Refer: Medium - Append to object

Describe

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

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

Test Cases

code snippetCopytypescript
import { 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>>,
]

Solution

code snippetCopytypescript
type AppendToObject<T extends Record<string, any>, U extends string, V> = {
  [P in keyof T | U]: P extends keyof T ? T[P] : V
}