javascript - 等待数组中每个项目的 promise

Question

我在 Node 中使用 ssh2-sftp-client 来下载一系列文件。下载文件后，我需要检查每个文件，看看它是否是 CSV，然后检查我的数据库，看看它是否是之前导入的。

我正在使用 Promise，但在排序方面遇到了麻烦。我想经过几个小时的研究我已经很接近了，但还不能完全理解。

代码如下:

const isFileCSV = (filename) => {

    return new Promise( (resolve, reject) => {
        if (filename.endsWith('.csv')) {
            result = 'true';
            resolve(result);
        } else {
            result = 'false';
            resolve(result);
        }
    });
}

const wasFileImported = (filename) => {     
    // Return a new promise
    return new Promise( (resolve, reject) => {
        // Core function
        con.query(`CALL wasFileImported('${filename}')`, (err, results) => {
            console.log('02 Checking if file was previously imported.');    
            if (err) {
                logEntry('error', `Error searching database: ${err}`);
                reject(Error('false'));
            } else {
                result = results[0][0]['importCheck'];
                resolve(result);
            }
        })
    });
}

const importFile = (file) => {
    isFileCSV(file).then((result) => {
        console.log(`01a File ${file} ends with csv? ${result}`);
        result = result.toLowerCase();
        return result;
    }).then((result) => {
        if (result === 'false') {
            console.log(`01b ${result} = Not csv, log it and skip to next.`);
            return;
        } else if (result === 'true') {
            console.log(`01b ${result} = It is csv, go to next step`);
            return result;
        }
    }).then((result) => {
        wasFileImported('file').then((result) => {
            console.log('03 checked import, result: ', result);
            return result.toLowerCase();
        }).then((result) => {
            switch(result) {
                case 'false':
                    console.log('04a download file and log it');
                    break;
                case 'true':
                    console.log('04b skip it and log the skip');
                    break;
                default:
                    console.log('04c unknown error, log the skip');                         
            }           
        })              
    })              
}

logEntry('info', `attempting to connnect to ${config.destination.host}`);
console.log('00 start');

sftp.connect(crmConfig).then(() => {
    logEntry('info', `connected to ${config.destination.host}`);
    return sftp.list(ordersUrl);
}).then((data) => {
    fileList = [];
    Object.entries(data).forEach(([key, val]) => {
        fileList.push(val['name']);
    });
    return fileList;
}).then((fileList) => {
    let promises = [];

    fileList.forEach((file) => {
        promises.push(importFile(file));
    });

    Promise.all(promises).then(function(results) {
        console.log('Done');
    }).catch(function(err) {
        console.log('Error');
    });
});

我期望看到的是以下内容:

00 start
01a File daily.csv ends with csv? true
01b true = It is csv, go to next step
02 Checking if file was previously imported.
03 checked import, result:  FALSE
04a download file and log it
01a File daily072418.csv ends with csv? true
01b true = It is csv, go to next step
02 Checking if file was previously imported.
03 checked import, result:  FALSE
04a download file and log it
01a File test.csw ends with csv? false
01b false = Not csv, log it and skip to next.
02 Checking if file was previously imported.
03 checked import, result:  FALSE
04a download file and log it
Done

我实际得到的是:

00 start
01a File daily.csv ends with csv? true
01a File daily072418.csv ends with csv? true
01a File test.csw ends with csv? false
01b true = It is csv, go to next step
01b true = It is csv, go to next step
01b false = Not csv, log it and skip to next.
Done
02 Checking if file was previously imported.
03 checked import, result:  FALSE
04a download file and log it
02 Checking if file was previously imported.
03 checked import, result:  FALSE
04a download file and log it
02 Checking if file was previously imported.
03 checked import, result:  FALSE
04a download file and log it

早期版本没有使用Promise.all，但仍然生成相同的结果。我还尝试更改解析以返回 wasFileImported 和 isFileCSV，但随后我只得到“00 Start”，然后是“Done”，中间没有执行。

显然，我没有按照我的意图正确地迭代数组。最好的方法是什么，以便一切都按顺序完成？

编辑对于任何需要它的人来说，这是最终的工作代码，感谢 @jordan-peterson 和 @spakmad:

const isFileCSV = (filename) => {
    return new Promise( (resolve, reject) => {
        if (filename.endsWith('.csv')) {
            result = 'true';
            resolve(result);
        } else {
            result = 'false';
            resolve(result);
        }
    });
}

const wasFileImported = (filename) => {     
    // Return a new promise
    return new Promise( (resolve, reject) => {
        // Core function
        con.query(`CALL wasFileImported('${filename}')`, (err, results) => {
            console.log('02 Checking if file was previously imported.');    
            if (err) {
                logEntry('error', `Error searching database: ${err}`);
                reject(Error('false'));
            } else {
                result = results[0][0]['importCheck'];
                resolve(result);
            }
        })
    });
}

const importFile = (file) => {
    return isFileCSV(file).then((result) => {
        console.log(`01a File ${file} ends with csv? ${result}`);
        result = result.toLowerCase();
        return result;
    }).then((result) => {
        if (result === 'false') {
            console.log(`01b ${result} = Not csv, log it and skip to next.`);
            return;
        } else if (result === 'true') {
            console.log(`01b ${result} = It is csv, go to next step`);
            return result;
        }
    }).then((result) => {
        return wasFileImported('file').then((result) => {
            console.log('03 checked import, result: ', result);
            return result.toLowerCase();
        }).then((result) => {
            switch(result) {
                case 'false':
                    console.log('04a download file and log it');
                    break;
                case 'true':
                    console.log('04b skip it and log the skip');
                    break;
                default:
                    console.log('04c unknown error, log the skip');                         
            }           
        })              
    })              
}

console.log('00 start');

sftp.connect(crmConfig).then(() => {
    logEntry('info', `connected to ${config.destination.host}`);
    return sftp.list(ordersUrl);
}).then((data) => {
    fileList = [];
    Object.entries(data).forEach(([key, val]) => {
        fileList.push(val['name']);
    });
    return fileList;
}).then((fileList) => {
    let promiseChain = Promise.resolve()

    fileList.forEach((file) => {
        promiseChain = promiseChain.then(() => {
            return importFile(file)
        })
    })
});

Answer 1

您正在连续三次调用异步方法 importFile，因此它会同时遍历这些方法。如果你想一个接一个地执行，我喜欢做的就是这个。

let promiseChain = Promise.resolve()
myFiles.forEach((file) => {
   promiseChain = promiseChain.then(() => {
      return importFile(file)
   }
}

循环上的每次迭代都会将另一个 then 附加到 promiseChain 中的 Promise，直到第一个 then 返回。附加的第一个 then 被提前到立即，因为此时 promiseChain 是 Promise.resolve()

这也可以通过 reduce 来完成。

myFiles.reduce((acc, file) => acc.then(() => importFiles(file))), Promise.resolve())