Javascript Runtime Async Review

JavaScript Runtime & Async Correctness Review

Purpose

async/await reads like synchronous code, which leads developers to reason about it as if it were synchronous — but the underlying microtask/macrotask scheduling model still governs actual execution order, and getting it wrong produces exactly the class of bug that's hardest to catch in normal testing: intermittent, timing-dependent, "works on my machine" race conditions. This skill traces real event-loop ordering, audits every async chain for unhandled-rejection paths, and verifies every listener/timer has a reachable cleanup path, so timing correctness is verified rather than assumed.

When to use

Use this skill when the user asks to:

• review JavaScript/TypeScript async code (Promises, async/await, timers) for correctness,
• audit event-listener, timer, or observer cleanup to prevent memory leaks,
• diagnose or prevent race conditions in UI patterns with rapid repeated async calls (search-as-you-type, polling, infinite scroll),
• check for unhandled Promise rejections, especially on security-relevant code paths,
• verify actual microtask/macrotask execution order for a specific code sequence.

Do not use this skill for:

• framework-specific effect/hook lifecycle review (React useEffect dependency arrays, Vue watchers) — use the matching framework-specific review skill; this skill covers the underlying runtime/scheduling layer those skills build on,
• bundler/build-tool configuration or transpilation-target correctness — out of scope,
• a claim that requires live production traffic or load-test evidence to confirm timing under real network jitter — this is static-review-only; label those findings as needing live verification, do not assert them as proven.

Context7 Documentation Protocol

• Resolve the docs source with resolve-library-id against /mdn/content (or the closest current MDN Context7 ID) before asserting any ordering, scheduling, or API-behavior claim.
• Before ruling on a specific ordering question (e.g., "does this setTimeout(fn, 0) run before or after this .then()"), call query-docs for that exact scenario — do not answer from memory or from a synchronous mental model, since ordering intuitions are a frequent source of subtly-wrong review comments.
• Trace microtask-vs-task distinctions explicitly: microtasks (Promise callbacks, queueMicrotask, async/await continuations) drain completely — including microtasks they themselves enqueue — before the next macrotask (setTimeout, setInterval, I/O, UI rendering) runs. Cite this distinction rather than assuming the reader already applies it correctly.
• For cancellation/lifecycle patterns (AbortController, removeEventListener), verify current API shape and browser support notes via query-docs before recommending a specific signature — do not invent options or assume universal support without checking.
• If Context7 is unavailable, fall back to the official_docs URLs in this skill's metadata.json and label the claim documentation-based, unverified against current release.

Lean operating rules

• Do not assume async/await makes code race-condition-safe by default — it only guarantees ordering within a single linear await chain, not between independently-triggered async operations.
• Trace the actual microtask/macrotask interleaving for any ordering claim rather than asserting it from a synchronous mental model; query current MDN event-loop/microtask docs before ruling, since this is a frequent source of subtly-wrong assumptions.
• Every Promise chain must terminate in a .catch or sit inside a try/catch around every await — flag any chain that doesn't as an unhandled-rejection risk, with extra weight if it touches an authorization/permission check.
• Every addEventListener/setInterval/non-one-shot setTimeout/Observer must be matched to a traced, reachable removeEventListener/clearInterval/clearTimeout/disconnect call, including on early-return and error paths.
• Any UI pattern with rapid repeated async calls (search-as-you-type, polling) must use AbortController, a request-generation counter, or equivalent sequencing — flag its absence as a race-condition risk, not a style preference.
• Do not accept "passed manual testing" as sufficient evidence for timing-sensitive code — manual testing rarely hits the interleavings that matter; flag as needing live/load-tested verification instead.
• Flag eval, new Function(), and string-argument timers as code-injection risks requiring explicit justification, not routine patterns.
• Label every ordering/timing claim as spec-traced, documentation-based, or needs live-runtime verification so reviewers know what's actually been verified.
• Never execute, build, or run application code as part of this review; this is a static-review skill (Read/Grep/Glob only, plus git diff for scoping and WebFetch for spec/docs grounding).

References

Load these only when needed:

• Event-loop ordering trace patterns — use when tracing the actual execution order of a specific microtask/macrotask/await sequence in review.
• Unhandled-rejection audit checklist — use when systematically auditing a file/module's Promise chains for missing .catch/try-catch coverage.
• Race-condition and cancellation patterns — use when reviewing rapid-repeated-async UI code for AbortController, generation-counter, or equivalent sequencing needs.

Response minimum

Return, at minimum:

• the event-loop/ordering verdict for each async chain reviewed, with the traced resolution order (not assumed),
• the unhandled-rejection audit result for every Promise chain in scope,
• the listener/timer/observer cleanup audit result, matching every registration to its cleanup call,
• race-condition risk flags for any rapid-repeated-call pattern lacking sequencing or cancellation,
• residual risk notes for anything requiring live or load-tested verification beyond this static trace.