如何在 node.js 中将函数与 Promise 同步

Question

嗨，我正在尝试将我的函数与将 callback 转换为 promise 同步。
我想通过 forEach 循环添加到所有帖子、post.authorName 字段并查询到 user collection。
首先我尝试使用回调，但这是async，我需要sync 工具。
所以我使用promise，但我的结果仍然像回调。
这是我的代码：

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

var listPosts = function(req, res) {
    find('post', {}, 10, {author: 1})
        .then(function(posts) {

            var myPosts = posts;

            const promises = [];

            myPosts.forEach(function(post) {

                console.log("hi i'm forEach" + '\n');
                console.log(post);
                console.log('\n');

                const promise = new Promise(function(resolve, reject){
                    getPostAuthorName(post.authorID)
                        .then(function(postAuthor){
                            post.authorName = postAuthor;
                        })
                        resolve(); 
                });

                console.log("i'm end of forEach and this is result:");
                console.log(post);
                console.log('\n');

                promises.push(promise);
            });

            Promise.all(promises).then(() => {

                console.log('i should print at end' + '\n');

            });
        });
}

var getPostAuthorName = function(authorID) {
    return new Promise(function(resolve, reject){
        findOne('user', {_id: new ObjectId(authorID)})
            .then(function(result){

                console.log("i'm getPostAuthorName" + '\n');

                resolve(result.name);
            })
    })
}

var find = function(collection, cond = {}, limit = 0, sort = {}) {
    return new Promise(function(resolve, reject){
        mongo.connect(url) 
            .then(function(db){
                db.collection(collection)
                    .find(cond).limit(limit).sort(sort).toArray()
                        .then(function(result){
                            resolve(result);
                        })
            })
    });
}

var findOne = function(collection, cond = {}){
    return new Promise(function(resolve, reject){
        mongo.connect(url)
            .then(function(db){
                db.collection(collection).findOne(cond)
                    .then(function(result){

                        console.log("i'm findOne" + '\n');

                        resolve(result);
                    })
            })
    })
}


listPosts();

最后我收到了这个结果：

hi i'm forEach

{ _id: 59888f418c107711043dfcd6,
  title: 'FIRST',
  content: 'this is my FIRST post',
  timeCreated: 2017-08-07T16:03:13.552Z,
  authorID: '5987365e6d1ecc1cd8744ad4' }


i'm end of forEach and this is result:
{ _id: 59888f418c107711043dfcd6,
  title: 'FIRST',
  content: 'this is my FIRST post',
  timeCreated: 2017-08-07T16:03:13.552Z,
  authorID: '5987365e6d1ecc1cd8744ad4' }


hi i'm forEach

{ _id: 598d60d7e2014a5c9830e353,
  title: 'SECOND',
  content: 'this is my SECOND post',
  timeCreated: 2017-08-07T16:03:13.552Z,
  authorID: '5987365e6d1ecc1cd8744ad4' }


i'm end of forEach and this is result:
{ _id: 598d60d7e2014a5c9830e353,
  title: 'SECOND',
  content: 'this is my SECOND post',
  timeCreated: 2017-08-07T16:03:13.552Z,
  authorID: '5987365e6d1ecc1cd8744ad4' }


i should print at end

i'm findOne

i'm getPostAuthorName

i'm findOne

i'm getPostAuthorName

为什么函数不能同步运行。 有什么解决办法？

Answer 1

如果你想将回调转换为承诺，你可以简单地做这样的事情：

function functionWithCallback(params, callback)
{
    [...]
    callback(true);
}

function functionWithPromise(params)
{
    return new Promise((resolve, reject) => {
        functionWithCallback(params, (done) => {
            if (done)
                return resolve(true);
            reject(false);
        });
    });
}

现在，您可以使用 await 关键字同步 Promise（不要忘记将您的函数 async）。示例：

async function main()
{
    const p1 = functionWithPromise('1');
    const p2 = functionWithPromise('2');

    await p1;
    await p2;
    console.log('End');
}

Answer 2

您的问题在于这个（严重缩进）代码

const promise = new Promise(function(resolve, reject){
    getPostAuthorName(post.authorID)
        .then(function(postAuthor){
            post.authorName = postAuthor;
        })
        resolve(); 
});

正确缩进看起来像

const promise = new Promise(function(resolve, reject){
    getPostAuthorName(post.authorID)
        .then(function(postAuthor){
            post.authorName = postAuthor;
        })
    resolve(); 
});

所以很明显，resolve 相对于 getPostAuthorName 被“同步”调用 - 但在 getPostAuthorName 的 .then 之前（异步调用）可能被调用 - 因此为什么你的 promises 数组都解决得太早了

所以，如果你移动它

const promise = new Promise(function(resolve, reject){
    getPostAuthorName(post.authorID)
        .then(function(postAuthor){
            post.authorName = postAuthor;
            resolve(); 
        })
});

现在，您的代码应该按预期运行

解决代码中的“promise 构造函数反模式” - 以上是一个示例

由于getPostAuthorName返回一个Promise，所以不需要这样做

const promise = new Promise(function(resolve, reject){
    getPostAuthorName(post.authorID)
        .then(function(postAuthor){
            post.authorName = postAuthor;
            resolve(); // resolves to "undefined"
        })
});

这相当于

const promise = getPostAuthorName(post.authorID).then(function(postAuthor){
    post.authorName = postAuthor;
    return; // returns "undefined", just like your resolve() results in
});

所以，删除所有这些反模式，并使用

Promise.all(posts.map(

而不是用 push 构建数组

会产生类似的代码

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

const listPosts = function(req, res) {
    find('post', {}, 10, {author: 1})
    .then(posts => 
        Promise.all(posts.map(post => 
            getPostAuthorName(post.authorID)
            .then(postAuthor => post.authorName = postAuthor)
        ))
    )
    .then(() => console.log('i should print at end' + '\n'));
}

const getPostAuthorName = authorID => 
    findOne('user', {_id: new ObjectId(authorID)})
    .then(result => result.name);


const find = (collection, cond = {}, limit = 0, sort = {}) => 
    mongo.connect(url) 
    .then(db => 
        db.collection(collection)
        .find(cond)
        .limit(limit)
        .sort(sort)
        .toArray()
    );

const findOne = (collection, cond = {}) => 
    mongo.connect(url)
    .then(db => 
        db.collection(collection)
        .findOne(cond)
    );

我想我又掉进了陷阱.. 我敢打赌 posts 不是 javacript 数组 - 在那种情况下我会做一个类似的函数

const makeArray = collection => {
    const ret = [];
    collection.forEach(item => ret.push(item));
    return ret;
};

改变

        Promise.all(posts.map(post => 

到

        Promise.all(makeArray(posts).map(post => 

Answer 3

如果您不需要，请不要创建承诺！相反，利用链式承诺的能力：

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

var listPosts = function () {
  return find('post', {}, 10, {author: 1})
    .then(function (posts) {
      var promises = posts.map(post => getPostAuthorName(post.authorID));
      return Promise.all(promises).then(names => names.map((name, index) => {
        var post = posts[index];
        post.authorName = name;
        return post;
      });
  });
};

var getPostAuthorName = function(authorID) {
  return findOne('user', {_id: new ObjectId(authorID)}).then(author => author.name);
}

var find = function(collection, cond = {}, limit = 0, sort = {}) {
  return mongo.connect(url)
    .then(db => db.collection(db)
      .find(cond)
      .limit(limit)
      .sort(sort)
      .toArray()
    );
};

var findOne = function(collection, cond = {}) {
  return mongo.connect(url).then(db => db.collection(db).findOne(cond));
};


listPosts().then(posts => console.log('Post:', post, ', author: ', post.authorName));

使用new Promise 构造函数创建不必要的承诺称为explicit-construction anti-pattern。

但这不是您代码中的唯一问题：在以下 sn-p 中不必要的承诺中使代码变得如此复杂，以至于您没有意识到在找到作者姓名之前您已解决了承诺：

const promise = new Promise(function(resolve, reject){
  getPostAuthorName(post.authorID)
    .then(function(postAuthor){
      post.authorName = postAuthor;  
    })
  resolve(); // why resolve immediately?
});

相反，它应该是这样的：

const promise = getPostAuthorName(post.authorID)
  .then(function(postAuthor){
    post.authorName = postAuthor;  
  });