Document undefined data cases for mutations (#6549)

Co-authored-by: Jerel Miller <[email protected]>

src/__tests__/mutationResults.ts

@@ -2,7 +2,7 @@ import { cloneDeep } from "lodash";
 import gql from "graphql-tag";
 import { GraphQLError } from "graphql";
 
-import { ApolloClient } from "../core";
+import { ApolloClient, FetchResult } from "../core";
 import { InMemoryCache } from "../cache";
 import { ApolloLink } from "../link/core";
 import {
@@ -1819,5 +1819,40 @@ describe("mutation results", () => {
           }, reject);
       }
     );
+
+    itAsync(
+      "data might be undefined in case of failure with errorPolicy = ignore",
+      async (resolve, reject) => {
+        const client = new ApolloClient({
+          cache: new InMemoryCache(),
+          link: new ApolloLink(
+            () =>
+              new Observable<FetchResult<{ foo: string }>>((observer) => {
+                observer.next({
+                  errors: [new GraphQLError("Oops")],
+                });
+                observer.complete();
+              })
+          ).setOnError(reject),
+        });
+
+        const ignoreErrorsResult = await client.mutate({
+          mutation: gql`
+            mutation Foo {
+              foo
+            }
+          `,
+          fetchPolicy: "no-cache",
+          errorPolicy: "ignore",
+        });
+
+        expect(ignoreErrorsResult).toEqual({
+          data: undefined,
+          errors: undefined,
+        });
+
+        resolve();
+      }
+    );
   });
 });

src/core/ApolloClient.ts

@@ -426,7 +426,8 @@ export class ApolloClient<TCacheShape> implements DataProxy {
   /**
    * This resolves a single mutation according to the options specified and returns a
    * Promise which is either resolved with the resulting data or rejected with an
-   * error.
+   * error. In some cases both `data` and `errors` might be undefined, for example
+   * when `errorPolicy` is set to `'ignore'`.
    *
    * It takes options as an object with the following keys and values:
    */

src/link/core/types.ts

@@ -85,6 +85,7 @@ export interface SingleExecutionResult<
   TContext = DefaultContext,
   TExtensions = Record<string, any>,
 > extends ExecutionResult<TData, TExtensions> {
+  // data might be undefined if errorPolicy was set to 'ignore'
   data?: TData | null;
   context?: TContext;
 }