class.js

var defineClass = function(classDefinition) {
  var constructor = function() {
    if ("initialize" in this) this.initialize(arguments)
  }

  if (classDefinition) classDefinition.call(constructor.prototype, constructor)

  return constructor
}

var inherit = function(childClass, parentClass) {
  var p, proto = parentClass.prototype

  for (p in proto)
    childClass.prototype[p] = proto[p]

  if (proto.initialize)
    proto.initialize.call(this)
}

example.js

var Class = defineClass(function() {
  // Public interface, "this" is the prototype
  this.initialize = function() {
    console.log("init")
  }

  this.foo = function() {
    bar()
  }

  // Private methods
  var bar = function() { console.log("baz") }
})

var SubClass = defineClass(function(constructor) {
  this.initialize = function() {
    inherit.call(this, constructor, Class)
    inherit.call(this, constructor, Mixin) // Mixins use the same mechanism
  }
})

var Mixin = defineClass(function() {
  this.toString = function() {
    return "MIXIN ENABLED{" + {}.toString.call(this) + "}"
  }
})