javascript

name	javascript
description	JavaScript and TypeScript language patterns — const/let, async/await, ESM imports, and typed unknowns. Use when writing or reviewing JS/TS in frontend or backend. Don't use for React component patterns (use react), general file-size or nesting standards (use code-standards), or test structure (use tests).

JavaScript and TypeScript

Apply these language patterns to frontend and backend JS/TS. For file size, nesting, and related shape rules, invoke code-standards in full. For React components and hooks, invoke react in full.

Reference — language rules

const by default

Declare with const whenever the binding is not reassigned. Use let only when reassignment is required. Mutating an object's contents does not require let — the binding stays const.

async/await

Drive async flow with async/await and explicit try/catch when errors must be handled. For independent work, combine with Promise.all:

const [user, settings] = await Promise.all([
  loadUser(userId),
  loadSettings(userId),
]);

ESM in Node

In backend Node code, use import/export. Use import type for type-only imports.

import express from 'express';
import type { Request, Response } from 'express';
import { getHealth } from './services/healthService';

export const app = express();

Typed values

Type every value with an explicit type, interface, or union, or a safe inference — leave no any (explicit, inferred, or via unchecked casts). For untrusted external data, take unknown and narrow before use. Reach for a shared contract type before an unchecked as cast.

function isUser(value: unknown): value is User {
  if (typeof value !== 'object' || value === null) return false;
  return 'id' in value && 'name' in value;
}

Reference — review gate

Before finishing a JS/TS change, confirm: const/let only, async/await for async flow, ESM imports in Node, no any, and every unknown narrowed before use.