From the es-discuss posts:
> I suspect that this only applies to builtin properties on objects, as I think JSC has a similar issue.
> Speaking for JSC (but i wouldn't be surprised if V8 did something similar) we will delay the creation of the majority of builtin properties until they're actually used, eg. until you actually access Array.prototype.reduce we won't reify the property. A side effect of this is that when you simply assign to the property we skip reification, and so the property attributes are the same as you would get if you were creating a new property.
> A simple test would be to see if
> Array.prototype.reduce = ...
> Results in the correct behavior.
I have created a test case here:
Related V8 issue: