javascript-conventions

Apply all rules below strictly to any JavaScript you write, generate, or review.

Scope

• Language-level rules for JavaScript only — modules, syntax, functions, data, asynchrony, errors, JSDoc types, naming, comments.
• HTML, CSS, accessibility, and design tokens are not in this skill — see web-conventions.
• Architecture, state management, routing, dependency and build policy, project layout, dev server, and verification loops are not in this skill — see web-components (SPAs) and web-sprinkles (static sites with bounded enhancements); web-static forbids JavaScript entirely.
• When a composed skill specifies a rule, the composed skill wins; this skill is the fallback baseline.

Language Level

• target the newest ECMAScript syntax and standard-library APIs the Baseline policy permits — the decision is per feature, never per year: there is no "ES2020 target" to configure, only a feature's Baseline status
• no build step to reach a language feature — no transpiler (Babel, SWC, esbuild), no polyfill bundle; if a feature needs compiling, its Baseline status already says not to use it
• no TypeScript, no JSX — plain ES modules with JSDoc type annotations
• modules are strict mode already — never write 'use strict'

Baseline Lookup for JavaScript

The web-conventions Baseline policy governs JavaScript exactly as it governs CSS:

• Widely Available — use freely
• Newly Available — requires feature detection with a graceful fallback
• Limited availability — must not be used

CSS detects with @supports; JavaScript's equivalent is an existence check before first use:

const groupBy = Object.groupBy ?? ((items, key) => items.reduce(intoGroups(key), {}));
if (!document.startViewTransition) return render(url);
if (!('URLPattern' in globalThis)) return matchWithRegExp(url);

• look up the feature id in web-conventions/references/baseline-snapshot.md — never from model memory; widely-known features can still be Limited (top-level await is, in the 2026-07-08 snapshot)
• a feature missing from the snapshot or newer than its header date is uncertain — verify against webstatus.dev before recommending it, or treat it as Limited
• core syntax may have no feature id at all — optional chaining (?.) returns 404 from the API (verified 2026-08-04), meaning untracked, not Limited; judge such syntax on its own rather than by lookup
• when recommending a newer API, cite its status so the reader can judge support