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Apply these opinionated JavaScript conventions whenever writing or reviewing JS or Node: ESM-first packaging, choosing between Promise combinators, error handling that doesn't swallow failures, AbortController for cancellation, keeping the event loop unblocked, and reaching for platform APIs before dependencies.

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JavaScript

House conventions for modern JavaScript and Node. Apply them to code you are writing or changing — don't refactor untouched files to match unless asked.

Conventions

ESM everywhere. New packages get "type": "module". Mixing CJS and ESM inside one package produces the dual-package hazard — two copies of a module with separate state, failing in ways that read like race conditions.
Match the combinator to the intent. Sequential await in a loop serialises independent work; Promise.all drops the whole batch on one rejection; allSettled is for when partial success is acceptable. The wrong choice is a latency bug or a swallowed-error bug, and both look fine in review.
Every catch handles or rethrows. A catch that logs and continues converts a failure into corrupt state further downstream, where the stack trace no longer points at the cause. If there is nothing useful to do, don't catch.
AbortController for anything cancellable — fetches, timers, streams, listeners ({ signal }). Without it an abandoned request keeps its handler and its closure alive.
Never block the event loop. No readFileSync, no long synchronous parse or crypto on a request path: Node serves every other request from that thread. CPU-bound work goes to a worker thread.
Don't mutate arguments. A function that edits its input changes the caller's object with no assignment in sight. Return new values — and reach for the non-mutating array methods that exist for exactly this: toSorted, toReversed, toSpliced, with. arr.sort() sorting the caller's array in place is one of the most common accidental mutations in JS.
node: prefix on builtins. Unambiguous, and immune to shadowing by a userland package of the same name.
Reach for the platform before a dependency. structuredClone, Intl, URL/URLSearchParams, crypto.randomUUID, Array.prototype.at/findLast, Object.groupBy cover most of what utility libraries were installed for.

Publishing a package

The exports map is the public surface — anything not listed is unreachable, which is the point: it lets you refactor internals without breaking consumers. Declare sideEffects: false (or list the files that have them) so bundlers can drop unused exports; without it, tree shaking has to assume importing your module does something and keeps all of it.

{
  "type": "module",
  "exports": {
    ".": { "types": "./dist/index.d.ts", "default": "./dist/index.js" },
    "./package.json": "./package.json"
  },
  "sideEffects": false
}

Two ESM facts that cost the most time when they bite: relative imports need the file extension (./utils.js, not ./utils), and there is no __dirname — derive it with fileURLToPath(import.meta.url), or createRequire(import.meta.url) when you genuinely need to load a CJS-only package.

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answer-buzzkill

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Write a short, fact-checked reply that answers a colleague's question in one meaning only, ready to copy and paste. The input is their pasted message. Check the claims against the code first, use the project glossary terms, and print only the reply in chat. Read-only — no writes to code, the issue tracker, GitHub, chat or any MCP. Use on "/cortex-communication:answer-buzzkill", "help me answer", "reply to this question", or when the user pastes a colleague's question and asks for an answer in the same manner.

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