javascript - Using promises with streams in node.js

Question

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);
  })
}

Answer 1

In this line

stream.on("end", resolve(stream.dests[0].path));

you are executing resolve immediately, and the result of calling resolve (which will be undefined, because that's what resolve returns) is used as the argument to stream.on - not what you want at all, right.

.on's second argument needs to be a function, rather than the result of calling a function

Therefore, the code needs to be

stream.on("end", () => resolve(stream.dests[0].path));

or, if you're old school:

stream.on("end", function () { resolve(stream.dests[0].path); });

another old school way would be something like

stream.on("end", resolve.bind(null, stream.dests[0].path));

No, don't do that :p see comments