type UnsafeJsonSerializableOrUndefined: JsonSerializable | undefined;Deprecated
Prefer using JsonSerializable directly in new code.
Any type that can be natively converted to a string with JSON.stringify, and also `undefined, which will not be converted to a string.
This type is used correctly as a nested value inside JsonArray and JsonObject, and also exists to reflect legacy
behavior from before the distinction between null and undefined had been made. If you call JSON.stringify on
a value with this type, it may return a valid JSON string, or it may return undefined.
If you are later attempting to parse a value serialized in this way in other parts of the application, it may not be safe to unwrap or parse that value.