러닝 타입스크립트 CHAPTER 9 - 타입 제한자

function greetComedian(name: any) {
  // 타입 오류 없음
  console.log(`Announcing ${name.toUpperCase()}!`)
}

greetComedian({ name: 'Bea Arthur' })
// Runtime error: name.toUpperCase is not a function

function greetComedian(name: unknown) {
  console.log(`Announcing ${name.toUpperCase()}!`)
  // Error: Object is of type 'unknown'.
}

function greetComedianSafety(name: unknown) {
  if (typeof name === 'string') {
    console.log(`Announcing ${name.toUpperCase()}!`) // OK
  } else {
    console.log("Well, I'm off.")
  }
}

function isNumberOrString(value: unknown) {
  return ['number', 'string'].includes(typeof value)
}

function logValueIfExists(value: number | string | null | undefined) {
  if (isNumberOrString(value)) {
    value.toString()
  } else {
    console.log('Value does not exist:', value)
  }
}

function typePredicate(input: WideType): input is NarrowType

function isNumberOrString(value: unknown): value is number | string {
  return ['number', 'string'].includes(typeof value)
}

function logValueIfExists(value: number | string | null | undefined) {
  if (isNumberOrString(value)) {
    // value: number | string의 타입
    value.toString()
  } else {
    // value: null | undefined의 타입
    console.log('Value does not exist:', value)
  }
}

interface Comedian {
  funny: boolean
}

interface StandupComedian extends Comedian {
  routine: string
}

function isStandupComedian(value: Comedian): value is StandupComedian {
  return 'routine' in value
}

function workWithComedian(value: Comedian) {
  if (isStandupComedian(value)) {
    // value: StandupComedian
    console.log(value.routine) // OK
  }

  console.log(value.routine)
  // Error: Property 'routine' does not exist on type 'Comedian'.
}

interface Ratings {
  audience: number
  critics: number
}

function getRating(ratings: Ratings, key: string): number {
  return ratings[key]
  // Error: Element implicitly has an 'any' type because expression
  // of type 'string' can't be used to index type 'Ratings'.
  // No index signature with a parameter of type 'string' was found on type 'Ratings'.
}

const ratings: Ratings = { audience: 66, critics: 84 }

getRating(ratings, 'audience') // OK
getRating(ratings, 'not valid') // 허용되지만 사용하면 안 됨

interface Ratings {
  audience: number
  critics: number
}

function getRating(ratings: Ratings, key: 'audience' | 'critics'): number {
  return ratings[key]
}

const ratings: Ratings = { audience: 66, critics: 84 }

getRating(ratings, 'audience') // OK
getRating(ratings, 'not valid')
// Error: Argument of type '"not valid"' is not
// assignable to parameter of type '"audience" | "critics"'.

interface Ratings {
  audience: number
  critics: number
}

function getRating(ratings: Ratings, key: keyof Ratings): number {
  return ratings[key]
}

const ratings: Ratings = { audience: 66, critics: 84 }

getRating(ratings, 'audience') // OK
getRating(ratings, 'not valid')
// Error: Argument of type '"not valid"' is not
// assignable to parameter of type '"audience" | "critics"'.

const ratings = {
  imdb: 8.4,
  metacritic: 82,
}

function logRating(key: keyof typeof ratings) {
  console.log(ratings[key])
}

logRating('imdb') // OK
logRating('invalid')
// Error: Argument of type '"invalid"' is not assignable to parameter of type '"imdb" | "metacritic"'.

const rawData = '["grace", "frankie"]'

// 타입: any
JSON.parse(rawData)

// 타입: string[]
JSON.parse(rawData) as string[]

// 타입: [string, string]
JSON.parse(rawData) as [string, string]

// 타입: ["grace", "frankie"]
JSON.parse(rawData) as ['grace', 'frankie']

try {
  // 오류 발생 코드
} catch (error) {
  console.warn('Oh no!', (error as Error).message)
}

try {
  // 오류 발생 코드
} catch (error) {
  console.warn('Oh no!', error instanceof Error ? erorr.message : error)
}

// 타입 유추: Date | undefined
let maybeDate = Math.random() > 0.5 ? undefined : new Date()

// 타입이 Date라고 간주됨
maybeDate as Date
maybeDate!

interface Entertainer {
  acts: string[]
  name: string
}

const declared: Entertainer = {
  // Error: Property 'acts' is missing in type '{ name: string; }' but required in type 'Entertainer'.
  name: 'Moms Mabley',
}

const asserted = {
  name: 'Moms Mabley',
} as Entertainer // 허용되지만 런타임 시 오류 발생

let myValue = 'Stella!' as number

let myValueDouble = '1337' as unknown as number // 허용되지만 이렇게 사용하면 안 됨

// 타입: (number | string)[]
;[0, ''][
  // 타입: readonly [0, '']
  (0, '')
] as const

// 타입: () => string
const getName = () => 'Maria Bamford'

// 타입: () => 'Maria Bamford'
const getNameConst = () => 'Maria Bamford' as const

function describePreference(preference: 'maybe' | 'no' | 'yes') {
  switch (preference) {
    case 'maybe':
      return 'I suppose...'
    case 'no':
      return 'No thanks.'
    case 'yes':
      return 'Yes please!'
  }
}

// 타입 { movie: string, standup: string }
const preferencesMutable = {
  movie: 'maybe',
  standup: 'yes',
}

describePreference(preferencesMutable.movie)
// Error: Argument of type 'string' is not assignable to parameter of type '"maybe" | "no" | "yes"'.

preferencesMutable.movie = 'no' // OK

// 타입 { readonly movie: 'maybe', readonly standup: 'yes' }
const preferencesReadonly = {
  movie: 'maybe',
  standup: 'yes',
} as const

describePreference(preferencesReadonly.movie) // OK

preferencesReadonly.movie = 'no'
// Error: Cannot assign to 'movie' because it is a read-only property.