[kaon_]

One positive note: I think the name "undefined" is actually very fitting for what it represents.

[DemocracyFTW2]

The thing is though that `{ a: 1, d: undefined, }` and `{ a: 1, }` make a difference that shouldn't be there at all, and in this case its name is not very fitting as it does define an property of the object. This can be extremely annoying and the only way out is to always have tests around `null`, `undefined` and missing keys and have coding standards like I for one never use `undefined` explicitly; whenever it does occur it's likely a mistake; even functions without return value I make return `null` ("see, I have nothing to say here") explicitly instead of relying on the implicit `undefined`.