index.js

import fs from 'fs'
import { join } from 'path';
import Promise, { coroutine, all } from 'bluebird';

const joinPath = init => tail => join(init, tail);
const readFileP = Promise.promisify(fs.readFile);
const readUtf8P = (file) => readFileP(file, { encoding: 'utf8' });
const pipe = (f, g) => x => g(f(x));

const concatFiles = coroutine(function* (dir) {
  const readInDir = pipe(joinPath(dir), readUtf8P);
  const index = yield readInDir('index.txt');
  const results = yield all(index.match(/^.*(?=\n)/gm).map(readInDir));
  return results.join('');
});

const main = process => concatFiles(process.argv[2])
  .then(s => process.stdout.write(s));

main(process);