map三层循环遍历，操作数据

let tempArr = this.oldCityList.map(item => {
    return {
        value: item.code,
        text: item.name,
        type: item.type,
        children: item.citys.map(subItem => {
            return {
                value: subItem.code,
                text: subItem.name,
                type: subItem.type,
                children: subItem.county.map(thirdItem => {
                    return {
                        value: thirdItem.code,
                        text: thirdItem.name,
                        type: thirdItem.type,
                    }
                })
            }
        })
    }
});
this.newCityList = tempArr;