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JavaScript (MDN)

現代JavaScript（ECMAScript）のベストプラクティスをMDNドキュメントから精選。コードの作成、レビュー、修正時に、組み込みAPIの選択や非推奨パターンの置き換えを支援します。

JavaScript (MDN) とは？

このスキルは、MDNのJavaScriptガイドとリファレンスを、開発者が陥りがちな落とし穴、意外なデフォルト動作、非推奨APIとその代替、MDN推奨のパターンに焦点を当てて圧縮したものです。各リファレンスファイルには、主張の根拠となるMDNページへのリンクが付与されています。カバー範囲は最新のECMAScript標準（Stage 3を含む）であり、等価比較（===, Object.is）、let/constと一時的デッドゾーン、strictモード、ASI（自動セミコロン挿入）、関数、クラスとプライベートフィールド、オブジェクト、プロトタイプ、Proxy、Reflect、Symbol、eval、オペレーター（??, ?., スプレッド, BigInt演算）、Promise、async/await、Promise.try、イテレーターとイテレーターヘルパー、ジェネレーター、using/await using、DisposableStack、WeakRef、Array（toSorted, at, groupBy, fromAsync）、MapやSetのメソッド、WeakMap、文字列とUnicode、encodeURIComponent、RegExp（vフラグ、RegExp.escape、lastIndex）、Number、Math、BigInt、JSON（rawJSON、revivers）、型付き配列、ArrayBuffer、base64、SharedArrayBuffer、Atomics、Temporal、Date、Intl（DateTimeFormat、NumberFormat、Collator、Segmenter、PluralRules）、エラー（cause、Error.isError）、ESモジュール、import属性、import.meta、ダイナミックインポートなどを含みます。各参照ファイルは独立しており、タスクに応じて必要なファイルを選択して参照できます。高い頻度で発生する間違いを防ぐための「最重要ルール」セクションも提供されています。

対応~Claude Code~Codex CLI~Cursor
npx skills add https://github.com/quentin-autret/claude-skills/tree/main/skills/javascript-docs

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ドキュメント

JavaScript (MDN) は何をしますか？

このスキルは、MDNのJavaScriptガイドとリファレンスを、習慣と異なる点（落とし穴、驚くべきデフォルト動作、非推奨APIとその代替、MDN推奨のパターン）に絞って凝縮したものです。参照ファイル内の各主張の末尾には、その根拠となったMDNページへのリンクが付いています。すべてのJavaScript開発者が知っている基本は省略されています。

対応バージョン

MDNはライブのECMAScript標準を追跡しており、バージョン管理されたパスはありません。また、TC39ステージ3の機能（最終確定前のもの）も文書化しているため、例の中にはまだ広く利用できない機能が含まれている可能性があります。 https://developer.mozilla.org/en-US/docs/Web/JavaScript

最近の機能は利用可能性が限られている場合があります。特定の機能（Temporal、イテレーターヘルパー、using、Promise.try、RegExp.escape、Error.isError、Math.sumPrecise、Uint8Array.fromBase64、Setメソッド、Map.prototype.getOrInsertなど）に依存する前に、ターゲットランタイムを確認してください。MDNが実験的とマークしている項目は、参照ファイル内でそのようにラベル付けされています。

使い方

  1. 以下の「最重要ルール」を読んでください。これらは最も頻繁に発生する間違いをカバーしています。
  2. 最後の表から、タスクに合った参照ファイルを開いてください。各ファイルは独立しています。タイムゾーン、DST、または期間に関するタスクには、両方のTemporalファイルが必要です。クリーンアップやusingに関するタスクには、イテレーターとリソース管理ファイルが必要です。
  3. 古いコードに触れる場合は、該当ファイルの末尾にある「古いパターン」の表を読んでください。

最重要ルール

参照ファイル

ファイル内容
references/language-core-and-equality.md== と ===、型強制、typeof、let/const/var とデッドゾーン、strictモード、ASI、8進数、実行モデル、廃止された構文
references/functions-and-classes.mdアロー関数と this、デフォルトパラメータとレストパラメータ、アクセサー、bind、クラスフィールド、#private、静的ブロック、extends、super
references/objects-prototypes-and-proxy.mdObject.hasOwn、assign、defineProperty、freeze、groupBy、プロトタイプ、nullプロトタイプオブジェクト、Proxy、Reflect、Symbol、globalThis、eval
references/statements-and-operators.md??、?.、論理代入、スプレッド、分割代入、delete、算術とビット演算のトラップ、BigInt演算子、比較、ループ、switch、ラベル、try/finally
references/async-and-promises.mdPromiseチェーン、コンビネーター、Promise.try、withResolvers、allKeyed、async/awaitの順序、未処理の拒否、Promiseメモリリーク
references/iterators-and-resource-management.mdイテレーションプロトコル、イテレーターヘルパー、Iterator.zip/concat、ジェネレーター、using、await using、DisposableStack、SuppressedError、WeakRef、FinalizationRegistry
references/arrays-and-collections.md配列のコピーメソッド、at、includes、findLast、sort、reduce、疎配列、Array.from、Map、Setメソッド、getOrInsert、WeakMap、WeakSet
references/strings.mdUTF-16、コードポイントと書記素、localeCompare、normalize、replaceパターン、replaceAll、matchAll、split、String.raw、URIエンコーディング
references/regexp.mdRegExp.escape、メソッドの選択、lastIndex、y と d フラグ、u と v モード、\p{...}、修飾子、アンカー、グループ、後方参照、後読み
references/numbers-math-json.md浮動小数点精度、parseInt、丸め、toFixed、BigInt、Math.random、Math.sumPrecise、JSON.stringifyルール、リバイバー、JSON.rawJSON
references/binary-data-and-shared-memory.md型付き配列、DataViewのエンディアン、リサイズ可能および転送された ArrayBuffer、Uint8Array でのbase64とhex、SharedArrayBuffer、Atomics
references/date-and-temporal.mdDate から Temporal への移行、Temporal型の選択、共有Temporalルール、プレーンな日付と時刻、Dateコードの保守
references/temporal-time-zones-and-durations.mdZonedDateTime、IANAタイムゾーン、DSTの曖昧性解消と offset、since/until、Durationのバランス調整と丸め、Temporal.Now
references/intl.mdロケールの選択とフォールバック、DateTimeFormat、NumberFormatの桁数オプション、Collator、Segmenter、PluralRules、RelativeTimeFormat、ListFormat、DurationFormat
references/errors-and-debugging.mdcause、Errorサブクラス、Error.isError、AggregateError、スタックトレース、一般的なランタイムエラーメッセージとその修正
references/modules.mdモジュールとスクリプトの違い、ライブバインディング、循環、import()、import.meta、import属性、import defer、import source、importマップ

クロールされたページ、バージョン情報、取得日は SOURCES.md に記載されています。

Individual skills in this repo

This repo contains 16 individual skills — each has its own dedicated page.

eslint-docs

Teaches current ESLint 10 practices drawn from the official documentation. Use when writing, reviewing or fixing an eslint.config.js (flat config, defineConfig, files, ignores, globalIgnores, extends, languageOptions, globals, parsers, processors), configuring ESLint rules and severities, disable comments and inline config, running the CLI in CI (--fix, --cache, --quiet, --max-warnings, bulk suppressions, formatters, feature flags, MCP server), writing custom rules (meta, schema, fixes, suggestions, selectors, scope, code paths) and testing them with RuleTester, building plugins, shareable configs, custom parsers, processors or formatters, using the Node.js API (ESLint, Linter), or migrating to ESLint 9 or 10 from eslintrc. Triggers on eslint, eslint.config.js, flat config, .eslintrc, .eslintignore, @eslint/js, js/recommended, eslint-disable, no-unused-vars, no-restricted-imports, no-restricted-syntax, RuleTester, eslint-plugin, eslint-config, FlatCompat, context.getScope is not a function, ESLint 10.

grafana-cloud-docs

Grafana Cloud pitfalls, defaults, limits and current patterns from the Grafana, Mimir, Loki and Tempo docs, each tied to its docs URL. Use when sending OpenTelemetry data to Grafana Cloud, writing PromQL, LogQL or TraceQL, controlling cardinality and cost, designing alert rules, routing notifications, provisioning alerting, building Synthetic Monitoring checks, using the /apis dashboard API, choosing access policy tokens or service accounts, or migrating off API keys, the Loki Exporter and legacy /api routes.

next-intl-docs

Teaches current next-intl 4 practices for internationalizing Next.js apps, drawn from the official documentation. Use when setting up next-intl with the App Router, writing i18n/request.ts, adding a [locale] segment with defineRouting, createMiddleware in proxy.ts (formerly middleware.ts), building locale switchers with createNavigation, translating with useTranslations or getTranslations, passing messages to NextIntlClientProvider, writing ICU messages (plurals, select, rich text), formatting dates, numbers and lists with useFormatter, translating metadata, Server Actions, Route Handlers, localized 404 and error pages, static rendering with next/root-params, type-safe messages through AppConfig, testing with Jest, Vitest, Storybook, or extracting inline messages with useExtracted. Triggers on next-intl, useTranslations, getTranslations, NextIntlClientProvider, getRequestConfig, i18n routing, locale prefix, localePrefix, NEXT_LOCALE, hreflang, setRequestLocale, ICU messages, App Router internationalization.

nodejs-docs

Teaches Node.js practice drawn from the official Node.js API documentation, with pitfalls, surprising defaults, deprecations and replacements, each backed by its nodejs.org URL. Use when writing, reviewing, debugging or upgrading Node.js code, or choosing a built-in module (fs, crypto, streams, http, process, CLI flags, node:test, ESM/CommonJS, package.json exports, TypeScript type stripping, child_process, worker_threads, node:sqlite, deprecations).

playwright-docs

Pitfalls, surprising defaults and recommended patterns of Playwright Test for JavaScript and TypeScript, distilled from the official docs. Use when writing, reviewing, debugging or configuring Playwright tests (@playwright/test, playwright.config.ts, npx playwright test), including flaky tests, locators (getByRole, filter, strict mode), web-first assertions (expect, toHaveText, toBeVisible, expect.poll, toPass), fixtures, storageState authentication, project dependencies, timeouts, retries, network mocking (page.route, routeFromHAR, routeWebSocket), API testing, clock, aria snapshots, toHaveScreenshot, axe accessibility scans, traces, UI mode, reporters, CI, Docker, sharding, test locks, component testing with stories and mount, codegen, Playwright MCP, playwright-cli and migration from Puppeteer, Protractor, Testing Library or older Playwright versions.

postgresql-docs

Pitfalls, surprising defaults, PostgreSQL 18 changes and recommended patterns from the official PostgreSQL 18 documentation, each backed by its postgresql.org URL. Use when writing or reviewing PostgreSQL SQL, schema design, migrations and ALTER TABLE locks, indexes, transactions, isolation levels and retries, locking, upsert and MERGE, COPY, JSONB, timestamptz, generated and identity columns, partitioning, EXPLAIN, vacuum, roles, row-level security, PL/pgSQL, full-text search, connection settings, and upgrades to PostgreSQL 18.

react-docs

Writes, reviews and upgrades React 19.3 code from the official react.dev documentation. Use when writing components and Hooks, removing unnecessary useEffect calls, fixing Effect dependencies with useEffectEvent, building forms with Actions (useActionState, useOptimistic, useFormStatus, form action), using Server Components, 'use client', 'use server' and Server Functions, Suspense, use(), useTransition, useDeferredValue, ViewTransition, Activity, adopting React Compiler and eslint-plugin-react-hooks, deciding on useMemo, useCallback and memo, server rendering and hydration (createRoot, hydrateRoot, renderToPipeableStream, prerender), or migrating from React 18 (forwardRef, Context.Provider, ReactDOM.render, propTypes, defaultProps). Triggers on React, JSX, hooks, useState, useEffect, useRef, useContext, Server Components, RSC, Actions, React Compiler, Strict Mode, hydration mismatch, React 19 upgrade.

rust-std-docs

Gives current guidance from the official Rust standard library docs (std 1.99.1 stable) on pitfalls, panics, platform differences, discouraged APIs and newer stable replacements. Use when writing, reviewing or refactoring Rust code that uses std, especially collections (Vec, HashMap, BTreeMap, BinaryHeap, entry, extract_if, sort), Option/Result/error handling (expect, ?, Error, ExitCode, panic), iterators (collect, flatten, Peekable, try_fold), Box/Rc/RefCell/Cow/Pin/mem::take, Arc/Mutex/RwLock/OnceLock/LazyLock/thread::scope/mpsc/Condvar/atomics, String/str/fmt/format!/OsString/CString, io/fs/path (BufWriter, OpenOptions, PathBuf::join), process/env/net/time (Command, set_var, TcpStream, Instant, Duration), From/TryFrom/AsRef/Ord/Drop/Deref/Clone, and unsafe raw pointers, MaybeUninit, transmute, NonNull or integer overflow and float ordering.

socketio-docs

Builds, configures and debugs Socket.IO 4.8 servers and clients from the official Socket.IO v4 documentation. Use when writing a Node.js Socket.IO server (Express, Fastify, Koa, Bun, uWebSockets.js) or a socket.io-client app, emitting events with acknowledgements, timeout() and emitWithAck, broadcasting to rooms and namespaces, authenticating in middleware with the auth option, configuring CORS, reverse proxies and sticky sessions, scaling with the Redis, Redis Streams, Postgres, MongoDB or Cluster adapter, enabling connection state recovery and delivery guarantees, tuning reconnection, writing tests, troubleshooting connection issues, or migrating from Socket.IO v2 or v3. Triggers on Socket.IO, socket.io, socket.io-client, io(), new Server, emit, emitWithAck, acknowledgements, rooms, namespaces, broadcast, io.use, middleware, CORS, Redis adapter, sticky sessions, connection state recovery, reconnection, connect_error, Session ID unknown. Not for plain WebSocket servers such as ws without Socket.IO.

tailwindcss-docs

Teaches current Tailwind CSS v4 practices drawn from the official documentation. Use for any task that touches Tailwind CSS v4, even a small one, because many v3 habits silently break in v4. Use when writing or reviewing Tailwind CSS v4 code (utility classes, variants, arbitrary values, theme variables in @theme, @utility, @custom-variant, @apply, @reference, @source), setting up Tailwind in a framework or build tool (Vite plugin, PostCSS plugin, CLI, Next.js, SvelteKit, Nuxt, Angular, Laravel, Rails), customizing the theme (colors, fonts, breakpoints, spacing, animations), building dark mode with a class or data attribute, or migrating a project from Tailwind v3 (tailwind.config.js, @tailwind directives, darkMode class, safelist, bg-opacity, renamed shadow and outline utilities). Triggers on tailwindcss, @import "tailwindcss", @tailwindcss/vite, @tailwindcss/postcss, @tailwindcss/cli, @theme, @custom-variant dark, npx @tailwindcss/upgrade, upgrade to Tailwind v4.

tauri-docs

Tauri 2 apps, tauri.conf.json, capabilities and permissions, commands and invoke, events and channels, plugins, bundling, signing and the updater, migration from Tauri 1. Guides Tauri 2 work from the official v2 documentation, including scopes, managed state, plugin authoring, CSP and the isolation pattern, testing and debugging, Linux, Windows and macOS packages, and mobile and store releases. Use when writing or reviewing a Tauri 2 app, adding a plugin or a capability, fixing a "not allowed" or "forbidden path" error, wiring IPC between Rust and the webview, packaging, signing or publishing a desktop or mobile build, or upgrading a Tauri 1 or 2.0 beta project.

tokio-docs

Gives current guidance from the official tokio documentation (tokio 1.54.1) on async Rust pitfalls, cancel safety, blocking code, surprising defaults and deprecated APIs. Use when writing, reviewing or debugging async Rust code that uses tokio, especially the runtime (#[tokio::main], #[tokio::test], Builder, enable_all, block_on, current_thread, LocalRuntime, shutdown, feature flags), tasks (spawn, JoinHandle, JoinSet, abort, spawn_blocking, block_in_place, LocalSet, spawn_local, task_local), select!, join!, try_join!, cancellation and graceful shutdown, sync primitives (Mutex across .await, RwLock, Semaphore, Notify, OnceCell), channels (mpsc backpressure, oneshot, broadcast lag, watch), time (sleep, interval, MissedTickBehavior, timeout, paused time, start_paused), I/O and networking (AsyncReadExt, BufWriter flush, split, io::copy, framing, TcpListener, TcpStream, UdpSocket), tokio::fs, process (Command, kill_on_drop, Child), signals (ctrl_c, Windows console events) and Windows named pipes.

typescript-docs

Configures, writes and upgrades TypeScript 6.0 code and tsconfig.json from the official TypeScript documentation. Use when upgrading from TypeScript 5.x to 6.0 (strict, types, rootDir, baseUrl, moduleResolution node, target es5, outFile, esModuleInterop), choosing module and moduleResolution, writing package.json exports and imports, fixing ESM and CommonJS interop, enabling verbatimModuleSyntax or erasableSyntaxOnly, setting up project references and tsc --build, writing .d.ts files, checking JavaScript with JSDoc, or debugging narrowing, generics, mapped types, decorators and enums. Triggers on TypeScript, tsc, tsconfig, TS5112, ignoreDeprecations, nodenext, bundler, .d.ts, @ts-check, TypeScript 6 upgrade.

typesense-docs

Pitfalls, surprising defaults and recommended patterns of the Typesense search engine, distilled from the official docs. Use when designing a Typesense collection schema, writing search parameters (q, query_by, filter_by, sort_by, facet_by, group_by), tuning typo tolerance and ranking, building vector, semantic or hybrid search, conversational RAG or natural language search, creating API keys and scoped search keys for multi-tenancy, indexing and import (JSONL, upsert, emplace, dirty_values), syncing from a primary database, reindexing through aliases, managing synonyms (synonym sets), curation sets and stopwords, joins, analytics rules and query suggestions, InstantSearch, migrating from Algolia, or running, clustering, backing up and upgrading a Typesense server.

vite-docs

Teaches current Vite 8 practices drawn from the official documentation. Use when writing or fixing a vite.config, migrating from Vite 7 (Rolldown and Oxc replacing Rollup and esbuild), configuring the dev server, proxy or HMR, handling import.meta.env and modes, tuning the production build, library mode, SSR, writing a Vite plugin, using the HMR or JavaScript API, or the Environment API. Triggers on vite.config, defineConfig, rolldownOptions, import.meta.env, loadEnv, optimizeDeps, ssrLoadModule, ModuleRunner, this.environment, upgrade to Vite 8.

vitest-docs

Teaches current Vitest 5 practices drawn from the official documentation. Use when writing, reviewing or fixing Vitest tests (test, describe, expect, test.for, fixtures with test.extend, hooks, snapshots), mocking with vi (vi.fn, vi.spyOn, vi.mock, vi.when, fake timers, stubGlobal, stubEnv), configuring vitest.config or projects, choosing an environment or pool, filtering and running tests in CI, measuring coverage with v8 or istanbul, debugging slow or flaky suites, testing components in browser mode with Playwright, or migrating from Jest, Mocha or Vitest 4. Triggers on vitest, vi.mock, vi.fn, vitest.config, toMatchSnapshot, expect.poll, coverage thresholds, browser mode, @vitest/browser-playwright, jsdom, happy-dom, migrate Jest to Vitest, upgrade Vitest 5.

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