nodejs arr.prototype.map 無法新增對象屬性

PageTagMapModel.getPageTagMapNum({ tag: single._id, status: 'normal' })返回的是mongoose查詢到的page_num
但是代碼中要是用注釋的三行，得到的數(shù)據(jù)中就沒有page_num屬性。深拷貝下single后再賦值可用=。=不知道為嘛

 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
}))