Response in ended before the catch in `RequestListener`

In the following example the server is not returning 500 Internal Server Error. Instead the server throws an error (Error [ERR_HTTP_HEADERS_SENT]: Cannot render headers after they are sent to the client) and the process is killed.

import http from 'http';
import { createHandler } from 'graphql-sse';

const handler = createHandler({
  ...
  onNext: () => {
    throw new Error('Not implemented');
  },
});

http.createServer(async (req, res) => {
  try {
    await handler(req, res);
  } catch (err) {
    res.writeHead(500, 'Internal Server Error').end();
  }
});

[asadhazara]

Thank you @enisdenjo! The example I have used is directly copied from the JSDoc for the Handler. I have submitted a PR for this.

[enisdenjo]

Since re-writing the header is not possible, you can catch the error in onNext and populate the errors GraphQL result field to transmit to the client through the next message.

Something around the edges:

import http from 'http';
import { createHandler } from 'graphql-sse';

const handler = createHandler({
  // ... your other options ...
  onNext: () => {
    try {
      mightThrowAnError();
    } catch (err) {
      return {
        errors: [new GraphQLError(err)],
      };
    }
  },
});

http.createServer(async (req, res) => {
  try {
    await handler(req, res);
  } catch (err) {
    if (!res.headersSent) {
      res.writeHead(500, 'Internal Server Error').end();
    } else {
      console.error('Internal Server Error', err);
    }
  }
});

But beware, I do NOT recommend this. You should absolutely make sure that nothing is thrown from onNext. Execution errors, errors that are a part of the next message, are not terminal - the client will not end and the operation will proceed blindly.