name: JavaScript Language Patterns description: Modern JavaScript (ES2022+) patterns for clean, maintainable code. metadata: labels: [javascript, language, es6, modern-js] triggers: files: ['/*.js', '/.mjs', '**/.cjs'] keywords: [const, let, arrow, async, await, promise, destructuring, spread, class]

JavaScript Language Patterns

Priority: P0 (CRITICAL)

Implementation Guidelines

• Variables: const default. let if needed. No var.
• Functions: Arrows for callbacks. Declarations for top-level.
• Async: async/await + try/catch.
• Objects: Destructuring, Spread ..., Optional Chain ?., Nullish ??.
• Strings: Template literals ${}.
• Arrays: map, filter, reduce. No loops.
• Modules: ESM import/export. Export only what is necessary.
• Classes: Use #private fields for true privacy.

Anti-Patterns

• No var: Block scope only.
• No ==: Strict ===.
• No new Object(): Use literals {}.
• No Callbacks: Promisify everything.
• No Mutation: Immutability first.

Code

// Modern Syntax
const [x, ...rest] = items;
const name = user?.profile?.name ?? 'Guest';

// Async + Error Handling
async function getUser(id) {
  const res = await fetch(`/api/${id}`);
  return res.json(); // Errors propagate
}

// Private Fields
class Service {
  #key;
  constructor(k) {
    this.#key = k;
  }
}

Reference & Examples

For advanced patterns and functional programming: See references/REFERENCE.md.

Related Topics

best-practices | tooling