Return value from callback function inside another function

Question

I have a function being called which needs to return a value. The value exists inside a callback function inside of the downloadOrders function. The issue is "go" (log in post request) is printed before "close" (log in downloadOrders). So I'm not able to return the encodedPdf value. What do I need to do?

app.post('/api/***', async (req, res) => {
    try {
        const result = await orderServices.downloadOrders(data);
        console.log("go")
        if(result){
            res.status(200).send(JSON.stringify(result));
        } else {
            res.status(400).send({err: 'Error creating PDF document.'});
        }
    } catch(err){
    }
});

async downloadOrders(data){
    try {

        let documents = ["sample.pdf"];
        const pdfStream = pdfmerger(documents);
        var writeStream = fs.createWriteStream('merged-pdf.pdf');
        pdfStream.pipe(writeStream);
        pdfmerger(documents, 'merged-pdf.pdf');
        pdfStream.on('close', function(code){
            console.log("close")
            let encodedPdf = base64.base64Encode('merged-pdf.pdf');
            return encodedPdf;
        });

    } catch(err){
        console.log(err)
        return false;
    }
}

I don't see where "done" is being logged in your posted code. — Jacob
– Jacob, Commented Sep 7, 2018 at 15:59

Jacob · Accepted Answer · 2018-09-07 16:01:14Z

1

Wrap the close event in a Promise so you can await it.

await new Promise((resolve, reject) => {
  pdfStream
    .on('error', reject)
    .on('close', function(code){
      let encodedPdf = base64.base64Encode('merged-pdf.pdf');
      resolve(encodedPdf);
    });
});

answered Sep 7, 2018 at 16:01

Jacob

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Comments

sd5869 · Accepted Answer · 2018-09-07 16:23:23Z

You can wrap the downloadOrders function in a promise too.

app.post('/api/***', (req, res) => {
    orderServices.downloadOrders(data).then((result)=> {
        console.log("go")
        if(result){
            res.status(200).send(JSON.stringify(result));
        } else {
            res.status(400).send({err: 'Error creating PDF document.'});
        }
    }).catch(err => {
        res.status(500).send({err: 'Internal Server Error.'});
    });
});

const downloadOrders = function(data){
    return new Promise((resolve, reject) => {
      try {
        let documents = ["sample.pdf"];
        const pdfStream = pdfmerger(documents);
        var writeStream = fs.createWriteStream('merged-pdf.pdf');
        pdfStream.pipe(writeStream);
        pdfmerger(documents, 'merged-pdf.pdf');
        pdfStream.on('close', function(code){
            console.log("close")
            let encodedPdf = base64.base64Encode('merged-pdf.pdf');
            resolve(encodedPdf);
        });
        } catch(err){
          reject(err);
        }
    });
}

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