v.optional output type is broken

[sbking]

I believe this bug was introduced in v1.0.0-beta.3. When wrapping a transform schema in v.optional with a default value, the inferred output type is now a union including the type of the default value. However, at runtime the default value is still parsed through the transform schema.

Tricky to explain, so here is a playground example.

The schema will always return a number, but now TypeScript says it will return number | "10"

[sbking]

As a sidenote, the behavior of v.optional is a bit confusing and not explained well in the docs. I would expect the default value to be the output type of the transform schema (or any arbitrary type and then return it unmodified with a union output type), but instead what valibot seems to do is expect the input type of the transform schema and then pass it through the wrapped transform schema.

Either way, the TypeScript types are incorrect because it expects TDefault to be assignable to the input type of the wrapped schema, but then still has TDefault as one of the union options of the overall output.

[fabian-hiller]

Thank you for reaching out! You are right! I am very sorry. I will fix this and add tests to make sure this cannot happen again.

optional allows you to define a default input to be used when the input is undefined. The behaviour you describe with the output as the "default" is similar to our fallback method.

[fabian-hiller]

Fixed 😎

[fabian-hiller]

What information do you miss in the docs? I am happy to improve it. Here is our guide and API reference:

• https://valibot.dev/guides/optionals/#default-values
• https://valibot.dev/api/optional/