conditional-types.ts

type AppleLiteral = 'Apple'

let appleName: AppleLiteral = 'Apple'

type LiteralIsStringType<Type> = Type extends string
  ? string
  : Type extends number
  ? number
  : Type extends boolean
  ? boolean
  : never

type AppleLiteralType = LiteralIsStringType<AppleLiteral> // string
type NonNullableApple = NonNullable<AppleLiteral> // 'Apple'

type ExcludeFromUnion<Type, Union> = Type extends Union ? never : Type
type FavoriteLetters = 'h' | 'a' | 'o'
type Vowels = 'a' | 'e' | 'i' | 'o' | 'u'
type NonFavoriteVowels = ExcludeFromUnion<Vowels, FavoriteLetters>

type ExtractFromUnion<Type, Union> = Type extends Union ? Type : never
type FavoriteVowels = ExtractFromUnion<Vowels, FavoriteLetters>

addClassNameToElement(document.getElementsByTagName('div')[0], 'fruit-list-div')

function addClassNameToElement(element: unknown, className: string) {
  // element.classList.add(className)
}

// for any functions, if the function corresponds to this type annotation,
// in which the type of the return value can be inferred automatically by TypeScript,
// return the inferred type of the return value, otherwise just return any
type ReturnFunctionType<Type extends (...args: any) => any> = Type extends (
  ...args: any
) => infer RType
  ? RType
  : any

// for any functions, if the function corresponds to this type annotation,
// in which the type of the second parameter can be inferred automatically by TypeScript,
// return the inferred type of the second parameter, otherwise just return any
type SecondParameterType<Type extends (...args: any) => any> = Type extends (
  arg1: any,
  arg2: infer SecondParam,
) => any
  ? SecondParam
  : any

type ClassNameSecondParam = SecondParameterType<typeof addClassNameToElement>

// for any types, if the type corresponding the annotation,
// which is an array and in which the type of the element can be inferred
// then return the inferred type of the element, otherwise return the original type
type UnwrapArray<Type> = Type extends (infer Element)[] ? Element : Type

// for any types, if the type corresponding the annotation,
// which is a promise and whose the type of the resolved value can be inferred
// then return the inferred type of the resolved value, otherwise return the original type
type UnwrapPromise<Type> = Type extends Promise<infer ResolvedValue>
  ? ResolvedValue
  : Type