Constitute

Minimalistic Dependency Injection (DI) for ES6

Why Dependency Injection?

There are lots of good resources out there on Dependency Injection (DI) and Inversion of Control (IoC). For JavaScript developers, Vojta Jina's ng-conf presentation is a fantastic primer.

For many smaller apps, using plain ol' Node.js modules works just fine. But eventually you want more control over when your components get instantiated. So you switch to classes and inject your dependencies via the constructor. But now you have annoying glue code like this to maintain:

function main () {
  const electricity = new Electricity()
  const grinder = new Grinder(electricity)
  const heater = new Heater(electricity)
  const pump = new Pump(heater, electricity)
  const coffeeMaker = new CoffeeMaker(grinder, pump, heater)
  coffeeMaker.brew()
}

Tools like constitute can turn that into:

function main () {
  const coffeeMaker = constitute(CoffeeMaker)
  coffeeMaker.brew()
}

Your classes remain easily testable and life is good.

Why this library?

Awesome Dependency Injection frameworks are on the way for JavaScript. Like the one in Angular 2. But I wanted a module which is independent from any framework and works in ES5/ES6/ES7 with or without transpiling.

Installation

npm install --save constitute

Usage

Let's look at an example. For this README I'm going to use ES6 modules syntax. If you need CommonJS (require) style, please look in example/es6-cjs.

Suppose we have three classes A, B and C. A depends on B and C. There are no other dependencies. We need to tell constitute that A depends on B and C. We also call the dependencies "constituents".

a.js

import B from './b'
import C from './c'

export default class A {
  static constitute () { return [ B, C ] }
  constructor (b, c) {
    this.b = b
    this.c = c
  }
}

If you are transpiling, you can also use an ES7-style decorator:

a.js (alternative with ES7 decorator)

import { Dependencies } from 'constitute'

@Dependencies(B, C)
export default class A {
  constructor (b, c) {
    this.b = b
    this.c = c
  }
}

The classes B and C are defined without any special sugar:

b.js

export default class B {}

c.js

export default class C {}

Because these classes do not have any dependencies, we don't need to annotate them.

So how do we instantiate our annotated class A?

main.js

import constitute from '../../'
import A from './a'

// Instantiate a class
// Calling constitute() creates a new dependency injection context
const a = constitute(A)

console.log(a.constructor.name) // => A
console.log(a.b.constructor.name) // => B
console.log(a.c.constructor.name) // => C

// Simple.

And that's all you need to know to get started. The rest of the documentation below is there when you need it.

Resolvers

When requesting dependencies, you can modify what kind of value is provided by using a resolver.

import { Lazy } from 'constitute'

class D {
  static constitute () { return [ Lazy.of(A) ] }
  constructor (getA) {
    this.getA = getA
  }
}

There are different types of resolvers:

• Instance - The default resolver. Resolves the dependency immediately and provides it as the value
• Lazy - Provides a function which resolves the dependency when called, returning the value
• All - Provides an array of values for all dependencies bound to the provided key (see Binding below)
• Optional - Injects a value only if the dependency already exists in the container; undefined otherwise

Constitutors

You can also change how your dependencies are instantiated. There are three built-in policies:

• Singleton - The default. Your dependency is instantiated once per container.
• Global - Like a singleton, except the same instance is used even across containers. Warning: Use of globals is generally discouraged. According to some, globals are ok for very specific use cases, such as loggers.
• Transient - Your dependency is instantiated every time it is resolved.

To use a different constitutor, simply return it from the constitute method:

import { Transient } from 'constitute'

class E {
  static constitute () { return Transient.with( [ A ] ) }
  constructor (a) {
    this.a = a
  }
}

Binding

By default, classes resolve to a new instance of themselves. But what if we want to remap what they resolve to?

Binding for tests

Let's say we're testing and we to replace our Database service with a MockDatabase service. But first, here's our database service:

(In the interest of brevity, we'll skip imports for this example.)

lib/database.js

class Database {
  static constitute () { return [ Config ] }
  constructor (config) {
    this.connection = config.get('db.uri')
  }
}

And our app itself:

lib/app.js

class App {
  static constitute () { return [ Database ] }
  constructor (db) {
    this.db = db
  }
}

Here are our tests where we instantiate the app using a mock database:

test/appSpec.js

describe('App', function () {
  beforeEach(function () {
    // Here is our mock database class
    class MockDatabase { ... }

    // First, let's get a fresh container
    this.container = new constitute.Container()

    // Then we tell it to bind the database to the mock database
    this.container.bind(Database, MockDatabase)

    // Finally we can instantiate the app
    this.app = this.container.constitute(App)

    // Simple.
  })

  // ...
})

The main difference you'll notice is that this time we used new constitute.Container and Container#constitute() instead of the short-hand constitute(). We also introduced the Container#bind() method, which takes a key as its first argument and a class or factory as its second argument.

Factories

So far, we've only dealt with class dependencies. But classes (more specifically, class constructors) are actually just one type of factory in constitute.

• Class(constructor, constitutor) - This is the default factory. If you try to instantiate a non-factory value, constitute will try to wrap it in a Class factory. What this factory does is to try to gather the dependency and constitutor settings from a static method called constitute. The constitutor will resolve the dependencies and finally, the Class factory will call the constructor with the new keyword and the resolved dependencies as arguments.
• Alias(key, constitutor) - Links to another key on the same container. You can use Alias to specify another key and when it is asked to instantiate a value it will call that other factory instead.
• Value(value) - Doesn't instantiate anything, it simply returns the same value every time.
• Clone(value, constitutor) - Creates a clone of the provided value.
• Method(fn, constitutor) - Allows you to specify a custom factory function.

Class factory

Normally, you never need to worry about the Class factory. Any classes you pass to constitute will automatically be wrapped in Class factories.

However, manually creating a Class factory allows you to pass in a constitutor. That can be useful, if you don't want to add a constitute method on the class itself.

In other words, this:

class A {
  static constitute () { return [ B ] }
  constructor (b) { ... }
}

const a = constitute(A)

Is the same as this:

import constitute, { Class } from 'constitute'

class ActualA {
  constructor (b) { ... }
}
const A = new Class(ActualA, [ B ])

const a = constitute(A)

Just make sure when you specify your dependencies to reference this Class as A, not as ActualA. Although you could of course bind ActualA to A:

myContainer.bindClass(ActualA, A)

After that, both A and ActualA would resolve to your Class factory with the correct dependencies.

To add metadata to existing classes, you can also use the container.bindClass convenience wrapper:

import { Container } from 'constitute'

class A {
  constructor (b) { ... }
}

const container = new Container()
// Bind the key A to a ClassFactory for A with a Singleton constitutor and a single dependency, B
container.bindClass(A, A, [ B ])

const a = container.constitute(A)

Alias factory

The Alias factory can be used to cause a lookup for another key in the current container and use that key's factory instead. By default, Alias factories will use the Transient constitutor, meaning the alias mapping will be resolved every time the aliased key is requested. The alias target uses its own constitutor as normal, so the target may still be a cached instance.

class A {}
class B extends A {}

const container = new Container()
container.bindAlias(A, B)
const instance = container.constitute(A)

console.log(instance instanceof B) // => true

// Note that the alias respects any later bindings of the target Key
container.bindValue(B, 65537)
console.log(container.constitute(A)) // => 65537

Value factory

Possibly the most boring constructor. It always returns the same value. Because the value is static anyway it also doesn't need a constitutor. But you can still rebind it, alias it and so on.

import constitute, { Value, Container } from 'constitute'

const V = new Value(42)

class A {
  static constitute () { return [ V ] }
  constructor (v) {
    console.log('The answer is ' + v)
  }
}

class B extends A {}

constitute(A) // => The answer is 42

// Like all factories, Value factories support binding, so we can override the value later
const container = new Container()
container.bindValue(V, undefined)
container.constitute(B) // => The answer is undefined

Clone factory

Similar to the Value factory, but returns a clone of the value (for objects and arrays) instead of the value itself. Defaults to the Transient constitutor.

import constitute, { Clone, Container } from 'constitute'

const V = new Clone({ foo: 'bar' })

class A {
  static constitute () { return [ V ] }
  constructor (v) {
    this.v = v
  }
}

class B extends A {}

const a = constitute(A)
const b = constitute(B)

a.v.foo = 'baz'

console.log(a.v.foo) // => 'baz'
console.log(b.v.foo) // => 'bar'

Method factory

With Method, you can define your own factory function. Wield this power wisely.

Your factory function is called with the dependencies as the parameters and the container as this.

import { Method } from 'constitute'

class C { }

const B = new Method(function (c) {
  return { c }
}, [ C ])

export default class A {
  static constitute () { return [ B ] }
  constructor (b) {
    this.b = b
  }
}

console.log(constitute(A).b.c instanceof C) // => true

Containers

All instances (except for dependencies using the Global constitutor) are isolated within Containers. To get the container your instance lives in, just request Container as a dependency:

import { Container } from 'constitute'

class A {
  static constitute () { return [ Container ] }
  constructor (container) {
    // container is the current container context
  }
}

Container hierarchy

You can create subcontainers to override dependencies locally without affecting upstream bindings.

import { Container } from 'constitute'

const masterContainer = new Container()
const subContainer = masterContainer.createChild()

class A {}
class B {}

subContainer.bindClass(A, B)

console.log(subContainer.constitute(A) instanceof B) // => true
console.log(masterContainer.constitute(A) instanceof A) // => true

Subcontainers also use an inheritance-aware cache. If a class has already been instantiated on the parent (and it is using a per-container caching constitutor, such as Singleton) it will be returned from cache.

import { Container } from 'constitute'

class A {}

const masterContainer = new Container()
const subContainer = masterContainer.createChild()

const a1 = masterContainer.constitute(A)
const a2 = subContainer.constitute(A)

console.log(a1 === a2) // => true

If a class has already been instantiated in the subcontainer, the subcontainer will continue to use that cached instance even if the parent container later creates an instance of its own.

Post-constructors

Suppose you have two classes that depend on each other—a circular dependency. Constitute has to instantiate A before B and B before A which is impossible. You can resolve the situation using a post-constructor:

class A {
  static constitute () { return [ Container ] }
  constructor (container) {
    // Assigning b in a post-constructor allows both objects to be constructed
    // first, resolving the cyclic dependency.
    //
    // Note that the post-constructor still runs synchronously, before this
    // object is returned to any third-party consumers.
    container.schedulePostConstructor(function (b) {
      this.b = b
    }, [ B ])
  }
}

class B {
  static constitute () { return [ A ] }
  constructor (a) {
    this.a = a
  }
}

When keeping your classes in separate files, you need to also watch out for circular requires.

An easy solution is to put your require directly before schedulePostConstructor:

a.js

class A {
  static constitute () { return [ Container ] }
  constructor (container) {
    const B = require('./b')
    container.schedulePostConstructor(function (b) {
      this.b = b
    }, [ B ])
  }
}

b.js

const A = require('./a')
class B {
  static constitute () { return [ A ] }
  constructor (a) {
    this.a = a
  }
}

Acknowledgements

This library borrows heavily from the fantastic DI component in the Aurelia framework. Awesome stuff.

Further inspiration comes from the DI features in Angular 2.