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