node async function with nested function

Question

Currently I am trying to have an async function which has another function inside like this:

processFile: async function(req, whatTo, callback){

    const lines = [];

    const lineReader = require('readline').createInterface({
        input: require('streamifier').createReadStream(req.file.buffer)
    });

    let errorPresent = false;

    lineReader.on('line', line => {
        lines.push(line.replace(/ /g,''));
    });

    lineReader.on('close', async () => {

        try {

            const results = await Promise.map(lines, async(line) => {

                return await Transform(line);

            }, { concurrency: 80 });

            return results.join("\r\n");

        }catch(err){
            throw err;
        }

    });

}

Then I have a route, which calls this function, like this:

const data = await tokenizer.processFile(req, 'tokenize');

res.setHeader('Content-Type', 'text/plain'); 
res.setHeader('Content-Disposition', 'attachment; filename=tokenized.txt');
res.write(data, 'binary');
res.end();

The return results.join("\r\n"); isn't 'returning' the processFile.

How can I achieve this?

Can you describe the exact behavior you are expecting? There seem to be several issues with your code and I can't tell what path you are actually trying to go down. For example, processFile doesn't actually return anything and doesn't really wait to return, the async is superfluous. Also, the mixing of eventful and asynchronous code doesn't make sense either. You're mixing callbacks, async/await, and explicit Promises in a way that doesn't make sense. — zero298
– zero298, Commented Jun 11, 2018 at 19:25
@CodyG. Well... how do you suggest I return results.join("\r\n") from within lineReader.on('close', async () ...? I did mention I'm new to Node? — Fede E.
– Fede E., Commented Jun 11, 2018 at 19:25
@CodyG. I edited the question removing callbacks and only left the async/await. Now, how do I get to return results.join("\r\n")? — Fede E.
– Fede E., Commented Jun 11, 2018 at 19:30
(Is this a big file?) I've given a really rough answer (it might not work as is) but if you have any questions let me know. — Cody Geisler
– Cody Geisler, Commented Jun 11, 2018 at 19:45

Cody Geisler · Accepted Answer · 2018-06-11 19:32:14Z

1

My suggestion is to split up your conversion of line reader from cb to a promise doing something like this:

processFile: (req, whatTo) => new Promise((res,rej)=>{

    const lines = [];

    const lineReader = require('readline').createInterface({
        input: require('streamifier').createReadStream(req.file.buffer)
    });

    let errorPresent = false;

    lineReader.on('line', line => {
        lines.push(line.replace(/ /g,''));
    });

    lineReader.on('close', function(){
      res(lines);
    });
    // UNCAUGHT ERRORS, USE rej() for others
});

Then handling your transform back in your async/await world like this:

let data;
let lines = await tokenizer.processFile(req, 'tokenize');
let results = await Promise.map(lines, async (line) => {
  return await Transform(line);
}, { concurrency: 80 });

answered Jun 11, 2018 at 19:32

Cody Geisler

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1 Comment

Fede E. Over a year ago

Thanks Cody, I followed this option.

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