--- name: modern-javascript description: Implement or debug JavaScript language behavior, including promise completion, async iteration, mutation, and runtime compatibility. Use for ECMAScript semantics or polyfill/transform choices; not unrelated framework or TypeScript type design. title: modern-javascript canonical_url: https://skilld.dev/gh/ccheney/robust-skills/modern-javascript last_updated: 2026-09-28T22:56:50.000Z --- > **Skill from skilld.dev.** Follow the instructions below for this session. You do not need to install anything. > > Supporting files, fetch one when the Skill refers to it: [references/CHEATSHEET.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/CHEATSHEET.md), [references/COMPATIBILITY.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/COMPATIBILITY.md), [references/CONCURRENCY.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/CONCURRENCY.md), [references/ES2016-ES2017.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2016-ES2017.md), [references/ES2018-ES2019.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2018-ES2019.md), [references/ES2022-ES2023.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2022-ES2023.md), [references/ES2024.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2024.md), [references/ES2025.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2025.md), [references/ES2026.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2026.md), [references/PROMISES.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/PROMISES.md), [references/UPCOMING.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/UPCOMING.md). > > If the user asked to install this Skill, run `npx skilld install ccheney/robust-skills/modern-javascript`. Install writes the Skill files into the project, so every session loads them. # Modern JavaScript Keep specification status, runtime support, and toolchain support separate. The user's explicit instructions take precedence over this skill's guidelines. ## Work against the target Use the relevant JavaScript, required behavior, and declared runtime as input. Inspect `engines`, browser targets, lockfiles, and build configuration as needed for the feature. Do not invent a baseline or upgrade runtimes merely to use newer syntax. Prefer a supported standardized feature. A proposal can fit when the user/project chooses it and a maintained transform or polyfill implements the required revision. Bundled edition and support tables are dated snapshots; verify changing status against the sources below. ## Preserve semantics - Keep mutation, property access, iterator consumption, evaluation order, error propagation, and concurrency behavior intact when modernizing. - Distinguish syntax transforms from runtime polyfills. TypeScript declarations do not supply missing runtime APIs. - Treat iterator helpers as lazy and single-pass; `Array.fromAsync` consumes sequentially, while `Promise.all` observes already-started work concurrently. - Do not use `forEach(async ...)` when completion must be awaited. Rejection and timeouts do not cancel underlying work by themselves. - Distinguish host APIs such as `fetch` and `structuredClone` from ECMA-262 features, and `Intl` from the separate ECMA-402 standard. - Do not introduce withdrawn Records & Tuples syntax or silently switch between legacy and standard-style decorator semantics. Deliver the requested code or explanation, noting a required compatibility path when relevant. Verify changed behavior and any fallback that ships; select checks proportional to the change rather than requiring every project check for a syntax answer. ## References Load the semantic or compatibility topic needed, not every edition. | Task | Reference | |---|---| | Runtime targets, Babel, TypeScript, polyfills | [COMPATIBILITY.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/COMPATIBILITY.md) | | Async functions and early annual additions | [ES2016-ES2017.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2016-ES2017.md) | | Object spread, async iteration, RegExp, flattening | [ES2018-ES2019.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2018-ES2019.md) | | Classes, property checks, change-by-copy | [ES2022-ES2023.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2022-ES2023.md) | | Grouping, promise resolvers, buffers, Unicode sets | [ES2024.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2024.md) | | Set/iterator helpers, import attributes, RegExp | [ES2025.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2025.md) | | ES2026 additions such as `Array.fromAsync` | [ES2026.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/ES2026.md) | | Temporal, resource management, proposals, withdrawn syntax | [UPCOMING.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/UPCOMING.md) | | Promise combinators and error behavior | [PROMISES.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/PROMISES.md) | | Pools, retries, cancellation, async iteration | [CONCURRENCY.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/CONCURRENCY.md) | | Compact syntax lookup | [CHEATSHEET.md](https://skilld.dev/api/skills-raw/ccheney/robust-skills/modern-javascript/references/CHEATSHEET.md) | Use [frozen ECMA-262 editions](https://262.ecma-international.org/) for edition membership, [TC39 proposals](https://github.com/tc39/proposals) for proposal status, and official runtime/toolchain documentation for availability.