NodeJS Design Patters (third edition) challenges

asyncMap.js

function mapAsync(iterable, callback, concurrency, results) {
  if (!iterable.length)
    return Promise.all(results).then(results => console.log(results));

  const queue = iterable.splice(0, concurrency - 1);

  while (queue.length) {
    const task = queue.shift();
    callback(task, iterable, callback, concurrency, results);
  }
}

function callback(task, iterable, callback, concurrency, results) {
  results.push(
    new Promise((resolve, reject) => {
      if (typeof task === 'function') {
        resolve(task());
      } else {
        resolve(task);
      }
    })
  );
  mapAsync(iterable, callback, concurrency, results);
}

// using funciton for final results
const funcs = [
  () => [1, new Date().getMilliseconds()],
  () => [2, new Date().getMilliseconds()],
  () => [3, new Date().getMilliseconds()],
  () => [4, new Date().getMilliseconds()],
  () => [5, new Date().getMilliseconds()],
];

// simple values
const ab = ['a', 'b', 'c', 'd', 'e', 'f', 'g'];

mapAsync(funcs, callback, 2, []);