Exploring the limitations of module state

Because module state resides outside React components, there's a limitation: the module state defined globally is a singleton, and you can't have different states for different component trees or subtrees.

Let's revisit our createStore implementation from Chapter 4, Sharing Module State with Subscription:

const createStore = (initialState) => {
  let state = initialState;
  const callbacks = new Set();
  const getState = () => state;
  const setState = (nextState) => {
    state = typeof nextState === 'function'
      ? nextState(state) : nextState;
    callbacks.forEach((callback) => callback());
  };
  const subscribe =(callback) => {
    callbacks.add(callback);
    return () => { callbacks.delete(callback); };
  ...