What causes memory leaks in Node.js applications?

Direct answer

Common leaks: global caches without eviction, orphaned event listeners, closures holding request scope, timers never cleared, unbounded in-memory queues — detect with heap snapshots, process.memoryUsage, and clinic/Chrome DevTools; fix by removing listeners, TTL caches, and streaming large data.

Node processes are long-lived — small per-request leaks compound into OOM kills under PM2/Docker restart loops. Production APIs need observability on heap growth.

Why interviewers ask

Cause	Example	Fix
Global array/map growth	cache[key] = res without TTL	LRU cache with max size
Event listeners	bus.on in every request (Q57)	off/once, scoped emitters
Closures	Handler captures huge req.body forever	Clear refs after response
Timers	setInterval never cleared	clearInterval on shutdown
Detached buffers	Accumulating chunks without end	Use pipeline, destroy streams

process.memoryUsage() — rss, heapUsed, external — log periodically in staging.
Heap snapshot — node --inspect, compare snapshots before/after load test.
WeakRef / WeakMap — optional for caches when GC should reclaim entries.

memory-leak-fix.mjs

import { LRUCache } from "lru-cache";

const topicCache = new LRUCache({
  max: 500,
  ttl: 1000 * 60 * 10, // 10 min
});

function getCachedTopic(slug) {
  if (topicCache.has(slug)) return topicCache.get(slug);
  const topic = loadTopic(slug);
  topicCache.set(slug, topic);
  return topic;
}

// Anti-pattern: globalLeaks.push(everyResponsePayload)