Dependency Injection library for TypeScript/JavaScript

Introduction

Dependency Injection is a code organization technique which, when used correctly, increases maintainability.

The intent behind dependency injection is to achieve separation of concerns of construction and use of objects. This can increase readability and code reuse.

Read more from Wikipedia.

Get started

Install the package

npm i loopus-injection

Register – Configure your services

import { service } from "loopus-injection";

class MyDependencyA {
  ...
}

class MyDependencyB {
  ...
}

class MyService {
  constructor(
    readonly depA: MyDependencyA,
    readonly depB: MyDependencyB) {}
  ...
}

// Configure injection
service(s => {
  s.scoped(); // 1 instance per scope
  return MyDependencyA;
});
service(s => {
  s.singleton(); // 1 shared instance
  return MyDependencyB;
});
service(s => {
  s.transient(); // many instances
  s.dependency(MyDependencyA);
  s.dependency(MyDependencyB);
  return MyService;
});

Begin a scope

• Create a root scope with default registrations (ie registrations from your class decorators) – root scope will keep track of all your services

  const rootScope = new Scope();

• Create a child scope (ex: let's say you develop an API – you would begin a child scope for each HTTP request)

  const childScope = rootScope.beginScope();

Resolve – Get instances of your services

const myScopedService = childScope.resolve(MyScopedService);

const mySingletonService = childScope.resolve(MySingletonService);

const myTransientService = childScope.resolve(MyTransientService);

Release – Cleanup your scopes

• Let's say you develop an API – you would begin a child scope for each HTTP request. In that case, you would release the child scope when the HTTP request has been handled in order to cleanup all scoped services:

  childScope.release();

Advanced concepts

Custom registrations

For scenarios where you want to control how you service is constructed, use Registrations class.

Registrations.defaults.set({
  constructor: ..., // The constructor function to use as identifier
  factory: (scope: Scope) => ..., // A callback that will be used to create instances of the service
  lifetime: Lifetime.Transient // Or Scoped, Singleton
});

Composition root

In dependency injection world, composition root is a central place in your app where you put all your injection configuration code.

When using this library, you could decide to put all the configuration code in a central place, or configure each service right next to the service itself. That's up to you.

Factory Service

Let's say you want to send emails on your production environment, but when developing on your local machine you just want to log a message to the console.

The trick is to inject Scope class in order to resolve the proper service at runtime based on some configuration.

// Create an abstract class to declare a contract for our email service
abstract class EmailService {
  abstract sendEmail(to: string, body: string);
}

// We want this one to be injected in DEV
class DevEmailService extends EmailService {
  sendEmail(to: string, body: string) {
    console.log("Not sending email:", to, body);
  }
}

// We want this one to be injected in PROD
class ProdEmailService extends EmailService {
  sendEmail(to: string, body: string) {
    // TODO: Send the email for real
  }
}

// This factory service will be responsible for selecting the actual email service implementation based on some environment variable (NODE_ENV)
class EmailServiceSelector() {

  // Scope is a special service which allows you to resolve other services
  constructor(private scope: Scope) {

  }

  // Note the return type: our abstract class
  select(): EmailService {
    if (process.env.NODE_ENV === "production") {
      return this.scope.resolve(ProdEmailService);
    } else {
      return this.scope.resolve(DevEmailService);
    }
  }
}

// Configure injection
service(s => {
  s.singleton();
  return DevEmailService;
});

service(s => {
  s.scoped(); // Notice how lifetimes can be different for each implementation
  return ProdEmailService;
});

service(s => {
  s.dependency(Scope);
  s.transient(); // Important: the service factory should not have a 'higher' scope than the actual services. Here it could be Transient or Scoped, but never Singleton.
  return EmailServiceSelector;
});

// Use the service factory
const scope = new Scope();
const selector = scope.resolve(EmailServiceSelector);
const emailService = selector.select();
emailService.sendEmail("[email protected]", "hello!");

Use with Node

Should work out of the box.

Use with React

Should work out of the box.