Vue报错：TypeError: Cannot create property 'xxx' on string 'xxxx'

报错详情：

[Vue warn]: 
Error in callback for immediate watcher "fileList": 
"TypeError: Cannot create property 'uid' on string 'string_url_1'"
...

报错原因：
TypeError: Cannot create property ‘xxx’ on string ‘xxxx’，此类错误是赋值的类型错误，如以上示例错误，在使用ElementUI的upload组件时，把字符串列表赋值给了fileList，而fileList需求的是对象的列表。

示例错误代码：

pictureList = ['string_url_1', 'string_url_2'];
this.fileList = pictureList;

示例正确代码：

pictureList = ['string_url_1', 'string_url_2'];
this.fileList = pictureList.map(item => {
       return {
         name: item,
         url: baseUrl + item
       }
});