javascript - 如何通过 promise 映射

我有一个博客，上面有一些帖子。
在帖子表中，我插入了具有authorId的帖子。
现在我想从另一个称为用户的表中将所有authorId转换为authorName。
要进行转换，我使用一个函数getPostAuthor，该函数将诺言返回给主函数。
但我无法使用地图功能从getPostAuthor收集返回的值
有什么问题

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();

最佳答案

您需要等待使用.map收到的承诺才能解析以查看值，如下所示：

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);
});