Simple JavaScript class implementation with inheritance.

class.js

// Class implementation with inheritance.
//
// Creates a constructor function with all the properties of ``params`` and
// keeps the ``constructor`` property of the prototype intact.
//
// Chains the object of special ``params`` key ``Extends`` on the prototype
// and prevents multiple calls to initialize in a MooTools Class compatible
// way. Property ``parent`` of the prototype is set to the chained object.
var Class = function(params) {
  this.createConstructor();
  
  if (params.hasOwnProperty("Extends")) {
    this.extend(params.Extends);
    delete params.Extends;
  }
  
  this.implement(params);
  
  return this.constructor;
};

Class.prototype.createConstructor = function() {
  var constructor = this.constructor = function() {
    if (constructor.$prototyping) return undefined;
    return this.initialize ? this.initialize.apply(this, arguments) : undefined;
  }
};

Class.prototype.extend = function(parent) {
  parent.$prototyping = true;
  this.constructor.prototype = new parent();
  delete parent.$prototyping;
  
  this.constructor.prototype.constructor = this.constructor;
  this.constructor.prototype.parent = parent;
};

Class.prototype.implement = function(params) {
  for (var k in params) {
    this.constructor.prototype[k] = params[k];
  }
};

usage.js

var Thing = new Class({
  initialize: function(name) {
    this.name = name;
  },
  
  describe: function() {
    console.log(this.name);
  }
});

var SpecialThing = new Class({
  Extends: Thing,
  
  describe: function() {
    console.log(this.name + ", but special");
  }
});

console.log(Thing.prototype); // Object with 'constructor' property intact
console.log(SpecialThing.prototype); // Object with Thing as prototype and 'parent' property pointing to it

var thing = new Thing("Anything");
thing.describe(); // Logs: 'Anything'

var specialthing = new SpecialThing("Something"); // 'initialize' is called just on object creation, not when prototyping
specialthing.describe(); // Logs: 'Something, but special'