如何将承诺传递给地图答案

【问题标题】：how to pass promise to map如何将承诺传递给地图
【发布时间】：2017-08-20 04:04:04
【问题描述】：

我有一个博客，上面有一些帖子。
在 post 表中，我插入了带有 authorId 的帖子。现在我想从另一个名为 user 的表中将所有 authorId 转换为 authorName。
为了进行转换，我使用了一个函数 getPostAuthor，它向主函数返回一个承诺。
但我无法使用 map 函数从 getPostAuthor 收集返回值
什么问题？

var db = require('../controllers/db');
var mongo = require('mongodb').MongoClient();

var url = "mongodb://localhost:27017/blog";
var ObjectId = require('mongodb').ObjectID;


//#1: this function lists all posts in my blog
const list = function(req, res){
    db.find('post', {}, 10, {timeCreated: 1})
    .then(posts => {
        var promises = posts.map(post =>
            getPostAuthor(post.author)
            .then(author => author /* this value doesn't go to promises*/)
        );
        console.log(promises); //printed [Promise {<pending>}, ...]
    })
}

//#2: this function gets authorId and gives authorName
const getPostAuthor = function(authorId){
    return db.findOne('user', {_id: new ObjectId(authorId)})
    .then(author => author.name);
}

//#3: finds from db
const find = function(collection, cond = {}, limit = 0, sort = {}){
        return mongo.connect(url)
        .then(db =>
            db.collection(collection)
            .find(cond)
            .limit(limit)
            .sort(sort)
            .toArray()
        )
}


//#4: finds one from db
const findOne = function(collection, cond = {}){
        return mongo.connect(url)
        .then(db =>
            db.collection(collection).findOne(cond)
        )
}


list();

【问题讨论】：

.then(posts => { 没有返回任何东西......所以你已经在变量 promises 中构建了一个承诺数组......并丢弃了它 - 事实上，list 函数本身什么都不返回，所以，不确定你对代码的期望是什么
你可能需要做Promise.all(promises).then(results => { /* do something with the resolved promises here */});
在这一步中我只想要打印承诺并查看一组作者姓名
它们是承诺，您需要等待它们解决，请参阅第二条评论：p
tnx，这工作正常。

标签： javascript arrays asynchronous promise

【解决方案1】：

您需要等待使用 .map 收到的承诺解析才能看到值 - 像这样：

const list = function(req, res){
    db.find('post', {}, 10, {timeCreated: 1})
    .then(posts => {
        var promises = posts.map(post =>
            getPostAuthor(post.author)
            .then(author => author /* this value doesn't go to promises*/)
        );
        console.log(promises); //printed [Promise {<pending>}, ...]
        // ********* 3 lines added to your code here
        Promise.all(promises).then(authors => {
            console.log(authors);
        });
    })
}

或者

const list = function(req, res){
    // added return below
    return db.find('post', {}, 10, {timeCreated: 1})
    .then(posts => {
        var promises = posts.map(post =>
            getPostAuthor(post.author)
            .then(author => author /* this value doesn't go to promises*/)
        );
        console.log(promises); //printed [Promise {<pending>}, ...]
        // *** added line below
        return Promise.all(promises);
    })
}

...

list().then(authors => {
    console.log(authors);
});

【讨论】：