Memory leaks

A Node "memory leak" is usually unbounded retention — objects stay reachable via references you forgot to release. RSS grows until OOM kill.

Common retainers:

• Global Maps caching per user/request without TTL
• Event listeners never removed (`EventEmitter` on long-lived bus)
• Closures holding large request context
• Timers and intervals
• Unbounded job queues

					// Leak: cache grows forever
const cache = new Map();
app.use((req, res, next) => {
  cache.set(req.user.id, heavyObject(req));
  next();
});
				

Diagnose with two heap snapshots under load — compare retained size. Fix with WeakMap where keys are ephemeral, TTL caches, `off`/`removeListener`, and clear timers on shutdown.

On interviews: explain reachability vs "memory leak" in GC languages; listener leak example; how you'd prove fix in staging.

Common pitfalls: conflating normal heap growth with leak; fixing symptoms by raising `--max-old-space-size` only.

The trade-off is balancing simplicity, performance, safety, and operability — name which axis you optimized and what cost you accepted.

Checklist:

• Bound every cache.
• Remove listeners on lifecycle end.
• Snapshot diff methodology.
• Monitor heap trend after deploy.