# err-server-errors: Handle Server Function Errors

## Priority: MEDIUM

## Explanation

Server function errors cross the network boundary. Handle them gracefully with appropriate error types, status codes, and user-friendly messages. Avoid exposing internal details in production.

## Bad Example

```tsx
// Throwing raw errors - exposes internals
export const createUser = createServerFn({ method: 'POST' })
  .validator(createUserSchema)
  .handler(async ({ data }) => {
    const user = await db.users.create({ data })  // May throw DB error
    return user
    // Prisma error with stack trace sent to client
  })

// Generic error handling - no useful info for client
export const getPost = createServerFn()
  .handler(async ({ data }) => {
    try {
      return await fetchPost(data.id)
    } catch (e) {
      throw new Error('Something went wrong')  // Too vague
    }
  })
```

## Good Example: Structured Error Handling

```tsx
// lib/errors.ts
export class AppError extends Error {
  constructor(
    message: string,
    public code: string,
    public status: number = 400
  ) {
    super(message)
    this.name = 'AppError'
  }
}

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

export class UnauthorizedError extends AppError {
  constructor(message = 'Unauthorized') {
    super(message, 'UNAUTHORIZED', 401)
  }
}

export class ValidationError extends AppError {
  constructor(message: string, public fields?: Record<string, string>) {
    super(message, 'VALIDATION_ERROR', 400)
  }
}
```

## Good Example: Server Function with Error Handling

```tsx
import { createServerFn, notFound } from '@tanstack/react-start'
import { setResponseStatus } from '@tanstack/react-start/server'

export const getPost = createServerFn()
  .validator(z.object({ id: z.string() }))
  .handler(async ({ data }) => {
    const post = await db.posts.findUnique({
      where: { id: data.id },
    })

    if (!post) {
      // Use built-in notFound for 404s
      throw notFound()
    }

    return post
  })

export const createPost = createServerFn({ method: 'POST' })
  .validator(createPostSchema)
  .handler(async ({ data }) => {
    try {
      const post = await db.posts.create({ data })
      return post
    } catch (error) {
      if (error instanceof Prisma.PrismaClientKnownRequestError) {
        if (error.code === 'P2002') {
          // Unique constraint violation
          setResponseStatus(409)
          throw new AppError('A post with this title already exists', 'DUPLICATE', 409)
        }
      }

      // Log full error server-side
      console.error('Failed to create post:', error)

      // Return sanitized error to client
      setResponseStatus(500)
      throw new AppError('Failed to create post', 'INTERNAL_ERROR', 500)
    }
  })
```

## Good Example: Client-Side Error Handling

```tsx
function CreatePostForm() {
  const [error, setError] = useState<string | null>(null)

  const createMutation = useMutation({
    mutationFn: createPost,
    onError: (error) => {
      if (error instanceof AppError) {
        setError(error.message)
      } else if (error instanceof ValidationError) {
        // Handle field-specific errors
        Object.entries(error.fields ?? {}).forEach(([field, message]) => {
          form.setError(field, { message })
        })
      } else {
        setError('An unexpected error occurred')
      }
    },
    onSuccess: (post) => {
      navigate({ to: '/posts/$postId', params: { postId: post.id } })
    },
  })

  return (
    <form onSubmit={handleSubmit}>
      {error && <Alert variant="error">{error}</Alert>}
      {/* form fields */}
    </form>
  )
}
```

## Good Example: Using Redirects for Auth Errors

```tsx
export const updateProfile = createServerFn({ method: 'POST' })
  .validator(updateProfileSchema)
  .handler(async ({ data }) => {
    const session = await getSessionData()

    if (!session) {
      // Redirect to login for auth errors
      throw redirect({
        to: '/login',
        search: { redirect: '/settings' },
      })
    }

    return await db.users.update({
      where: { id: session.userId },
      data,
    })
  })
```

## Error Response Best Practices

| Scenario | HTTP Status | Response |
|----------|-------------|----------|
| Validation failed | 400 | Field-specific errors |
| Not authenticated | 401 | Redirect to login |
| Not authorized | 403 | Generic forbidden message |
| Resource not found | 404 | Use `notFound()` |
| Conflict (duplicate) | 409 | Specific conflict message |
| Server error | 500 | Generic message, log details |

## Context

- Use `notFound()` for 404 errors - integrates with router
- Use `redirect()` for auth-related errors
- Set status codes with `setResponseStatus()`
- Log full errors server-side, sanitize for client
- Create custom error classes for consistent handling
- Validation errors from `.validator()` are automatic