var store = store = db.transaction(storename,'readwrite').objectStore(storename),request

请问这条语句什么意思啊

或者说下面为啥可以防止添加数据

//添加数据，重复添加会报错

    var store = store = db.transaction(storename,'readwrite').objectStore(storename),request;
    for(var i = 0 ; i < data.length;i++){
        request = store.add(data[i]);
        request.onerror = function(){
            console.error('add添加数据库中已有该数据')
        };
        request.onsuccess = function(){
            console.log('add添加数据已存入数据库')
        };
    }