In Map Async, we wrote a function that accepts an array of items and maps each element with an asynchronous mapping function and returns a Promise which resolves to the mapped results.

Practically, this can be used for mapping an input array with the results of calling an API where the input element is the argument to the API. However, if your array has a large amount of items, you'd be making that many API calls at the same time which will almost certainly get you rate limited by the API service. We want to execute our tasks concurrently so that it is more efficient, yet stay within the rate limits of the API.

Implement a mapAsyncLimit that takes in an optional parameter size, the maximum number of ongoing async tasks so that the input array can be processed in chunks of size, achieving parallelism and staying within the provided limit. If size is not specified, the chunk size is unlimited.

Examples

async function fetchUpperCase(q: string) {
  // Fake API service that converts a string to uppercase.
  const res = await fetch('https://uppercase.com?q=' + q);
  return await res.text();
}

// Only a maximum of 2 pending requests at any one time.
const results = await mapAsyncLimit(
  ['foo', 'bar', 'qux', 'quz'],
  fetchUpperCase,
  2,
);
console.log(results); // ['FOO', 'BAR', 'QUX', 'QUZ'];