filter.js

const noop = () => {};

const once = (fn) => {
  let called = false;

  return (...args) => {
    if (called) return;
    called = true;
    fn(...args);
  };
};

const filter = (args, func, cb = noop) => {
  if (args.length === 0) {
    cb(null, []);
    return;
  }
  const oncedCallback = once(cb);
  const results = [];
  args.forEach(arg => func(arg, (err, files) => {
    if (err) {
      oncedCallback(err);
    } else {
      results.push([arg, files, args.indexOf(arg)]);
    }
    if (results.length === args.length) {
      const sortedRes = results.sort((a, b) => a[2] > b[2]);
      const check = sortedRes.reduce((acc, element) => element[1] ? [...acc, element[0]] : acc, []);
      cb(null, check);
    }
  }));
};

export default filter;