Data values can be “optional” - the meaning of this varies in practice depending on the implementation language, but in JavaScript these are generally serialized as null.
Optionals are denoted by the suffix ?.
Some languages, like Rust and Swift, have a concept of monadic nestable optionals implemented with sum types. This is extremely powerful and robust, but unfortunately does not map directly to JSON which only has a single null value. We are exploring the implications of this idea and tracking it in Unions & Monadic Optionals.