simple js factories using builder pattern

jsfactories.js

/**
* Builder pattern for factories, mimic use of FactoryGirl traits.
* usage: factories.boostedBoost({retweets: 1}).twitter().withdrawn().geo(11217).generate();
* must call `generate` as last step
* may pass options to initial method call, as well as subsequent method calls
*/
function addFactory(factoryName, config) {
  // add trait function to generated Factory
  function addTrait(k, Factory, config) {
    Factory.prototype[k] = function() {
      config.traits[k].apply(this.options, arguments);
      return this;
    };
  }

  var Factory = function(options) {
    this.options = options;
  };

  // last step in generating an object
  Factory.prototype.generate = function() {
    return config.constructor.call(this);
  };

  for (var k in config.traits) {
    if (config.traits.hasOwnProperty(k)) {
      addTrait(k, Factory, config);
    }
  }

  // attach Factory to global object
  f[factoryName] = function(options) {
    return new Factory(options);
  };
}

var boostedBoost = {
  constructor: function() {
    return new Application.Models.BoostedBoost(this.options);
  },
  traits: {
    twitter: function() {
      this.network_ids = [1];
      this.networks = 'Twitter';
      this.original_post = {
        id: 1
      };
    },
    facebook: function() {
      this.network_ids = [2];
      this.networks = 'Facebook';
      this.original_post = {
        id: 2
      };
    },
    withdrawn: function() {
      this.withdrawn = true;
      this.withdrawn_at = "2015-03-05 18:01:46 -0500";
      this.allow_withdraw = false;
    },
    geo: function(zip) {
      this.geo = zip;
    }
  }
};
addFactory('boostedBoost', boostedBoost);