concat.js

function concat(args, func, cb) {
  const results = [];
  const errors = [];
  args.forEach(arg => func(arg, (err, files) => {
    if (err) {
      errors.push(err);
    } else {
      results.push(files);
    }

    if (results.length + errors.length === args.length) {
      cb(errors.length ? errors : null, results.reduce((a, i) => a.concat(i), []));
    }
  }));
}

function expect(a, b) {
  console.log(a, b);
}

const callOrder = [];
const iteratee = (x, cb) => {
  setTimeout(() => {
    callOrder.push(x);
    let y = x;
    const r = [];
    while (y > 0) {
      r.push(y);
      y -= 1;
    }
    cb(null, r);
  }, x * 25);
};
concat([1, 3, 2], iteratee, (err, results) => {
  expect(results, [1, 2, 1, 3, 2, 1]);
  expect(callOrder, [1, 2, 3]);
  expect(err, null);
});