javascript

JavaScript fails by continuing. A mismatched type is coerced, a missing property is undefined, a rejected promise nobody watched is a warning. None of it reports itself, so the discipline is to make a failure loud at the earliest point it can be seen. Three biases decide most calls:

• Prefer the construct that refuses over the one that converts — === over ==, ?? over ||, Number(s) over parseInt(s), a thrown error over a silent undefined.
• The declared engine baseline decides what may be written, not the ECMAScript edition a feature belongs to.
• Reach for the built-in globals first. A dependency earns its place by doing what Intl, URL, structuredClone, AbortController, and the iterator helpers do not.

Language Edition

Read the project's engine baseline before writing code: engines in package.json, browserslist, target and lib in tsconfig.json or jsconfig.json, .nvmrc, and the CI matrix. Where a transpiler stands between source and runtime, its target governs the output and the source may run ahead; where none does, the engine baseline's oldest engine governs the source.

An edition number is not an availability claim. Engines ship features years before ratification and lag on others, and the gap runs both ways inside a single edition: Array.fromAsync and Math.sumPrecise are both ES2026, yet the first shipped in Chrome 121 and reached Baseline widely available on 2026-07-25, while the second reached Baseline newly available on 2026-04-10 and is absent from Node.js 26. Check the feature, never the edition.

Floor edition per feature this skill's rules reference. Anything from ES2019 or earlier is available in every engine a project can realistically target.

• ES2020 — ?., ??, globalThis, BigInt, Promise.allSettled, String.prototype.matchAll, dynamic import(), import.meta, specified for...in order
• ES2021 — ??=, ||=, &&=, String.prototype.replaceAll, Promise.any, AggregateError, numeric separators
• ES2022 — class fields, #private fields and methods, #x in obj, static {} blocks, top-level await, Object.hasOwn, .at(), Error cause, RegExp d flag
• ES2023 — toSorted, toReversed, toSpliced, with, findLast, findLastIndex
• ES2024 — Object.groupBy, Map.groupBy, Promise.withResolvers, RegExp v flag, String.prototype.isWellFormed
• ES2025 — iterator helpers, Set methods (union, intersection, difference, …), Promise.try, RegExp.escape, RegExp modifiers (?i:), duplicate named capture groups, import attributes, JSON modules
• ES2026 — Map.prototype.getOrInsert and getOrInsertComputed, Iterator.concat, Array.fromAsync, Error.isError, Math.sumPrecise, Uint8Array base64 and hex, JSON.rawJSON
• Stage 4, unratified — using and await using with Symbol.dispose and DisposableStack; Temporal; Iterator.zip. No Safari 26.x release implements any of them, so none is safe for an engine baseline that includes Safari 26. using requires Chrome 134, Firefox 141, or Node.js 24; Temporal requires Chrome 144, Firefox 139, or Node.js 26. Atomics.pause is the exception, reaching Chrome 133, Firefox 137, and Safari 18.4.
• Not shippable — decorators are Stage 2.7 and no engine implements them. A project using them runs TypeScript or Babel, and TypeScript's experimentalDecorators is the older, incompatible design.

Read [${CLAUDE_SKILL_DIR}/references/versions/es20NN.md] — one file per ratified edition, es2020.md through es2026.md, plus [${CLAUDE_SKILL_DIR}/references/versions/stage4-queue.md] for the unratified set — when writing against a feature near the engine baseline, and whenever raising it. Each carries what its edition added, what behavior it changed, and the traps it introduced.