Extend constructors using backbone's .extend signature

Usage no npm install needed!

<script type="module">
  import extendible from 'https://cdn.skypack.dev/extendible';



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Extend your JavaScript constructor in the same way as would extend Backbone.js Models using the extend method. Extending and inheritance has never been easier and more developer friendly. While the original version was straight port of the Backbone's source it has been re factored and rewritten many times to the project it is today. It features:

  • Backbone compatible extend API.
  • Support for EcmaScript 5 getters and setters.
  • Understands the difference between constructors that are created in use strict mode.


The module is intended for server-side and browser usage. We use feature detection to ensure compatibility with EcmaScript 3 based environments. The module is distributed through npm and can be installed using:

npm install --save extendible


In all example code we assume that you've already required the extendible module and saved it as the extend variable as shown in the following example:

var extend = require('extendible');

The extend method should be on the constructor as .extend method:

function Word() {
  this.foo = 'bar';

// It should be added on the constructor, not as property on the prototype!
Word.extend = extend;

To create a new Foo class of your own you call the Foo.extend method with 2 arguments:

  1. Object properties and methods that should be added to your extended class prototype. These will override existing properties, but it would not override the properties on the parent/root class that you extend on.
  2. Object properties and methods that should added on the constructor directly. So instead of being introduced on the .prototype it's directly added to the returned Function.

As the properties of the prototype and constructor are inherited from the parent/root constructor you can further extended using the same extend method:

var Hello = Word.extend({
  name: 'hello',
  say: function update() {
    console.log('the word is: '+ this.name);

var World = Hello.extend({
  name: 'world'

var world = new World();
world.say(); // 'the word is: world'