Using promises with streams in node.js

user_2652763 0
Viewed 38967

I've refactored a simple utility to use promises. It fetches a pdf from the web and saves it to disk. It should then open the file in a pdf viewer once saved to disk. The file appears on disk and is valid, the shell command opens the OSX Preview application, but a dialog pops up complaining that the file is empty.

What's the best way to execute the shell function once the filestream has been written to disk?

// download a pdf and save to disk
// open pdf in osx preview for example
download_pdf()
  .then(function(path) {
    shell.exec('open ' + path).code !== 0);
  });

function download_pdf() {
  const path = '/local/some.pdf';
  const url = 'http://somewebsite/some.pdf';
  const stream = request(url);
  const write = stream.pipe(fs.createWriteStream(path))
  return streamToPromise(stream);
}

function streamToPromise(stream) {
  return new Promise(function(resolve, reject) {
    // resolve with location of saved file
    stream.on("end", resolve(stream.dests[0].path));
    stream.on("error", reject);
  })
}
promise node.js javascript
5 Answers

In the latest nodejs, specifically, stream v3, you could do this:

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

const rs = fs.createReadStream('archive.tar');

async function run() {
  await finished(rs);
  console.log('Stream is done reading.');
}

run().catch(console.error);
rs.resume(); // Drain the stream.

https://nodejs.org/api/stream.html#stream_event_finish

user_23052430

The other solution can look like this:

const streamAsPromise = (readable) => {
  const result = []
  const w = new Writable({
    write(chunk, encoding, callback) {·
      result.push(chunk)
      callback()
    }
  })
  readable.pipe(w)
  return new Promise((resolve, reject) => {
    w.on('finish', resolve)
    w.on('error', reject)
  }).then(() => result.join(''))
}

and you can use it like:

streamAsPromise(fs.createReadStream('secrets')).then(() => console.log(res))
user_19079020

This can be done very nicely using the promisified pipeline function. Pipeline also provides extra functionality, such as cleaning up the streams.

const pipeline = require('util').promisify(require( "stream" ).pipeline)

pipeline(
  request('http://somewebsite/some.pdf'),
  fs.createWriteStream('/local/some.pdf')
).then(()=>
  shell.exec('open /local/some.pdf').code !== 0)
);
user_26350090
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