Holoscope NPM | npm.io

Dependency injection tool for Typescript projects.

Inspired by Awilix and AWS CDK Constructs.

Installation

With npm:

npm install holoscope

Getting Started

If you're familiar with Awilix or similar IoC libraries, you should check out this section

Holoscope has two key concepts: Scopes and Resolvers. You wrap your dependencies (service classes, configuration objects, etc) in an appropriate resolver and pass them to a Scope, which manages passing peer dependencies to each resolver. Take this simple example:

import { asClass, Scope } from 'holoscope'

class PostService {
  constructor(private container: { database: Database }) {}
  async getPosts() {
    return this.container.database.posts.getAll()
  }
}

const scope = new Scope({
  database: asClass(Database, { cached: true }),
  postService: asClass(PostService, { cached: true }),
})

const posts = await scope.container.postService.getPosts()

Scope.container is a proxy that handles dependency resolution at access-time. It is passed to each resolver's resolve method. asFunction and asClass resolvers pass the container to the provided factory function or class respectively.

In the example above, the database is instantiated inside the getPosts method of PostService. After that, it gets cached, and reused in any subsequent access through the container.

Note: values that are not a Resolver are wrapped in asValue() at registration. This allows to register plain values without wrapping them.
If you want to create your own resolver with custom logic, look here.

Building your scopes

The Scope class should be extended to build your scopes:

type GeneralContainer = {
  database: Database
  postService: PostService
}

class GeneralScope extends Scope<GeneralContainer> {
  constructor() {
    super({
      database: asClass(Database, { cached: true }),
      postService: asClass(PostService, { cached: true }),
    })
  }
}

const scope = new GeneralScope()

Extending scopes

Notice that the values in the container type provided to scope's generic parameter (GeneralContainer above) don't have to be specific implementations, but interfaces instead. This allows to extend and swap out the dependencies:

class DevelopmentScope extends GeneralScope {
  constructor() {
    super()
    this.register({
      // DevelopmentDatabase satisfies Database
      database: asClass(DevelopmentDatabase, { cached: true }),
    })
  }
}

Expanding scopes

To expand a scope (extend and add dependencies, not just swap them out), consider using ExtendedInjection generic type as a constructor input in the following pattern:

interface BaseContainer {
  name: string
}

class BaseScope<TExtended extends ExampleContainer> extends Scope<TExtended> {
  constructor(extended: ExtendedInjection<ExampleContainer, TExtended>) {
    const registrations: Injection<ExampleContainer> = {
    }
    super({
      name: 'example',
      ...extended,
    } as Injection<TExtended>)
  }
}

class ExtendedScope extends BaseScope<ExtendedContainer> {
  constructor() {
    super({
      greeting: asFunction(({ name }) => `Hello, ${name}!`),
    })
  }
}

new ExtendedScope().container.greeting // 'Hello, World!'

Disposing scopes and their resolvers

For asFunction and asClass resolvers (as well as possibly for custom resolvers), there is an option to provide a disposer function.

Scope.dispose() is an async method, which internally awaits every resolver's disposer. This is useful to close database connections, write logs, etc. at the end of a process.

For asFunction and asClass resolvers with cached: true, disposers are only called if the dependency was resolved before.

Comparison with Awilix

Different terminology for similar concepts/types: awilix.Container -> holoscope.Scope; awilix.Container.cradle -> holoscope.Scope.container.
All registration must be provided at Scope init, ensuring type-safety.
Factory resolvers (asFunction, asClass) are not bound to the scope. Cache is handled inside the resolvers themselves.
No auto-loading modules.
Instead of "child" containers, scopes can be extended, overwriting and adding new dependencies in a flat internal structure.

Built-in resolvers and helper functions

Holoscope includes the following general-purpose resolvers:

asValue - used internally to wrap non-resolver values. Can be used explicitly.
asFunction, asClass - factory resolvers. Optionally handle cache and disposing. Allow injecting per-dependency peers with inject option.
aliasTo - returns a dependency of a passed name from the container. Beware recursive loops - do not pass the name this resolver is registered itself.
asResolvers - pass an object of resolvers. Used to nest dependencies.

There are also helper functions asCachedFunction and asCachedClass, that are simple shorthands for as___(factory, { cached: true })

Custom resolvers

You can create your own resolvers with custom logic, e.g.:

import { Container, IS_RESOLVER, Resolver } from 'holoscope'
class CustomResolver<T = unknown> implements Resolver<T> {
  readonly [IS_RESOLVER] = true
  resolve(container: Container): T {
    // custom logic, that returns T
  }
}

Resolvers have to:

have a resolve method that accepts a container and returns the value
have IS_RESOLVER property set to true. Resolver doesn't necessarily have to be a class instance.
NOTE: IS_RESOLVER is a Symbol imported from the library — this prevents prop name conflicts. Make sure you are setting [IS_RESOLVER] = true and not IS_RESOLVER = true.

A resolver can have an optional dispose method that accepts the entire container. However, it is recommended to only interact with the dependency the resolver represents in custom disposers, to avoid accessing a peer dependency after it already was disposed.