One of the biggest flaws of JSON is that it doesn't support "undefined". This makes translating Javascript structures to and from JSON actually not preserve the original value. Sigh.
But undefined is specific to Javascript… there are lots of other Javascript things that JSON doesn't handle either, like Set or Map objects. It's not intended to serialize arbibtrary JS objects — it's intended to serialize a useful least-common-denominator which has proven useful through experience.
