# typescript

Guides Cursor to enforce TypeScript best practices across JavaScript and JSX files using JSDoc, strict compiler flags, and runtime type guards.

- rule · `type=rule` · Cursor · ★ 3580
- Author: sanjeed5
- License: CC0-1.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/typescript.mdc
- Updated: 2026-05-19
- HTML: https://affpapa.org/en/nevibecode/rules/sanjeed5-typescript

## What it does

This rule directs Cursor when editing `.js` and `.jsx` files to enforce TypeScript-grade type checking without migrating everything to `.ts`. It requires strict compiler options in `tsconfig.json`—such as `strict`, `strictNullChecks`, `noImplicitAny`, and `checkJs`—paired with interface definitions imported into JSDoc annotations.

It bans `any` in favor of `unknown`, mandating runtime type guards and `typeof` checks before accessing properties on incoming data. It also steers the model away from numeric enums toward string literal union types and `const enum`.

Because the rule glob specifically targets `**/*.{js,jsx}`, its instructions trigger strictly on JavaScript and JSX source files.

## Install

### Cursor (native platform)

**Manually: put the files in place**
- File: `.cursor/rules/typescript.mdc` — Frontmatter: description, globs, alwaysApply

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/typescript.md` — The paths frontmatter field limits the files: move globs from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### GitHub Copilot

**Manually: put the files in place**
- File: `.github/instructions/typescript.instructions.md` — The applyTo field sets which files it applies to: move globs from the source into applyTo

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/typescript.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/typescript.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/typescript.mdc

## When to use it
- type-check JavaScript and JSX files via JSDoc without renaming them to `.ts`
- replace untyped `any` signatures with `unknown` and runtime type guards
- declare reusable data contracts in `.d.ts` files for JSDoc imports
- prevent numeric enum hazards by using string literal unions

## Example prompts
- Document `processData` parameters with JSDoc and narrow `unknown` data with type guards
- Configure `tsconfig.json` for strict type checking on `.js` and `.jsx` files
- Create a custom type guard to validate API payloads in `user-service.js`

## Source text · typescript.mdc · CC0-1.0

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BEGIN THIRD-PARTY FILE d135db8d4a372c7f
````text
---
description: Enforce modern TypeScript best practices for robust, type-safe JavaScript applications, focusing on strictness, clear type definitions, and runtime validation.
globs: **/*.{js,jsx}
---
# TypeScript Best Practices (for JS/JSX Type-Checking)

This guide outlines essential TypeScript best practices for teams working with JavaScript or JSX files that are type-checked by TypeScript (e.g., via `tsconfig.json` with `allowJs` and/or JSDoc annotations). While many examples use native TypeScript syntax for clarity and conciseness, the underlying principles and type-safety benefits apply directly to your `.js`/`.jsx` codebase.

## 1. Enable Strict Mode in `tsconfig.json`

This is the single most impactful change you can make. `strict: true` enables a suite of crucial checks that catch the vast majority of common type-related bugs at compile time.

**Action**: Ensure your `tsconfig.json` includes:

```json
// tsconfig.json
{
  "compilerOptions": {
    "strict": true, // Enables all strict type-checking options
    "noImplicitAny": true, // Catches untyped variables/parameters
    "strictNullChecks": true, // Prevents `null`/`undefined` access without checks
    "strictPropertyInitialization": true, // Ensures class properties are initialized
    "allowJs": true, // Crucial for type-checking .js/.jsx files
    "checkJs": true, // Enables type-checking in .js/.jsx files
    // ... other options
  },
  "include": ["**/*.js", "**/*.jsx"] // Ensure your JS/JSX files are included
}
```

## 2. Define Clear Type Contracts

Use interfaces and type aliases to describe the shape of your data, especially for API payloads, component props, and complex objects. For `.js`/`.jsx` files, leverage JSDoc to apply these types.

### ✅ GOOD: Interfaces for Object Shapes & Classes

Interfaces are ideal for defining the shape of objects and for implementing explicit contracts when working with classes. Define these in `.d.ts` or `.ts` files, then reference them in JSDoc.

```typescript
// types.d.ts or types.ts
interface UserProfile {
  id: string;
  name: string;
  email: string;
  age?: number; // Optional property
}

interface Point {
  readonly x: number;
  readonly y: number;
}
```

```javascript
// user-service.js
/**
 * @typedef {import('./types').UserProfile} UserProfile
 */

/**
 * @param {UserProfile} user
 * @returns {string}
 */
export function greetUser(user) {
  return `Hello, ${user.name}!`;
}
```

### ✅ GOOD: Type Aliases for Complex Types & Unions

Type aliases are powerful for defining unions, intersections, primitive aliases, and tuples. Define these in `.d.ts` or `.ts` files, then reference them in JSDoc.

```typescript
// types.d.ts or types.ts
type ID = string | number;
type UserRole = "admin" | "user" | "guest"; // Strict literal type
type Coords = [number, number];
```

```javascript
// auth-service.js
/**
 * @typedef {import('./types').ID} ID
 * @typedef {import('./types').UserRole} UserRole
 */

/**
 * @param {ID} userId
 * @param {UserRole} role
 * @returns {void}
 */
export function assignRole(userId, role) {
  console.log(`User ${userId} assigned role: ${role}`);
}
```

## 3. Avoid `any` and Prefer `unknown` for Untyped Data

`any` completely bypasses TypeScript's checks, reintroducing JavaScript's runtime errors. `unknown` is a safer alternative that forces you to narrow the type before use.

### ❌ BAD: Using `any`

```javascript
// data-processor.js
/**
 * @param {any} data // Bypasses all type checks
 */
export function processDataBad(data) {
  // No type safety here, data.foo might not exist
  console.log(data.foo.bar);
}
```

### ✅ GOOD: Using `unknown` (and Type Guards)

When dealing with data from external sources (e.g., API responses), use `unknown` and then narrow its type using runtime checks.

```javascript
// data-processor.js
/**
 * @param {unknown} data // Forces runtime checks
 */
export function processDataGood(data) {
````
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