Debug: Node.js event loop blocking detection

Node.js application becomes unresponsive or slow. Requests queue up and timeouts increase.

Detect and fix event loop blocking:

1. Detect with built-in diagnostics:

const { monitorEventLoopDelay } = require('perf_hooks');
const h = monitorEventLoopDelay({ resolution: 20 });
h.enable();

setInterval(() => {
  console.log(`Event loop delay: p50=${h.percentile(50)/1e6}ms p99=${h.percentile(99)/1e6}ms`);
  h.reset();
}, 5000);

1. Simple lag detection:

let lastCheck = Date.now();
setInterval(() => {
  const now = Date.now();
  const lag = now - lastCheck - 1000;
  if (lag > 100) console.warn(`Event loop lag: ${lag}ms`);
  lastCheck = now;
}, 1000);

1. Common causes and fixes:

• JSON.parse/stringify on large objects -> use streaming parser
• Synchronous file I/O -> use async versions
• CPU-intensive computation -> use worker_threads
• Large array operations -> chunk with setImmediate
• Regular expressions on large strings -> use RE2 or limit input

Node.js runs JavaScript on a single thread. Any synchronous operation that takes more than a few milliseconds blocks all other requests.

Node.js applications experiencing latency or unresponsiveness