Option<T>

Option<T> is accepted wherever the page for T lists Option<T> as supported: as an argument or return value of exported and imported functions, as a public field of an exported struct, and as a closure argument or return value.

Going from Rust to JS, None is undefined. Going from JS to Rust, both undefined and null are None, for every T.

The two directions differ because only one of them makes a choice. Sending a None has to pick a JS value, and that is undefined. Reading a JS empty decides nothing: a T | null value loses no information as an Option<T>, so a binding can read T | null as Option<T>. An exported optional argument is typed T | null | undefined in TypeScript, or arg?: T | null when trailing, and an optional return value T | undefined.

When a slot has to carry null out, or when both empties come in and the caller has to tell them apart, use Nullable<T> for T | null and Option<Nullable<T>> for T | null | undefined. The Option layer of Option<Nullable<T>> treats only undefined as None, since null has to reach the Nullable layer.