Javascript Idioms

JavaScript Idioms and Patterns

Modern JavaScript (ES2024+) rewards modules, async/await, and functional patterns. Idiomatic JS = strict mode, modular, well-tested. When TypeScript is available, prefer it — see typescript-idioms.

Scope: Plain JavaScript idioms. For TypeScript-specific patterns, load @.gemini/skills/typescript-idioms/SKILL.md.

Modern Features

1. ES modules over CommonJS:

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

2. const by default, let when reassignment needed, never var.

3. Optional chaining and nullish coalescing:

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

4. Destructuring for clean parameter handling:

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

5. structuredClone for deep copies (not JSON.parse(JSON.stringify())).

Async/Await

1. async/await over raw promises:

   // ✅
   const user = await fetchUser(id);
   const tasks = await fetchTasks(user.id);
   
   // ❌ Promise chains for sequential ops
   fetchUser(id).then(user => fetchTasks(user.id)).then(tasks => ...);
   

2. Promise.all for parallel I/O:

   const [user, tasks] = await Promise.all([fetchUser(id), fetchTasks(id)]);
   

3. Always handle promise rejections — never unhandled.

Error Handling

1. Domain error classes:

   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.

Naming

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

Testing

Vitest or Jest. Testing Library for DOM.

Formatting and Static Analysis

Related

• TypeScript Idioms @.gemini/skills/typescript-idioms/SKILL.md
• Code Idioms and Conventions GEMINI.md § Code Idioms and Conventions
• Testing Strategy GEMINI.md § Testing Strategy