如何在节点 js 中正确使用等待/异步与 for 循环

Posted 2023-03-07

【中文标题】如何在节点 js 中正确使用等待/异步与 for 循环

【英文标题】：How to properly use await/async with for loops in node js

【发布时间】：2020-12-02 20:02:17

【问题描述】：

我正在尝试提出一个函数，该函数将目录中的所有歌曲与文件路径、持续时间和上次访问时间一起作为列表提供。虽然循环内的日志确实打印了所需的内容，但响应是在循环完成之前发送的。

observation0：最后的日志发生在循环内的日志之前

router.get('/', function (req, res) 

    let collection = new Array();

    // glob returns an array 'results' containg the path of every subdirectory and file in the given location
    glob("D:\\Music" + "/**/*", async (err, results) => 

        // Filter out the required files and prepare them to be served in the required format by
        for (let i = 0; i < results.length; i++) 
            if (results[i].match(".mp3$") || results[i].match(".ogg$") || results[i].match(".wav$")) 

                // To get the alst accessed time of the file: stat.atime
                fs.stat(results[i], async (err, stat) => 
                    if (!err) 

                        // To get the duration if that mp3 song
                        duration(results[i], async (err, length) => 
                            if (!err) 
                                let minutes = Math.floor(length / 60)
                                let remainingSeconds = Math.floor(length) - minutes * 60

                                // The format to be served
                                let file = new Object()
                                file.key = results[i]
                                file.duration = String(minutes) + ' : ' + String(remainingSeconds)
                                file.lastListend = moment(stat.atime).fromNow()

                                collection.push(file)
                                console.log(collection) //this does log every iteration
                            
                        )
                    
                )
            
        
        console.log(collection); //logs an empty array
    )

    res.json(
        allSnongs: collection
    );
);

我无法在一定程度上理解文档，从而使我能够自己纠正代码:(

感谢您的帮助和建议

【问题讨论】：

问题不在于循环与async/await 的组合，它工作得很好，问题是你传递的是普通的旧异步回调而不是使用（和awaiting ) 承诺。

【参考方案1】：

此答案不会修复您的代码，而只是为了消除任何误解：

fs.stat(path, callback); // fs.stat is always asynchronous.
                         // callback is not (normally), 
                         // but callback will run sometime in the future.

await 像 fs.stat 这样使用回调而不是承诺的函数的唯一方法是自己做出承诺。

function promiseStat( path )
    return new Promise( ( resolve, reject ) => 
        fs.stat( path, ( err, stat ) => 
            if( err ) reject( err );
            else resolve( stat );
        ;
    );
 

现在我们可以：

const stat = await promiseStat( path );

【参考方案2】：

有大量时间来玩弄您的代码。 我会这样写，我做了很多改变。它有效

srv.get('/', async function (req, res) 

    // glob returns an array 'results' containg the path of every subdirectory and file in the given location
    let results = await new Promise((res, rej) =>
        glob("D:\\Music" + "/**/*", (err, results) => 
            if (err != null) rej(err);
            else res(results);
        )
    );

    // Filter out the required files and prepare them to be served in the required format by
    results = results.filter(
        (result) =>
            /\.mp3$/.test(result) ||
            /\.ogg$/.test(result) ||
            /\.wav$/.test(result)
    );

    const collection = await Promise.all(
        results.map(async (result) => 
            const stat = await new Promise((res, rej) =>
                fs.stat(result, (err, stat) => 
                    if (err != null) rej(err);
                    else res(stat);
                )
            );

            const length = await new Promise((res, rej) =>
                duration(result, (err, length) => 
                    if (err != null) rej(err);
                    else res(length);
                )
            );

            const minutes = Math.floor(length / 60);
            const remainingSeconds = Math.floor(length) - minutes * 60;

            // The format to be served
            return 
                key: result,
                duration: `$minutes : $remainingSeconds`,
                lastListend: moment(stat.atime).fromNow(),
            ;
        )
    );

    console.log(collection);
    res.json(
        allSnongs: collection,
    );
);

给予，

[
  
    key: 'D:\Music\1.mp3',
    duration: '0 : 27',
    lastListend: 'a few seconds ago'
  ,
  
    key: 'D:\Music\1.mp3',
    duration: '0 : 27',
    lastListend: 'a few seconds ago'
  ,
  
    key: 'D:\Music\1.mp3',
    duration: '0 : 52',
    lastListend: 'a few seconds ago'
  ,
  
    key: 'D:\Music\1.mp3',
    duration: '2 : 12',
    lastListend: 'a few seconds ago'
  
]

【讨论】：

不要在回调中使用collection.push(…)，而是使用return的值并使用const collection = await Promise.all(…)

@Bergi +1，这样更好