Validate and throws a custom error when attempting to register invalid hook functions

[jhhom]

Handles this issue, a hook is invalid if it is not of type function because it needs to bind with the fastify instance.

This PR will add validation for the hook functions passed in the options argument of route registration functions.

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

[cesarvspr]

lgtm

[delvedor]

Thanks for the pr!
Can you use/create a custom error like we are doing for every other Fastify custom error? See here.
Thanks!

[delvedor]

It seems that we have this error already:

fastify/lib/errors.js

Lines 98 to 103 in acba25a

	FST_ERR_HOOK_INVALID_HANDLER: createError(
	'FST_ERR_HOOK_INVALID_HANDLER',
	'The hook callback must be a function',
	500,
	TypeError
	),

[mcollina]

lgtm

[jhhom]

@delvedor I think the error message I added (e.g "onSend hook should be a function, instead got undefined") can be more informative than the error message in the existing error "The hook callback must be a function", it also provides the following additional information which may helps in debugging:

• which hook is invalid
• the actual type passed

In my latest commit, I modified the error message of the existing error instead.
Just that I'm not sure if modifying the error message of the existing error might have any backwards-compatibility issues. I think its probably not an issue because error checking should be done using the error code.

[delvedor]

I see your point, then I'll suggest to update the existing error message :)

Just that I'm not sure if modifying the error message of the existing error might have any backwards-compatibility issues. I think its probably not an issue because error checking should be done using the error code.

This is a valid point, but we have error codes for a reason. Error messages are the human-friendly version of them which is subjected to change.

[jhhom]

I see your point, then I'll suggest to update the existing error message :)

the existing error message is updated in my last commit 👍

[mcollina]

lgtm

[mcollina]

@delvedor PTAL

[cesarvspr]

Nice

[delvedor]

LGTM

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