T.undefinable

An absence primitive: a value that is either the wrapped inner or literally undefined. An ordinary schema, usable anywhere — a top-level Fabricator, an array element, or an object field that stays required.

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

A weight of 0 disables that outcome ({ undefined: 0 } always produces the inner value). Every weight zero throws; negative weights, NaN, and Infinity are invalid. Zeroing changes the distribution, not the type: the value is still T | undefined.

This is a fact about the value, not about whether a key exists. For a field that sometimes isn't on the object at all, use T.omittable. For omitted / undefined / value as a three-way split, use T.optional.