showProgress = () => {
  readline.cursorTo(process.stdout, 0);
  const toMB = i => (i / 1024 / 1024).toFixed(2);

  process.stdout.write(`Audio  | ${(tracker.audio.downloaded / tracker.audio.total * 100).toFixed(2)}% processed `);
  process.stdout.write(`(${toMB(tracker.audio.downloaded)}MB of ${toMB(tracker.audio.total)}MB).${' '.repeat(10)}\n`);

  process.stdout.write(`Video  | ${(tracker.video.downloaded / tracker.video.total * 100).toFixed(2)}% processed `);
  process.stdout.write(`(${toMB(tracker.video.downloaded)}MB of ${toMB(tracker.video.total)}MB).${' '.repeat(10)}\n`);

  process.stdout.write(`Merged | processing frame ${tracker.merged.frame} `);
  process.stdout.write(`(at ${tracker.merged.fps} fps => ${tracker.merged.speed}).${' '.repeat(10)}\n`);

  process.stdout.write(`running for: ${((Date.now() - tracker.start) / 1000 / 60).toFixed(2)} Minutes.`);
  readline.moveCursor(process.stdout, 0, -3);
}
Reencode audio & video without creating files first

Requirements: ffmpeg, ether via a manual installation or via ffmpeg-static

If you need more complex features like an output-stream you can check the older, more complex example:
https://github.com/fent/node-ytdl-core/blob/cc6720f9387088d6253acc71c8a49000544d4d2a/example/ffmpeg.js