nodejs arr.prototype.map 无法新增对象属性

Question

PageTagMapModel.getPageTagMapNum({ tag: single._id, status: 'normal' })返回的是mongoose查询到的page_num
但是代码中要是用注释的三行，得到的数据中就没有page_num属性。深拷贝下single后再赋值可用=。=不知道为嘛

 result = await Promise.all(result.map(async (single) => {
    // single.page_num = await PageTagMapModel.getPageTagMapNum({ tag: single._id, status: 'normal' })
    // console.log(single)
    // return single
    let tag = JSON.parse(JSON.stringify(single))
    tag.page_num = await PageTagMapModel.getPageTagMapNum({ tag: single._id, status: 'normal' })
    return tag
}))

Answer 1

因为single是一个document

single = single.toObject()