[TypeScript] Model a DataStore type

Given this code as starter:

export interface DataEntity {
  id: string
}
export interface Movie extends DataEntity {
  director: string
}
export interface Song extends DataEntity {
  singer: string
}
 
export type DataEntityMap = {
  movie: Movie
  song: Song
}
 
export class DataStore {}

This DataEntityMap object should drive a lot of what happens to DataStore.

Ultimately, DataStore should end up with methods like

const ds = new DataStore()
ds.addSong({ id: "song-123", singer: "The Flaming Lips" })
ds.addMovie({
  id: "movie-456",
  director: "Stephen Spielberg",
})
ds.getSong("song-123") // returns the song
ds.getMovie("movie-456") // returns the movie
ds.getAllSongs() // array of all songs
ds.getAllMovies() // array of all movies
ds.clearSongs() // clears all songs
ds.clearMovies() // clears all movies

It’s ok to define these explicitly in the DataStore class, but they should be type-checked against the DataEntityMap type in some way.

Requirements

• If you mis-name a method on the class (e.g., getSongs instead of getAllSongs), you should get some sort of type error that alerts you that you’ve broken the established pattern
• If you add a new entity like Comic (shown below) and make no other changes to your solution, you should get some sort of type error that alerts you to the absence of a clearComics, getAllComics and getAllSongs method.

• There should be no externally-visible properties on an instance of DataStore beyond the required methods
• Your code, and the test suite should type-check

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Answer

export interface DataEntity {
  id: string
}
export interface Movie extends DataEntity {
  director: string
}
export interface Song extends DataEntity {
  singer: string
}

export interface Comic extends DataEntity {
  issueNumber: number
}

export type DataEntityMap = {
  movie: Movie
  song: Song
}

/**
 * if you change DataEntityMap from type to interface
 * Then you can have the DataEntityMap defined in multi files and TS will combine those automaticlly
// model/song.model.ts
export interface DataEntityMap {
  song: Song;
}  
// model/movie.model.ts
export interface DataEntityMap {
  movie: Movie;
}
 */

export type DataStoreMethods = {
  [K in keyof DataEntityMap as `getAll${Capitalize<K>}s`]: () => DataEntityMap[K][]
} & {
  [K in keyof DataEntityMap as `get${Capitalize<K>}`]: (id: string) => DataEntityMap[K]
} & {
  [K in keyof DataEntityMap as `clear${Capitalize<K>}s`]: () => void
}
& {
  [K in keyof DataEntityMap as `add${Capitalize<K>}`]: (item: DataEntityMap[K]) => DataEntityMap[K]
}

function isDefined<T>(x: T | undefined): x is T {
  return typeof x !== "undefined";
}

export class DataStore implements DataStoreMethods {
  #data: {[K in keyof DataEntityMap]: Record<string, DataEntityMap[K]>} = {
    movie: {},
    song: {}
  }

  getSong(id: string): Song {
    const song =  this.#data.song[id];
    if (!song) {
      throw new Error(`Could not find a song with id ${id}`);
    }
    return song;
  }
  getAllSongs(): Song[] {
    return Object.keys(this.#data.song).map(id => this.#data.song[id]).filter(isDefined)
  }
  clearSongs(): void {
    this.#data.song = {}
  }

  addSong(song: Song) {
    this.#data.song[song.id] = song;
    return song;
  }

  getMovie(id: string): Movie {
    const movie =  this.#data.movie[id];
    if (!movie) {
      throw new Error(`Could not find a movie with id ${id}`);
    }
    return movie;
  }
  getAllMovies(): Movie[] {
    return Object.keys(this.#data.movie).map(id => this.#data.movie[id]).filter(isDefined)
  }
  clearMovies(): void {
    this.#data.movie = {}
  }
  addMoive(movie: Movie) {
    this.#data.movie[movie.id] = movie;
    return movie;
  }
}