Validation transformer throwing TS error on nested data access

[m-kutnik]

Hi there! I'd like to create a following schema util:

const unwrapResponse = <T extends v.GenericSchema>(schema: T) =>
  v.pipe(
    v.object({
      body: v.object({
        data: schema,
      }),
    }),
    v.transform((input) => {
      return input.body.data;
    }),
  );

This helper is meant to:

• Take any validation schema
• Extend it to match the API response structure ({ body: { data: T } })
• Return just the inner data after validation

The runtime validation works perfectly, but TypeScript is throwing an error in the transform function:
Property 'data' does not exist on type '{ [TKey in keyof OutputWithReadonly<{ readonly data: T; }, OutputWithQuestionMarks<{ readonly data: T; }, InferEntriesOutput<{ readonly data: T; }>>>]: OutputWithReadonly<...>[TKey]; }'

Here is the playground example

Any suggestions on how to fix this?
Thanks!

[fabian-hiller]

Without further investigation, it appears that TS has problems resolving a generic schema in this case. In the long run, we should investigate whether this is due to our implementation or a limitation of TS.

A simple workaround is to keep only the input (and optionally the output) generic, but not the pass schema itself:

const unwrapResponse = <TInput, TOutput>(
  schema: v.GenericSchema<TInput, TOutput>
) =>
  v.pipe(
    v.object({
      body: v.object({
        data: schema,
      }),
    }),
    v.transform((input) => {
      return input.body.data;
    })
  );

[m-kutnik]

Great, works fine, thanks!

[fabian-hiller]

Good news! I fixed some types in v1.0.0-beta.14. Your initial code example should work now.