TypeScript Strict Patterns

# TypeScript Strict Patterns

## When to Use
Apply in every TypeScript file across any project.

## Rules
1. **No `any` type** — use `unknown` with type guards, or proper interfaces.
2. **Use `interface`** for object shapes, `type` for unions and intersections.
3. **Mark arrays and properties `readonly`** when they shouldn't be mutated.
4. **Use discriminated unions** for state machines and variant types.
5. **Prefer named exports** — easier to refactor and tree-shake.

## Code Examples

```typescript
// Good — discriminated union
type Result<T> =
  | { readonly success: true; readonly data: T }
  | { readonly success: false; readonly error: string };

// Good — type guard
function isString(value: unknown): value is string {
  return typeof value === 'string';
}

// Good — readonly arrays
interface Config {
  readonly tags: readonly string[];
  readonly allowedOrigins: readonly string[];
}
```

## Anti-Patterns
- Using `any` to "fix" type errors
- Not using strict mode in tsconfig
- Type assertions (`as`) without validation
- Mutable arrays in public APIs

## Checklist
- [ ] No `any` in codebase
- [ ] Strict mode enabled in tsconfig
- [ ] Readonly where appropriate
- [ ] Type guards for unknown data