[SigmundA]

There are two common "nulls" in this situation, the "unset" one and the "unknown" one. Unset would imply no one has ever set the value, unknown means someone has set the value to unknown which is sometimes an important distinction.

Javascript this is easy undefined can represent unset, and null can represent unknown. This is very useful for updating the state as well, undefined means do not change the value while null means update the value to unknown.

I have used code (enums) for yes/no/unknown that allow null value for unset as well in relational db's. If I had a choice I prefer JavaScripts data model of a boolean that can be undefined or null as well.

A nullable boolean in general has been better for a tristate than enums, but like I said many times I need a quadstate.