What does it mean to "block the event loop," and how do you avoid it?

Answer: Because your JavaScript runs on one thread, any long-running synchronous operation prevents the event loop from handling other events. Requests queue up and latency spikes for all clients.

Common culprits:

• Tight CPU loops (image processing, encryption, big data transforms).
• Synchronous FS calls (fs.readFileSync) on hot paths.
• JSON.parse/JSON.stringify on very large payloads.
• Complex synchronous regexes (ReDoS).
• crypto.pbkdf2Sync / bcrypt sync variants.

// ❌ Blocks the loop — no other request is served during this
app.get('/hash', (req, res) => {
  const result = expensiveSyncHash(req.query.data);
  res.send(result);
});

Ways to avoid blocking:

1. Use async APIs — fs.promises.readFile, async crypto (crypto.pbkdf2), etc.
2. Offload CPU work to worker_threads (or a job queue / separate microservice):

const { Worker } = require('worker_threads');
app.get('/hash', (req, res) => {
  const worker = new Worker('./hash-worker.js', { workerData: req.query.data });
  worker.once('message', result => res.send(result));
});

1. Chunk long loops — break work across ticks with setImmediate so I/O gets turns.
2. Stream large data instead of buffering it all in memory.

Diagnosing: watch event-loop lag (e.g., perf_hooks.monitorEventLoopDelay), and profile with --prof or clinic.js.