Metaskills — скилл

---
name: js-conventions
description: Metarhia JavaScript and TypeScript conventions, project lint setup, formatting, naming, object shape and hidden class rules, named intermediate variables, split-then-compose, contracts, module and export order, ternaries, lookup tables, dynamic dispatch, iteration, async/await, cognitive complexity, performance, and benchmarking. Apply automatically whenever writing, editing, refactoring, simplifying, optimizing, formatting, cleaning up, or reviewing .js, .mjs, .cjs, .ts, or .d.ts files, or when the user asks about code style or linting
---

# JavaScript conventions

Apply on every JavaScript or TypeScript change. Prefer readability unless the
task has a measured reason to do otherwise. See [examples.md](examples.md) to
compare code before and after applying most rules. Read
[performance.md](performance.md) for library or system code (runtimes, parsers,
protocols, I/O) or a measured hot path, and benchmark exceptions on the target
runtime.

## Preparations

- Add these dev dependencies to `package.json` when missing:
  `eslint-config-metarhia`, `eslint`, and `prettier`
- Add these scripts to `package.json` when missing or update to following readable format:

```json
"lint": "eslint . && prettier --check \"**/*.{js,jsx,mjs,cjs,ts,tsx,json,md,mdx,yaml,yml}\"",
"fix": "eslint . --fix && prettier --write \"**/*.{js,jsx,mjs,cjs,ts,tsx,json,md,mdx,yaml,yml}\""
```

The extension list groups JavaScript, TypeScript, data, docs, configs.

- Run `npm t` before and after changing code with ai. Finish only when lint, format,
  and test checks pass
- Run `npm run fix` before fixing remaining lint or format issues by hand
- Preserve the existing module system, source and declaration file types, and
  package layout unless the task asks to change them
- Resolve runtime packages from the registry (or the project's declared source of truth), not from sibling `file:` links in a gitignored lockfile that other machines will not have

## Formatting

For writing new code use single quotes, semicolons, trailing commas in multiline arrays, objects, and parameters, and one blank line between semantic blocks. Keep lines within 80 characters except URLs. Let Prettier and `eslint-config-metarhia` handle spacing, braces, and indentation.

## Naming

- Use `camelCase` for variables and parameters
- Use `UpperCamelCase` for classes and types
- Use `UPPER_SNAKE_CASE` for global constants
- Use `new` only with capitalized constructor names
- Prefer full words for all identifiers (`error`, not `err`). Use a short or