Sack

An Inversion-of-Control container for all your dependency injection needs.

Installation

npm install sack

Introduction

This is a simple Inversion of Control Container. It provides the mechanism you need to have a centralized store of lazily-created, dynamically resolved object instances that other types can passively request get injected.

This means that you can have objects use, access, or compose other objects without having to explicitly know how to create/setup/initialize them.

It lets you have a highly-decoupled and framework-independent domain objects.

Sack is used to power the service-oriented application harness Billy.

Philosophy

As with any IoC container, you want to only have your code be aware of the container the topmost/single entry point that bootstraps the rest of your application.

It is very much so an antipattern to have traces of the IoC container all over your codebase. The idea is to allow your business logic and domain classes to be totally unaware that they are getting injected by a container, keeping them free of Sack-specific code.

This is done by embracing and reifying the pattern of dependency injection via constructor parameters. Instead of having all of your objects be aware of how to find, access, setup, initialize, and manage other objects, they simply implicitly state their need for a dependency as a constructor parameter.

It allows your business logic classes to go from this:

class UserController()
{
  constructor()
  {
    const connection = new DbConnection(global.settings.conConfig);
    this.users = connection.selectUsers();
  }
}

To this:

class UserController()
{
  constructor(users)
  {
    this.users = users;
  }
}

This removes the knowledge and logic from UserController on how to connect to a database.

In large applications, things like model stores, application configs, service handles, etc. are all used across several different parts of an application. Relying on every consumer object to manage its dependencies violates DRY and SRP, making your codebase difficult to maintain as it scales.

The goal is to reduce how tightly coupled your various objects are by removing the knowledge of how to create other dependency objects, and simply rely on expressing what kind of objects we need.

This facilitates testing, as complex dependencies that are injected can be substituted with mocks, and the consuming classes are not tied to a specific implementation.

Usage

All dependencies should be managed from a single Container instance:

import Container from 'sack';

const container = new Container();

Registering Objects

Register a class constructor that will get executed every time the dependency is resolved, creating a new instance:

container.register('service', MyService);

Register a constructor function that will get executed one time when the first time the dependency is resolved, and then re-used after that (singleton pattern, lazily created):

container.register('service', MyService).asSingleton();

Register an existing object instance as a dependency:

container.register('service', someService).asInstance();

Register a (lazily evaluated) callback to provide the dependency on every request:

container.register('service', () => new MyService());

Registered a callback to provide the dependency the first time it is requested, and then re-use it all subsequent times (lazily-created singleton via callback):

container.register('service', () => new MyService()).asSingleton();

Resolving Objects

You can create / request objects via the make() function by passing in the string tag used during registration:

const service = container.make('service');

Not that you should typically not be using Sack this way, but rather expressing dependencies as explained below.

Expressing Dependencies

Whenever Sack creates an object, it satisfies that object's dependencies by resolving them out of the container as well.

An object expresses its dependency as a constructor parameter, whose name must match a registered object.

class UserEditController
{
  constructor(users)
  {
    this.users = users;
  }

  refreshUsers()
  {
    this.users.refresh();
  }
}

Assuming we have registered some implementation for users:

container.register('users', LocalStorageUsers).asSingleton();

Then making UserEditController via the container will resolve the dependency automatically.

const controller = container.make(UserEditController);

Strong vs Weak Dependencies

By default, all registered dependencies are considered "strong", that is, they cannot be overridden. Attempting to register a dependency with the same name as another one will result in an error:

container.register('server', express());
container.register('server', http.createServer());

> Error: Cannot override: server

Registering a dependency as weak allows it to be overriden later:

container.register('config', {}).asWeak();
container.register('config', new ConfigStore());

// No error

Documentation

This will generate the HTML documentation under ./doc:

$ npm run doc

Testing

$ npm test

License

MIT