#409 mapLimit

medium

javascript

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);

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);