Configuring slice‑specific filtering for Redux store subscriptions

Answer

The official way to receive notifications only when a chosen slice of the Redux state changes is to wrap store.subscribe with a selector function and a reference‑equality check. This pattern is documented in the Redux FAQ and is the basis for useSelector in React‑Redux.

Confirmed facts

• store.subscribe fires after every dispatch; the core API has no built‑in filtering.
• The Redux maintainers intentionally leave slice‑specific filtering to subscribers or enhancers.
• React‑Redux’s useSelector (or connect) already implements selector‑based equality checks, so manual subscription filtering is rarely needed in React apps.

Likely explanation / recommended steps

1. Define a selector that returns the slice you care about, e.g. const selectSlice = state => state.mySlice;
2. Create a small helper that compares the previous and current slice using Object.is (or a custom equality function) and only calls the listener when they differ.
3. Pass that helper to store.subscribe instead of the raw listener.
4. If you are using React, replace the manual subscription with useSelector(selectSlice, equalityFn) (or connect) which does the same work automatically.

function observeStore(store, select, onChange) {
  let current = select(store.getState());
  return store.subscribe(() => {
    const next = select(store.getState());
    if (!Object.is(current, next)) {
      const prev = current;
      current = next;
      onChange(prev, next);
    }
  });
}

// usage
const unsubscribe = observeStore(store, state => state.users, (prev, next) => {
  console.log('users slice changed', prev, next);
});

Verification

• Dispatch an action that does not modify the selected slice and confirm the listener is not called.
• Dispatch an action that does modify the slice and confirm the listener runs exactly once.
• If using React, open React DevTools and check that components using useSelector do not re‑render on unrelated dispatches.

Missing diagnostic detail

Are you building a plain Redux application or a React‑Redux application? The recommendation shifts to useSelector for React projects, while the observeStore helper is the direct solution for plain Redux.