GraphQL compatibility

Sanity's GraphQL API generates one GraphQL type per _type, so every _type in a schema must map to exactly one shape. verifyUniqueTypeNames(schema) walks the schema and returns it unchanged, or throws a DuplicateTypeNameError if the same _type literal appears in more than one place. Wrapping a schema in it turns a duplicate into an error as soon as the module loads, in tests and in CI, instead of when you deploy the GraphQL API.

import * as s from '@sanity-labs/typed-schema'

const Post = s.verifyUniqueTypeNames(
  s.document({
    _type: s.literal('post'),
    author: s.object({_type: s.literal('person'), name: s.string()}),
  }),
)

type Post = s.Infer<typeof Post>

A second object with _type: 'person' anywhere in the schema makes it throw a DuplicateTypeNameError:

import * as s from '@sanity-labs/typed-schema'

const Post = s.verifyUniqueTypeNames(
  s.document({
    _type: s.literal('post'),
    author: s.object({_type: s.literal('person'), name: s.string()}),
    reviewer: s.object({_type: s.literal('person'), email: s.string()}),
  }),
)

The error message names the duplicated _type and the paths where it appears:

Duplicate type names in schema:
  _type "person" at author, reviewer

To use a type in more than one place, declare it once and reuse the schema. Reusing the same schema counts as one type, while two separate declarations of person are rejected even if their fields match, so every type has one definition:

import * as s from '@sanity-labs/typed-schema'

const Person = s.object({_type: s.literal('person'), name: s.string()})

type Person = s.Infer<typeof Person>

const Post = s.verifyUniqueTypeNames(
  s.document({
    _type: s.literal('post'),
    author: Person,
    reviewer: Person,
  }),
)

type Post = s.Infer<typeof Post>