types: nullish error types in callback function's parameter for after and ready method

[nokazn]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

I think err type definition in the first parameter of the callback function for after and ready methods are insufficient.
The one for after method can be nullish and the one for ready method is optional.

See also an small example for Fastify & TS project if needed.
Thank you.

[Eomm]

Why 2 different approaches?

[climba03003]

error should be null by default. If it doesn't, better to fix it inside avvio.
https://github.com/fastify/avvio/blob/master/boot.js#L447-L448

[mcollina]

Thanks for opening a PR! Can you please add a unit test? We use tsd for typescript.

[nokazn]

@Eomm

Why 2 different approaches?

The both are depends on current implementation.
The parameter type of ready method seem to pass undefined to callback function as the first argument when no errors.👀

ex. https://github.com/nokazn/fastify-ts-minimum-sample

import Fastify from 'fastify';

const fastify = Fastify({
  logger: true,
});

fastify.register((server, options, done) => {
  // If you uncomment the line below, you can see error on `after` event.
  // done(new Error('something occurred'));
  done();
});

fastify.after((error) => {
  console.error({ error }, 'error has occurred on `after` event');
});
fastify.ready((error) => {
  console.error({ error }, 'error has occurred on `ready` event');
});

fastify.get('/', async (request, reply) => {
  reply.type('application/json').code(200);
  return { hello: 'world' };
});

fastify.listen({ port: 3000 }, (err, address) => {
  if (err) throw err;
  fastify.log.debug(`Server is now listening on ${address}`);
});

↓

(I also think the both should be nullish not optional as @climba03003 said)

[nokazn]

@mcollina

Thanks for opening a PR! Can you please add a unit test? We use tsd for typescript.

I've added some type tests for after & ready methods in 8d1e75a. Please check them and feel free to point out if something goes wrong. Thank you.

[mcollina]

lgtm

[Uzlopak]

LGTM