Challenge 1: Building a typed data store

The Challenge

Let’s get some hands on experience working with template literal types by building a data store, whose method names are based on the type of entities we’re managing.

If you look at the starting point code for this exercise, you’ll see the following

ts
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 {}Try

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

Ultimately, DataStore should end up with methods like

ts
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.

diff
+export interface Comic extends DataEntity {
+  issueNumber: number
+}
export type DataEntityMap = {
  movie: Movie
  song: Song
+  comic: Comic
}

• 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
• All pre-existing tests should pass

Setup

First, if you haven’t done so already, clone the workshop project for this course

sh
git clone https://github.com/mike-north/making-typescript-stick
cd making-typescript-stick

Make sure you have Volta installed. If you haven’t done so already, just run the following to install it

sh
curl https://get.volta.sh | bash

Next, let’s install our dependencies

sh
yarn

and finally, let’s navigate to the folder containing this specific challenge

sh
cd challenges/data-layer

You can now run the test suite by running yarn test

Your job is to modify the code in ./src/index.ts until all of the requirements of this exercise are met

Hints

• Brush up on indexed access types
• Brush up on mapped types, and in particular the new “key remapping” feature that landed in TS 4.1
• Brush up on template literal types