Mobx 6 + TypeScript

classes.ts

export class PlayerStore {
  song?: Song;
  isPlaying = false;

  constructor() {
    makeAutoObservable(this);
  }

  playSong(song: Song) {
    this.song = song;
    this.isPlaying = true;
  }
  
  pause() {
    this.isPlaying = false;
  }
}

functions-manual-typing.ts

export type PlayerStore = {
  song?: Song;
  isPlaying: boolean;
  playSong: (song: Song) => void;
  pause: () => void;
}

export const createPlayerStore = (): PlayerStore => {
  return makeAutoObservable({
    song: undefined,
    isPlaying: false,
    playSong(song: Song) {
      this.song = song;
      this.isPlaying = true
    },
    pause() {
      this.isPlaying = false;
    }
  })
}

functions-typescript-type-inference.ts

const value = <T extends any>(value: T): T => value;

export const createPlayerStore = () => {
  return makeAutoObservable({
    song: value<Song | undefined>(undefined),
    isPlaying: false,
    playSong(song: Song) {
      this.song = song;
      this.isPlaying = true
    },
    pause() {
      this.isPlaying = false;
    }
  })
}
  
export type PlayerStore = ReturnType<typeof createPlayerStore>;