Typescript errors for context object in Apollo gateway

Package Versions: @apollo-gateway: 0.10.4

Using the code below (from: [https://www.apollographql.com/docs/apollo-server/federation/implementing/]) I wouldn’t expect to see any errors when using typescript.

const gateway = new ApolloGateway({
  serviceList: [
    { name: 'products', url: 'http://localhost:4001' },
    // other services
  ],
  buildService({ name, url }) {
    return new RemoteGraphQLDataSource({
      url,
      willSendRequest({ request, context }) {
        // pass the user's id from the context to underlying services
        // as a header called `user-id`
        request.http.headers.set('user-id', context.userId);
      },
    });
  },
});

However, typescript warns me that “Property ‘userId’ does not exist on type ‘TContext’.”

Looking at the underlying code it appears that willSendRequest should accept any key value pair with a string but that doesn’t appear to be the case. Although Im not sure where the error could be.

Sorry if I have missed anything, Im still learning some of the advanced methods in typescript.

[migueloller]

Adding to this issue, if a type of { request: GraphQLRequest, context: Context } is added, TypeScript will complain that the provided Context isn’t compatible with TContext.

[LucienLee]

It looks the author misunderstands the way to use generic type in the class. It should pass generic type from class to it’s method, but in code it only set generic type on the method.

The correct one would be:

export class RemoteGraphQLDataSource<TContext> implements GraphQLDataSource { // <- pass generic from class
  public willSendRequest?<TContext>( // <- so we could pass our own context type here
    requestContext: Pick<
      GraphQLRequestContext<TContext>,
      'request' | 'context'
    >,
  ): ValueOrPromise<void>;
}

I can make a PR later.