Export `ActionFailure` to allow `instanceof`

[probablykasper]

Describe the problem

I have some shared logic for parsing user input, but it's not that convenient to deal with success/error types.

Describe the proposed solution

ActionFailure is a class, but SvelteKit exposes it as an interface. It would be nice to simply return fail() and do instanceof ActionFailure:

function parse(value: T | null) {
	if (!value) {
		return fail(400, { message: 'Too high' })
	}
	// ...
	return value
}

// Action:
const result = parse('test')
if (result instanceof ActionFailure) {
	return result
}

Alternatives considered

• Creating my own class
• Returning a result type like { value: T, error: null } | { value: null, error: string }
• Use the Error class message as an intermediate, if I only need an error string

Importance

nice to have

Additional Information

No response

[teemingc]

Does adding a check such as below solve your issue?

if ('status' in result && typeof result.status === 'number' && result.status >= 400) {
  return result;
}

or simply throw the ActionFailure so the return type is always T

function parse(value: T | null) {
	if (!value) {
		throw fail(400, { message: 'Too high' })
	}
	// ...
	return value
}

// Action:
try {
  const result = parse('test')
} catch (error) {
  if ('status' in error) {
    return error as ActionFailure;
  }
}

Using instanceof to check the type comes with it's own set of problems in monorepo setups that may have multiple SvelteKits installed and therefore multiple ActionFailure class instances. See #12597

[probablykasper]

No, I don't feel like those are any simpler than the workarounds I mentioned. The second is also not type safe so I would highly discourage using that

[dummdidumm]

We already have isError etc methods exposed from @sveltejs/kit - we should just add isActionFailure there

[probablykasper]

Isn't it just as convenient and more performant to call instanceof directly?

[teemingc]

We already have isError etc methods exposed from @sveltejs/kit - we should just add isActionFailure there

I don't think we do 🤔 but for some reason that sounds really familiar to me.

[dummdidumm]

oops it's called isHttpError

Isn't it just as convenient and more performant to call instanceof directly?

I don't think performance matters here. What matters more is consistency, and we decided a while ago we want this to exposed via functions (gives more flexibility for the underlying implementation) and so it makes sense to keep it that way for new additions.