How DI Works Under the Hood

NestJS reads constructor types from metadata. Our runnable example has no reflect-metadata, so we pass the tokens explicitly with an @Inject() parameter decorator (NestJS also has a real @Inject() for exactly this case). The container creates each class once and reuses it.

Example

A mini NestJS-style container with legacy decorators:

type Ctor<T = unknown> = new (...args: any[]) => T;

const injectable = new Set<Ctor>();
const paramTokens = new Map<Ctor, Ctor[]>();

function Injectable(): ClassDecorator {
  return (target) => { injectable.add(target as unknown as Ctor); };
}
function Inject(token: Ctor): ParameterDecorator {
  return (target, _key, index) => {
    const list = paramTokens.get(target as unknown as Ctor) ?? [];
    list[index] = token;
    paramTokens.set(target as unknown as Ctor, list);
  };
}

class Container {
  private instances = new Map<Ctor, unknown>();
  get<T>(cls: Ctor<T>): T {
    if (this.instances.has(cls)) return this.instances.get(cls) as T;
    if (!injectable.has(cls)) throw new Error(cls.name + ' is not @Injectable');
    const deps = (paramTokens.get(cls) ?? []).map((t) => this.get(t));
    console.log('creating', cls.name, 'with', deps.map((d) => (d as object).constructor.name));
    const instance = new cls(...deps);
    this.instances.set(cls, instance);
    return instance;
  }
}

@Injectable()
class PricingService {
  withGst(p: number) { return Math.round(p * 1.18); }
}

@Injectable()
class ProductsRepository {
  findPrice(id: number) { return id === 7 ? 1299 : 999; }
}

@Injectable()
class ProductsService {
  constructor(
    @Inject(ProductsRepository) private readonly repo: ProductsRepository,
    @Inject(PricingService) private readonly pricing: PricingService,
  ) {}
  finalPrice(id: number) { return this.pricing.withGst(this.repo.findPrice(id)); }
}

const app = new Container();
const service = app.get(ProductsService);
console.log('final price of 7:', service.finalPrice(7));
console.log('same instance again?', app.get(ProductsService) === service);

Output

creating ProductsRepository with []
creating PricingService with []
creating ProductsService with [ 'ProductsRepository', 'PricingService' ]
final price of 7: 1533
same instance again? true

Build a Tiny DI Container

Example

What to Notice