jerkface

I swore to myself I'd never do this. I don't know how it even happened. It kind of snuck up on me, and before I even realized what was happening I had written a dependency injection library for Node.js. Now here it is. It's fairly opinionated. It suits my purposes, and perhaps it will fit your use case. If not, don't worry, there's a bazillion other DI libraries out there all doing it a different way.

Usage

Add jerkface to your project. Or don't.

$ npm install jerkface -S

The jerkface DI library is primarily intended to work with constructible objects. The idea is that you design your app around application boundaries. Each boundary is represented by an abstraction. Each module in your application is defined as a class whose constructor accepts a parameter that contains all of the class' dependencies.

An example:

const elv = require('elv');

class EventStream {
  constructor(dependencies) {
    this.driver = elv.coalesce(dependencies.driver, () => new Driver());
  }

  write(message) {
    // use this.driver to write a message to the event stream
  }
}

class Storage {
  constructor(dependencies) {
    this.fs = dependencies.fs;
  }

  save(file) {
    // execute some business logic, and use this.fs to persist the file
  }
}

class Uploads {
  constructor(dependencies) {
    this.eventStream = dependencies.eventStream;
    this.storage = dependencies.storage;
  }

  save(file) {
    const self = this;
    // assume everything uses promises
    this.storage.save(file)
      .tap((info) => {
        return self.eventStream.write({
          topic: '/uploads/file/saved',
          payload: info,
        });
      });
  }
}

This design has a number of advantages over relying on CommonJS. Not the least of which is that it makes unit testing extremely easy; as, dependencies can be very easily mocked out, and injected.

This has the beginnings of a very basic service-oriented-architecture. Each class in the example above is basically a service. It's fairly common that instances of services are managed as singletons in applications. However, ensuring there is a single instance of each service in an app, and that each module can efficiently access that lone instance is the tricky part.

That's where jerkface comes in. Simply configure each service as though it were a separate package in your app, and define its dependencies.

// When your application starts up:

const Container = require('jerkface').Container;

Container.shared = new jerkface.Container();

event-stream.js

// This class manages its own dependencies outside of jerkface, but can still be
// declared as a binding that is injectable into other services.

Container.shared.bind('event-stream', EventStream);

storage.js

// Bindings do not have to be a constructor.  If a binding is an object, that
// instance will be returned.

Container.shared.bind('fs', require('fs'));

Container.shared.bind('storage', EventStream, {
  dependencies: { fs: 'fs' },
});

uploads.js

// Tie it all together:

Container.shared.bind('uploads', Uploads, {
  dependencies: {
    eventStream: 'event-stream',
    storage: 'storage',
  },
});

Now when you want instance of Uploads:

const uploads = Container.shared.resolve('uploads');

This method can be called over and over again, from multiple different modules, and you will always get the exactly same instance (there are caveats to this related to how various CommonJS implementations, or whatever module system you're using, handles different versions of packages).

Compatibility

The jerkface module was built with Node.js in mind, though it may work in the browser just fine (as long as you are using a CommonJS module system). I have not gotten around to testing this, so use it in the browser at your own risk.

It's also worth nothing that jerkface was built using ECMAScript 2015, and, so, will require a Node.js version and configuration capable of using class, Map, Set, for...of, etc.

API

The base module is an object with the following keys:

• Container: a reference to the Container class.

• errors: an object with the following keys:

  • BindingError: a reference to the BindingError class.

  • CircularReferenceError: a reference to the CircularReferenceError class.

  • ResolveError: a reference ot the ResolveError class.

• Lifetime: a reference to the Lifetime enum.

Class: Container

The heart of the jerkface project. Each Container functions independently from one another. All bindings and object instances are not shared across Container instances.

• Properties

  • Container.shared: a static, convenience property for sharing an instance of Container across an application. This property is by default null, and will throw a TypeError if you attempt to set it to anything other than null or an instance of Container.

• Method

  • Container.prototype.bind(name, target [, options]): binds name to target.

    Dependencies do not have to be bound in a specific order. The jerkface module only requires that the entire dependency graph be configured before Container.prototype.resolve() is called.

    Additionally, the Container.prototype.bind() method will traverse the dependency graph, and ensure that no circular references are created. If a circular dependency is detected, a CircularReferenceError is thrown.

    Subsequent calls to Container.prototype.bind() with an existing name, will overwrite the previously configured binding. However, any already constructed and resolved dependent bindings will not be modified.

    This method returns its instance of Container. This allows multiple calls to be easily chained together.

    Parameters

    • name: (required) the name by which this binding is know. All other bindings, will reference this name when declaring dependencies to other bindings.

    • target: (required) the object to which name is bound. This can be either a constructor function, or any type. In the event a constructor function is provided, it is new'ed up when name is resolved.

    • options: (optional) an object that can be provided to further customize how jerkface treats a binding. Keys include:

      • dependencies: (optional) an object where each key maps to the name of a key on the dependencies object argument on a bound constructor. See Bound Constructors for more information. If target is not a constructor function then setting the dependencies key will result in a BindingError being thrown. This key defaults to null.

      • lifetime: (optional) how the lifetime of constructed object is to be managed. This value is always a string, and can be either "singleton" or "transient". If lifetime is set to "transient", then an instance of target is new'ed up every time resolve() is called. If lifetime is set to "singleton", which is the default, then a single instance is created. If the target argument is not a constructor function, and this option is set, then BindingError is thrown.

      • params: (optional) an array of additional parameters expected by the constructor function specified by target. See Bound Constructors for more information.

  • Container.prototype.bindAll(base, dependencies): supplements all bindings where a target constructor function is derived from base with additional dependencies.

    For example, lets assume ClassA extends ClassB, and ClassA is bound to the name a. ClassA also has a dependency to a binding named foo. ClassB is configured using bindAll(), and specifies the binding bar as a dependency. When the binding a is resolved, ClassA will be new'ed with both foo and bar in its dependencies map.

    If there are any key collisions on dependencies defined by Container.prototype.bindAll() between base and any bindings that extend from base, those defined at the binding level are preferred.

    Further, if there are multiple base objects defined with Container.prototype.bindAll(), then jerkface will build out the inheritence order. In the event that any depenency keys collide in the inheritence chain, preference is given to derived classes.

    Subsequent calls to Container.prototype.bindAll() with an existing base, will overwrite the previously configured binding. However, any already constructed and resolved dependent bindings will not be modified.

    This method returns its instance of Container. This allows multiple calls to be easily chained together.

    Parameters

    • base: (required) the target super class.

    • dependencies: (required) an object where each key maps to the name of a key on the dependencies object argument on a bound constructor. See Bound Constructors for more information.

  • Container.prototype.resolve(name): returns an instance of the object bound to name. If the bound object is a constructor function, it is invoked with all configured parameters and dependencies, and returned.

    If the binding is managed as a singleton, it is lazily constructed when resolve() is called, and all subsequent calls to resolve() for the same name return the same instance.

    If the binding specified by name has a dependency that has not been configured, then ResolveError is thrown.

    Parameters

    • name: (required) a string that specifies the name of the binding to resolve.

Bound Constructors

When a target constructor is added as a binding using jerkface, it is important to keep in mind how the Container class injects arguments into the function. This is largely dictated by the options provided to Container.prototype.bind().

When options.dependencies is provided, an object with identical keys is provided to the constructor. However, each key value is replaced by the resolved binding name.

When options.params is provided, the constructor is called with each item in the array provided as a separate argument. In this scenario, if the binding was configured with options.dependencies, then the dependencies object will be the last argument provided to the constructor.

Example

const Container = require('jerkface').Container;
const container = new Container();

class Test {
  constructor(a, b, dependencies) {
    console.log(a);
    console.log(b);
    console.log(dependencies.c);
  }
}

container.bind('test', Test, {
  dependencies: {
    c: 'c',
  },
  params: [1, 2],
});

container.bind('c', 3);

const test = container.resolve('test');
// Written to the console:
//   1
//   2
//   3

Class: BindingError

Thrown when options provided to the Container.prototype.bind() method are invalid.

This class is extends the builtin Error class.

Class: CircularReferenceError

Thrown when attempting to create a binding with a dependency that has a dependency to the original binding. Calling Container.prototype.bind() will cause jerkface to walk the entire dependency chain, and CircularReferenceError will be thrown even if the dependency is several times removed from the original binding.

For example, a depends on b, and b depends on c. If either b or c depends on a, then a circular reference is created, and an error is thrown.

This class is extends the builtin Error class.

Class: ResolveError

Thrown when Container.prototype.resolve() is called on a binding that does not exist in the Container instance.

Enum: Lifetime

An enum value that specifies the possible lifetime values that can be used with jerkface:

• Lifetime.singleton: instances are managed as singletons.

• Lifetime.transient: the lifetime of instances are not managed by jerkface. Once an instance is returned by Container.prototype.resolve() it is entirely up to the consuming code how it lives on past the calling scope.

Performance Considerations

Calling Container.prototype.bind() and Container.prototype.bindAll() should not be called in hot code paths, as the extensive validation routines that must be run are computationally expensive (especially the later). Generally, these methods should only be called during application initialization.