How to Build an Async Concurrency Limiter (Task Pool) in JavaScript

In production web applications, firing hundreds of concurrent HTTP requests simultaneously can overwhelm browser network pools or hit API rate limits. Building an Async Concurrency Limiter (Task Pool) is a classic senior live coding interview task.

Problem Statement

Write a function mapConcurrent(items, limit, asyncFn) that executes asyncFn for every item in items, ensuring at most limit promises execute concurrently.

Optimal Solution

async function mapConcurrent(items, limit, asyncFn) {
  const results = [];

for (const [index, item] of items.entries()) { const p = Promise.resolve().then(() => asyncFn(item, index)); results[index] = p; executing.add(p);

const clean = () => executing.delete(p); p.then(clean, clean);

if (executing.size >= limit) { await Promise.race(executing); } }

return Promise.all(results); } ```

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Try this Problem Live on Pairlet Practice the [Concurrency Limiter coding challenge](https://www.pairlet.dev/problems/concurrency-limiter) live in a collaborative interview session.