TypeScript tricks

typed-merge.ts

type OptionalOf<T> = { [K in keyof T]?: T[K] }

function typedMerge<T extends object>(target: T, ...sources: OptionalOf<T>[]): T {
  return Object.assign(target, ...sources)
}

interface IUser {
  name: string;
  age: number;
  hobbie: HOBBIES;
}

enum HOBBIES {
  PLAY_FOOTBOOL = 'PLAY_FOOTBOOL',
  WATCH_TV = 'WATCH_TV'
}

const user: IUser = {
  name: 'John',
  age: 28,
  hobbie: HOBBIES.PLAY_FOOTBOOL
}

typedMerge(user, {
  age: 35,
  hobbie: HOBBIES.WATCH_TV,
})