CHANGELOG.md

Changelog

useSelector(atom, selector, equals?) React hook (chem-rx/react): derive any value from an atom with a selector function and re-render only when the selected value changes. The optional equals comparator (defaults to Object.is) dedupes re-renders by content, so an atom that churns references on every update only re-renders the component when the selected slice is not equal. See useSelector.

Removed useSelectAtom. Migrate to useSelector with a selector function: useSelectAtom(atom, 'key') becomes useSelector(atom, (s) => s.key).

Optional equals comparator on every atom creation path: Atom(value, { equals }), atom.derive(fn, { equals }), and atom.select(key, { equals }). Supply a content comparator to dedup notifications by data instead of identity. See Equality & change detection.
Atom()'s second argument now also accepts an options object ({ readOnly?, equals? }) in addition to the legacy readOnly boolean.
Exported Equals<T> and AtomFactoryOptions<T> types.

Every atom now decides whether to notify subscribers using an equality comparator that defaults to Object.is:
- A base atom no longer re-emits when next() is called with an Object.is-equal value (previously every next() notified).
- derive no longer re-emits when its computed output is Object.is-equal (previously it re-emitted on every parent update).
- select and combine are unchanged in practice (already Object.is).
Migration: pass equals: () => false to restore the previous always-notify behavior.