Type-safe Providing

In Angular, object literals are used to declare injectables of an injection context:

providers: [
  {
    provide: TitleStrategy,
    useClass: SuffixedTitleStrategy,
  },
  {
    provide: DIALOG_GLOBAL_CONFIG,
    useValue: { closable: false } satisfies DialogGlobalConfig,
  },
  {
    provide: APP_INITIALIZER,
    multi: true,
    useFactory: () => () => console.log('Initialized'),
  },
];

However, although the ProviderToken already carries type information, the object literals cannot perform type checks to ensure type-safety, requiring the developer to manually make sure that the provided value is valid.

In the snippet above, the developer:

1. Must make sure SuffixedTitleStrategy correctly implements TitleStrategy.
2. Must use the satisfies keyword to manually restrict the type of the provider of DIALOG_GLOBAL_CONFIG.
3. Cannot appropriately type the APP_INITIALIZER since there is no such AppInitializer type exported.

As a solution, Angularity offers provide and provideMulti as helper functions for type-safe provider declarations:

providers: [
  provide({
    token: TitleStrategy,
    useClass: SuffixedTitleStrategy,
  }),
  provide({
    token: DIALOG_GLOBAL_CONFIG,
    useValue: { closable: false },
  }),
  provideMulti({
    token: APP_INITIALIZER,
    useFactory: () => () => console.log('Initialized'),
  }),
];

In the example above, all the providers' types are automatically inferred and restricted from the token. By elevating the multi option to the function name, it also helps prevent the developer from forgetting the multi option.