error-handling

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# Error Handling

---

## Recoverable vs Fatal

Not all errors are equal. Classify before you handle.

| Category | Description | Example | Action |
|----------|-------------|---------|--------|
| **Recoverable** | Expected failure, caller can adapt | File not found, validation failure, network timeout | Return error value or throw typed error |
| **Transient** | Temporary condition, retry may succeed | Rate-limit 429, DB connection blip | Retry with backoff, then escalate |
| **Fatal** | System cannot continue safely | Missing required env var on startup, corrupted state | Log + exit (or crash the request, not the process) |
| **Programming error** | Bug in the code itself | TypeError: Cannot read property of undefined | Let it propagate — do not catch bugs |

**Rule:** Never catch a `TypeError`, `RangeError`, or `ReferenceError` unless you specifically expect it. These indicate bugs, not runtime conditions.

---

## Typed Error Classes

Give errors machine-readable codes. Avoid stringly-typed `message` checks.

```typescript
// ✅ CORRECT — typed, discriminable, chainable
export class AppError extends Error {
  constructor(
    message: string,
    public readonly code: string,
    public readonly statusCode: number = 500,
    options?: ErrorOptions,
  ) {
    super(message, options); // passes { cause } through
    this.name = this.constructor.name;
    // Fix prototype chain for instanceof checks in transpiled code
    Object.setPrototypeOf(this, new.target.prototype);
  }
}

export class NotFoundError extends AppError {
  constructor(resource: string, id: string, options?: ErrorOptions) {
    super(`${resource} with id "${id}" not found`, 'NOT_FOUND', 404, options);
  }
}

export class ValidationError extends AppError {
  constructor(
    message: string,
    public readonly fields: Record<string, string[]>,
    options?: ErrorOptions,
  ) {
    super(message, 'VALIDATION_FAILED', 422, options);
  }
}

export class ConflictError extends AppError {
  constructor(message: string, options?: ErrorOptions) {
    super(message, 'CONFLICT', 409, options);
  }
}
```

```typescript
// ❌ WRONG — string-matching error messages is fragile
try {
  await getUser(id);
} catch (err) {
  if ((err as Error).message.includes('not found')) {
    return null;
  }
  throw err;
}

// ✅ CORRECT — discriminate by type or code
try {
  await getUser(id);
} catch (err) {
  if (err instanceof NotFoundError) return null;
  throw err;
}
```

---

## Result Pattern for Expected Failures

When a function has a known failure mode that the caller must handle, prefer returning a `Result<T, E>` over throwing.

```typescript
// Result type (no library needed)
type Result<T, E = AppError> =
  | { ok: true; value: T }
  | { ok: false; error: E };

const ok = <T>(value: T): Result<T, never> => ({ ok: true, value });
const err = <E>(error: E): Result<never, E> => ({ ok: false, error });
```

```typescript
// ✅ CORRECT — caller is forced to handle both cases
async function findUser(id: string): Promise<Result<User, NotFoundError>> {
  const user = await db.users.findUnique({ where: { id } });
  if (!user) return err(new NotFoundError('User', id));
  return ok(user);
}

const result = await findUser(userId);
if (!result.ok) {
  // TypeScript knows result.error is NotFoundError here
  return res.status(404).json({ error: result.error.code });
}
// TypeScript knows result.value is User here
doSomethingWith(result.value);
```

```typescript
// ❌ WRONG — caller can forget to handle the failure
async function findUser(id: string): Promise<User | null> {

From Nadav011/apex-skills, MIT.

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