class Store{  constructor (options = {}) {  // 1.部分2個‘斷言函數'判斷條件  assert(Vue, `must call Vue.use(Vuex) before creating a store   instance.`) // 在Store實例化之前一定要確保Vue的存在  assert(typeof Promise !== 'undefined', `vuex requires a Promise polyfill in this browser.`)  //確保promise存在    // 2.結構賦值拿到options里面的state，plugins和strict  const {  state = {}, //rootState  plugins = [], // 插件  strict = false //是否嚴格模式   } = options     // 3.Store internal state創建store內部屬性  this._options = options //存儲參數  this._committing = false //標識提交狀態，保證修改state只能在mutation里面，不能在外部隨意修改  this._actions = Object.create(null) //存儲用戶定義的actions  this._mutations = Object.create(null) //存儲用戶定義的mutations  this._wrappedGetters = Object.create(null) //存儲用戶定義的getters  this._runtimeModules = Object.create(null) //存儲運行時的modules  this._subscribers = [] //存儲所有堵mutation變化的訂閱者  this._watcherVM = new Vue() //借用Vue實例的方法，$watch來觀測變化    // 4.將dispatch和commit的this指向當前store實例  const store = this  const { dispatch, commit } = this  this.dispatch = function boundDispatch (type, payload) {  return dispatch.call(store, type, payload)}  this.commit = function boundCommit (type, payload, options) {  return commit.call(store, type, payload, options)}}