index.js

// Usage:
//  node index.js ${input} ${output}
// Require:
//  Node.js
//  ffmpeg
const fs = require('fs');
const path = require('path');
const exec = require('child_process').execSync;

const originDir = process.argv[2];
const targetDir = process.argv[3];

const walkDir = (dir, callback) => {
  fs.readdirSync(dir).forEach( f => {
    const dirPath = path.join(dir, f);
    const isDirectory = fs.statSync(dirPath).isDirectory();
    isDirectory ?
      walkDir(dirPath, callback) : callback(path.join(dir, f));
  });
};

const getCommand = (input, dist) => `ffmpeg "${dist}" -i "${input}" -codec:a libmp3lame -qscale:a 1`;

if (!originDir || !targetDir) {
  console.error(`
    invalid command argument
    $ node index.js [input_dir] [output_dir]
      `);
}

const inputFiles = [];
walkDir(originDir, (filePath) => { inputFiles.push(filePath); });
const files = inputFiles.filter(file => /^.*\.(m4a)$/.test(file))

console.log(`target file ${files.length}`);

for (let i=0;i<files.length;i++) {
  const input = files[i];
  // ex) origin/artist/album_name/song.m4a => song
  const inputFileName = path.basename(input, '.m4a');

  // ex) origin/artist/album_name/song.m4a => origin/artist/album_name
  const inputDir = path.dirname(input);
  // ex) origin/artist/album_name/song.m4a => artist/album_name
  const dirForFile = path.join(...inputDir.split(path.sep).slice(1));
  // ex) origin/artist/album_name/song.m4a => target/artist/album_name
  const distDir = path.join(targetDir, dirForFile);
  // ex) origin/artist/album_name/song.m4a => target/artist/album_name/song.mp3
  const dist = path.join(distDir, `${inputFileName}.mp3`);

  if (!fs.existsSync(distDir)) {
    console.log(`mkdir ${distDir}`);
    fs.mkdirSync(distDir, { recursive: true });
  }


  if (!fs.existsSync(dist)) {
    console.log(`in progress of converting ${input} to ${dist}`);
    exec(getCommand(input, dist));
  } else {
    console.log(`[SKIP] ${dist} already exists`);
  }
}

console.log(`converted ${files.length} songs! Enjoy :)`);