A Serializer's deserialize() function receives the raw serialized JSON value as its first argument and this registration function as the second. Functions registered with whenDone will be run last-in-first-out (stack order) when the raw JSON has been processed into the final object instance. WhenDone functions are needed whenever the serialized data is more complex than simple JSON values, for example when referential equality needs to be maintained or when the serialize function returns anything that needs to be re-serialized by some other serializer. Referenced objects may not be fully initialized when the registered function is called, but its instance will be instantiated so that references can be fixed.

Credit goes to json-dry for the whenDone concept.