javascript-idioms

JavaScript Idioms and Patterns

Modern JavaScript (ES2024+) rewards modules, async/await, and functional patterns. Idiomatic JS = strict mode, modular, well-tested.

Scope: Plain JavaScript only — projects with no tsconfig.json. This skill is intentionally thin: most JS guidance (coercion traps, async pitfalls, scope bugs, security, collections, performance) is shared with TypeScript and lives in one place to avoid divergence.

Loading guards:

• If tsconfig.json exists, do NOT load this skill — load @.agents/skills/typescript-idioms/SKILL.md instead. TypeScript is preferred whenever available.
• Coercion / async / scope / security / collection / performance pitfalls: see @.agents/skills/typescript-idioms/references/ts-patterns-and-anti-patterns.md — despite the filename, it is scoped to "TypeScript/JavaScript" and covers runtime pitfalls that apply to plain JS verbatim.
• Project layout: see @.agents/skills/typescript-idioms/references/project-structure.md (the generic backend/library/monorepo layouts are language-level, not TS-only).

Module System

1. ES modules over CommonJS for all new code:

   // ✅ ESM
   import { createTask } from './task-service.js';
   export function handler(req, res) { ... }
   
   // ❌ CommonJS (legacy only)
   const { createTask } = require('./task-service');
   

2. Always use .js extensions in ESM relative imports — Node.js ESM requires them; bundlers accept them.
3. package.json "type": "module" to enable ESM by default; use .cjs for any CommonJS holdouts.
4. CJS/ESM interop — import default from './cjs.cjs' works; named exports from CJS are not statically analyzable, prefer module.exports = { named } + const { named } = require(...) or migrate to ESM.

Declarations and Strict Mode

1. const by default, let when reassignment needed, never var. (var hoisting/closure bugs: see ts-patterns-and-anti-patterns.md §3.)
2. Enable strict mode — 'use strict'; at file top in scripts. Strict mode is implicit in ESM modules ("type": "module" in package.json) and class bodies.
3. Optional chaining and nullish coalescing (prefer ?? over || to avoid 0/'' falsy traps):

   const title = task?.title ?? 'Untitled';
   const count = config?.scoring?.default ?? 0;
   

4. Destructuring with defaults for clean parameter handling:

   function createTask({ title, priority = 'medium', tags = [] }) { ... }
   

Runtime Without Types

Since there is no compiler to catch mistakes, lean harder on:

1. Runtime validation at boundaries — use Zod (works in plain JS) or ajv to validate external input. Don't trust unvalidated I/O.
2. Defensive narrowing — typeof, Array.isArray(), Object.hasOwn() checks before property access. (Coercion traps: see ts-patterns-and-anti-patterns.md §1.)
3. JSDoc for public APIs — @param, @returns, @throws give editors and tools type hints without a TS toolchain.
4. structuredClone() for deep copies (not JSON.parse(JSON.stringify())) — lossy and throws on cycles).

Error Handling

For universal error handling principles, see @.agents/rules/error-handling-principles.md. Below: JS-specific only.

1. Domain error classes (never throw primitives — loses stack trace):

   class DomainError extends Error {
       constructor(message) { super(message); this.name = this.constructor.name; }
   }
   class NotFoundError extends DomainError {
       constructor(resource, id) {
           super(`${resource} '${id}' not found`);
           this.resource = resource;
           this.resourceId = id;
       }
   }
   

2. Never catch without handling. Empty catch blocks are forbidden.
3. Always handle promise rejections — never floating/unhandled (see ts-patterns-and-anti-patterns.md §2 for the full async pitfall catalog).

Naming

1. camelCase for functions, variables. PascalCase for classes.
2. UPPER_SNAKE_CASE for constants.
3. Prefix booleans: isActive, hasPermission, canEdit.

Testing

Vitest (preferred — ESM-native, zero-config) or Jest. Testing Library for DOM. (TypeScript skill mandates Vitest; in plain JS either is acceptable but Vitest is recommended.)

Toolchain and Formatting

Tool	Purpose	Command
Prettier	Formatting	npx prettier --write .
ESLint	Linting	npx eslint .
npm audit / pnpm audit	CVE scanning	npm audit

Default package manager: pnpm (matches the TypeScript skill). Use npm only if the project already has a package-lock.json.

Related

• TypeScript Idioms @.agents/skills/typescript-idioms/SKILL.md (load this instead if tsconfig.json exists)
• TS/JS Patterns & Anti-Patterns @.agents/skills/typescript-idioms/references/ts-patterns-and-anti-patterns.md (shared coercion/async/scope/security pitfalls)
• Code Idioms and Conventions @.agents/rules/code-idioms-and-conventions.md
• Testing Strategy @.agents/rules/testing-strategy.md
• Error Handling Principles @.agents/rules/error-handling-principles.md