Prevent undefined mutation result in useMutation

CHANGELOG.md

@@ -8,6 +8,9 @@
 - During server-side rendering, allow initial `useQuery` calls to return final `{ loading: false, data }` results when the cache already contains the necessary data. <br/>
   [@benjamn](https://github.com/benjamn) in [#7983](https://github.com/apollographql/apollo-client/pull/7983)
 
+- Prevent `undefined` mutation result in useMutation <br/>
+  [@jcreighton](https://github.com/jcreighton) in [#8018](https://github.com/apollographql/apollo-client/pull/8018)
+
 ## Apollo Client 3.3.14
 
 ### Improvements

src/react/data/MutationData.ts

@@ -74,8 +74,17 @@ export class MutationData<
         return response;
       })
       .catch((error: ApolloError) => {
+        const { onError } = this.getOptions();
         this.onMutationError(error, mutationId);
-        if (!this.getOptions().onError) throw error;
+        if (onError) {
+          onError(error);
+          return {
+            data: undefined,
+            errors: error,
+          };
+        } else {
+          throw error;
+        }
       });
   };
 
@@ -128,8 +137,6 @@ export class MutationData<
   }
 
   private onMutationError(error: ApolloError, mutationId: number) {
-    const { onError } = this.getOptions();
-
     if (this.isMostRecentMutation(mutationId)) {
       this.updateResult({
         loading: false,
@@ -138,10 +145,6 @@ export class MutationData<
         called: true
       });
     }
-
-    if (onError) {
-      onError(error);
-    }
   }
 
   private generateNewMutationId(): number {
@@ -152,13 +155,14 @@ export class MutationData<
     return this.mostRecentMutationId === mutationId;
   }
 
-  private updateResult(result: MutationResultWithoutClient<TData>) {
+  private updateResult(result: MutationResultWithoutClient<TData>): MutationResultWithoutClient<TData> | undefined {
     if (
       this.isMounted &&
       (!this.previousResult || !equal(this.previousResult, result))
     ) {
       this.setResult(result);
       this.previousResult = result;
+      return result;
     }
   }
 }

src/react/hooks/__tests__/useMutation.test.tsx

@@ -252,6 +252,58 @@ describe('useMutation Hook', () => {
     });
 
     describe('mutate function upon error', () => {
+      itAsync('resolves with the resulting data and errors', async (resolve, reject) => {
+        const variables = {
+          description: 'Get milk!'
+        };
+
+        const mocks = [
+          {
+            request: {
+              query: CREATE_TODO_MUTATION,
+              variables
+            },
+            result: {
+              data: CREATE_TODO_RESULT,
+              errors: [new GraphQLError(CREATE_TODO_ERROR)],
+            },
+          }
+        ];
+
+        let fetchResult: any;
+        const Component = () => {
+          const [createTodo] = useMutation<{ createTodo: Todo }>(
+            CREATE_TODO_MUTATION,
+            {
+              onError: error => {
+                expect(error.message).toEqual(CREATE_TODO_ERROR);
+              }
+            }
+          );
+
+          async function runMutation() {
+            fetchResult = await createTodo({ variables });
+          }
+
+          useEffect(() => {
+            runMutation();
+          }, []);
+
+          return null;
+        };
+
+        render(
+          <MockedProvider mocks={mocks}>
+            <Component />
+          </MockedProvider>
+        );
+
+        await wait(() => {
+          expect(fetchResult.data).toEqual(undefined);
+          expect(fetchResult.errors.message).toEqual(CREATE_TODO_ERROR);
+        }).then(resolve, reject);
+      });
+
       it(`should reject when errorPolicy is 'none'`, async () => {
         const variables = {
           description: 'Get milk!'