Typescript Best Practices

---
name: typescript-best-practices
description: Use when reading or writing TypeScript or JavaScript files (.ts, .tsx, .js, tsconfig.json).
---

# TypeScript Best Practices

Follows the type, error, and assertion law in `code-law`. This skill covers language-specific idioms only.

## Make Illegal States Unrepresentable

Use the type system to prevent invalid states at compile time.

**Discriminated unions for mutually exclusive states:**
```ts
// Good: only valid combinations possible
type RequestState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

// Bad: allows invalid combinations like { loading: true, error: Error }
type RequestState<T> = {
  loading: boolean;
  data?: T;
  error?: Error;
};
```

**Branded types for domain primitives:**
```ts
type UserId = string & { readonly __brand: 'UserId' };
type OrderId = string & { readonly __brand: 'OrderId' };

// Compiler prevents passing OrderId where UserId expected
function getUser(id: UserId): Promise<User> { /* ... */ }
```

**Const assertions for literal unions:**
```ts
const ROLES = ['admin', 'user', 'guest'] as const;
type Role = typeof ROLES[number]; // 'admin' | 'user' | 'guest'

// Array and type stay in sync automatically
function isValidRole(role: string): role is Role {
  return ROLES.includes(role as Role);
}
```

**Exhaustive switch with never check:**
```ts
type Status = "active" | "inactive";

function processStatus(status: Status): string {
  switch (status) {
    case "active":
      return "processing";
    case "inactive":
      return "skipped";
    default: {
      const _exhaustive: never = status;
      throw new Error(`unhandled status: ${_exhaustive}`);
    }
  }
}
```

**Restrict who can do it** — when only one scope may perform an effect, keep its functions unexported and export only the scope that performs it; a branded capability that the scope alone creates makes an out-of-scope call a type error. Hold module boundaries with ESLint's shipped `no-restricted-imports` rule. Correct types are the proof.

## Runtime Validation with Zod

- Define schemas as single source of truth; infer TypeScript types with `z.infer<>`. Avoid duplicating types and schemas.
- Use `safeParse` for user input where failure is expected; use `parse` at trust boundaries where invalid data is a bug.
- Compose schemas with `.extend()`, `.pick()`, `.omit()`, `.merge()` for DRY definitions.
- Add `.transform()` for data normalization at parse time (trim strings, parse dates).

```ts
import { z } from "zod";

const UserSchema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
  name: z.string().min(1),
  createdAt: z.string().transform((s) => new Date(s)),
});

type User = z.infer<typeof UserSchema>;

// Strict parsing at trust boundaries — throws if API contract violated
export async function fetchUser(id: string): Promise<User> {
  const response = await fetch(`/api/users/${id}`);
  if (!response.ok) {
    throw new Error(`fetch user ${id} failed: ${response.status}`);
  }
  return UserSchema.parse(await response.json());
}

// Caller handles both success and error from user input
const result = UserSchema.safeParse(formData);
if (!result.success) {
  setErrors(result.error.flatten().fieldErrors);
  return;
}
```

## Test Doubles

`toHaveBeenCalledWith` / `toHaveBeenNthCalledWith` on a `vi.fn()` standing in for your own module assert the call sequence, not the result — the change-detector shape in `testing-best-practices`. Mock at the network edge (`msw`) and assert the value the caller receives.

## Optional: type-fest

For advanced type utilities beyond TypeScript builtins, consider [type-fest](https://github.com/sindresorhus/type-fest):

- `Opaque<T, Token>` - cleaner branded types than manual `& { __brand }` pattern
- `PartialDeep<T>` - recursive partial for nested objects
- `ReadonlyDeep<T>` - recursive readonly for immutable data
- `SetRequired<T, K>` / `SetOptional<T, K>` - targeted field modifications
- `Simplify<T>` - flatten complex intersection types in IDE tooltips

```ts
import type { Opaque, PartialDeep } from 'type-fest';

type UserId = Opaque<string, 'UserId'>;
type UserPatch = PartialDeep<User>;
```