Extensible constructor pattern (ES5 version) inspired by Backbone.js – NOTE: Using `this.super` multiple times in the inheritance chain creates a problem since it results in a loop. There hasn't yet been an elegant solution to this, but a proper `super` keyword will be supported in ES6. In the meantime, coupling with the super's name can be avoi…

Class.js

function Class(properties) {
  // This base constructor can be left empty, but a nice boilerplate might look like this
  if (properties) {
    // Add properties to the instance
    Object.getOwnPropertyNames(properties).forEach(function (property_name) {
      var descriptor = Object.getOwnPropertyDescriptor(properties, property_name);
      Object.defineProperty(this, property_name, descriptor);
    }, this);
  }
}

// Extensible constructors pattern inspired by Backbone.js
Class.extend = function (prototype) {
  var constructor;
 
  // Use constructor function from passed in prototype or from superclass
  if (prototype && prototype.hasOwnProperty("constructor") && typeof prototype.constructor === "function") {
    constructor = prototype.constructor;
    delete prototype.constructor;  // This property is re-attached later
  }
  else {
    var self = this;
    constructor = function Subclass() { return self.apply(this, arguments); };
  }
  
  // Set up the prototype chain
  constructor.prototype = Object.create(this.prototype);

  // Locked down references
  Object.defineProperties(constructor.prototype, {
    "constructor": { value: constructor },
    "super": { value: this.prototype }
  });
  
  if (prototype) {
    // Add extended prototype properties to the subclass
    Object.getOwnPropertyNames(prototype).forEach(function (property_name) {
      var descriptor = Object.getOwnPropertyDescriptor(prototype, property_name);
      Object.defineProperty(constructor.prototype, property_name, descriptor);
    });
  }
 
  // Add extend capability and reference to super constructor
  constructor.extend = this.extend;
  constructor.super = this;
 
  return constructor;
};

/* ... continued from above */

var Mammal = Animal.extend({
  constructor: function Mammal() {
    // Instance field
    this.endothermic_amniote = true;

    console.log("Mammal, Ma'am");

    // Also call super's constructor function in this context
    this.super.constructor.apply(this, arguments);
  }
});

var my_cow = new Mammal("mootable");  // "Mammal, Ma'am" "I'm mootable!"