helper to derive fragment types

src/__tests__/fragments.test-d.ts

@@ -0,0 +1,31 @@
+import { assertType, test } from 'vitest';
+import { Introspection } from '../introspection';
+import { Document } from '../parser';
+import { FragmentType } from '../typed-document/fragments';
+import { schema } from './introspection.test-d';
+
+type Intro = Introspection<typeof schema>;
+const any = {} as any;
+
+test('creates a type for a given fragment', () => {
+  const unionQuery = `
+  fragment Fields on Todo {
+    id
+    text
+    complete
+    __typename
+  }
+`;
+
+  type doc = Document<typeof unionQuery>;
+  type typedDoc = FragmentType<doc, Intro>;
+
+  const actual = any as typedDoc;
+
+  assertType<{
+    id: string | number;
+    text: string;
+    complete: boolean | null;
+    __typename: 'Todo';
+  }>(actual);
+});

src/typed-document/fragments.ts

@@ -0,0 +1,23 @@
+import { FragmentDefinitionNode, Kind } from '@0no-co/graphql.web';
+import { Selection, ObjectLikeType, FragmentMap } from './index';
+import { Introspection as IntrospectionType } from '../introspection';
+
+export type FragmentType<
+  Document extends { kind: Kind.DOCUMENT; definitions: any[] },
+  Introspection extends IntrospectionType<any>,
+  Fragments extends Record<string, FragmentDefinitionNode> = FragmentMap<Document>
+> = Document['definitions'][0] extends {
+  kind: Kind.FRAGMENT_DEFINITION;
+  typeCondition: { name: { value: infer TypeName } };
+}
+  ? TypeName extends keyof Introspection['types']
+    ? Introspection['types'][TypeName] extends ObjectLikeType
+      ? Selection<
+          Document['definitions'][0]['selectionSet']['selections'],
+          Introspection['types'][TypeName],
+          Introspection,
+          Fragments
+        >
+      : never
+    : never
+  : never;

src/typed-document/index.ts

@@ -20,7 +20,7 @@ import type {
 } from '../introspection';
 import type { Obj, ObjValues } from '../utils';
 
-type ObjectLikeType = {
+export type ObjectLikeType = {
   kind: 'OBJECT' | 'INTERFACE' | 'UNION';
   name: string;
   fields: { [key: string]: IntrospectionField };
@@ -119,7 +119,7 @@ type TypenameOfType<X extends ObjectLikeType> = X extends {
 // i.e. excluding `PossibleFragmentsSelection<...>['__typename']` when we're on a GraphQL union or interface?
 // TODO: For the interface case, do we need to type-union `FieldSelectionContinue<...>` into each possible
 // intersection type of `PossibleFragmentsSelection<...>`?
-type Selection<
+export type Selection<
   Selections extends readonly any[],
   Type extends ObjectLikeType,
   Introspection extends IntrospectionType<any>,