Allow defining error responses

[jsumners]

🚀 Feature Proposal

As discussed in #1888 (comment) we have several points in the framework where it is assumed a response will be content-type: application/json. However, people use Fastify for more than just that one content type. So it would be appropriate to allow them to define how these responses should be constructed and sent.

[mcollina]

I’m definitely +1 on this

[alemagio]

Hi, I would like to work on this.
I believe, following the comment mentioned in the description, that this is already happening on some parts of the framework, so I will take those as examples.

[mcollina]

Awesome @alemagio, we'll wait for your PR!

[alemagio]

After some investigation i found that the default content-type is used a lot of times.
Do you think it should be a config parameter?
For errors, for example, should the user be able to define a default content-type header?

[delvedor]

@alemagio yes, the user should be able to customize the response completely :)
The best-case scenario is that all these cases were we are returning a json will call the defaultErroHandler, so the user can decide what to do.
The problem (you can find many discussions about this) is that often we can't have access to the default error handler.

A good strategy could be to have an option, something like frameworkErrors, which by default returns the JSON we are building, otherwise, the user can override it and do what they prefer. Clearly, this method will not support encapsulation.

const fastify = require('fastify')({
  frameworkErrors (error, req, reply) {
    // custom handling
  }
})

[alemagio]

I created a draft PR to show my idea.
Hope it is useful.