Object.prototype.__delegated__ = function(x) {
  function f() { }
  f.prototype = this
  var r = new f()
  if (x)
    for (var prop in x)
      if (x.hasOwnProperty(prop))
        r[prop] = x[prop]
  return r
}

C = {
  initialize: function(args) { ... },
  x: undefined,
  y: undefined
}

D = C.__delegated__({
  initialize: function(args) { C.initialize.apply(this, args)
                                          z = 5 },
  z: undefined
})

x = {a: 1, b: 2}
y = x.__delegated__({c: 3})

y.a1
y.b2
y.c 3