Typescript: Conditional Types

interface Animal {
  live(): void
}
interface Dog extends Animal {
  woof(): void
}

type Example1 = Dog extends Animal ? number : string // type Example1 = number

type Example2 = RegExp extends Animal ? number : string // type Example2 = string

// 예시 1
type MessageOf<T> = T extends { message: unknown } ? T["message"] : never

interface Email {
  message: string
}

interface Dog {
  bark(): void
}

type EmailMessageContents = MessageOf<Email> // string

type DogMessageContents = MessageOf<Dog> // never

// 예시 2
type Flatten<T> = T extends any[] ? T[number] : T

// Extracts out the element type.
type Str = Flatten<string[]> // type Str = string

// Leaves the type alone.
type Num = Flatten<number> // type Num = number

// 예시1
type Flatten<Type> = Type extends Array<infer Item> ? Item : Type

//예시2
type GetReturnType<Type> = Type extends (...args: never[]) => infer Return
  ? Return
  : never

type Num = GetReturnType<() => number> // number

type Str = GetReturnType<(x: string) => string> // string

type Bools = GetReturnType<(a: boolean, b: boolean) => boolean[]> // boolean[]

type ToArray<Type> = Type extends any ? Type[] : never

type StrArrOrNumArr = ToArray<string | number> // string[] | number[]