Implement p-limit — concurrency limiter for async tasks

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Accepted Answer

**Approach:** Queue + active count. `limit(fn)` enqueues and tries to run. `runNext` dequeues if `active < concurrency`, increments active, awaits fn, then decrements and recurses.

```javascript
function pLimit(concurrency) {
  let active = 0;
  const queue = [];
  const next = () => {
    if (active >= concurrency || queue.length === 0) return;
    active++;
    const { fn, resolve, reject } = queue.shift();
    (async () => fn())().then(resolve, reject).finally(() => { active--; next(); });
  };
  return function limit(fn) {
    return new Promise((resolve, reject) => {
      queue.push({ fn, resolve, reject });
      next();
    });
  };
}

// Demo
const limit = pLimit(2);
const sleep = (ms, v) => new Promise(r => setTimeout(() => r(v), ms));
async function demo() {
  const start = Date.now();
  const tasks = [1,2,3,4].map(i => limit(() => sleep(200, i).then(v => {
    console.log(`task ${v} done at ${Date.now()-start}ms`); return v;
  })));
  console.log(await Promise.all(tasks));

Implement p-limit — concurrency limiter for async tasks

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Understand the problem

Attempt it yourself

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Join the discussion

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KEEP THESTREAK ALIVE.

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