对象 key 值扩展

import type { Equal, Expect } from '@type-challenges/utils'

type test1 = {
  key: 'cat'
  value: 'green'
}

type testExpect1 = {
  key: 'cat'
  value: 'green'
  home: boolean
}

type test2 = {
  key: 'dog' | undefined
  value: 'white'
  sun: true
}

type testExpect2 = {
  key: 'dog' | undefined
  value: 'white'
  sun: true
  home: 1
}

type test3 = {
  key: 'cow'
  value: 'yellow'
  sun: false
}

type testExpect3 = {
  key: 'cow'
  value: 'yellow'
  sun: false
  isMotherRussia: false | undefined
}

type cases = [
  Expect<Equal<AppendToObject<test1, 'home', boolean>, testExpect1>>,
  Expect<Equal<AppendToObject<test2, 'home', 1>, testExpect2>>,
  Expect<Equal<AppendToObject<test3, 'isMotherRussia', false | undefined>, testExpect3>>,
]

type AppendToObject<T extends object, U extends string, V> = {
  [k in keyof T | U]: k extends keyof T ? T[k] : V
}

获取一个对象字面量类型的 value 值

const RestfulMethod = {
  get: 'GET',
  post: 'POST',
  put: 'PUT',
  delete: 'DELETE'
} as const


type IRestfulMethod = typeof RestfulMethod
type TypeRestfulMethod = IRestfulMethod[keyof IRestfulMethod] // GET | POST | PUT | DELETE