edit for clarity: You're assuming that the application code isn't doing something with each key that it reflectively sees in the object, e.g. creating database fields to match them, or launching missiles towards those destinations, etc.. If you wouldn't automatically add dummy elements to a hashmap or dictionary in Java or Python, then you shouldn't add keys in a javascript object, unless you control the source to the program that will processing the data. Even then you shouldn't, because it will become a habit to add comments this way, and that will bite you when an extra key does matter.
edit for clarity: You're assuming that the application code isn't doing something with each key that it reflectively sees in the object, e.g. creating database fields to match them, or launching missiles towards those destinations, etc.. If you wouldn't automatically add dummy elements to a hashmap or dictionary in Java or Python, then you shouldn't add keys in a javascript object, unless you control the source to the program that will processing the data. Even then you shouldn't, because it will become a habit to add comments this way, and that will bite you when an extra key does matter.