app.js

const fs = require('fs');
const ytdl = require('./lib/index');

ytdl('https://youtu.be/fxlbIS8CoW8')
  .pipe(fs.createWriteStream('video.flv'));

node-youtube-core

in util 
 case 'baseline': 
      format = formats.find(function(format) {return format.profile == 'baseline'})
      break;

request(url, {}, function(err, res, body) {
    if (err) {
      format = util.chooseFormat(info.formats, {quality: 'baseline'});
      url = format.url
    }
    if (format.live) {
    var req = m3u8stream(url, {
      chunkReadahead: +info.live_chunk_readahead,
      requestOptions: options.requestOptions,
    });
    req.on('error', stream.emit.bind(stream, 'error'));
    stream.destroy = req.end.bind(req);
    req.pipe(stream);

  } else {
    if (options.begin) {
      url += '&begin=' + util.fromHumanTime(options.begin);
    }
    doDownload(stream, url, options, {
      trys: options.retries || 5,
      range: {
        start: options.range && options.range.start ? options.range.start : 0,
        end: options.range && options.range.end ? options.range.end : -1,
      },
      downloaded: 0,
    });
    }
  })