baseClass.js

// The base constructor (can be left empty, but a nice boilerplate might look like this)
function Class(properties) {
  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 new prototype or from superclass
  if (prototype && prototype.hasOwnProperty("constructor") && typeof prototype.constructor === "function") {
    constructor = prototype.constructor;
    delete prototype.constructor;  // This property is handled further down
  }
  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;
};