Dependency Injector

Here's an example of a module that has two parts: a service and a db. The service depends on the db.

Here's the db.js:

module.exports = Db;

function Db() {
}

Db.$inject = [];

Db.prototype.answer = function () {
  return 42;
};

And here's service.js:

module.exports = Service;

function Service(db) {
  this._db = db;
}

Service.$inject = [
  './db'
];

Service.prototype.answer = function () {
  return this._db.answer();
};

Note how the service has one special property $inject. It is an array of paths of modules that are to be "injected". Use this just as you use require(). The exact same rules that Node.js uses for resolving the modules apply.

The only difference is that if the $injected module is a "class" (function) which has an $inject property that is an array, a new instance of that class will be created and provided as an argument of the constructor of the class that $injected it. This happens recursively. Thus, if the injected class had an $inject of its own, that would be resolved first.

For example, in this case the Db has no dependencies to inject, so a new instance of the Db class will be created and provided as the first argument in Service's constructor.

Finally, here's index.js that gets a fresh new instance of the Service class.

var DI = require('areus-di'),
  di = DI(__dirname);

exports.create = function () {
  return di.create('./service');
};

The __dirname is the root directory, relative to which ./service is locataed.

Providing Existing Packages

Use the .provide() method to provide existing packages. Any package that you give to .provide() will be available in $inject. Here's an example:

index.js

var DI = require('areus-di'),
  mongo = require('mongoskin'),
  di = DI(__dirname);

di.provide({
  db: mongo.db(process.env.MONGO_URI)
});

di.create('./article_service');

article_service.js

module.exports = ArticleService;

function ArticleService(db) {
  this._articles = db.collection('articles');
}

ArticleService.$inject = [
  'db'
];

ArticleService.prototype.findOne = function (id, cb) {
  this._articles.findOne(id, cb);
};

Why Use a Dependency Injector?

• Isolation In the example above Service did not need to know how Db was created. It did not have to be aware of the dependencies of Db or care whether it was backed by LevelDB, MongoDB or PostgreSQL. This is important because this implies that Db can change independently from Service.

• Testability to test Service a new instance can be created with a fake Db as the first argument. For example:

var Service = require('../service'),
  assert = require('assert'),
  fakeDb, service;

fakeDb = {
  answer: function () {
    return 7;
  }
};

service = new Service(fakeDb);
assert.equal(7, service.answer());

Guiding Principles

• No new knowledge needed for basic operation.

  • Path resolution follows the require() algorithm to a T.

• In addition to require(), $inject instantiates the "class".

License

MIT