Differentiate between error and standard terminal log with ffmpeg - nodejs

Question

I'm using ffmpeg in node js. Both the standard terminal output and the error seems to be sent to stdout, so I don't know how to differentiate between error and success... Here's my code:

var convertToMp3 = function(filePath) {
  var ffmpeg = child_process.spawn('ffmpeg',['-i', filePath, '-y', 'output.mp3']);
  var err = '';
  ffmpeg.stderr
    .on('data', function(c) { err += c; })
    .on('end', function() { console.log('stderr:', err); });
  var d = '';
  ffmpeg.stdout
    .on('data', function(c){d +=c;})
    .on('end', function(){ console.log('stdout', d); });
}

wether the conversion succeeds or fails, stdout is empty and stderr contains what I'd get if I'd run the corresponding command in the terminal

Answer 1

Inspired by @aergistal comment, here is a solution that works for me, where callback is to be executed at the end of the task, with the signature function(error, success), as usual.

var convertToMp3 = function(filePath, callback) {
  var ffmpeg = child_process.spawn('ffmpeg',['-i', filePath, '-y', 'output.mp3']);
  var err = '';
  ffmpeg.stderr.on('data', function(c) { err += c; }).on('end', function() { console.log('stderr:', err); });

  ffmpeg.on('exit', function(code) {
    if (code) {
      callback({code: code, message: err});
    } else {
      callback(null, {success: true, message: err});
    }
  });
}

or in the new js style:

const convertToMp3 = (filePath) => new Promise((resolve, reject) {
  const ffmpeg = child_process.spawn('ffmpeg', ['-i', filePath, '-y', 'output.mp3']);
  let output = '';
  ffmpeg.stderr
    .on('data', c => { output += c; });

  ffmpeg.on('exit', code => {
    if (code) {
      reject({ code: code, message: output });
    } else {
      resolve(output);
    }
  });
});

const ffmpegOutput = await convertToMp3('./video.mp4');
...