T.nullable

An absence primitive: a value that is either the wrapped inner or null. An ordinary schema, usable anywhere.

T.nullable(T.string); // string | null, 50/50
T.nullable(inner).weighted({ null: 1, value: 9 }); // null 10% of the time

For null or undefined as well as a value, use T.nullish — not T.nullable(T.undefinable(inner)), which would give 50/25/25. For a value that is always exactly null, use T.null.