ffmpeg.js

// Install fluent-ffmpeg before running this!
const readline = require('readline');
const path   = require('path');
const fs     = require('fs');
const ytdl   = require('..');
const ffmpeg = require('fluent-ffmpeg');

const url = 'https://www.youtube.com/watch?v=TGbwL8kSpEk';
const audioOutput = path.resolve(__dirname, 'sound.mp4');
const mainOutput = path.resolve(__dirname, 'output.mp4');

console.log('downloading audio track');
ytdl(url, {
  filter: format => format.container === 'm4a' && !format.encoding
}).on('error', console.error)
  .on('progress', (chunkLength, downloaded, total) => {
    const percent = downloaded / total;
    readline.cursorTo(process.stdout, 0);
    process.stdout.write(`${(percent * 100).toFixed(2)}% downloaded `);
    process.stdout.write(`(${(downloaded / 1024 / 1024).toFixed(2)}MB of ${(total / 1024 / 1024).toFixed(2)}MB)`);
  })

  // Write audio to file since ffmpeg supports only one input stream.
  .pipe(fs.createWriteStream(audioOutput))
  .on('finish', () => {
    console.log('\ndownloading video');
    ffmpeg()
      .input(ytdl(url, { filter: format => {
        return format.container === 'mp4' && !format.audioEncoding; } }))
      .videoCodec('copy')
      .input(audioOutput)
      .audioCodec('copy')
      .save(mainOutput)
      .on('error', console.error)
      .on('progress', progress => {
        readline.cursorTo(process.stdout, 0);
        readline.clearLine(process.stdout, 1);
        process.stdout.write(progress.timemark);
      }).on('end', () => {
        fs.unlink(audioOutput, err => {
          if(err) console.error(err);
          else console.log('\nfinished downloading');
        });
      });
  });