無関係な React コンポーネント間での状態の共有

export const state: { count: number } = { count: 0 };

const stateMap = new WeakMap<object, object>();
const subscribers: (() => void)[] = [];

export function useCommonState<T extends object>(stateObj: T) {
  // more efficient than `useEffect` since we don't have any deps
  React.useInsertionEffect(() => {
    const cb = () => {
      const val = stateMap.get(stateObj);
      _setState(val!);
    };
    // subscribe to events
    subscribers.push(cb);

    return () => {
      subscribers.slice(subscribers.indexOf(cb), 1);
    };
  }, []);
}

  // all instances of hook will point to same object reference
  const [state, _setState] = React.useState<typeof stateObj>(() => {
    const val = stateMap.get(stateObj) as T;
    if (!val) {
      stateMap.set(stateObj, stateObj)
      return stateObj
    }
    return val
  });

  const setState = React.useCallback((newVal: object) => {
    // update value
    stateMap.set(stateObj, newVal);
    // notify all other hook instances
    subscribers.forEach((sub) => sub());
  }, []);

  return { state, setState };

import { state as myState } from './state';
//...

const { state, setState } = useCommonState(myState);

<button
  onClick={() => setState({ count: state.count + 1 })}
  className="p-2 border"
>
  +
</button>
// ...
Component A<div>Count: {state.count}</div>