> For the complete documentation index, see [llms.txt](https://program.hackyourfuture.dk/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://program.hackyourfuture.dk/course-content/backend/node/week2/session-materials/validation-and-error-models.md).

# 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:

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

or

```json
{
  "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.


---

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