gistfile1.js

var sys = require('sys');

function Child() {}
// Defined on the prototype.
Object.defineProperty(Child.prototype, 'foo', { enumerable: false, value: 'test', writable: true });
Object.defineProperty(Child.prototype, 'bar', { enumerable: false, value: 'test2', writable: true });

var test = new Child;
// Defined local to the object "instance".
Object.defineProperty(test, 'baz', { enumerable: false, writable: true });
test.foo = 'bar';   // "overwrites" enumerable value of 'foo'.
test.baz = 'test3';


for (var key in test) {
  sys.puts(key);             // -->  foo
}

sys.puts('----');
sys.puts(test.foo);          // -->  bar
sys.puts(test.bar);          // -->  test2
sys.puts(test.baz);          // -->  test3