How can I get "string & {}" in the inferred output type?

[jack-allocate]

I have a schema like this:

const fruits = v.union([
  v.literal('apple'),
  v.literal('banana'),
  v.pipe(v.string(), v.uuid()),
]);

and the output type infers to string which makes sense. I'd love it if there was a way for it to infer as "apple" | "banana" | (string & {}).

Playground

[EltonLobo07]

Since string and string & {} represent the same values at runtime, I recommend using a type assertion with the transform action.

import * as v from 'valibot';

const fruits = v.union([
  v.literal('apple'),
  v.literal('banana'),
  v.pipe(
    v.string(),
    v.uuid(),
    v.transform((v) => v as typeof v & {}),
  ),
]);

type Out = v.InferOutput<typeof fruits>;

[jack-allocate]

Hmm, this seems to break once used within an object

const Schema = v.object({
  source: v.union([
    v.literal('wallet'),
    v.literal('manual-wire'),
    v.pipe(
      v.string(),
      v.uuid(),
      v.transform((v) => v as typeof v & {}),
    ),
  ]),
})

type Out = v.InferOutput<typeof Schema>

[EltonLobo07]

This is a bug. I have created a PR that fixes this.

[fabian-hiller]

I found a workaround:

import * as v from 'valibot';

interface HideMe {}

const fruits = v.union([
  v.literal('apple'),
  v.literal('banana'),
  v.pipe(
    v.string(),
    v.uuid(),
    v.transform((v) => v as string & HideMe),
  ),
]);

type Output = v.InferOutput<typeof fruits>;

[fabian-hiller]

I will soon introduce a new flavor action to solve this issue:

import * as v from 'valibot';

const fruits = v.union([
  v.literal('apple'),
  v.literal('banana'),
  v.pipe(v.string(), v.uuid(), v.flavor('UUID')),
]);

[fabian-hiller]

Thanks for reaching out! What's the idea behind string & {}? Do you look for a branded type? We have brand for that.

[jack-allocate]

Yeah exactly this. That way I still get intellisense for the validated literals, but can also pass any string (e.g. the uuid)

[fabian-hiller]

Thanks for the info! Does this have a common name? If so, we could add a new Valibot action to transform the output type to T & {}.

[jack-allocate]

Not that I know of, whenever I need to look it up I just search "typescript string hack"

[ryanagillie]

I think it works for any "literal" type (string, number, bigint) ts playground

[fabian-hiller]

I will soon introduce a new flavor action to solve this issue:

import * as v from 'valibot';

const fruits = v.union([
  v.literal('apple'),
  v.literal('banana'),
  v.pipe(v.string(), v.uuid(), v.flavor('UUID')),
]);