How to use ES8 async/await with streams?

Question

In https://stackoverflow.com/a/18658613/779159 is an example of how to calculate the md5 of a file using the built-in crypto library and streams.

var fs = require('fs');
var crypto = require('crypto');

// the file you want to get the hash    
var fd = fs.createReadStream('/some/file/name.txt');
var hash = crypto.createHash('sha1');
hash.setEncoding('hex');

fd.on('end', function() {
    hash.end();
    console.log(hash.read()); // the desired sha1sum
});

// read all file and pipe it (write it) to the hash object
fd.pipe(hash);

But is it possible to convert this to using ES8 async/await instead of using the callback as seen above, but while still keeping the efficiency of using streams?

Answer 1

async / await only works with promises, not with streams. There are ideas to make an extra stream-like data type that would get its own syntax, but those are highly experimental if at all and I won't go into details.

Anyway, your callback is only waiting for the end of the stream, which is a perfect fit for a promise. You'd just have to wrap the stream:

var fd = fs.createReadStream('/some/file/name.txt');
var hash = crypto.createHash('sha1');
hash.setEncoding('hex');
// read all file and pipe it (write it) to the hash object
fd.pipe(hash);

var end = new Promise(function(resolve, reject) {
    hash.on('end', () => resolve(hash.read()));
    fd.on('error', reject); // or something like that. might need to close `hash`
});

Now you can await that promise:

(async function() {
    let sha1sum = await end;
    console.log(sha1sum);
}());

Answer 2

If you are using node version >= v10.0.0 then you can use stream.pipeline and util.promisify .

const fs = require('fs');
const crypto = require('crypto');
const util = require('util');
const stream = require('stream');

const pipeline = util.promisify(stream.pipeline);

const hash = crypto.createHash('sha1');
hash.setEncoding('hex');

async function run() {
  await pipeline(
    fs.createReadStream('/some/file/name.txt'),
    hash
  );
  console.log('Pipeline succeeded');
}

run().catch(console.error);

Answer 3

Node V15 now has a promisfiy pipeline in stream/promises . This is the cleanest and most official way.

const { pipeline } = require('stream/promises');

async function run() {
  await pipeline(
    fs.createReadStream('archive.tar'),
    zlib.createGzip(),
    fs.createWriteStream('archive.tar.gz')
  );
  console.log('Pipeline succeeded.');
}

run().catch(console.error);

We all should appreciate how much works it's done here:

• Capture errors in all the streams.
• Destroy unfinished streams when error is raised.
• Only return when the last writable stream is finished .

This pipe thing is one of the most powerful feature Node.JS has. Making it fully async is not easy. Now we have it.

Answer 4

Something like this works:

for (var res of fetchResponses){ //node-fetch package responses
    const dest = fs.createWriteStream(filePath,{flags:'a'});
    totalBytes += Number(res.headers.get('content-length'));
    await new Promise((resolve, reject) => {
        res.body.pipe(dest);
        res.body.on("error", (err) => {
            reject(err);
        });
        dest.on("finish", function() {
            resolve();
        });
    });         
}

Answer 5

I would comment, but don't have enough reputation.

A WORD OF CAUTION: If you have an application that is passing streams around AND doing async/await, be VERY CAREFUL to connect ALL pipes before you await. You can end up with streams not containing what you thought they did. Here's the minimal example

const { PassThrough } = require('stream');

async function main() {
    const initialStream = new PassThrough();

    const otherStream = new PassThrough();
    const data = [];
    otherStream.on('data', dat => data.push(dat));
    const resultOtherStreamPromise = new Promise(resolve => otherStream.on('end', () => { resolve(Buffer.concat(data)) }));

    const yetAnotherStream = new PassThrough();
    const data2 = [];
    yetAnotherStream.on('data', dat => data2.push(dat));
    const resultYetAnotherStreamPromise = new Promise(resolve => yetAnotherStream.on('end', () => { resolve(Buffer.concat(data2)) }));

    initialStream.pipe(otherStream);
    initialStream.write('some ');

    await Promise.resolve(); // Completely unrelated await

    initialStream.pipe(yetAnotherStream);
    initialStream.end('data');
    const [resultOtherStream, resultYetAnotherStream] = await Promise.all([
        resultOtherStreamPromise,
        resultYetAnotherStreamPromise,
    ]);

    console.log('other stream:', resultOtherStream.toString()); // other stream: some data
    console.log('yet another stream:', resultYetAnotherStream.toString()); // yet another stream: data
}
main();

Answer 6

2021 Update:

New example from Node documentation:

async function print(readable) {
  readable.setEncoding('utf8');
  let data = '';
  for await (const chunk of readable) {
    data += chunk;
  }
  console.log(data);
}

see https://nodejs.org/api/stream.html#stream_readable_symbol_asynciterator

Answer 7

I believe it will be helpful for someone:

async function readFile(filename) {
    let records = []
    return new Promise(resolve => {
        fs.createReadStream(filename)
            .on("data", (data) => {
                records.push(data);
            })
            .on("end", () => {
                resolve(records)
            });
    })
}