redux&&createStore

const createStore = (reducer,presetState, enhancer) =>  {
    if (typeof presetState === "function") {
        enhancer = presetState
        presetState = undefined
    }
    if (typeof enhancer === 'function') {
       return enhancer(createStore)(reducer,presetState)
    }
     if (typeof reducer  !== "function") {
         throw new Error('传入的reducer不是一个函数')
     }
    let state = presetState
    let listeners = []
    const getState = () => state
    const dispatch = (action) => {
      state =  reducer(state, action)
        listeners.forEach(item => item())
    }
    const subscribe = (fn) => {
        listeners.push(fn)
    }
    dispatch({type:'@@react/Init'})
    return {
        getState,
        dispatch,
        subscribe
    }
}