TypeScript Strict Rules

---
name: cursor-typescript-strict
description: |
  Cursor rules enforcing strict TypeScript patterns. Always active when editing
  TypeScript files in Cursor. No any types, explicit returns, exhaustive checks.
---

# TypeScript Strict Rules for Cursor

## Core rules

1. **No `any` type.** Use `unknown` for truly unknown values, then narrow
   with type guards. Use specific types everywhere else.

2. **Explicit return types** on all exported functions and all functions longer
   than 3 lines. Inferred returns are OK for one-liners.

3. **Exhaustive switch/if checks.** Use `satisfies never` in the default case
   to catch unhandled union members at compile time.

4. **Readonly by default.** Use `readonly` arrays and `Readonly<T>` for objects
   that should not be mutated.

5. **Discriminated unions over optional fields.** Instead of
   `{ type?: string; data?: T }`, use `{ type: "a"; data: T } | { type: "b" }`.

## Example: exhaustive check

```typescript
type Status = "active" | "inactive" | "pending";

function getLabel(status: Status): string {
  switch (status) {
    case "active": return "Active";
    case "inactive": return "Inactive";
    case "pending": return "Pending";
    default: {
      const _exhaustive: never = status;
      throw new Error(`Unhandled status: ${_exhaustive}`);
    }
  }
}
```

## tsconfig.json recommendations

```json
{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noFallthroughCasesInSwitch": true
  }
}
```

## Requirements

- TypeScript 5+
- `strict: true` in tsconfig.json