#409 mapLimit

Implement a mapLimit function that behaves like Array.map, but for asynchronous iteratees with a concurrency limit.

Requirements

• The returned promise resolves to an array of results in the same order as inputs.
• No more than limit iteratees run concurrently.
• If any iteratee calls back with a non-null error, the promise rejects immediately.
• Each iteratee must call its callback exactly once.

Example

mapLimit([1, 2, 3, 4, 5], 3, (n, cb) => {
  setTimeout(() => {
    cb(null, n * 2);
  }, 2000);
})
  .then(res => console.log(res))   // [2, 4, 6, 8, 10]
  .catch(console.error);