gistfile1.txt

var Config = function(){};
Config.prototype.a = 100;
Config.prototype.b = 200;

var Flags = function() {};
Flags.prototype = new Config();
Flags.constructor = Flags;
Flags.prototype.a = 3;
Flags.prototype.c = 5;

var options = new Flags();

js> options.a
3
js> options.b
200
js> options.c
5
js> options instanceof Flags
true
js> options instanceof Config
true