# Option conventions

- `match()` takes **2 callbacks**: `(onSome, onNone)`
- `filter()` takes ONLY a predicate — **no errorFactory** (unlike Result)
- `mapErr()` and `flatMapErr()` do **NOT exist** on Option
- `none()` returns a frozen singleton
- `Option.from()` treats only `null | undefined` as None — `0`, `false`, `""` become `Some`
- `flatMap` callback must return `Option<any>`
- Async return type: `OptionPromise<T>` — never `Promise<Option<T>>`

See `/native-monad-option` skill for full API reference.