how to use async/await with fs.createReadStream in node js

Question

I have node js service which reads csv file and and filter records based on parameters provided. I need to wait till file reading is processed.but its not waiting. I tried using async/await and promise but no luck.

How to wait till file is processed and return json response.

app.get('/getEmpInfo/:emailId/:unique_code', async (req,res) => {
    console.log("req.params.emailId :"+req.params.emailId);
    console.log("req.params.unique_code :"+req.params.unique_code);
    const response = await getEmpDetailsByEmailIdAndUniqueCode(req.params.emailId,req.params.unique_code);
    res.send(response)
}); 

async function getEmpDetailsByEmailIdAndUniqueCode(emailId,uniqueCode){
    console.log("inside getEmpDetailsByEmailIdAndUniqueCode()::"+emailId+"::"+uniqueCode);
    var fetchData = [];

    // try 1
/*  await fs.createReadStream(investorFileName1)
      .pipe(csv())
      .on('data', (row) => {
        if(row.unique_code != null && row.unique_code == uniqueCode && row.investor_email_id == emailId) {
            fetchData.push(row);
            console.log(row);
        }
      })
      .on('end', () => {
        console.log('CSV file successfully processed');
        console.log(fetchData);
    }); */

  // try 2
    var response = fs.createReadStream(fileName).pipe(csv());

    return new Promise(function(resolve,reject){
        response.on('end', () => resolve(fetchData));
        response.on('error', reject); 
    })
}

If you want to check the same file for each request, maybe it would be better to parse the data once and keep it in some variable, instead of re-reading the file over and over again? — ahwayakchih
– ahwayakchih, Commented Oct 17, 2019 at 11:38

ahwayakchih · Accepted Answer · 2019-10-17 11:38:30Z

You were very close. Just combine your 2 implementations into a third one:

async function getEmpDetailsByEmailIdAndUniqueCode(fileName,emailId,uniqueCode){
    console.log("inside getEmpDetailsByEmailIdAndUniqueCode()::"+emailId+"::"+uniqueCode);

    return new Promise(function(resolve,reject){
        var fetchData = [];
        fs.createReadStream(fileName)
            .pipe(csv())
            .on('data', (row) => {
                if(row.unique_code != null && row.unique_code == uniqueCode && row.investor_email_id == emailId) {
                    fetchData.push(row);
                    console.log(row);
                }
            })
            .on('end', () => {
                console.log('CSV file successfully processed');
                console.log(fetchData);
                resolve(fetchData);
            })
            .on('error', reject); 
    })
}

Note that i added fileName as first parameter, because i could not see it defined anywhere in your code.

I also did not test it, just glued together your code :).

Ahmed Waqas · Accepted Answer · 2019-10-17 11:41:15Z

3

Please use the code below

const getEmpDetailsByEmailIdAndUniqueCode = () => {
 return new Promise((resolve, reject) => {
  const fetchData = [];
let stream = fs.createReadStream(investorFileName1);
stream.on("error", err => reject(err));
stream.on('data', (row) => {
    if(row.unique_code != null && row.unique_code == uniqueCode && 
        row.investor_email_id == emailId) {
        fetchData.push(row);
        console.log(row);
    }
  })
stream.on("end", () => resolve(fetchdata));
 });
}

answered Oct 17, 2019 at 11:41

Ahmed Waqas

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