---

name: typescript-and-javascript description: Writes TypeScript and JavaScript code following best practices and style rules. Use when writing TS/JS code or reviewing code style.

TypeScript/JavaScript Protocol

Type Checking

When iterating solutions (not reviewing), run type check:

Copypnpm tsc --noEmit
# monorepo: cd apps/demo-app && pnpm tsc --noEmit

Code Block Types

• Pure TypeScript: ts
• TypeScript with JSX: tsx

Style Rules

• Early returns over nested conditionals
• Named functions over arrows (except callbacks/one-liners)
• Strict equality: ===, !== only
• const for non-mutated variables
• async/await over .then()
• Never use any — find specific types
• import type for type-only imports
• Always use curly braces for if
• Never barrel exports in index.ts (harms tree-shaking)
• Never write code in index.ts(x) — use specific names
• satisfies never in switch default for exhaustiveness
• satisfies <Type> over type casting for traceability

Examples

Copy// ✓ Early return
function process(data: Data | null) {
  if (!data) {
    return;
  }
  // process
}

// ✗ Nested
function process(data: Data | null) {
  if (data) {
    // process
  }
}

Copy// ✓ Named function
function add(a: number, b: number): number {
  return a + b;
}

// ✓ One-liner arrow acceptable
const add = (a: number, b: number): number => a + b;

// ✗ Multi-line arrow
const add = (a: number, b: number): number => {
  return a + b;
};

Copy// ✓ Type imports
import type { MyType } from './types';
import { type ReactNode } from 'react';

Copy// ✓ Exhaustiveness + satisfies
function factory(target: 'a' | 'b'): A | B | null {
  switch (target) {
    case 'a':
      return new A();
    case 'b':
      return new B();
    default:
      target satisfies never;
      return null;
  }
}

// ✓ satisfies for traceability
const obj = { key: 'value' } satisfies Record<string, string>;

// ✗ Type casting loses inference
const obj: Record<string, string> = { key: 'value' };

Copy// ✓ Always curly braces
if (!condition) {
  return;
}

// ✗ No braces
if (!condition) return;