[Data masking] Ensure masking algorithm properly handles nulls

.size-limits.json

@@ -1,4 +1,4 @@
 {
-  "dist/apollo-client.min.cjs": 41255,
-  "import { ApolloClient, InMemoryCache, HttpLink } from \"dist/index.js\" (production)": 33943
+  "dist/apollo-client.min.cjs": 41282,
+  "import { ApolloClient, InMemoryCache, HttpLink } from \"dist/index.js\" (production)": 33969
 }

src/core/__tests__/masking.test.ts

@@ -56,6 +56,40 @@ describe("maskOperation", () => {
     );
   });
 
+  test("returns null when data is null", () => {
+    const query = gql`
+      query {
+        foo
+        ...QueryFields
+      }
+
+      fragment QueryFields on Query {
+        bar
+      }
+    `;
+
+    const data = maskOperation(null, query, new InMemoryCache());
+
+    expect(data).toBe(null);
+  });
+
+  test("returns undefined when data is undefined", () => {
+    const query = gql`
+      query {
+        foo
+        ...QueryFields
+      }
+
+      fragment QueryFields on Query {
+        bar
+      }
+    `;
+
+    const data = maskOperation(undefined, query, new InMemoryCache());
+
+    expect(data).toBe(undefined);
+  });
+
   test("strips top-level fragment data from query", () => {
     const query = gql`
       query {
@@ -177,6 +211,83 @@ describe("maskOperation", () => {
     });
   });
 
+  test("handles nulls in child selection sets", () => {
+    const query = gql`
+      query {
+        user {
+          profile {
+            id
+          }
+          ...UserFields
+        }
+      }
+      fragment UserFields on User {
+        profile {
+          id
+          fullName
+        }
+      }
+    `;
+
+    const nullUser = maskOperation(
+      deepFreeze({ user: null }),
+      query,
+      new InMemoryCache()
+    );
+    const nullProfile = maskOperation(
+      deepFreeze({ user: { __typename: "User", profile: null } }),
+      query,
+      new InMemoryCache()
+    );
+
+    expect(nullUser).toEqual({ user: null });
+    expect(nullProfile).toEqual({
+      user: { __typename: "User", profile: null },
+    });
+  });
+
+  test("handles nulls in arrays", () => {
+    const query = gql`
+      query {
+        users {
+          profile {
+            id
+          }
+          ...UserFields
+        }
+      }
+      fragment UserFields on User {
+        profile {
+          id
+          fullName
+        }
+      }
+    `;
+
+    const data = maskOperation(
+      deepFreeze({
+        users: [
+          null,
+          { __typename: "User", profile: null },
+          {
+            __typename: "User",
+            profile: { __typename: "Profile", id: "1", fullName: "Test User" },
+          },
+        ],
+      }),
+      query,
+      new InMemoryCache()
+    );
+
+    expect(data).toEqual({
+      users: [
+        null,
+        { __typename: "User", profile: null },
+        { __typename: "User", profile: { __typename: "Profile", id: "1" } },
+      ],
+    });
+  });
+
   test("deep freezes the masked result if the original data is frozen", () => {
     const query = gql`
       query {
@@ -1710,6 +1821,39 @@ describe("maskOperation", () => {
 });
 
 describe("maskFragment", () => {
+  test("returns null when data is null", () => {
+    const fragment = gql`
+      fragment Foo on Query {
+        foo
+        ...QueryFields
+      }
+
+      fragment QueryFields on Query {
+        bar
+      }
+    `;
+
+    const data = maskFragment(null, fragment, new InMemoryCache(), "Foo");
+
+    expect(data).toBe(null);
+  });
+
+  test("returns undefined when data is undefined", () => {
+    const fragment = gql`
+      fragment Foo on Query {
+        foo
+        ...QueryFields
+      }
+
+      fragment QueryFields on Query {
+        bar
+      }
+    `;
+
+    const data = maskFragment(undefined, fragment, new InMemoryCache(), "Foo");
+
+    expect(data).toBe(undefined);
+  });
   test("masks named fragments in fragment documents", () => {
     const fragment = gql`
       fragment UserFields on User {
@@ -1764,6 +1908,75 @@ describe("maskFragment", () => {
     });
   });
 
+  test("handles nulls in child selection sets", () => {
+    const fragment = gql`
+      fragment UserFields on User {
+        profile {
+          id
+        }
+        ...ProfileFields
+      }
+      fragment ProfileFields on User {
+        profile {
+          id
+          fullName
+        }
+      }
+    `;
+
+    const data = maskFragment(
+      deepFreeze({ __typename: "User", profile: null }),
+      fragment,
+      new InMemoryCache(),
+      "UserFields"
+    );
+
+    expect(data).toEqual({ __typename: "User", profile: null });
+  });
+
+  test("handles nulls in arrays", () => {
+    const fragment = gql`
+      fragment UserFields on Query {
+        users {
+          profile {
+            id
+          }
+          ...ProfileFields
+        }
+      }
+      fragment ProfileFields on User {
+        profile {
+          id
+          fullName
+        }
+      }
+    `;
+
+    const data = maskFragment(
+      deepFreeze({
+        users: [
+          null,
+          { __typename: "User", profile: null },
+          {
+            __typename: "User",
+            profile: { __typename: "Profile", id: "1", fullName: "Test User" },
+          },
+        ],
+      }),
+      fragment,
+      new InMemoryCache(),
+      "UserFields"
+    );
+
+    expect(data).toEqual({
+      users: [
+        null,
+        { __typename: "User", profile: null },
+        { __typename: "User", profile: { __typename: "Profile", id: "1" } },
+      ],
+    });
+  });
+
   test("deep freezes the masked result if the original data is frozen", () => {
     const fragment = gql`
       fragment UserFields on User {

src/core/masking.ts

@@ -40,6 +40,11 @@ export function maskOperation<TData = unknown>(
     "Expected a parsed GraphQL document with a query, mutation, or subscription."
   );
 
+  if (data == null) {
+    // Maintain the original `null` or `undefined` value
+    return data;
+  }
+
   const context: MaskingContext = {
     operationType: definition.operation,
     operationName: definition.name?.value,
@@ -100,6 +105,11 @@ export function maskFragment<TData = unknown>(
     fragmentName
   );
 
+  if (data == null) {
+    // Maintain the original `null` or `undefined` value
+    return data;
+  }
+
   const context: MaskingContext = {
     operationType: "fragment",
     operationName: fragment.name.value,
@@ -132,6 +142,10 @@ function maskSelectionSet(
     let changed = false;
 
     const masked = data.map((item, index) => {
+      if (item === null) {
+        return null;
+      }
+
       const [masked, itemChanged] = maskSelectionSet(
         item,
         selectionSet,
@@ -155,7 +169,7 @@ function maskSelectionSet(
 
           memo[keyName] = data[keyName];
 
-          if (childSelectionSet) {
+          if (childSelectionSet && data[keyName] !== null) {
             const [masked, childChanged] = maskSelectionSet(
               data[keyName],
               childSelectionSet,