mapLimit: async map with a concurrency limit

Problem

You have 500 image URLs to process, but firing 500 requests at once will exhaust sockets and trip rate limits. You want to process them N at a time — start the next as soon as one finishes — while still getting results back in the original order.

Implement mapLimit(inputs, limit, iteratee) that:

• Calls iteratee(input) (returns a promise) for each input.
• Keeps at most limit calls in flight at any moment.
• Resolves with an array of results in input order.
• Rejects if any iteratee rejects.

Input

const asyncDouble = (n) =>
  new Promise((res) => setTimeout(() => res(n * 2), 100));

mapLimit([1, 2, 3, 4, 5], 2, asyncDouble).then(console.log);

Expected output

• [2, 4, 6, 8, 10] — with never more than 2 running at once.

Implement from scratch:

function mapLimit(inputs, limit, iteratee) {
  // Your code here
}