[Typescript] 98. Medium – Append to object

Implement a type that adds a new field to the interface. The type takes the three arguments. The output should be an object with the new field.

For example

type Test = { id: '1' }
type Result = AppendToObject<Test, 'value', 4> // expected to be { id: '1', value: 4 }

/* _____________ Your Code Here _____________ */
type Merge<T extends Record<PropertyKey, any>> = {
  [Key in keyof T]: T[Key]
} 
type AppendToObject<T extends Record<PropertyKey, any>, U extends string | number | symbol, V extends any> = Merge<T & {
  [Key in U]: V  
}>

/* _____________ Test Cases _____________ */
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>>,
]

原文地址：http://www.cnblogs.com/Answer1215/p/16884174.html