A class extender

Usage no npm install needed!

<script type="module">
  import extendMe from 'https://cdn.skypack.dev/extend-me';



A class extender

Update: This module is essentially obsoleted by ES6 class syntax, which is recommended going forward when possible (when your target browsers support it or your code is transpiled for unsupported browsers).


Node.js / Browserify:

npm install --save extend-me
var extend = require('extend-me');


<script src="http://fin-hypergrid.github.io/extend-me/extend-me.js"></script>


<script src="http://fin-hypergrid.github.io/extend-me/extend-me.min.js"></script>

Do not confuse this extend function with Underscore-style .extend() which is something else entirely. I've used the name "extend" here because other packages (like Backbone.js) use it this way. You are free to call it whatever you want when you "require" it, such as var inherits = require('extend').


First establish a base class

  1. Use either the provided extend.Base (which provides super support):
var Base = extend.Base;
  1. Roll your own base class:
var MyBase() { ... }
MyBase.extend = extend;

Then extend it

var MyClass = Base.extend(extendedClassName, prototypeAdditions);


  • Base is the base class being extended from. This could also be any descendant class (any class previously extended in this way).
  • extendedClassName Optional. This value, if provided, is copied to the prototype as $CLASS_NAME and is useful in debugging to identify the derived class, the name of which is otherwise (unfortunately) not displayed by the debugger. Could also be useful in your code. You can also name a class by including $CLASS_NAME or simply name in prototypeAdditions.
  • prototypeAdditions Required. A prototype object for the new class. The members of this object are added to the new constructor's prototype.


var MyClass = Base.extend({
    initialize: function () { ... },
    member1: ...,
    member2: ...

var MyChildClass = MyClass.extend({
    preInitialize: function () { ... },  // called before base class's initialize() */
    initialize: function () { ... },     // called after base class's initialize() and before derived class's initialize() */
    postInitialize: function () { ... }, // called after this class's initialize() */
    member1: ..., // overrides base class's definition of member1
    member3: ...

var a = new MyClass(), b = new MyChildClass();


var Parabola = Base.extend({
    initialize: function (a, b) {
        this.a = a;
        this.b = b;
    calculate: function(x) {
        return this.a * Math.pow(x, 2) + (this.b * x);

var ParabolaWithIntercept = Parabola.extend({
    initialize: function(a, b, c) {
        this.c = c;
    calculate: function(x) {
        var y = this.super.calculate.apply(this, arguments);
        return y + this.c;

var parabola = new ParabolaWithIntercept(3, 2, 1),
    y = parabola.calculate(-3); // yields 22


You may optionally supply an initialize method to be called as your practical constructor. It will be called upon object instantiation with the same parameters as passed to the actual constructor.

Initialization Chain

There may be initialize methods at each level of inheritance. Instantiating a derived class will automatically call initialize on all ancestor classes that implement it, starting with the most distant ancestor all the way up to and including the derived class in question. Each initialize method is called with the same parameters as passed to the constructor.

In the example above, on instantiation (var parabola = new ParabolaWithIntercept(3, 2, 1)), Parabola.prototype.initialize is called first; then ParabolaWithIntercept.prototype.initialize.

To add initialization code to be executed before and/or after this chain of initialize calls, you can define methods preInitialize and/or postInitialize, respectively. These are not part of the initialization chain. They are only called on the object being instantiated; they are not called when a derived class is being instantiated. For example, in the sample usage above, if MyClass had had a preInitialize method, it would be called on a's instantiation but not b's.


A base class is provided in extend.Base. This base class contains the methods described below.

Use of Base is not required, however, as you can also create your own base class simply by adding extend to it (see Syntax above).

function MyBase() {}
MyBase.extend = extend;

The following methods are available in the prototype of extend.Base.


Reference to the immediate ancestor in the prototype chain. Implemented as a getter on the Base's prototype. See example above.

NOTE: super is not a reference to ancestral constructor; ancestral constructors are always called automatically, as described above.


Find member on prototype chain beginning with super class.


Find method on prototype chain beginning with super class.

callSuperMethod(methodName, arg1, arg2, arg3, ...)

Find method on prototype chain beginning with super class and call it with remaining args.


A demo/test using the ParabolaWithIntercept class described above can be found on github.io.

Update history


  • Simplified repo as it no longer has any dependencies (since 2.7.0)
  • Removed gulpfile.js and its dependencies
  • Added build.sh
  • Removed vapid test
  • Removed custom jsdoc template
  • Moved to new home in the fin-hypergrid Github organization


  • Added getClassName as both static method to extended class and as prototype method to Base.
  • Removed overrider dependency.


  • Added postExtend, an optional static method of the base "class" (constructor). When defined, it is called at the end of extend() with the new "class" (new constructor) as its sole parameter. This permits miscellaneous tweaking and cleanup of the new class.


  • Added parent(/*optional*/ancestorConstructorName) to constructor to get the parent class's constructor or the named ancestor class's constructor.
  • Now resets returned constructor's name to extendedClassName OR prototypeAdditions.$CLASS_NAME in the prototype OR prototypeAdditions.name


Previously, on instantiation, the preInitialize and postInitialize methods were called if and only if they were defined on the subclass's (extended object's) own prototype. This has been changed so that the "top" such methods on the prototype chain are now called, whether defined on the extended class or on an ancestor class. As before, these methods are called before and after the initialize cascade, respectively. Unlike initialize, however, there is no cascade; only the top most method is ever called.

This was an oversight and the workaround has been to forward the calls by redefining new methods with these names whose sole function was to forward the call to this.super. This change is backwards compatible with that workaround; it will simply call the forwarding method as before. However, it is now safe to remove the forwarding methods altogether and the call will be made for you.

This can be considered a breaking change because previously without the workarounds, such ancestor methods were not executed. If you were dependent on this unlikely scenario, you can restore that behavior by defining new methods with these names as no-ops.