Limits the number of concurrently running async operations. Useful for rate-limited APIs or controlling resource usage.

p-limit.js

export function pLimit(concurrency) {
  const queue = [];
  let activeCount = 0;

  const next = () => {
    if (queue.length === 0 || activeCount >= concurrency) return;

    activeCount++;
    const { fn, resolve, reject } = queue.shift();

    fn()
      .then(resolve, reject)
      .finally(() => {
        activeCount--;
        next();
      });
  };

  return (fn) =>
    new Promise((resolve, reject) => {
      queue.push({ fn, resolve, reject });
      next();
    });
}

Use case:

const limit = pLimit(3);

await Promise.all(
  urls.map(url => limit(() => fetch(url)))
);