# Typescript Strict

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- Skill: `nadav011/typescript-strict` (Agent Skill)
- Install (CLI): `npx skillmds@latest add nadav011/typescript-strict`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nadav011/typescript-strict/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- Author: Nadav011 (https://skillmd.com/u/nadav011)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nadav011/typescript-strict

---


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# TypeScript Strict Mode

Every type decision is a contract. Weak types are silent bugs.

---

## Rule 1 — Never `: any`

`: any` disables the type checker. It is a lie that compiles.

**Before**
```typescript
function processUser(user: any) {
  return user.name.toUpperCase(); // crashes if name is undefined — no warning
}

async function fetchData(): Promise<any> {
  const res = await fetch('/api/data');
  return res.json(); // caller gets zero guidance
}
```

**After**
```typescript
function processUser(user: unknown): string {
  if (!isUser(user)) throw new TypeError('Invalid user shape');
  return user.name.toUpperCase(); // safe — narrowed
}

async function fetchData(): Promise<UserData> {
  const res = await fetch('/api/data');
  const raw = await res.json();
  return UserDataSchema.parse(raw); // validated at runtime boundary
}
```

Use `unknown` when the type truly isn't known yet — then narrow it before use.

---

## Rule 2 — Required tsconfig Flags

Minimum strict configuration for production TypeScript:

```jsonc
// tsconfig.json
{
  "compilerOptions": {
    "strict": true,                         // enables all strict checks
    "noUncheckedIndexedAccess": true,       // arr[0] is T | undefined, not T
    "exactOptionalPropertyTypes": true,     // {x?: string} ≠ {x: string | undefined}
    "noImplicitReturns": true,              // all code paths must return
    "noFallthroughCasesInSwitch": true,    // exhaustive switch cases
    "forceConsistentCasingInFileNames": true,
    "moduleResolution": "bundler"           // or "node16" for Node projects
  }
}
```

`strict: true` alone is not enough. `noUncheckedIndexedAccess` alone prevents a whole class of runtime crashes.

---

## Rule 3 — Branded Types for IDs

A `UserId` string and a `PostId` string are both `string` at runtime. TypeScript won't stop you passing one where the other is required — unless you brand them.

**Before**
```typescript
type UserId = string;
type PostId = string;

// TypeScript allows this — bug compiles silently
function deletePost(postId: PostId) { /* ... */ }
deletePost(userId); // wrong ID type, no error
```

**After**
```typescript
// Branded type pattern
type Brand<T, B> = T & { readonly __brand: B };
type UserId = Brand<string, 'UserId'>;
type PostId = Brand<string, 'PostId'>;

// Smart constructors validate + brand at system boundary
function toUserId(raw: string): UserId {
  if (!raw || raw.length < 1) throw new Error('Invalid user ID');
  return raw as UserId;
}
function toPostId(raw: string): PostId {
  if (!raw || raw.length < 1) throw new Error('Invalid post ID');
  return raw as PostId;
}

function deletePost(postId: PostId) { /* ... */ }
deletePost(userId); // Type error: UserId is not assignable to PostId
```

Apply branded types to: user IDs, resource IDs, currency amounts, percentages, validated emails.

---

## Rule 4 — Type Narrowing Patterns

### Type Guard (`is*`)
```typescript
interface User { name: string; email: string; }

// Reusable, testable guard
function isUser(value: unknown): value is User {
  return (
    typeof value === 'object' &&
    value !== null &&
    typeof (value as Record<string, unknown>).name === 'string' &&
    typeof (value as Record<string, unknown>).email === 'string'
  );
}

// Usage
if (isUser(rawData)) {
  console.log(rawData.name); // fully typed, safe
}
```

### Assertion (`assert*`)
```typescript
function assertUser(value: unknown): asserts value is User {
  if (!isUser(value)) {
    throw new TypeError(`Expected User, got: ${JSON.stringify(value)}`);
  }
}

// Usage — throws on invalid, narrows on success
assertUser(rawData);
console.log(rawData.name); // TypeScript knows it's a User here
```

### Discriminated Unions
```typescript
type Result<T> =
  | { success: true; data: T }
  | { success: false; error: string };

function handleResult<T>(result: Result<T>): T {
  if (!result.success) throw new Error(result.error); // narrowed to failure branch
  return result.data; // narrowed to success branch
}
```

---

## Rule 5 — Zod for Runtime Validation

TypeScript types exist only at compile time. At runtime boundaries (API responses, form inputs, env vars, WebSocket messages), the data is `unknown`. Use Zod to enforce types at runtime.

```typescript
import { z } from 'zod';

const UserSchema = z.object({
  id: z.string().uuid(),
  name: z.string().min(1).max(100),
  email: z.string().email(),
  role: z.enum(['user', 'admin']),
  createdAt: z.coerce.date(),
});

// Infer TypeScript type from schema — single source of truth
type User = z.infer<typeof UserSchema>;

// At the boundary (API handler, form submit, message receiver)
const result = UserSchema.safeParse(req.body);
if (!result.success) {
  return res.status(400).json({ error: result.error.flatten() });
}
const user = result.data; // fully typed User
```

**Never** declare a TypeScript type for external data and then separately write a validator. Define the Zod schema, derive the type from it.

---

## Anti-Patterns

| Pattern | Problem | Fix |
|---------|---------|-----|
| `: any` | Disables type checker | Use `unknown` + narrowing |
| `as T` without guard | Lying to compiler | Use type guard or Zod |
| `// @ts-ignore` | Silences real errors | Fix the root cause |
| Array access without check | Runtime crash on undefined | Enable `noUncheckedIndexedAccess` |
| `JSON.parse()` typed as known shape | Runtime mismatch | `z.parse(JSON.parse(...))` |
| Parallel type + validator definitions | Out-of-sync drift | Derive type from Zod schema |