如何在 node.js 中将函数与 Promise 同步

Posted 2023-03-07

【中文标题】如何在 node.js 中将函数与 Promise 同步

【英文标题】：how to synchronize functions with promises in node.js

【发布时间】：2018-01-19 16:08:11

【问题描述】：

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？

【问题讨论】：

能否将问题简化为具体问题并提供minimal reproducible example？

你只能回答一个问题：promise能保证同步编程吗？

不，当然不是。 Promise 是处理异步的一种更好的方法。您不应该尝试使异步任务同步。

避免使用 Promise 构造函数反模式 - getPostAuthorName 返回一个 Promise，无需将调用包装在一个 Promise 中……反过来，findOne 和 mongo.connect 也返回 Promise，所以，也不需要将它们包装在 Promise 构造函数中

does promise guarantee sync programing - 恰恰相反，承诺保证异步编程

【参考方案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');

【参考方案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 => 

【参考方案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;  
  );