simplied.js

const reducer = (oldState, action) => {
  if (typeof action === "function") {
    return action(oldState) 
  } else {
    return action;
  }
}

// useState is simplified useReducer
const useState = (initialState, init) => {
  // it returns [state, dispatch]
  // dispatch === setState in this case, because of the way we implemented reducer
  return useReducer(reducer, initialState, init);
}

// useCallback is simplified useMemo
const useCallback = (callback, dependencies) => {
  return useMemo(() => callback, dependencies);
}