const fs = require('fs'),
  url = process.argv[2],
  ytdl = require('ytdl-core'),
  ffmpeg = require('fluent-ffmpeg'),
  stream = new ffmpeg(ytdl(url, { filter: 'audioonly' }))

stream.on('error', function (err) {
  console.log(err)
})
.format('mp3').pipe(fs.createWriteStream(__dirname + '/' + url.replace(/^.*v=/, '') + '.mp3'))