Skip to content
Low-level type-centric utility functions for Javascript. Provides inheritance, mixins, and function reflection helpers.
Find file
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


Build Status

Typedef is a lightweight library (1.7 KB, 4.7 KB with underscore.js bundled in) that brings some much-needed sanity to object-oriented programming in Javascript. These features will feel very familiar and refreshing to anyone experienced in a more civilized staticly-typed language like C# or Java.

Typedef introduces two interesting concepts for Javascript: class member decorations, and the idea of "define-time" (semi-static) checking. In addition it rolls in classical-style inheritance, interfaces, and mixins.

The goal is to let you build scalable web apps with a rich class hierarchy using proven object-oriented concepts and semantics-enforcing constructs. Wed with the dynamic nature of Javascript-- it's a potent mix.

The Javascript romantic in you may recoil and shout back about the virtues of embracing the native prototype-style inheritance model. This is understandable-- but if you can unburden yourself from the limitations you've built around the language and keep an open mind... you're in for a wild ride.

Feature Overview

  • Class inheritance. Classical inheritance goodness in Javascipt. Create a rich class hierarchy that feels sane, with all the power and common sense you've come to expect in an object-oriented language.

  • Mixins. Add member functions to a class without enforcing a strict single-inheritance relationships. The before, after, and wrapped "advice" decorations can be used to modify existing functions.

  • Interfaces. Classical interface pattern lets you write code that is self-documenting and forces define-time checks on code contracts.

  • Member decorations. Use modifiers on class members, mixin members, or interface members to change their behavior or enforce inheritance and access semantics, or use custom decorations and reflection to amp up your own classes and reduce boilerplate.

  • Define-time semantics checking. Use the virtual, abstract, and override member decorations to ensure predictable inheritance behavior, and get feedback in the form of define-time errors when implementation contracts are broken.

Getting Started

Typedef is packaged with the UMD pattern, so it can be used in the browser via a global with a <script> tag, on the server with Node, as an AMD module with RequireJS, or with CommonJS via Browserify.


Install all of the required modules to build and use grunt to build the files into the bin/ directory:

npm install
grunt release

Defining Classes

Easy peasy. Defining a class involves passing a hash of all the corresponding member functions to the typedef function, as well as (optionally) naming the class.

// Base class definition example ....

It works exactly as you might imagine.

// Base class instantiation and use example ...

Notice that the (optional) constructor function is provided via the __constructor__ property in the hash.

Classes are constructor functions that leverage the native prototypical inheritance model.

Extending Classes

Typedef allows for single-inheritance from a base class via chaining the .extends() function in your class definition. Child constructors are implicitly called, from the base up, when instantiating child classes.

// Child class definition example

Child classes can be thought of as inheriting the members of the base class-- though really what is going on is simply building up the native prototype chain.

// Child class use example

Using Mixins

Mixins provide a way to add or augment existing member functions in a class. Multiple mixins can be used during a class definition, and the advice decorations allow for modifying existing functions.

Mixins are added after the class is setup, meaning any wrapping augmentations will be applied on top of defined members of the class.

To add a mixin to your class definition, chain the .uses() method in your definition.

// Mixin definition ...

Implementing Interfaces

Interfaces give you a way of specifying a required set of member functions be present in a class implementation. Though this may seem superfluous for a duck-typed language like Javascript, this allows you run-time inspection of an object's abilities with a greater degree of clarity.

In addition, using interface patterns allows for your code to be more self-documenting. Any discrepancy between a class and an interface it implements will cause define-time exception, informing you of the issue.

Specify that a class implements a specific interface by chaining the implements() method in your class definition.

// Interface definition ...

Inheritance Decorations

The inheritance decorations are used to govern and check the inheritance semantics of your classes.

All inheritance decorations only add additional checking overhead during define time; no run-time overhead is incurred after the initial load and definition.


The virtual decoration follows its classical use, in that indicates a class member may be overridden in child class. By default, all members are non-virtual, and thus cannot be overridden. This effectively makes any sort of member hiding in Typedef explicit.

Attempting to override a base member that isn't virtual will result in a define-time error.


An abstract member is the same as a virtual member, with the difference that a derived class must override the abstract member.

A class with at least one abstract member is considered an abstract class. An exception is thrown when attempting to instantiate an abstract class.


The override decoration is required when overriding a base member in a child class. The base member must be virtual.


The new decoration indicates that the previous implementation of a member is to be disregarded. This can be used to explicitly hide a base class member that isn't set to virtual, for example.


A sealed member indicates that it cannot be override at all, even if using the new decoration. This provides a way to very clearly indicate that a member should not be changed in derived implementations of a class.

Attempting to override a sealed member will result in a define-time exception.

Accessor Decorations


Using the readonly decoration will define the property with the writable flag set to false. Any attempts to update a readonly property will silently fail (unless you use strict).

Note that this only applies for the values of members; if a member set as readonly is initialized with an object, that object can still be mutated.


Mixin Decorations




Behavior-enforcing Decorations


This decoration allows you to signal in your public API that method is designed to returned the this pointer. This allows for the elegant "fluent API" style of method chaining.

The define-time check uses function#toString to check all return statements to ensure that they are returning this. Returning something else (or not having any return statements) will result in a define-time warning.


Typedef makes liberal use of ES5 features such as Object.defineProperty and Object.create, and thus will work with any modern browser:

  • Chrome 7+
  • Firefox 4+
  • Safari 5.1+
  • Opera 4+
  • Internet Explorer 9+


Testing is powered by QUnit and can be run from the command line via grunt-contrib-qunit. To install all modules and test:

npm install
grunt test



Copyright 2013 Brandon Valosek

Typedef is released under the MIT licenses.

Something went wrong with that request. Please try again.