# Modern Javascript

> Implement or debug JavaScript language behavior, including promise completion, async iteration, m…

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.

- Page: https://ss33.ai/skills/ccheney/modern-javascript-56495282347
- Publisher: ccheney
- Category: [Software Development](https://ss33.ai/skills/category/software-development-t44z8z) › [Languages & frameworks](https://ss33.ai/skills/category/software-development-t44z8z/languages-and-frameworks-rgvrso)
- License: MIT
- Source: https://github.com/ccheney/robust-skills/blob/0eea7b060d12e3520556a73b15fbfa79480987d1/skills/modern-javascript/SKILL.md

## Skill

SKILL.md: https://ss33.ai/api/v1/ss33/download/ccheney/modern-javascript-56495282347

# 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- Do not introduce withdrawn Records & Tuples syntax or silently switch between legacy and standard-style decorator semantics.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](references/COMPATIBILITY.md) |
| Async functions and early annual additions | [ES2016-ES2017.md](references/ES2016-ES2017.md) |
| Object spread, async iteration, RegExp, flattening | [ES2018-ES2019.md](references/ES2018-ES2019.md) |
| Classes, property checks, change-by-copy | [ES2022-ES2023.md](references/ES2022-ES2023.md) |
| Grouping, promise resolvers, buffers, Unicode sets | [ES2024.md](references/ES2024.md) |
| Set/iterator helpers, import attributes, RegExp | [ES2025.md](references/ES2025.md) |
| ES2026 additions such as `Array.fromAsync` | [ES2026.md](references/ES2026.md) |
| Temporal, resource management, proposals, withdrawn syntax | [UPCOMING.md](references/UPCOMING.md) |
| Promise combinators and error behavior | [PROMISES.md](references/PROMISES.md) |
| Pools, retries, cancellation, async iteration | [CONCURRENCY.md](references/CONCURRENCY.md) |
| Compact syntax lookup | [CHEATSHEET.md](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.