Handling Graphql errors with Apollo

[DongusJr]

Im setting up subscriptions with apollo using SSELink.
We have an express server.
I followed the instructions for the server in the documentation and it works as intended.

For the client side I setup an ApolloLink like instructed in the Recipes

import {
  ApolloLink,
  Operation,
  FetchResult,
  Observable,
} from '@apollo/client/core';
import { print, GraphQLError } from 'graphql';
import { createClient, ClientOptions, Client } from 'graphql-sse';
 
class SSELink extends ApolloLink {
  private client: Client;
 
  constructor(options: ClientOptions) {
    super();
    this.client = createClient(options);
  }
 
  public request(operation: Operation): Observable<FetchResult> {
    return new Observable((sink) => {
      return this.client.subscribe<FetchResult>(
        { ...operation, query: print(operation.query) },
        {
          next: sink.next.bind(sink),
          complete: sink.complete.bind(sink),
          error: sink.error.bind(sink),
        },
      );
    });
  }
}
 
export const link = new SSELink({
  url: 'http://where.is:4000/graphql/stream',
  headers: () => {
    const session = getSession();
    if (!session) return {};
    return {
      Authorization: `Bearer ${session.token}`,
    };
  },
});

I defined the link this way, but additionally added singleConnection: true and onMessage: console.log.

To subscribe, I'm using an apollo useSubscription hook

const TEST_SUBSCRIPTION = gql`
  subscription TestSubscription {
    testSubscription {
      testField
    }
  }
`;

export const useTestSubscription = () => {
  const { client } = useContext(ApplicationContext);
  return useSubscription<TestSubscription>(TEST_SUBSCRIPTION, {
    client,
    onSubscriptionData: (options) => {
      console.log('options:', options);
    },
  });
};

When I test the "TestSubscription" event with valid data, everything is fine, I get the data passed down to the hook.

However, when I try to propegate an error in the server, in my case returning testField = undefined when it can only be of a string type, the response is not sent down to the useSubscription hook.

Preferably, we would like to get the error down to the onSubscriptionData options object so we can properly handle an error when it occurs.

Is there some way for me the configure the link so that is passes the error down?

[enisdenjo]

This looks like an Apollo Client specific question. Sorry, but I don't have any experience with Apollo so I cannot help you out here.