How to limit concurrent async operations in Node.js

Short answer

Use libraries like p-limit or implement a semaphore pattern to restrict how many promises run at the same time.

Steps

1. Install p-limit: npm install p-limit.
2. Create a limiter: const pLimit = require('p-limit'); const limit = pLimit(5);.
3. Map your tasks through the limiter: const results = await Promise.all(tasks.map(t => limit(() => t())));.
4. Alternatively, implement a worker pool or queue pattern manually.
5. Adjust the concurrency limit based on the downstream service capacity.

Tips

• Rate limiting and concurrency limiting are complementary; use both when calling external APIs.
• Batch processing with limited concurrency keeps memory usage stable compared to unbounded Promise.all.

Common issues

• Setting concurrency too high can trigger rate limits or memory issues.
• Setting it too low underutilizes resources and slows down batch jobs unnecessarily.