fix: strengthen MockedResponse.newData type (#11584)

.changeset/dirty-insects-return.md

@@ -0,0 +1,5 @@
+---
+"@apollo/client": patch
+---
+
+Strengthen `MockedResponse.newData` type

src/testing/core/mocking/__tests__/mockLink.ts

@@ -1,7 +1,60 @@
 import gql from "graphql-tag";
-import { MockLink } from "../mockLink";
+import { MockLink, MockedResponse } from "../mockLink";
 import { execute } from "../../../../link/core/execute";
 
+describe("MockedResponse.newData", () => {
+  const setup = () => {
+    const weaklyTypedMockResponse: MockedResponse = {
+      request: {
+        query: gql`query A { a }`,
+      },
+    };
+
+    const stronglyTypedMockResponse: MockedResponse<{ a: string }, { input: string }> = {
+      request: {
+        query: gql`query A { a }`,
+      }
+    };
+
+    return {
+      weaklyTypedMockResponse,
+      stronglyTypedMockResponse,
+    };
+  }
+
+  test("returned 'data' can be any object with untyped response", () => {
+    const { weaklyTypedMockResponse } = setup();
+
+    weaklyTypedMockResponse.newData = ({ fake: { faker } }) => ({
+      data: {
+        pretend: faker,
+      },
+    });
+  });
+
+  test("can't return output that doesn't match TData", () => {
+    const { stronglyTypedMockResponse } = setup();
+
+    // @ts-expect-error return type does not match `TData`
+    stronglyTypedMockResponse.newData = () => ({
+      data: {
+        a: 123
+      }
+    });
+  });
+
+  test("can't use input variables that don't exist in TVariables", () => {
+    const { stronglyTypedMockResponse } = setup();
+
+    // @ts-expect-error unknown variables
+    stronglyTypedMockResponse.newData = ({ fake: { faker } }) => ({
+      data: {
+        a: faker
+      }
+    });
+  });
+});
+
 /*
 We've chosen this value as the MAXIMUM_DELAY since values that don't fit into a 32-bit signed int cause setTimeout to fire immediately
 */

src/testing/core/mocking/mockLink.ts

@@ -35,7 +35,7 @@ export interface MockedResponse<
   error?: Error;
   delay?: number;
   variableMatcher?: VariableMatcher<TVariables>;
-  newData?: ResultFunction<FetchResult>;
+  newData?: ResultFunction<FetchResult<TData>, TVariables>;
 }
 
 export interface MockLinkOptions {