Memory Leak Audit

Memory Leak Audit

Find and fix memory leaks in JavaScript/TypeScript codebases. This skill is based on the 60 leak patterns catalogued by the Memory Leak Laboratory (js-leak-lab).

When to use this skill

Use it when the user:

• reports a process whose memory grows over time, gets OOM-killed, or restarts
• has a browser tab that gets slow/janky the longer it runs
• asks "why does this leak?", "is this a leak?", or for a memory audit/optimization
• adds long-lived state, timers, listeners, caches, or subscriptions and wants a check

What a leak actually is

A memory leak is memory that stays retained (reachable by the GC) but is never used again. Before flagging something, confirm both halves:

1. Something keeps a reference to it (an array, a closure, a listener, a cache).
2. That thing only ever grows, or outlives the data's usefulness.

A bounded cache or a fixed-size pool is not a leak. An array that only ever gets .push()-ed is.

Audit method

1. Identify the runtime. Server (Node/Next/Nuxt) leaks accumulate across requests in a long-lived process; client (browser/React/Vue) leaks accumulate across component mounts/unmounts and user navigation. The fix differs.
2. Scan for the 8 categories below. For each, grep for the signature, then check whether the retained thing is ever released.
3. Rank by severity — critical (fast unbounded growth, process-killer), high (unbounded, OOM under load), medium (slow growth), low (subtle).
4. Apply the smallest fix that bounds or releases the memory (see below and reference.md for all 60 concrete patterns).
5. Verify. Server: sample process.memoryUsage().heapUsed over time. Browser: performance.memory.usedJSHeapSize (Chromium) or compare heap snapshots.

The 8 leak categories

1. Collections & buffers that only grow

Signature: an array / Map / Set / Buffer at module or instance scope that only ever gets .push() / .set() / .add() — never .shift() / .delete() / cleared. Fix: bound it — circular buffer, max-size with eviction, or don't retain at all.

2. Timers & intervals

Signature: setInterval / setTimeout / requestAnimationFrame whose handle is discarded, or kept but never clear* / cancelAnimationFrame-ed. Every pending timer pins its callback's entire closure. Fix: keep the handle; clear it on teardown (unmount, request end, stop()).

3. Listeners & observers

Signature: addEventListener / emitter.on / new MutationObserver / IntersectionObserver called repeatedly with no matching removeEventListener / .off / .disconnect. Fix: remove/disconnect on teardown; register once, not per-render or per-request.

4. Closures holding large scope

Signature: a long-lived function (event handler, memoized callback, stored closure) that closes over a large object it does not fully need. Fix: capture only the small value actually used; drop references when done.

5. Async — promises & callbacks

Signature: promises that never settle, resolve functions stashed in an array, pub/sub callback registries with no unsubscribe. Fix: always settle or time out promises; expose and call an unsubscribe.

6. Caching & retention

Signature: a cache — often module-scope, so it survives across every request in Next.js/Nuxt — keyed by unbounded or dynamic values, with no eviction (or eviction code with a broken condition). Fix: bounded LRU with a correct eviction check; request-scoped state instead of module-scope; WeakMap/WeakRef when keys are objects.

7. Detached DOM

Signature: DOM nodes or <iframe>s removed from the document but still referenced by JavaScript — the whole subtree (and an iframe's JS realm) stays alive. Fix: drop the JS references when removing nodes; bound any node pool.

8. Framework state & lifecycle

React signature: useEffect with no cleanup return; intervals/subscriptions started in effects and never cleared; state arrays that only grow; context values or trees retained in refs. Vue signature: watch/watchEffect created outside setup scope (so it is not auto-disposed); event-bus handlers not removed in onBeforeUnmount; module-scope reactive() stores; unbounded <KeepAlive>. Fix: return a cleanup from useEffect; create watchers inside setup or stop them in onBeforeUnmount; bound caches, keep-alive sets, and state arrays.

Reporting findings

For each finding report:

• file:line — where it is
• category (1–8 above) and severity (low/medium/high/critical)
• why it leaks — what holds the reference and why it is never released
• the fix — show the corrected code; prefer the smallest change that bounds or releases the memory

If asked to fix, apply the fixes and then re-scan to confirm nothing was missed.

Full pattern catalogue

reference.md (next to this file) lists all 60 concrete patterns grouped by category, each with its bad signature and standard fix. Load it when you need a specific pattern or a more exhaustive scan.