combineReducers

function combineReducers(reducers) {
  if(!reducers) {
    throw new Error('reducers错误')
  }
  const reducerKeys = Object.keys(reducers)
  return (state = {}, action) => {
    // 派发动作是否引起了状态的改变
    let isChange = false;
    const nextState = {};
    for(const key of reducerKeys) {
      const prevStateKey = state[key];
      const reducer = reducers[key];
      const nextStateKey = reducer(prevStateKey, action);
      nextState[key] = nextStateKey
      isChange = isChange || nextStateKey !== prevStateKey;
    }
    return isChange ? nextState : state;
  }
}
export default combineReducers