feat(react): modernize useSyncExternalStore usage for React 18+ (#1021)

- Drop use-sync-external-store shim; use native useSyncExternalStore
  from react and inline the with-selector logic (mirrors React's
  reference implementation).
- Remove initializeUseStoreState indirection — no longer needed since
  React 18+ is the floor (per peerDependencies).
- Migrate useLocalStore from raw useState/subscribe to useSyncExternalStore
  to prevent tearing in concurrent mode.
- Add 'use client' banner to ESM and CJS bundles via Rollup so the
  package works correctly in React Server Components environments
  (Next.js App Router etc).
- Remove use-sync-external-store from runtime dependencies.

Refs #1004. Deferred React 18/19 primitive work tracked in #1020.

Author: ctrlplusb

package.json

@@ -97,8 +97,7 @@
     "immer": "^9.0.21",
     "redux": "^5.0.1",
     "redux-thunk": "^3.1.0",
-    "ts-toolbelt": "^9.6.0",
-    "use-sync-external-store": "^1.4.0"
+    "ts-toolbelt": "^9.6.0"
   },
   "devDependencies": {
     "@babel/core": "^7.22.9",

rollup.config.js

@@ -8,10 +8,17 @@ const external = (id) => !id.startsWith('.') && !id.startsWith(root);
 
 const extensions = ['.js'];
 
+const useClientBanner = `'use client';`;
+
 function createESMConfig(input, output) {
   return {
     input,
-    output: { sourcemap: true, file: output, format: 'esm' },
+    output: {
+      sourcemap: true,
+      file: output,
+      format: 'esm',
+      banner: useClientBanner,
+    },
     external,
     plugins: [
       resolve({ extensions }),
@@ -28,7 +35,13 @@ function createESMConfig(input, output) {
 function createCommonJSConfig(input, output) {
   return {
     input,
-    output: { sourcemap: true, file: output, format: 'cjs', exports: 'named' },
+    output: {
+      sourcemap: true,
+      file: output,
+      format: 'cjs',
+      exports: 'named',
+      banner: useClientBanner,
+    },
     external,
     plugins: [
       resolve({ extensions }),

src/hooks.js

@@ -1,16 +1,96 @@
-import { useContext, useDebugValue, useEffect, useState } from 'react';
+import {
+  useContext,
+  useDebugValue,
+  useEffect,
+  useMemo,
+  useRef,
+  useState,
+  useSyncExternalStore,
+} from 'react';
 import EasyPeasyContext from './context';
 
-const useNotInitialized = () => {
-  throw new Error('uSES not initialized!');
-};
-let useSyncExternalStoreWithSelector = useNotInitialized;
-export const initializeUseStoreState = (fn) => {
-  useSyncExternalStoreWithSelector = fn;
-};
-
 const refEquality = (a, b) => a === b;
 
+function useSyncExternalStoreWithSelector(
+  subscribe,
+  getSnapshot,
+  getServerSnapshot,
+  selector,
+  isEqual,
+) {
+  const instRef = useRef(null);
+  let inst;
+  if (instRef.current === null) {
+    inst = { hasValue: false, value: null };
+    instRef.current = inst;
+  } else {
+    inst = instRef.current;
+  }
+
+  const [getSelection, getServerSelection] = useMemo(() => {
+    let hasMemo = false;
+    let memoizedSnapshot;
+    let memoizedSelection;
+    const memoizedSelector = (nextSnapshot) => {
+      if (!hasMemo) {
+        hasMemo = true;
+        memoizedSnapshot = nextSnapshot;
+        const nextSelection = selector(nextSnapshot);
+        if (isEqual !== undefined && inst.hasValue) {
+          const currentSelection = inst.value;
+          if (isEqual(currentSelection, nextSelection)) {
+            memoizedSelection = currentSelection;
+            return currentSelection;
+          }
+        }
+        memoizedSelection = nextSelection;
+        return nextSelection;
+      }
+
+      const prevSnapshot = memoizedSnapshot;
+      const prevSelection = memoizedSelection;
+
+      if (Object.is(prevSnapshot, nextSnapshot)) {
+        return prevSelection;
+      }
+
+      const nextSelection = selector(nextSnapshot);
+
+      if (isEqual !== undefined && isEqual(prevSelection, nextSelection)) {
+        memoizedSnapshot = nextSnapshot;
+        return prevSelection;
+      }
+
+      memoizedSnapshot = nextSnapshot;
+      memoizedSelection = nextSelection;
+      return nextSelection;
+    };
+
+    const getSnapshotWithSelector = () => memoizedSelector(getSnapshot());
+    const getServerSnapshotWithSelector =
+      getServerSnapshot == null
+        ? undefined
+        : () => memoizedSelector(getServerSnapshot());
+
+    return [getSnapshotWithSelector, getServerSnapshotWithSelector];
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [getSnapshot, getServerSnapshot, selector, isEqual]);
+
+  const value = useSyncExternalStore(
+    subscribe,
+    getSelection,
+    getServerSelection,
+  );
+
+  useEffect(() => {
+    inst.hasValue = true;
+    inst.value = value;
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [value]);
+
+  return value;
+}
+
 export function createStoreStateHook(Context) {
   return function useStoreState(mapState, equalityFn = refEquality) {
     if (process.env.NODE_ENV !== 'production') {
@@ -31,19 +111,10 @@ export function createStoreStateHook(Context) {
 
     const store = useContext(Context);
 
-    /*
-    function useSyncExternalStoreWithSelector<Snapshot, Selection>(
-        subscribe: (onStoreChange: () => void) => () => void,
-        getSnapshot: () => Snapshot,
-        getServerSnapshot: undefined | null | (() => Snapshot),
-        selector: (snapshot: Snapshot) => Selection,
-        isEqual?: (a: Selection, b: Selection) => boolean,
-    ): Selection;
-    */
     const selectedState = useSyncExternalStoreWithSelector(
       store.subscribe,
       store.getState,
-      store.getState, // getServerSnapshot = getState
+      store.getState,
       mapState,
       equalityFn,
     );

src/index.js

@@ -1,13 +1,3 @@
-// React 18 requires the use of the useSyncExternalStore hook for external
-// stores to hook into its concurrent features. We want to continue supporting
-// older versions of React (16/17), so we are utilsing a shim provided by the
-// React team which will ensure backwards compatibility;
-import { useSyncExternalStoreWithSelector } from 'use-sync-external-store/shim/with-selector';
-
-import { initializeUseStoreState } from './hooks';
-
-initializeUseStoreState(useSyncExternalStoreWithSelector);
-
 export * from './hooks';
 export * from './create-store';
 export * from './create-context-store';

src/use-local-store.js

@@ -1,4 +1,4 @@
-import { useEffect, useRef, useState } from 'react';
+import { useRef, useSyncExternalStore } from 'react';
 import { useMemoOne } from './lib';
 import { createStore } from './create-store';
 
@@ -24,17 +24,11 @@ export function useLocalStore(
     return _store;
   }, dependencies);
 
-  const [currentState, setCurrentState] = useState(() => store.getState());
-
-  useEffect(() => {
-    setCurrentState(store.getState());
-    return store.subscribe(() => {
-      const nextState = store.getState();
-      if (currentState !== nextState) {
-        setCurrentState(nextState);
-      }
-    });
-  }, [store]);
+  const currentState = useSyncExternalStore(
+    store.subscribe,
+    store.getState,
+    store.getState,
+  );
 
   return [currentState, store.getActions(), store];
 }