Error Handling

# Error Handling Patterns

Consistent, robust error handling patterns for production applications.

## When to Activate

- Designing error types or exception hierarchies for a new module or service
- Adding retry logic or circuit breakers for unreliable external dependencies
- Reviewing API endpoints for missing error handling
- Implementing user-facing error messages and feedback
- Debugging cascading failures or silent error swallowing

## Core Principles

1. **Fail fast and loudly** — surface errors at the boundary where they occur; don't bury them
2. **Typed errors over string messages** — errors are first-class values with structure
3. **User messages ≠ developer messages** — show friendly text to users, log full context server-side
4. **Never swallow errors silently** — every `catch` block must either handle, re-throw, or log
5. **Errors are part of your API contract** — document every error code a client may receive

## TypeScript / JavaScript

### Typed Error Classes

```typescript
// Define an error hierarchy for your domain
export class AppError extends Error {
  constructor(
    message: string,
    public readonly code: string,
    public readonly statusCode: number = 500,
    public readonly details?: unknown,
  ) {
    super(message)
    this.name = this.constructor.name
    // Maintain correct prototype chain in transpiled ES5 JavaScript.
    // Required for `instanceof` checks (e.g., `error instanceof NotFoundError`)
    // to work correctly when extending the built-in Error class.
    Object.setPrototypeOf(this, new.target.prototype)
  }
}

export class NotFoundError extends AppError {
  constructor(resource: string, id: string) {
    super(`${resource} not found: ${id}`, 'NOT_FOUND', 404)
  }
}

export class ValidationError extends AppError {
  constructor(message: string, details: { field: string; message: string }[]) {
    super(message, 'VALIDATION_ERROR', 422, details)
  }
}

export class UnauthorizedError extends AppError {
  constructor(reason = 'Authentication required') {
    super(reason, 'UNAUTHORIZED', 401)
  }
}

export class RateLimitError extends AppError {
  constructor(public readonly retryAfterMs: number) {
    super('Rate limit exceeded', 'RATE_LIMITED', 429)
  }
}
```

### Result Pattern (no-throw style)

For operations where failure is expected and common (parsing, external calls):

```typescript
type Result<T, E = AppError> =
  | { ok: true; value: T }
  | { ok: false; error: E }

function ok<T>(value: T): Result<T> {
  return { ok: true, value }
}

function err<E>(error: E): Result<never, E> {
  return { ok: false, error }
}

// Usage
async function fetchUser(id: string): Promise<Result<User>> {
  try {
    const user = await db.users.findUnique({ where: { id } })
    if (!user) return err(new NotFoundError('User', id))
    return ok(user)
  } catch (e) {
    return err(new AppError('Database error', 'DB_ERROR'))
  }
}

const result = await fetchUser('abc-123')
if (!result.ok) {
  // TypeScript knows result.error here
  logger.error('Failed to fetch user', { error: result.error })
  return
}
// TypeScript knows result.value here
console.log(result.value.email)
```

### API Error Handler (Next.js / Express)

```typescript
import { NextRequest, NextResponse } from 'next/server'

function handleApiError(error: unknown): NextResponse {
  // Known application error
  if (error instanceof AppError) {
    return NextResponse.json(
      {
        error: {
          code: error.code,
          message: error.message,
          ...(error.details ? { details: error.details } : {}),
        },
      },
      { status: error.statusCode },
    )
  }

  // Zod validation error
  if (error instanceof z.ZodError) {
    return NextResponse.json(
      {
        error: {
          code: