Moog provides powerful module subclassing.
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


Build Status

Moog creates objects, with rich support for subclassing and "implicit subclassing" (like "categories" in Objective C). Moog implements the "self pattern," so you never have to worry about using .call, .apply or .bind.

var moog = require('moog')();

moog.define('baseclass', {
  color: 'blue',
  // sync constructor
  construct: function(self, options) {
    self._options = options;

    self.jump = function(howHigh) {
      return 'I jumped ' + howHigh + ' pixels high';

moog.define('subclass', {
  color: 'red',
  // async constructor
  construct: function(self, options, callback) {
    return goGetTheCandy(function(err, results) {
      if (err) {
       return callback(err);
      self.candy = results;
      return callback(null);

moog.create('subclass', { age: 20 }, function(err, obj) {
  assert(obj._options.color === 'red');
  assert(obj.jump(5) === 'I jumped 5 pixels high');

moog synthesizes objects with full support for inheritance. You can define them with any combination of synchronous and asynchronous constructors, specify default options easily, and modify options before they are seen by base classes.

Factory function

To create an instance of moog:

var moog = require('moog')();

You may also pass options:

var moog = require('moog')({
  defaultBaseClass: 'superclass'

moog.define(type, definition)

Defines a new type. type is a string.

moog.define('baseclass', {
  // Set the default value of an option
  color: 'red',
  // Simple synchronous constructor
  construct: function(self, options) {
    self._options = options;

The definition object can contain the properties construct, beforeConstruct, and afterConstruct, which are functions invoked by moog.create, as described below. The extend property allows for subclassing. All other properties are treated as defaults for the options object provided when constructing an instance of the type.

To subclass another type, just extend it by name in the definition of your subclass:

moog.define('subclass', {
  // Change the default value of an option
  color: 'blue',
  extend: 'baseclass'

Default base class

If you set the defaultBaseClass option of moog and do not explicitly extend anything for a particular type, then that type will extend the defaultBaseClass. If you wish to override this behavior for a specific type, just set extend to false.

Implicit subclassing

If you define the same class twice without setting extend the second time, an implicit subclass is created.

The new version subclasses the old one, effectively "patching" it with new options and behavior without having to redefine everything. All other types that subclass that name now subclass the new version.

Fallback base classes

If you are not sure if there is an existing definition for the type, you can use extendIfFirst to specify a fallback base class. This is helpful when encouraging third-party developers to optionally define a type themselves.

Defining many types at once

For convenience, you may pass an object containing properties that define many different types:

  // This is equivalent to calling moog.define for each of these types
  'baseclass': {
    // See above for example of a definition
  'subclass': {
    // See above for example of a definition

moog.redefine(type, definition)

Explicitly replaces any previous definition of type with a new one. Does not subclass the old type. If there was no old definition, this method is equivalent to moog.define.

moog.isDefined(type, options)

Returns true if the type is defined, whether explicitly or via the autoloader option. That is, moog.create will succeed for type, provided that the constructor does not signal an error. If the type is available via the autoloader, this method returns true but does not leave the definition in place.

The options argument may be omitted entirely. If options.autoload is explicitly set to false, no attempt to test for the ability to load the type via the autoloader is made.

moog.create(type, options, /* callback */)

Creates an object of the specified type, passing options to override any default options set in moog.define.

moog.define('myObject', {
  color: 'blue',
  construct: function(self, options) {
    self.color = options.color;

var myObject = moog.create('myObject', { color: 'purple' });
alert("My object is " + myObject.color); // "My object is purple"

When create is called, moog will first call beforeConstruct, starting with the deepest subclass first. Then the construct methods are called, if present, starting with the base class and ending with the final subclass. Finally the afterConstruct methods are called, if present, starting with the base class and ending with the final subclass.

In the above example, moog.create is called synchronously, but could be called asynchronously as follows:

moog.create('myObject', { color: 'purple' }, function(err, myObject) {
  alert("My object is " + myObject.color); // "My object is purple"

If it's called asynchronously, the callback receives the arguments err, obj where obj is the object created. If it's called synchronously, an exception is thrown in the event of an error, otherwise the object is returned. If you call moog.create synchronously but have asynchronous beforeConstruct, construct, or afterConstruct methods, moog will throw an exception. You may, however, call moog.create asynchronously, even if your constructor functions are synchronous.

obj will always have a __meta property, which contains an array of metadata objects describing each module in the inheritance chain, starting with the base class. The metadata objects will always have a name property. moog-require also provides dirname and filename. This is useful to implement template overrides, or push browser-side javascript and styles defined by each level.

moog.createAll(globalOptions, specificOptions, /* callback */)

Creates one object of each type that has been defined via moog.define or via the definitions option given when configuring moog. Only types explicitly defined in this way are created, but they may extend types available via the autoloader option given when configuring moog.

The options passed for each object consist of globalOptions extended by specificOptions[type].

If you pass a callback, it will receive an error and, if no error, an object with a property for each type name. If you do not pass a callback, such an object is returned directly. If you do not pass a callback, then you must not define any types that have asynchronous construct and beforeConstruct methods.

Using moog in the browser

moog works in the browser, provided that async and lodash are already global in the browser. moog defines itself as window.moog. Currently it is not set up for use with browserify but this would be trivial to arrange.

moog.mirror(meta, suffix)

Often, it is useful for the same type hierarchy to exist in two separate instances of moog — for instance, on the server side and the browser side. It is also often useful to recreate the same type hierarchy, but with a suffix appended to each type name.

If your server-side application has a "nifty-blog" type that inherits from "blog" which inherits from "pieces", you may want to ensure that the same types are defined on the browser side, and then take advantage of implicit subclassing to supply actual code for some or all of the types in the hierarchy.

To pull this off, invoke moog.mirror with the __meta property of an instance of any type:


If any of the types in the hierarchy already exist, they are left alone. This allows you to safely use moog.mirror to patch any gaps in the type hierarchy, making sure any types that were not explicitly defined are filled in implicitly.

You may also optionally pass a suffix as the second argument. This is helpful if you wish to define types like this:

nifty-blog-editor : blog-editor : pieces-editor nifty-blog-manager : blog-manager : pieces-manager

That code looks like:

moog.mirror(niftyBlog.__meta, '-editor')

If you are using moog-require, the __meta property can contain information about where your types are defined in the filesystem. You might not want to disclose that information to the web browser. To avoid that, you may filter the __meta object:

var meta = {
  chain: []
_.each(object.__meta.chain, function(entry) {
  meta.chain.push({ name: });

Before making it available to the browser as JSON for use in a moog.mirror call.


All tests passing.

1.0.3: nudging past npm not making 1.0.2 available for some reason; no code changes.

1.0.2: in addition to the shallow top-level clone, we must also clone the __meta property and its __meta.chain subproperty to ensure they are not reused between instances of moog. No other properties of the definition are objects subject to modification.

1.0.1: shallowly clone each definition to avoid numerous problems when multiple instances of moog intended to be separate worlds wind up sharing the same definition objects due to the require cache. A shallow clone gives us independent __meta properties, which are all we need to solve the problem. Definitions are few, instances are many and the clone is shallow, so this is not a significant performance hit.

Also, use the apostrophe eslint test configuration. This required various syntax updates and flushed out a few oddities although no bugs.

1.0.0: updated lodash and mocha to satisfy npm audit. Code is still compatible with lodash 3.x as well, for those using the @sailshq/lodash fork for bc, and for Apostrophe's frontend which uses that version and shouldn't push multiple versions.

0.3.1: new instanceOf method. Given an object and a type name, this method returns true if the object is of the given type or a type that extends it.

0.3.0: new options argument to isDefined, which may contain an autoloader: false property to prevent isDefined from attempting to test whether the type can be defined by the autoloader.

0.2.4: throw the proper exception when synchronously creating a type that extends an undefined type. (Previously an exception was thrown, but it wasn't informative. It was an accidental benefit of trying to invoke a nonexistent callback.)

0.2.3: exceptions thrown for attempts to synchronously create types with asynchronous beforeConstruct/construct/afterConstruct methods now include the correct name of the type or ancestor type that requires the call to be asynchronous.

0.2.2: if afterConstruct expects a callback, calling create synchronously should throw an error. This is a bug fix, so no minor version bump is required.

0.2.1: __meta property is available in beforeConstruct. I regard this as a bug fix as the idea was always to have this information be available as early as possible.

0.2.0: added support for mirror, which allows browser-side type hierarchies to match those used on the server side. To add actual code for those types, take advantage of the implicit subclassing feature of moog.define.

0.1.5: added support for extendIfFirst, useful when you don't know if there is an existing definition of the type. report certain errors synchronously when creating objects synchronously.

0.1.4: allow setting extend to false to explicitly turn off defaultBaseClass for a particular type. Also corrected the unit test for defaultBaseClass (the feature worked, but the test was wrong).

0.1.3: never pass options to afterConstruct. We formerly were correctly leaving it off in the async case, but passing it in the sync case.

0.1.2: Updated some documentation.

0.1.1: added afterConstruct, another optional method which is invoked after construct. Like beforeConstruct and construct it can be sync or async. Unlike those methods it DOES NOT take the options parameter.

0.1.0: bc break: __meta is now an object with chain and name properties. chain is the array of subclass metadata objects as before. name is the class name being instantiated. Also, __meta is fully populated before any constructors are called.