Read original post at Kraig Brockschmidt's Blog
In WinRT, a number of APIs return a Windows.Foundation.Collections.PropertySet object. Two examples are the localSettings and roamingSettings objects within Windows.Storage.ApplicationData.current, whose values propert is a PropertySet.
In JavaScript, doing a simple for…in iteration on a PropertySet will give you not only the properties in that set, but also its methods, which is often unexpected.