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DI in Typescript

Sangwon Lee

What is DI?

Dependency Injection

In software engineering, dependency injection is a technique in which an object receives other objects that it depends on, called dependencies. Typically, the receiving object is called a client and the passed-in ('injected') object is called a service. The code that passes the service to the client is called the injector. -- wikipedia

#dependencies #client #service #inejcted #injector

What is Dependencies?

  • Nothing depends on each other

w:440

What is Dependencies?

  • AppRunner has dependencies on
    • GithubRepository
    • DockerProvisioner

w:740

What is Dependencies?

  • In code,
// runner.ts
import GithubRepository from 'libs/GithubRepository';
import DockerProvisioner from 'libs/DockerProvisioner';

class AppRunner {
  githubRepository = new GithubRepository(...);
  dockerProvisioner = new DockerProvisioner(...);

  run(appId: string) {
    ...
  }
}

Problem?

"Requirements changed.."

  • App(Source) Repository

    • Github --> Gitlab

  • Provision Target

    • Docker --> Kubernetes

Problem solved

// runner.ts
import GitlabRepository from 'libs/GitlabRepository';
import KubeProvisioner from 'libs/KubeProvisioner';

class AppRunner {
  gitlabRepository = new GitlabRepository(...);
  kubeProvisioner = new KubeProvisioner(..);

  run(appId: string) {
    ...
  }
}

So far so good.

Problem?

"Need to distribute AppRunner package to 3rd-parties"

  • with encapsulating its implementation
  • so that 3rd-parties cannot modify AppRunner core logic
  • but allowing them to extend repository and provisioner

Yes, Dependency Injection

w:740

Declare interface

// runner.ts
export interface Repository {
  pull(repo: string, rev: string): void;
}

export interface Provisioner {
  provision(key: string, options: ProvisionOptions): void;
}

Decoupling dependencies by interface

// runner.ts
export class AppRunner {
  constructor(
    private repository: Repository,
    private provisioner: Provisioner
  )

  run(appId: string) {
    ...
  }
}

Define service(injected) by implementing interfaces

// third.ts
import { Repository, Provisioner }  from 'runner';

export class GitlabRepository implements Repository {
  pull(repo: string, rev: string): void {
    ...
  }
}

export class KubeProvisioner implements Provisioner {
  provision(key: string, options: ProvisionOptions): void {
    ...
  }
}

Inject dependencies to client

// main.ts
import { GitlabRepository, KubeProvisioner } from 'third';

// const runner = new AppRunner(
//   new GithubRepository(...),
//   new DockerProvisioner(...)
// );
const runner = new AppRunner(
  new GitlabRepository(...),
  new KubeProvisioner(...)
);

runner.run('sangwonl/my-app');

DIP(SOLID) vs DI

SOLID - Dependency Inversion Principle

  • High-level modules should not import anything from low-level's
  • Abstractions should not depend on details

So, DI is a technique or practice of DIP

Other definition of DI

One of the best definition by James Shore:

"Dependency Injection" is a 25-dollar term for a 5-cent concept. Dependency injection means giving an object its instance variables.

class TeslaModel3 {
  constructor(color: string) { ... }

  setBatteryCapacity(cap: number): void { ... }
}

DI(IoC) Container

w:900

Easier Testing (Mock)

describe('AppRunner', () => {
  class MockRepository implements Repository { ... }
  class MockProvisioner implements Provisioner { ... }

  it('runs app with mock', () => {
    const runner = new AppRunner(
      new MockRepository(),
      new MockProvisioner()
    )
    runner.run('test/app');
  });
});

Easier Testing (Mock)

With mock helper such like ts-mockito

const mockedRepository: Repository = mock<Repository>();
const mockRepo:Foo = instance(mockedRepository);

const mockedProvisioner: Provisioner = mock<Provisioner>();
const mockProv:Foo = instance(mockedProvisioner);

const runner = new AppRunner(mockRepo, mockProv);
runner.run('test/app');

verify(mockedRepo.pull('test/app', 'HEAD')).called();
verify(mockedProv.provision(anything(), anything())).called();

DI Tools for Typescript

  • inversify/InversifyJS

    • Reflection in runtime
    • Using decorator, relatively verbose

  • wessberg/DI

    • Reflection in compile(transform)-time
    • Less injection codes, it just works

Examples - Inversify

Installation

$ npm install inversify reflect-metadata --save

Decorator related options tsconfig.json

// Important! InversifyJS requires
// TypeScript >= 4.4 and the experimentalDecorators, emitDecoratorMetadata, types and lib in your typescript compilation options.
{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "moduleResolution": "node",
    "lib": ["es6"],
    "types": ["reflect-metadata"],
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

Examples - Inversify

Define symbols

// types.ts
const TYPES = {
  Repository: Symbol.for('Repository'),
  Provisioner: Symbol.for('Provisioner'),
};

export { TYPES };

Examples - Inversify

Define concrete classes

// third.ts
import { injectable } from 'inversify';
import { Repository, Provisioner }  from 'runner';

@injectable()
export class GitlabRepository implements Repository {
  pull(repo: string, rev: string): void { ... }
}

@injectable()
export class KubeProvisioner implements Provisioner {
  provision(key: string, options: ProvisionOptions): void { ... }
}

Examples - Inversify

Declare dependencies (to be injected) in the constructor

// runner.ts
import { injectable, inject } from 'inversify';
import { TYPES } from 'types'

@injectable()
export class AppRunner {
  constructor(
    @inject(TYPES.Repository) private repository: Repository,
    @inject(TYPES.Provisioner) private provisioner: Provisioner
  )
}

Examples - Inversify

Registration to DI container

// di.ts
import 'reflect-metadata';
import { Container } from 'inversify';
import { TYPES } from 'types';
import { Repository, Provisioner, AppRunner } from 'runner';
import { GithubRepository, KubeProvisioner } from 'third';

const container = new Container();
container.bind<Repository>(TYPES.Repository).to(GithubRepository).inSingletonScope();
container.bind<Provisioner>(TYPES.Provisioner).to(KubeProvisioner).inSingletonScope();
container.bind<AppRunner>(AppRunner).toSelf().inSingletonScope();

export default container

Examples - Inversify

Get dependencies injected instance

// main.ts
import container from 'di';
import { AppRunner } from 'runner';

const runner = container.get(AppRunner);
runner.run('sangwonl/my-app');

Examples - wessberg/DI

Installation

$ npm install @wessberg/di

Declare dependencies (to be injected) in the constructor

// runner.ts
export class AppRunner {
  constructor(
    private repository: Repository,
    private provisioner: Provisioner
  )
}

Examples - wessberg/DI

Registration to DI container

// di.ts
import { DIContainer } from '@wessberg/di';
import { Repository, Provisioner, AppRunner } from 'runner';
import { GithubRepository, KubeProvisioner } from 'third';

const container = new DIContainer();
container.registerSingleton<Repository, GithubRepository>();
container.registerSingleton<Provisioner, KubeProvisioner>();
container.registerSingleton<AppRunner>();

export default container

Examples - wessberg/DI

Get dependencies injected instance

// main.ts
import container from 'di';
import { AppRunner } from 'runner';

const runner = container.get<AppRunner>();
runner.run('sangwonl/my-app');

References