Fix formatResponse typings

This typing was added in #3431; it looks like we put the `| null` in the wrong
place. Fortunately, a function that's OK with accepting a null argument and will
never return null can still typecheck with the new type.

Fixes #5009.

CHANGELOG.md

@@ -13,6 +13,7 @@ The version headers in this history reflect the versions of Apollo Server itself
 
 - `apollo-server-core`: Add optional argument to `ApolloServer.executeOperation` allowing the caller to manually specify an argument to the `config` function analogous to that provided by integration packages. [PR #4166](https://github.com/apollographql/apollo-server/pull/4166) [Issue #2886](https://github.com/apollographql/apollo-server/issues/2886)
 - `apollo-server-cache-redis`: New `BaseRedisCache` class which takes an `ioredis`-compatible Redis client as an argument. The existing classes `RedisCache` and `RedisClusterCache` (which pass their arguments to `ioredis` constructors) are now implemented in terms of this class. This allows you to use any of the `ioredis` constructor forms rather than just the ones recognized by our classes. This also fixes a long-standing bug where the Redis cache implementations returned a number from `delete()`; it now returns a number, matching what the `KeyValueCache` interface and the TypeScript types expect. [PR #5034](https://github.com/apollographql/apollo-server/pull/5034) [PR #5088](https://github.com/apollographql/apollo-server/pull/5088) [Issue #4870](https://github.com/apollographql/apollo-server/issues/4870) [Issue #5006](https://github.com/apollographql/apollo-server/issues/5006)
+- `apollo-server-core`: Fix type for `formatResponse` function. It never is called with a `null` argument, and is allowed to return `null`. [Issue #5009](https://github.com/apollographql/apollo-server/issues/5009) [PR #?](https://github.com/apollographql/apollo-server/pull/?)
 
 ## v2.22.2
 

docs/source/api/apollo-server.md

@@ -301,7 +301,7 @@ Provide this function to transform the structure of error objects before they're
 </td>
 <td>
 
-Provide this function to transform the structure of GraphQL response objects before they're sent to a client. The function takes a [`GraphQLResponse`](https://github.com/apollographql/apollo-server/blob/main/packages/apollo-server-types/src/index.ts#L77-L82) object and a [`GraphQLRequestContext`](https://github.com/apollographql/apollo-server/blob/main/packages/apollo-server-types/src/index.ts#L95-L130) object, and it should return a `GraphQLResponse` object.
+Provide this function to transform the structure of GraphQL response objects before they're sent to a client. The function takes a [`GraphQLResponse`](https://github.com/apollographql/apollo-server/blob/main/packages/apollo-server-types/src/index.ts#L77-L82) object and a [`GraphQLRequestContext`](https://github.com/apollographql/apollo-server/blob/main/packages/apollo-server-types/src/index.ts#L95-L130) object, and it should return a `GraphQLResponse` object, or null to preserve the existing structure.
 </td>
 </tr>
 

packages/apollo-server-core/src/graphqlOptions.ts

@@ -50,9 +50,9 @@ export interface GraphQLServerOptions<
   validationRules?: Array<(context: ValidationContext) => any>;
   executor?: GraphQLExecutor;
   formatResponse?: (
-    response: GraphQLResponse | null,
+    response: GraphQLResponse,
     requestContext: GraphQLRequestContext<TContext>,
-  ) => GraphQLResponse
+  ) => GraphQLResponse | null;
   fieldResolver?: GraphQLFieldResolver<any, TContext>;
   debug?: boolean;
   tracing?: boolean;