a Type Helper

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Support Function Type Constraints with Variable Arguments

Functions as Constraints for Type Helpers

Leverage an essential TypeScript concept to constrain types to anything but null or undefined.

Exclude null and undefined from the Maybe Type

Constraining Types for Anything but null or undefined

Using TypeScript’s built-in types allows you to add constraints when working with arrays.

Enforce a Minimum Array Length in a Type Helper

Constraining Type Helpers to Non-Empty Arrays

Explainer: Type Helpers in Redux

Type Helpers

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Compare and Return Values with Extends and the Ternary Operator

Add Conditional Logic to a Type Helper

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Prevent Unwanted Type Scenarios from Happening

Refine Conditional Logic in a Type Helper

Daniel Rosenwasser discusses how new features are added to TypeScript, using the example of conditional types inspired by Knockout.js.

How TypeScript Added Conditional Types

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Infer Elements Inside a Conditional with Infer

Introducing infer for Conditional Logic

infer can extract out all the members of the type arguments without needing to dive deep into the element itself to understand its structure.

Use infer with Generics to Extract Types from Arguments

Extract Type Arguments to Another Type Helper

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Pattern Matching on Template Literals with Infer

Extract Parts of a String with a Template Literal

The benefits of using template literal types for expressing string requirements over regexes, as explained by TypeScript's program manager.

Template Literal Types Were Nearly Regexes

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Optionally Infer the Return Type of a Function

Extract the Result of an Async Function

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Two Methods for Extracting the Result of Multiple Possible Functions

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Using Generic Context to Avoid Distributive Conditional Types

Distributivity in Conditional Types

Conditional Types and Infer

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Map Over a Union to Create an Object

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Map Over the Keys of an Object

Mapped Types with Objects

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Remapping Object Keys in a Mapped Type

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Selective Remapping with Conditional Types and Template Literals

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Two Techniques for Mapping a Discriminated Union to an Object

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Create a Union of Tuples by Reindexing a Mapped Type

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Map an Object to a Union of Template Literals

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Mapped Types

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Extract the Discriminator from a Discriminated Union

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