It isn't always possible to know all the keys to the ExtensionData after it's been set, for example, if there's a parser function that may be included multiple times on a page and thus stores multiple sets of data via function hooks (T121068). We could store the keys in some kind of cache, but that would defeat the entire purpose of using ExtensionData. Instead there should be a way to retrieve all the ExtensionData at once (without specifying any keys).