onError is NOT being called when an Error is thrown

Expected Behaviour When an Error is thrown in a resolver onError passed to useServer should be called.

Actual Behaviour When an Error is thrown in a resolver onNext is called, instead of onError.

[enisdenjo]

I won’t add a customFormatErrorFn because graphql-ws does not (and should not) care about the contents of an operation result.

Nevertheless, you can format your errors directly in the onNext hook as well as the onError hook, roughly:

import { GraphQLObjectType, GraphQLSchema, GraphQLString } from 'graphql';
import { useServer } from 'graphql-ws/lib/use/ws';
import { WebSocketServer } from 'ws';

const Query = new GraphQLObjectType({
  name: 'Query',
  fields: {
    hello: {
      type: GraphQLString,
      resolve() {
        throw new Error('Something went wrong');
      },
    },
  },
});

const schema = new GraphQLSchema({
  query: Query,
});

useServer(
  {
    schema,
    onError: (ctx, _msg, errors) => {
      return yourErrorFormatter(ctx, errors);
    },
    onNext: (ctx, _msg, _args, result) => {
      if (result.errors) {
        // if there are errors in the operation result, format them and return the altered result
        result.errors = yourErrorFormatter(ctx, result.errors);
        return result;
      }
      // otherwise do nothing
    },
  },
  new WebSocketServer({ port: 4000 }),
);

[asadhazara]

Okay, thank you @enisdenjo for the quick replies!