Generic Mapping in Typescript Example

examples.ts

import { Merge } from 'type-fest'

type MergeGenerics<TDefaultGenerics, TUserGenerics> = Merge<
  Required<TDefaultGenerics>,
  TUserGenerics
>


interface Person {
  name: string
}

interface DefaultGenerics {
  User?: Person
  Friend?: Person
  Frienemy?: Person
}


interface NewUser<TGenerics extends DefaultGenerics> {
  user: TGenerics['User']
  friend: TGenerics['Friend']
  frienemy: TGenerics['Frienemy']
}

interface User<TGenerics extends DefaultGenerics> extends NewUser<TGenerics> {
  created: number
}

function createUser<
  TUserGenerics extends DefaultGenerics = DefaultGenerics,
  TGenerics extends MergeGenerics<
    DefaultGenerics,
    TUserGenerics
  > = MergeGenerics<DefaultGenerics, TUserGenerics>
>(user: NewUser<TGenerics>): User<TGenerics> {
  return { ...user, created: Date.now() }
}

// Usage

interface MyFrienemy extends Person {
  age?: number
}

const user = createUser<{ Frienemy: MyFrienemy }>({
  user: { name: 'tanner' },
  friend: { name: 'Derek' },
  frienemy: { name: 'Matt', age: 21 },
})

console.log(user.frienemy.age) // 21