Map Async Limit

mediumcodingJavaScriptPromisesConcurrency~25 min

Asked at Google, Stripe, Amazon

Overview

Implement mapAsyncLimit(array, limit, asyncFn) that runs at most limit async operations concurrently.

Constraints

Max limit concurrent operations at any time
Order of results matches input order

Examples

await mapAsyncLimit([1,2,3,4], 2, async x => x * 2); // [2,4,6,8]

Solution

Reveal solution

async function mapAsyncLimit(array, limit, asyncFn) {
  const results = new Array(array.length);
  let idx = 0;
  async function worker() {
    while (idx < array.length) {
      const i = idx++;
      results[i] = await asyncFn(array[i]);
    }
  }
  const workers = Array.from({ length: Math.min(limit, array.length) }, () => worker());
  await Promise.all(workers);
  return results;
}

map-async-limit.js

Map Async Limit

mediumcodingJavaScriptPromises

Overview

Implement mapAsyncLimit(array, limit, asyncFn) that runs at most limit async operations concurrently.

Constraints

Max limit concurrent operations at any time
Order of results matches input order

Examples

await mapAsyncLimit([1,2,3,4], 2, async x => x * 2); // [2,4,6,8]

Solution

Reveal solution

async function mapAsyncLimit(array, limit, asyncFn) {
  const results = new Array(array.length);
  let idx = 0;
  async function worker() {
    while (idx < array.length) {
      const i = idx++;
      results[i] = await asyncFn(array[i]);
    }
  }
  const workers = Array.from({ length: Math.min(limit, array.length) }, () => worker());
  await Promise.all(workers);
  return results;
}

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