javascript - 类型错误 : Cannot read property 'then' of undefined on Promise in Express route

Question

我的 MEEN 应用程序中有一条路线，它接受文件上传，然后将数据传递到解析文件并将其存储在数组中的辅助模块，一旦此辅助模块完成，我想将其传递给另一个模块然后将处理返回的数组。由于某种原因，即使我在帮助程序末尾有一个 return 语句，then 方法也是未定义的。

路线:

router.route('/tools/sku/reactivate').post(upload.single('csvdata'),function(req,res){
        console.log('handing request over to helper.csv');
        csv.reader(req,res).then(sku.reactivate(data));
    });

csv.reader:

var csv = require('csv-parse');
var multer = require('multer');
var fs = require('fs');

module.exports.reader = function(req,res){
  //define array for holding csv data in this case skus
    const csvArray = [];
    //max number of columns in csv
    const maxColumns = parseInt(req.body.maxColumns);
    //create an array of column headers to check chunks against so we dont parse headers
    let columnHeader = req.body.moduleTarget.split(',');
    //loopThrough Array to create arrays inside container array for each column
    for(var i = 0; i < maxColumns; i++){
        csvArray.push([]);
    }
    //define filesystem readstream from uploaded file
    let readStream = fs.createReadStream(req.file.path).pipe(csv());
    //push csv data to array ignoring headers to csvArray
    readStream.on('data', function(chunk){
        //get number of keys in the dataChunk
        let chunkLength = Object.keys(chunk).length;
        //check column count on csv if greater than expected throw error
        if(chunk[maxColumns]){
            throw '[ERROR] More columns than expected in CSV, please fix and try again';
        }else{
            //loop through chunk keys and store each one in csvArray by index
            for(var i = 0; i < chunkLength; i++){
                //if chunk at this index doesnt equal column header and chunk at this index exists push to array
                if(chunk[i] !== columnHeader[i] && chunk[i]){
                    csvArray[i].push(chunk[i]);
                }
            }
        }
    });
    //error handling
    readStream.on('error',function(err){
        console.log('Error while reading file stream '+ err);
        res.json({message:err,errorType:'1'});
    });
    readStream.on('end',function(){
        console.log('finished reading csv returning array back to router to process next middleware');
        return csvArray;
    });
}

我在 readStream 结束监听器上收到 console.log 消息后立即收到错误

Answer 1

来自 Express 网站。 https://expressjs.com/en/advanced/best-practice-performance.html

TL;博士

尝试将阅读器模块的内容包装在 promise 中并返回 promise 。

...但是，有两个注意事项:所有异步代码都必须返回 Promise(发射器除外)。如果特定库不返回 Promise，请使用 Bluebird.promisifyAll() 等辅助函数转换基础对象。事件发射器(如流)仍然可能导致未捕获的异常。因此，请确保您正确处理错误事件；例如:

app.get('/', wrap(async (req, res, next) => {
  let company = await getCompanyById(req.query.id)
  let stream = getLogoStreamById(company.id)
  stream.on('error', next).pipe(res)
}))

@示例:::

module.exports.reader = function(req,res){
    return new Promise((resolve, reject)=>{
      //define array for holding csv data in this case skus
        const csvArray = [];
        //max number of columns in csv
        const maxColumns = parseInt(req.body.maxColumns);
        //create an array of column headers to check chunks against so we dont parse headers
        let columnHeader = req.body.moduleTarget.split(',');

            ...
        //error handling
        readStream.on('error',function(err){
            console.log('Error while reading file stream '+ err);
            res.json({message:err,errorType:'1'});
        });
        readStream.on('end',function(){
            console.log('finished reading csv returning array back to router to process next middleware');
            return csvArray;
        });
    })
}