Inheritance support

Needle DI offers extensive support for inheritance, allowing for abstractions and interfaces.

Auto-binding using @injectable()

NOTE

Auto-binding with @injectable() has some limitations, see the note below.

Given the following class structure:

abstract class ExampleService {
  /* ... */
}

@injectable()
class FooService extends ExampleService {
  /* ... */
}

@injectable()
class BarService extends ExampleService {
  /* ... */
}

This will automatically bind FooService and BarService, but it will also automatically bind two multi-providers for the token ExampleService:

const fooService = container.get(FooService);
const barService = container.get(BarService);

const myServices = container.get(ExampleService, { multi: true });
//    ^? Type will be inferred as `ExampleService[]`
//        and will be the same instances as "fooService" and "barService'

IMPORTANT

If you inject something using a parent class as token, make sure your subclasses are referenced somewhere. Otherwise, the auto-binding might not work since their decorators are not invoked. Even worse, your subclasses might not even appear in your final bundle due to tree-shaking.

To prevent this, consider to register your subclasses explicitly using manual binding:

container.bindAll(FooService, BarService);

Manual binding

If you bind something that has a parent class, a multi-provider for the parent class will be registered automatically.

Given the following example:

abstract class ExampleService {
  /* ... */
}

class FooService extends ExampleService {
  /* ... */
}

class BarService extends ExampleService {
  /* ... */
}

container.bindAll(
  FooService,
  BarService,
);

The container will automatically register the following bindings internally:

container.bindAll(
  {
    provide: ExampleService,
    useExisting: FooService,
    multi: true,
  },
  {
    provide: ExampleService,
    useExisting: BarService,
    multi: true,
  }
);

This enables you to inject all instances of ExampleService using multi-injection:

const fooService = container.get(FooService);
const barService = container.get(BarService);

const myServices = container.get(ExampleService, { multi: true });
//    ^? Type will be inferred as `ExampleService[]`
//        and will be the same instances as "fooService" and "barService'

This even works with multiple levels of inheritance.

What about interfaces?

If you're using TypeScript interfaces instead, you should use injection tokens instead. This is because TypeScript interfaces don't exist at runtime and therefore cannot be used as tokens.

interface Logger {
  info(): void;
}

class FileLogger implements Logger {
  info(): void {
    //
  }
}

class ConsoleLogger implements Logger {
  info(): void {
    //
  }
}

const LOGGER = new InjectionToken<Logger>('LOGGER');

container.bindAll(
  {
    provide: LOGGER,
    multi: true,
    useClass: FileLogger,
  },
  {
    provide: LOGGER,
    multi: true,
    useClass: ConsoleLogger,
  },
);