# Javascript Patterns > JavaScript language patterns, modern syntax, async handling, module systems (ESM/CJS), error handling, and review checklist for JavaScript code without TypeScript. Use whenever the project contains `.js`, `.mjs`, `.cjs`, or `.jsx` files (and no `tsconfig.json`), `package.json` without TypeScript dependencies, OR the user asks about JavaScript, JS, ECMAScript, async/await, promises, modules, ESM, CommonJS, package.json, npm, pnpm, yarn, bun, or any work in JavaScript code without explicit TypeScript usage, even if "JavaScript" is not mentioned by name. Use when this capability is needed. - Skill: `tomevault-io/javascript-patterns` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/javascript-patterns` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/javascript-patterns/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/javascript-patterns --- # JavaScript patterns Default assumption: a project running on the current Node.js Active LTS (Node 24, "Krypton") or a modern browser baseline. If the project also uses TypeScript, the type-aware skill applies on top of these language patterns. Adapt advice to the Node version in the project's `.nvmrc`, `.tool-versions`, or `engines` field in `package.json`. ## Severity rubric - `failure`: a concrete defect or violation that should not ship. - `warning`: a smell or pattern that compounds with other findings. - `info`: a hardening opportunity or note, not a defect. ## Reference files | File | Covers | | ---------------------------------------------------------------------- | -------------------------------------------------------------------------------- | | [reference/modules.md](reference/modules.md) | ESM and CJS, `package.json` `"type"`, dynamic `import()`, dual-package hazard | | [reference/syntax.md](reference/syntax.md) | Modern syntax: optional chaining, nullish coalescing, logical assignment, `at()` | | [reference/async.md](reference/async.md) | `async`/`await`, `Promise.*`, `for await...of`, top-level `await` | | [reference/errors.md](reference/errors.md) | Error subclasses, `Error.cause`, unhandled rejection, `using` (advisory) | | [reference/equality-and-numbers.md](reference/equality-and-numbers.md) | `===`, `Number.isNaN`, `Array.isArray`, IEEE 754, `BigInt` | | [reference/collections.md](reference/collections.md) | `Map`, `Set`, `WeakMap`, `WeakSet` and when to reach for each | | [reference/jsdoc.md](reference/jsdoc.md) | `// @ts-check` plus JSDoc tags for editor-checked types in `.js` files | | [reference/anti-patterns.md](reference/anti-patterns.md) | Twelve review-time anti-patterns with severity calls | ## When to load this skill - Any task touching `.js`, `.mjs`, `.cjs`, or `.jsx` files. - Any task involving `package.json`, npm / pnpm / yarn / bun workflows, or Node runtime configuration. - Code review where the diff includes module structure, async control flow, or error handling in JavaScript. - Migrations between ESM and CJS, or between Node major versions. ## When not to load this skill - TypeScript projects that already load type-aware patterns (this skill still applies for the language-level rules, but `any`-vs-`unknown`, generics, and tsconfig live elsewhere). - Projects with no JavaScript surface (pure backends in another language). ## References - MDN JavaScript reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference - Node.js packages: https://nodejs.org/api/packages.html - Node.js ESM: https://nodejs.org/api/esm.html - Node.js process: https://nodejs.org/api/process.html - Node.js previous releases (LTS schedule): https://nodejs.org/en/about/previous-releases - TC39 proposals: https://github.com/tc39/proposals - 2ality (Axel Rauschmayer): https://2ality.com/ - You Don't Know JS, 2nd ed. (Kyle Simpson): https://github.com/getify/You-Dont-Know-JS ## Maintenance note ECMAScript ships a new edition each June. New syntax tends to land in V8 and Node within a release cycle of being standardized; TC39 stage 3 proposals can sometimes land in Node behind a flag before reaching stage 4. When new syntax appears in a code review, check this skill against the current MDN page and the current Node Active LTS before relying on legacy guidance here. Dropping support for an older Node major in `engines.node` is a breaking change for downstream consumers; treat it as such. --- > Source: [ku5ic/dotfiles](https://github.com/ku5ic/dotfiles) — distributed by [TomeVault](https://tomevault.io).