injector.js

/**
 * @constructor
 */
function Injector() {

  /**
   * @type {!Object.<string, function(Injector=): !Object>}
   */
  this.factories = {};

  /**
   * @type {!Object.<string, !Object>}
   */
  this.services = {
    '$injector': this
  };
}

/**
 * Adds a service factory.
 * @param {string} key A service key.
 * @param {function(Injector=): !Object} factory A service factory.
 */
Injector.prototype.addService = function (key, factory) {
  this.factories[key] = factory;
};

/**
 * Returns a service by its key.
 * @param {string} key The key of the service to get.
 * @return {!Object} The service.
 */
Injector.prototype.getService = function (key) {
  var service = this.services[key];
  if (!service) {
    var factory = this.factories[key];
    if (!factory) {
      return null;
    }
    service = factory();
    this.services[key] = service;
  }
  return service;
};

/**
 * Instantiates the given constructor providing it with its dependencies.
 * @param {Function} Constructor The constructor function to use.
 * @return {!Object} An instance of the constructor.
 */
Injector.prototype.create = function (Constructor) {
  var Dependant = function () {};
  Dependant.prototype = Contructor.prototype;

  var instance = new Dependant();
  this.inject(Constructor, instance);

  return instance;
};

/**
 * Injects dependencies to a constructor in the context of the given instance.
 * @param {Function} Constructor The constructor function to use.
 * @param {!Object} instance The instance to use.
 */
Injector.prototype.inject = function (Constructor, instance) {
  var keys = Constructor.prototype.$deps || [];
  var deps = keys.map(this.getService, this);

  Constructor.apply(instance, deps);
};