alert(Object.getOwnPropertyNames(Array.prototype).join(","));
alert(Object.getOwnPropertyNames(Array.__proto__).join(","));
Типо так?
var trololo = {}
Object.defineProperty(trololo, "ololo", {enumerable: false})
for (var key in trololo) alert(key)
alert(Object.getOwnPropertyNames(trololo).join(","));