Cannot Inject Dependency Into Derived Class

Question

I have a relatively simple setup with three classes. I am using inversify for dependency injection. But when injecting the class MessageBroker into the derived class Repository the MessageBroker is undefined:

import 'reflect-metadata';
import { injectable, inject, Container, unmanaged } from 'inversify';

const container = new Container();

const registerProviders = (...providers: any[]) =>
  providers.forEach(provider => container.bind(provider.name).to(provider));

const getProvider = (provider): any => container.get(provider.name);

@injectable()
export class MessageBroker {
  start = () => console.log('init message broker');
}

@injectable()
export abstract class Repository {
  @inject(MessageBroker.name) private mb: MessageBroker;

  constructor(@unmanaged() protected readonly user: any) {}

  // this.mb is undefined
  initialize = () => this.mb.start();
}

@injectable()
export class UserRepository extends Repository {
  constructor() {
    super({ user: 'some object' });
    this.initialize();
  }
}

registerProviders(UserRepository, Repository, MessageBroker);

const repo: UserRepository = getProvider(UserRepository);

You can try it yourself. I've created a small GitHub repository: https://github.com/flolude/stackoverflow-inversify-injected-service-undefined

When running the script, I get this error:

/project/index.ts:22
  initialize = () => this.mb.start();
                             ^
TypeError: Cannot read property 'start' of undefined
    at UserRepository.Repository.initialize (/project/index.ts:22:30)
    at new UserRepository (/project/index.ts:29:10)
    at _createInstance (/project/node_modules/inversify/lib/resolution/instantiation.js:21:12)
    at Object.resolveInstance (/project/node_modules/inversify/lib/resolution/instantiation.js:41:18)
    at /project/node_modules/inversify/lib/resolution/resolver.js:72:42
    at Object.resolve (/project/node_modules/inversify/lib/resolution/resolver.js:96:12)
    at /project/node_modules/inversify/lib/container/container.js:319:37
    at Container._get (/project/node_modules/inversify/lib/container/container.js:310:44)
    at Container.get (/project/node_modules/inversify/lib/container/container.js:230:21)
    at getProvider (/project/index.ts:9:50)

PS I get pretty much the same error when compiling the code to Javascript

Answer 1

Your MessageBroker has only been set in memory but has never been instantiated, which is how it is getting the undefined error. In your constructor you will need to set

this.mb = new MessageBroker();

Another way you can do this without the above line is to add a empty parameter signatured constructor into the MessageBroker class.