formatResponse typings: null response?

[nwalters512]

In #3431, the typings for formatResponse were set such that the response argument could be null. However, this causes problems when trying to write a basic implementation that, for instance, adds a property to the object:

new ApolloServer({
  formatResponse: (response) => {
    if (response) {
      if (!response.extensions) {
        response.extensions = {};
      }
      response.extensions.code_version = VERSION;
    }
    return response;
  }
})

This produces a TypeScript error, as response is typed as GraphQLResponse | null but the expected return type is just GraphQLResponse (non-nullable):

Type '(response: GraphQLResponse | null) => GraphQLResponse | null' is not assignable to type '(response: GraphQLResponse | null, requestContext: GraphQLRequestContext<object>) => GraphQLResponse'.
  Type 'GraphQLResponse | null' is not assignable to type 'GraphQLResponse'.
    Type 'null' is not assignable to type 'GraphQLResponse'.

It's not clear to me if response ever actually can be null here, but I would expect this function to be typed such that the response argument and the return value have the same type. If response will not ever be null, I'd like the types to reflect that.

This is mentioned in a comment on #4186, though I don't think that's actually the issue that OP encountered there, so I opted to lift this into a new issue.

[glasser]

Thanks for the report. Looks like we put the | null in the wrong place in #3431. Looks like #5089 should fix it.

[glasser]

I've released a prerelease with this fix, version 2.23.0-alpha.0. Try out the alpha and see if it works for you! Please provide any feedback in #5094.

[glasser]

This is released in Apollo Server 2.23.0.