8. Validation and error models

This file supports the Week 2 segment on error model and validation design.

Goals for this segment

• Design a consistent error format for the Snippets API.
• Decide which validations to enforce and where (middleware vs per-endpoint).
• Make sure the design is reflected in both:

  • Your Express implementation.
  • Your OpenAPI specification.

Error format

Recommend something simple such as:

{ "error": "Human-readable message" }

or

{
  "error": {
    "code": "SNIPPET_NOT_FOUND",
    "message": "Snippet with id 42 was not found"
  }
}

The exact format is less important than being consistent across endpoints.

Validation examples

For the Snippets API, talk through validations like:

• POST /api/snippets

  • title is required and non-empty.
  • contents is required and non-empty.
  • Optional fields like is_private have sensible defaults.
• PUT /api/snippets/:id

  • Reject updates with empty title or contents.
  • Return 404 when id does not exist.
• Any /:id route

  • Validate that id is a number before hitting the database.

Connect these decisions back to status codes from Week 1:

• 400 Bad Request for invalid inputs.
• 404 Not Found when a resource is missing.
• 500 Internal Server Error for unexpected issues.