為什麼封裝Storage
  Storage本身有API，但是隻是簡單的key/value形式
  storage只儲存字串，需要人工轉換json物件
  storage只能一次性清空，不能單個清空
  mall  ｛“user”:{“userName”:’JAVA’,”age”:30 ,”sex”:1}｝
  const STORAGE_KET = ‘mall’;
  conmonjs規範
es6規範

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import storage from ‘./storage’
 


export default{
    //儲存值
setItem(Key，value,module_name){
    if(module_name){
let val = this.getItem(module);
  val[key]= value;
    this.setItem(module,val);
   }else{
    ley val = this.getStorage();
    val[key] = value;
window.sessionStorage.setItem(STORAGE_SET.JSON.stringify(val));
   }
   
   },
   //獲取某一個模組下面的屬性user下面的username
   getItem(key,module_name){
       if(module_name){
        let val = this.getItem(module);
        if(val) return val[key];
      }
       return this.getStorage()[key];
   },
   //getStorage(){
     return json.parse(window.sessiongStorage.getItem( STORAGE_KET) ||‘{}’);
   },
   clear(key,module_name){
        let val = this.getStorage();
        if(module_name){
          delete val[module_name][key];

       }else{
        delete val[key];
      }
      this.setItem(val);
   }
}