T.omittable

An absence primitive: a field that's sometimes missing from the enclosing object entirely. Only meaningful inside T.object(...) — it describes whether a key exists, not a value in isolation.

T.object({
  name: T.string.whereby({ length: { max: 20 } }),
  nickname: T.omittable(T.string.whereby({ length: { max: 20 } })),
});

A 50/50 roll by default: nickname either doesn't appear on the fabricated object at all, or holds a fabricated string. Reweight either side:

T.omittable(inner).weighted({ omitted: 1, value: 4 }); // present 80% of the time

Force the field off (e.g. in .override() or .fabricate(overrides)), use the exported Omitted sentinel:

import { Omitted } from "@ghostry/fabricator";
 
WidgetFabricator.fabricate({ nickname: Omitted });

For a three-way split across omitted / present-as-undefined / present with a value, use T.optional — not T.omittable(T.undefinable(inner)), which would give 50/25/25. For a key that's always present with a possibly-undefined value, use T.undefinable.