Add this types to decorator functions

[matthyk]

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

Just added the correct type for the this binding in the decorator functions. Additionally I added a note in the documentation that an arrow function in the decorate function would break the this binding.

[mcollina]

lgtm

[climba03003]

I think any of the type merge with any will become any.

[climba03003]

If it is FastifyInstance | any, I think it would become any at the end?

[matthyk]

I think you're right. But then the tests are actually useless, aren't they?

[RafaelGSS]

We can't do expectType<FastifyRequest>(this)?

[matthyk]

For me, the test then fails. I guess the problem is that the type of the decorator value is types as any.

  decorate(property: string | symbol, value: any, dependencies?: string[]): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;

[climba03003]

AFAIK, any overload on top of any the type will be merged to becomes any. So, either change the any to something generic without Function or this PR should be no actual gain.

[matthyk]

I have added generics to the 3 decorator functions. This can now be used to check whether the value to be decorated is a function, and if so, the correct this type is added.

const server = fastify()

server.decorate<(n: number) => void>('myDecorator', function(n: number): void {
  this // has FastifyInstance type now
})

[climba03003]

I like the idea of Generic types for solving this issue. Can you add an test-case for not specify the types? Generic suppose to works without explicit specification.

For example the below test should works.

server.decorate('test', function (x: string): void {
  expectType<FastifyInstance>(this)
})

[mcollina]

lgtm

[RafaelGSS]

LGTM. Amazing approach!

[climba03003]

Just one more thing. Add both test case for decorateRequest and decorateReply without Generic specification. Then it is all good.

Just to ensure three of them can use with or without explicit specification.

[climba03003]

LGTM. Thanks a lot.

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